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Geometry, kinematics, and a mechanical analysis of a strip of the Lewis allochthon from Peril Peak to Bison Mountain, Glacier National Park, Montana

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Geometry, kinematics, and a mechanical analysis of a strip of the Lewis allochthon from Peril Peak to Bison Mountain, Glacier National Park, Montana

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Content GEOMETRY, KINEMATICS, AND A MECHANICAL ANALYSIS OF A STRIP OF THE LEWIS ALLOCHTHON FROM PERIL PEAK TO BISON MOUNTAIN, GLACIER NATIONAL PARK, MONTANA by An Yin A D is s e r ta tio n Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In P a r tia l F u lf il lm e n t of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Geological Sciences) August 1988 UMI Number: DP28583 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Phishing UMI DP28583 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 UNIVERSITY OF SOUTHERN CAUFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CAUFORNIA 90089 This dissertation, written by An Yin under the direction of .. Dissertation Committee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillment of re quirements for the degree of DOCTOR OF PHILOSOPHY Dean of Graduate Studies ^ . May 23, 1988 D ate ...... DISSERTATION COMMITTEE Chairperson TO MY BELOVED GRAND UNCLE AND GRAND AUNT Mr. Tjong T j h i t Ton Mrs. Jong Wai W a AND UNCLE AND AUNT Mr. Phan Feong Tai Mrs. Tjhin Liong Piet ACKNOWLEDGEMENTS I am in debt to many people who helped me s ta rt and f in is h this project* First and foremost, I would lik e to thank Dr. Greg Davis, my d is s e rta tio n advisor, for his guidance, foreseeing the p ro je c t, and constructive c r itic is m on the d is s e r ta tio n . The s tru ctu ral in te r p r e t a tio n for the formation of the frontal zone and its r e la tio n with the present Lewis thrust is an outgrowth of Greg's original in te r p r e ta tio n and observations. I'd also like to thank Greg for his moral support, t r u s t , and jencouragement during the years of my graduate study at the i i University of Southern C a l if o r n i a . I could not fin ish this project without him. I am extremely grateful to Dr. jRachel Burks, Dr* Robert Osborne, Dr. Tom Henyey, and Dr* ! Vincent Lee for th e ir service in my d is s e rta tio n committee. Their comments and suggestions helped improve the d is s e r ta tio n , and provided many insights for this research. The f i e l d work was carried out while I was working for the U.S. Geological Survey. I would like to thank Project Chief James W. Whipple, for l o g is t ic a l assistance i i and for teaching me the Belt stratigraphy in G lacier Park. ( I would li k e to thank my friends and colleagues, Tom j Kelty, John (Jack) Fisher Dunn, Jay Jackson, and Michael Winn, George (George Lia) Steward for providing moral, i i i i te c h n ic a l, and imaginative support during my graduate ' study at the University of Southern C a lifo r n ia . I owe a ! special thank to Tom Kelty. His experience in f ie ld ! mapping and backcountry camping helped me greatly in the 1 f i e l d . Discussions with Tom Kelty contributed to many I ideas presented in this d is s e r ta tio n . I would lik e to thank Anthony Allen, Evie E in ste in, i Robert Hansen, and Lisa Kaplan for t h e i r able assistance | in the f i e l d . I am p a r t i c u l a r l y grateful to Anthony Allen. His cheerful s p i r i t and good sense of humor kept i me motivated a ll the time in the f i e l d . I could not have studied in the United States without jfinancial support from my grand uncle and grand aunt, jMr.Tjong T j h i t Ton and Mrs. Jong Wai Wa, and my uncle and iaunt Mr. Phan Feong Tain and Mrs. T j h i t Liong Piet. They ^paid t u i t i o n and liv in g expenses during my f i r s t year and ipart of the second at the University of Southern C a l if o r n i a . Their love accompanied me throughout the years I spent in graduate school. Their expectations . motivated me whenever I suffered f a i l u r e and f r u s t r a t i o n . I I would lik e to dedicate this d iss e rtatio n to them to express my love and appreciation for support during the most important stage of my l i f e . | I would li k e to thank Susan Roberts (Desper), Sue Orel 1 , LiLi Mezger, Kathi Beraton, Sandy Steacy, Mary iv j X Droser, John Faulkner, all in the Department of Geological Sciences at the University of Southern C a l ifo r n ia . The friendship they offered made my study at the U niv e rsity of Southern C a lifo rn ia an unforgettable experience. F i n a l l y , I would lik e to thank my parents for t h e ir moral support and encouragement. v TABLE OF CONTENTS Page DEDICATION....................... ii ACKNOWLEDGEMENTS....................................... iii List of Figures............................ ix List of.............Tables................. ,xyi List of Plates............................. xvii ABSTRACT................................................ xviii CHAPTER 1: INTRODUCTION............................... 1 General Statement.......... 1 Previous Work..................................... 2 Geography......................................... 9 Field Work....................................... 12 CHAPTER 2: STRATIGRAPHY............................... 13 General Statement................................ 13 Allochthonous Units: the Belt Supergroup............................ 13 Regional Correlations...................... 14 Stratigraphic Descriptions................. 15 Altyn Formation....................... 23 Appekunny Formation................... 28 Grinnell Formation.................... 39 Empire Formation...................... 42 Helena Formation...................... 43 Autochthonous Rocks.............................. 44 CHAPTER 3: GEOMETRY OF THE LEWIS THRUST SYSTEM.............................. 46 General Statement................................ 46 The Lewis Thrust Fault.......... 47 Scenic Point Structural Complex................. 54 Frontal Zone...................................... 62 Eastern Structural Belt.......................... 75 Elk Mountain Imbricate System........ 85 Brave Dog Fault........ 87 Mount Henry Imbricate System.................... 91 Rockwell Fault.................................... 97 Two Medicine Pass Fault........................... 106 Akamina Sync line................................... 107 Structures on Lone Walker Mountain............... 110 Minor Contractional Faults on Cloudcroft Peak.............................. 113 Lone Walker Fault........................... 118 CHAPTER 4: DE FORMATI0NAL HISTORY OF THE LEWIS THRUST SYSTEM................. 122 General Statement.................................. 122 Lewis Thrust Fault................................. 123 Frontal Zone....................................... 127 Eastern Structural Belt........................... 148 Scenic Point Structural Complex.................. 162 Brave Dog Fault.................................... 165 Mount Henry Imbricate System..................... 168 v i i Elk Mountain Imbricate System................... 178 Rockwell Fault.................................... 178 Two Medicine Pass Fault.......................... 182 Lone Walker Fault................................ 185 Deformational History of the Lewis Allochthon.................................. 186 Discussions on the Sequence of Thrust Fault Development................... 201 CHAPTER 5: A MECHANICAL ANALYSIS OF THE LEWIS THRUST SYSTEM................ 210 General Statement................................ 210 Mechanical Paradox of Overthrusts.......... 210 Controls on Thrust Fault Geometry.............. 221 Mechanical Model................................. 230 Formulations................ 230 Directions of Principal Stresses and Geometries of Potential Faults.......................... 242 Maximum Length of Over thrust Blocks........................ 254 Discussions............................... 268 Maximum Length of Overthrusts........ 268 Alternation of Low-angle Faults and Imbricate Thrusts....... 269 Extensional v.s. Contractional Faults............................... 277 CHAPTER 6: SUMMARY..................................... 279 REFERENCES.............................................. 284 LIST OF FIGURES ........ , ......_____ Page Figure 1-1. Location of the Lewis thrust sheet and the.study area ( a f t e r Ki ng, 196 9; Price, 1981; Harrison; 1981). LT, the Lewis th r u s t; FBF, F Ia th e a d /B la c k ta i1 normal f a u lt system; NKP, North Kootenay Pass; MP, Marias Pass....................................3 Figure 1-2. Location of the study area in Glacier Park. KS, K a lis p e ll ; WG, West Glacier; EG, East G lac ier; LM, Lake McDonald; SM, Saint Mary; LS, Lake Sherburne; LSK, Lake Saint Mary....................................10 Figure 2-1. S im plified stratigraphy in the study area....................................................................................................19 |Figure 2-2. Generalized stratigraphy of the Altyn i Formation in the study a r e a ..................................................................25 'Figure 2-3. Generalized stratigraphy of the Appekunny i Formation in the study a rea ..................................................................31 Figure 3-1. a. Sim plified geologic map of the Lewis allochthon in the study area. b. Geologic cross-section through lin e AA'A1 'in a. No v e rtic a l exaggeration............................................................................. . . 4 8 Figure 3-2. View to the north-northwest along the trace of the Lewis thrust f a u l t , southeastern side of Glacier Park. The trace of the fa u lt (LT) follows the break of slope and separates i upper-plate Proterozoic Belt rocks ( l e f t ) j from low er-plate Cretaceous rocks ( r i g h t ) . In the foreground, a thin orange layer of Altyn carbonate rocks lies d i r e c t ly above the Lewis thrust and brush-covered slopes of lower-plate Cretaceous s tr a t a . Appekunny rocks o verlie the Altyn and exhibit considerable complexity in the v i c i n i t y of the prominent valley bordered by green vegetated pattern (c f. F i g . 3 - 6 ) ................................. 51 Figure 3-3. Scenic Point Structural Complex as viewed to the south from the north base of Scenic Point. Scenic Point f a u l t (SPF) lies beneath a whitish gray quartz a ren ite bed and separates l i t t l e deformed strata in its upper plate from highly deformed rock packages in its lower p la te . MBC is marker bed C of a ix quartz--a re-n-i-t e- bed. Ya, the A1 tyji. Fo rmat i on ; ____ Yap, the Appekunny Formation. The Lewis th ru s t is covered by talus deposits at the base of the c l i f f . Figure 3-4 shows d e tailed structures on the north c l i f f of Scenic Poi n t ..............................................................................................................................55 Figure 3-4. Sketch of Scenic Point s tru c tu ra l complex on the north side of Scenic Point. View from no rthe a s t...................................................................................................................58 Figure 3-5. A ttitudes of fa u lts in the Scenic Point Structural Complex. o shows a ttitude s of s tria e measured on f a u l t s . Dashed lines show a ttitude s of extensional f a u l t s , solid lines show a ttitu d e s of contracti onal f a u lt s . n = 8.......................63 Figure 3-6. a. View to the northwest of structures in the frontal zone on the east side of Bison Mountain, b. A geologic cross-section based on Figure 3-6a. See text for descriptions of structures in the frontal zone. Kinematic in te r p r e ta tio n s of these frontal zone structures are presented in Chapter 4..66 Figure 3-7. A small-scale drag fold developed in the foot wall of a west-dipping high-angle extensional f a u lt on the east c l i f f of Bison Mountain. Picture is viewed from southeast to northwest. Clark Davis (gray s h i r t ) and the w rite r (red s h i r t ) fo r s c a l e ................................................................................................ 71 Figure 3-8. A ttitudes of fa u lts in the frontal zone measured on the east side of Bison Mountain and Head Mountain, data including both mappable fa u lts (o ffs e t > 40 f t or 13 m) and unmappable f a u l t s . Solid lines represent contractional f a u l t s , dashed lines represent extensional f a u l t s . n = 8...........................................................................73 Figure 3-9. Rose diagram for s trik es of contractional faults in the eastern s tru c tu ra l b e lt . n = 3 2 . ...................76 Figure 3-10. Stereographic projectio n of contractional and extensional f a u lt s the eastern s tru ctu ral b e lt. Ncon=32, Nex=5. Neon and Nex are sample sizes of contractional fa u lts and extensional f a u l t s . The data set for the contractional faults is the same as that plotted in Figure 3 - 9 ...............................78 Figure 3-11. A ttitu d e s of sm all-scale fold hinges in x the east e rn _ s t ru etu r al^be11 . n = 5 9. Contouring done b / hand. For contouring method see Davis (p .8 2 - 8 6, 1 9 8 5 ) . . . . . . . . . . * . .................. Figure 3-12. An east-dipping contractional f a u l t in j the eastern s tru ctu ral belt is truncated by the I Brave Dog f a u l t (BDF). The picture is viewed j from northwest to southeast on the northwest j side of Scenic Point. The Brave Dog f a u l t is p a r a lle l to bedding of its upper plate and truncates a northeast-dipping minor f a u l t . The drag fold in the hanging wall of the minor f a u l t indicates that th is fa u lt is a reve rse f aul t ............................................................................................ Figure 3-13. a. Truncational re lationsh ip between the thrust f a u lt s of Mount Henry imbricate ! system and the Brave Dog f a u l t . This picture is viewed from north on the ! northern side of Squaw Mountain. About ; 200 m across the l e f t side to the right side of the p ic tu r e . b. Sketch of major structures shown in Figure 13a............................................................89 Figure 3-14. a. Mount Henry imbricate system at Mount Henry. b. Sketch of structures shown in Figure 3 -14a. Yap, the Appekunny Formation; Ygl, the ! Grinnell Formation.............................................................................................92 > iFigure 3-15. A ttitudes of mesoscopic c o n tra c tio n a 1 fa u lts in the Mount Henry imbricate system. All the ! measurements shown here were taken within member i 4 of the Appekunny Fm. and Grinnell Fm.........................................94 I [Figure 3-16. The Rockwell f a u l t (RF) cuts down • section to the east on the southern c l i f f of ! Cloudcroft Peaks. Viewed from southeast to northwest. Note that two minor c o n tra c tio n a 1 faults on the eastern side of the picture f l a t t e n downward into the Two Medicine Pass f a u l t (TMPF), but they steepen upward and appear to be truncated by the Rockwell f a u l t . On westernmost and central part of the pictu re , southwest-dipping contractional fa u lts o ffs e t the Two Medicine Pass f a u l t which are terminated above by the Rockwell f a u l t .......................................99 Figure 3-17. The Rockwell f a u lt (RF) at Mount Rockwell. Viewed from southeast to northwest. Note that the f a u l t ramps up to the east on the 81 ; 83 xi sou thwestern side of Mt. Rockwe 11. The Rockwell f a u lt is offs e t by the Lone Walker f a u l t (LWF) f o r about 150 m........... 101 (Figure 3-18. Gouge zone along the Rockwell f a u l t j exposed on the southern c l i f f of Caper Peak... 103 ;Figure 3-19. General geology of the Flathead area (from Dahls t rom, 1970) 108 .Figure 3-20. View to the northwest of highly folded G rinnell Formation at Lone Walker Mountain, i Robert Hansen provides scale. Beds are mostly overturned. Folds verge northeastward........................... Ill Figure 3-21. Highly folded Grinnell quartz a ren ite and a r g i l l i t e appear to be truncated by the planar Rockwell f a u l t above. The Rockwell f a u l t surface (RF) is concealed by talus and vegetation. Viewed from southwest to northeast at Lone Walker Mountain....................................................................................................................114 Figure 3-22. A minor reverse f a u l t cuts across the Two Medicine Pass f a u l t (TMPF) and terminates by | the Rockwell f a u l t (RF) on the southeastern ! side of Cloudcroft Peaks. Note that a white | quartz a ren ite bed is truncated from below by | the Rockwell f a u l t . View is to the northwest......................116 (Figure 3-23. View of the Lone Walker f a u l t (LWF) to i the eastsoutheast on the northwestern side of , Mount Rockwell. Note that the Lone Walker f a u l t offsets the contact between the Grinnell Formation (Ygl) and Empire Formation (Yem). It also offsets the Rockwell f a u lt (RF). The Lone Walker f a u l t dips steeply (ca 75 degrees) to the southwest...............................................................119 Figure 4-1. Stereographic p rojec tio n of s t r i a t io n s ; (n=17) measured on the Lewis th ru s t surface and on shear surfaces above and below the f a u l t along the eastern side of Bison Mountain and northwestern side of Scenic Point (see de ta ile d descriptions in the | t e x t ) . Note that the trend of s t r i a e measured from the upper plate is about 20-30 degrees more j northerly than those measured from lower j p la te . The trend of s tria e measured from the ! upper p late is more-or-less perpendicular to 1 the average trend of fold hinges measured from the frontal zone (Cf. Fig. 4 - 2 ) .........................................125 j xi i Figure 4-2. Stereographic p r o je c tio n of drag fold hinges ( n =17) measured from the frontal zone.....................128 Figure 4-3. Pa 1 inspastic re constructions of the frontal zone. Figure 4 - 3a to 4 - 3j shows possible deformational episodes responsible for the development of structures in the frontal zone. MBA, MBB, and MBC are marker beds in the Appekunny F o r m a t i o n . . . . .................................................................137 Figure 4-4. Sketch of the re la tio n s h ip between some sm all-scale folds and mesoscopic folds. Measurements taken on the western slope of Scenic Point. Note that sm all-s c ale folds verge to the hinge of the a n t i c l i n e . Numbers shown on the sketch represent approximate positions of the measured small folds (Cf. Fig. 4 - 4 ) ....................................................................................150 Figure 4-5. Stereographic p ro je c tio n of hinges and axial planes of the small folds shown in Figure 4 -3. Note that hinges and the axial planes were measured independently in the f i e l d . This has introduced errors and causes hinges to depart s l i g h t l y from the corresponding a x ia l-p la n e great c ir c le s . Solid lines represent measurements from east-dipping fold limbs, the dashed lines represent measurements from west-dipping fold limbs................................... 152 iFigure 4-6. Lineations of mineral fibers along bedding j of the Appekunny Formation at Scenic Point. Steps facing to the u p p e r-le ft corner on the ! bedding surface indicates th a t the sense of shear is top to the right. Such steps are present across the fold hinge. This suggests that fold is la t e and has warped a striated bedding s u rfa c e .................................................................................................155 Figure 4-7. Stereographic p ro jec tio n of s t r ia t io n s along bedding planes in the Appekunny Formation in the eastern s tru c tu ra l belt (n = 1 8 ) ..................................................157 Figure 4-8. Stereographic p ro je c tio n of "drag" fold hinges along f a u lt s in the eastern structural belt ( n = 30 ) .........................................................................................................................160 Figure 4-9. Stereographic p ro je c tio n of hinges of mesoscopic drag folds along the Brave Dog f a u l t collected at the west side of Scenic x i i i I Poi nt (n = 8) ... . - ............... . . . . ^ ..............1 66 > i “ ' | ; Figure 4-10. Stereographic p rojectio n of hinges of ! "drag" folds measured from the Mount Henry imbricate system (n=23 ) . . . .. . . . . .................................................. .170 Figure 4-11. A kinematic model for a forward development of the Mount Henry imbricate system..............173 i Figure 4-12. A kinematic model for a hindward development of the Mount Henry imbricate system...........175 Figure 4-13. Stereographic projectio n of fold hinges | and s tr ia e along the Rockwell f a u l t (see te x t fo r sample l o c a l i t i e s ) .........................................................................................179 I Figure 4-14. Stereographic projection of drag fold ' j hinges along the Two Medicine Pass f a u l t , n = 22...................183 i I Figure 4-15. Possible kinematic processes responsible j for the formation of the Lewis thrust system, j Explanations see t e x t ...................................................................................189 Figure 4-16. a. Geometry of s t r a i l i n g edge imbricate system or duplex f a u l t zone as defined by Dahlstrom (1970). b. A kinematic model for the development of a duplex f a u l t zone (Dahlstrom, 19 70 )........................................................................................................................... 204 Figure 4-17. A kinematic model fo r the development of a duplex system (Boyer and E l l i o t t , 1982 ) .................... 207 | Figure 5-1. Rectangular model for calc u la tin g the maximum length of the overthrust blocks (Hubbert and Rubey, 1 9 5 9 )......................................................................................... ..214 Figure 5-2. Geologic cross sections and palinspastic reconstruction of the Belt basin ( F r i t t s and Klipping, 1987a,b). a. Regional geologic map of the Belt Basin and locations of geologic cross sections in Figure 5 -2b. b. Geologic cross sections across I the Belt basin. Locations of the cross-sections j see 5-2b. c. Tectonic model for the development of I the Lewis thrust sheet on a regional s c ale ............................216 \ Figure 5-3. Two possible kinematic processes for the 1 formation of l i s t r i c thrusts in the imbricate j systems in the Lewis allochthon. a. Imbricate j thrusts began to develop before the Lewis thrust ! f a u l t reach the surface. b. Imbricate thrusts xiv j began to develop a f t e r the Lewis thrust reach the s u rfa ce...........................................................................................................224 Figure 5-4. Mechanical models corresponding to the two kinematic models proposed in Figure 5-3. a. Mode I I of a cracked body, s lid in g mode (Kanninen and Popelar, 1985). b. A wedge-shaped body pushing from the rear of the wedge.................................227 Figure 5-5. Geometry of a t r i a n g u l a r wedge and the framework of reference used in the c a lc u l a t i o n ..............231 Figure 5-6. Sign convention for solving e l a s t i c p robl ems...................................................................................................................2 33 Figure 5-7. D ire ction of the prin cipal stresses, is positive in the d ire c tio n shown in the f i gure.......................................................................................... 243 Figure 5-8. T r a je c to r ie s of prin cip a l stresses and the potential f a u l t pattern fo r =0.6, =0.5, = 0.7, =tan30 , =7 , = 1 ...............................................................................................................247 Figure 5-9. T r a je c to r ie s of p rin cip a l stresses and the potential f a u l t pattern for =0.6, =0.5, = 0.7, = t a n 3 0 , =10, =5............................................250 Figure 5-10. T r a je c to r ie s of p rin cip a l stresses and the potential f a u l t pattern fo r =0.6, =0.5, = 0.7, =ta n30 , =1 0, =5 ..................................252 Figure 5-11. Maximum length of thrust wedge is defined by L. At the point (L, Ltan ), the state of stress reaches the c r i t i c a l value of the Coulomb frac ture c r i t e r i o n ......................................................................................... 256 Figure 5-12. Plot of the maximum length of the overthrust wedge L as a function of the dip angle and cohesive strength. See te x t for exp la n a tio n s ..........................................260 Figure 5-13. Plot of the maximum length of the overthrust wedge L as a function of the dip angle and the p o r e - f l u i d pressure r a t i o along the thrus t f a u l t . See text for e x p la n a tio n s ................................. 264 Figure 5-14. Plot of the maximum length of the overthrust wedge L as a function of the surface slope and cohesive strength. See text for explanations.......................................................................................................266 XV LIST OF TABLES ............ Page Table la. St r a tig r a p h ic c o rre la tio n s of the Belt Supergroup in the northwest United States ( a f t e r j Harrison, 197 2 ) .................................................................................................16 i ! Table lb. St r a tig r a p h ic c o rre la tio n s of the Belt | Supergroup in G lacier National Park ( a f t e r Whipple and others, 1 984)........................................................................ 16 Table 2. C l a s s i f i c a t i o n of thickness of s t r a t i f i c a t i o n I units (from Ingram, 1 9 5 3 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 ! Table 3. Mineral compositions of arenite beds in the Appekunny F o r m a t i o n . . . .................................................................................36 LIST OF PLATES Plate I. Geologic map of a s t r i p of the Lewis allochthon i from Head Mountain to Peril Peak, Glacier j National Park, Montana (in p o c k e lt). Ip l a t e I I . A geologic cross section of the Lewis | allochthon from Head Mountain to Peril Peak (in pocket). xv i i r " ~ ABSTRACT Geologic mapping of an E-W s trip of the Lewis thrust Isystem from Bison Mountain to Peril Peak in southern G la c ie r National Park, Montana, has shown that its a rch ite ctu re is extremely complex. It consists of i symmetric and asymmetric concentric fo ld s , high-angle and low-angle c o ntra c tiona l and extensional f a u l t s , zones of complex s tru c tu re s , and imbricate thrust systems. 1 M ultiply deformed structures of the fro n ta l zone lalong the eastern edge of the study area are truncated i ifrom below by the Lewis thrust f a u l t , suggesting a complex | deformational h is to ry of the Lewis allochthon before the : i jformation of the present Lewis thrust f a u l t . Two major lim'bricate thrust systems, the Elk Mountain and the Mount Henry, and two major low-angle extensional f a u l t s , the jBrave Dog and the Rockwell, were developed during iemplacement of the Lewis allochthon along the present Lewis th ru s t. The two systems developed in a forward progression from west (Elk Mountain) to east (Mount Henry). However, temporally interspersed low-angle extensional f a u l t s (Brave Dog and Rockwell f a u l t s ) I i developed hindward, with the younger of the two (Rockwell) j developed at a higher l e v e l. The compressional imbricate systems and the two low-angle extensional faults , xviii alternated in the times of t h e i r development: (1) Elk Mountain imbricate system; (2) Brave Dog f a u l t ; (3) Mount Henry imbricate system; and (4) Rockwell f a u l t . This i . a lt e r n a t io n of extensional and compressional f a u l t i n g w ith in the Lewis allochthon can be best explained by temporal in te ra c tio n s between the evolving geometry of ith ru s t wedges and internal stress d is tr ib u tio n s as proposed by Dahlen (1984), P la tt (1986), and am plified in th is study. | A simple e la s t ic wedge model was developed in this i Istudy to in v e s tig a te possible mechanical controls on the i I geometry and location of imbricate fa u lts within the Lewis i 'allochthon. The model shows that the maximum length of a i J !wedge-shaped thrust sheet is not only controlled by the j istrength of the thrust sheet and the c o e ffic ie n t of basal i ( f r i c t i o n , but also by the basal dip angle and surface i jslope of the thrust wedge. The geometries of l i s t r i c th ru s t faults and imbricate systems can be also explained i iby the e la s tic wedge model. CHAPTER 1: INTRODUCTION General Statement This study investigates the s tructural geology of a portion of the Lewis thrust system in southern Glacier National Park, Montana (Fig. 1 -1 ). The study area is an E-W s trip across the Lewis allochthon from Peril Peak to Bison Mountain (Fig. 1-2). It covers an area of approximately 200 square kilometers. The purposes of th is research are th r e e -f o ld : (1) to examine the geometric framework of the Lewis thrust system in the study area, (2) to reconstruct the deformational history of the system, and (3) to explore possible mechanical controls on i t s development. Structural analyses of the Lewis thrust system presented in th is study w ill be in the follow ing order: (1) geometric, (2) kinematic, and (3) mechanical. In order to in v e s tig a t e the geometry and the kinematics of the system, a s tra tig r a p h ic framework is necessary. Chapter 2, which describes the s tra tig ra p h y in the study area and presents geometrical lines and planes of reference, serves th is purpose. Chapter 3 defines the major s tru c tu ra l elements in the study area. Chapter 4 investigates the kinematics of the Lewis thrus t system based on sm all-scale structures formed during its 1 development, cross-cutting re la tio n s h ip s between major s tru c tu ra l components, and geometric reconstructions . Chapter 5 discusses possible mechanical controls for the evolution of the Lewis thrust system. A simple mechanical model is developed in this chapter in order to explore possible dynamic processes associated with the development of the thrust system. Chapter 6 summarizes the di s se r t a t i o n . Previous Work The Lewis thrust f a u lt is a classical thrus t f a u l t in the C o rd ille ra n foreland fold and thrust b e lt . The f a u l t was f i r s t recognized and named by Bailey W i l l i s (1902) of the U.S. Geological Survey along the east side of Glacier National Park. Since then, the f a u l t has been mapped from Steamboat Mountain of west-central Montana to Kidd Mountain in southern Alberta ( e . g . , Campbell, 1914; B i l l i n g s , 1938; Douglas, 1952; Price, 1958, 1959, 1965; Ross 1959; Dahlstrom and others, 1962; C hilde rs , 1963; Mudge and Ea rha rt, 1980). The t o t a l length of the thrus t f a u l t is about 450 km (Fig. 1 -1 ). The trace of the Lewis thrus t can be divided into three segments (Fig. 1-1; Mudge and Earhart, 1980). The northern segment extends from Kidd Mountain to North Kootenay Pass. The central segment lies between North Figure 1-1, Location of the Lewis thrust sheet and the study area ( a f t e r King, 196 9; Price, 1981; Harrison; 1981), LT, the Lewis th r u s t; FBF, F la th e a d /B la c k ta il normal f a u l t system; NKP, North Kootenay Pass; MP, Marias Pass. 3 100 km ewis Salient ^ ^ n a d a ____ r \ l U S A Study Area Lewis thrust sheet Upper Jurassic-Paleocene Granitoid intrusions Thrust Fault Normal Fault Kootenay Pass and Marias Pass, and formed a major east- directed s a lie n t which w ill be ca lle d the Lewis s a lie n t in th is paper. Within most of the s a l i e n t , the Lewis thrust f a u lt juxtaposes the middle Proterozoic B e lt-P u r c e ll Supergroup atop upper Cretaceous molasse (nonmarine, synorogenic c l a s t i c ) deposits (Price, 196 5; Mudge and Earhart, 1980). The study area to be discussed in this paper lie s near the south edge of the s a lie n t in southern G lac ier National Park. The southern segment of the Lewis thrust is from Marias Pass to Steamboat Mountain. The in te rn a l geometry of the Lewis thrust sheet has been mapped and studied by numerous geologists. In his pioneering work, W i lli s (1902) noted that minor thrust fa u lt s dipping about 30 degrees in Yellow Mountain and Chief Mountain along the northeastern edge of Glacier National Park, are bounded at the top by a major bedding- p a r a lle l f a u l t and at the bottom by the Lewis th ru s t f a u l t ( W i l l i s , 1902, Figs. 5 and 6). Immediately north of the in te rn a tio n a l boundary in the Waterton area of Canada, 16 km northwest of Chief Mountain, the Lewis thrust sheet was mapped by Douglas (1952). Douglas recognized that a suite of more steeply dipping minor thrust fa u lts are bounded above by the Mount Crandell th ru s t and below by the Lewis th ru s t, a s tru c tu ra l association s im ila r to th a t observed by W i l l i s (1902) at Yellow Mountain and Chief Mountain in 5 G lac ier Park, This s tru ctu ral association, described by W i l l i s and Douglas, was l a t e r cited as examples of duplex f a u l t zones (Dahlstrom, 1 970) or duplex systems (Boyer and E l l i o t t , 1982), The formation of duplex structures is widely in terp re te d as an important mechanism for shortening w ithin a thrust system ( c f . , Boyer and E l l i o t t , 1 982 ) , The Lewis thrust f a u l t and its associated structures at the north edge of the Lewis s a lie n t near North Kootenay Pass ( F i g , 1-1) were mapped by Price (1959, 1962, 1965), and Fermor and Price (1976, 1987), Their mapping shows th a t the Lewis th ru s t sheet is highly deformed w ithin a zone approximately 500-1500 feet (156-468 m) thick immediately above the Lewis t h r u s t. This zone of complex structures consists of imbricate th r u s ts , low-angle thrus t f a u l t s , duplex s tru c tu re s , and low-angle f a u lt s along which younger s tr a t a o v e r lie older s tra ta with the omission of the intervening s tr a tig r a p h ic un its . Above th is zone of complex structures is the main mass of the Lewis thrust sheet, several kilometers t h i c k , is characterized by broad open folds in r e l a t i v e l y l i t t l e de fo rmed rocks. The southern segment of the Lewis thrust f a u l t and the structures in its upper and lower plates have been mapped by Mudge and Earhart from Marias Pass to Sun River area (Mudge and Earhart, 1980, 1983), Their mapping shows that the Lewis th ru s t sheet is broadly folded and l i t t l e f a u lte d . In c ontra s t, structures in the lower plate of the Lewis thrus t f a u l t are very complicated, consisting of closely-spaced imbricate thrust f a u lt s and folds ( l o c a l l y ove rt u rned ) . The Lewis th ru s t sheet is o ffs e t by the FI athead- B lacktail normal f a u l t zone in North Kootenay Pass (P ric e , 1965) and Marias Pass areas (C h ild e rs , 1963; K e lty , 1986), Surface mapping (Constenius, 1982) and seismic r e fle c tio n studies (Bally et a ! . , 196 6) suggest that the Flathead- B la c kta il normal f a u l t zone consists of a family of l i s t r i c normal fa u lt s that probably merge at depth with the Lewis th r u s t. Displacement along the Lewis thrust f a u l t varies along its length from zero at both ends (Dahlstrom et a l . , 1962; Mudge and Earhart, 1980) to at least 65 km near the In tern a tio n a l Boundary (P ric e , 196 5). The minimum displacement of the Lewis thrust f a u l t along the southern edge of G lacier Park is 15 miles (24 km; Campbell, 1914). The age of the Lewis thrus t f a u lt is generally believed to be Late Cretaceous to ea rly T e r t ia r y (Mudge and Earhart, 1980). Bentonites within Cretaceous rocks d i r e c t l y beneath the Lewis thrust f a u l t y ie ld K-Ar ages between 56 to 70 m.y. B.P. These ages are in te rp re te d by 7 Hoffman and others (1976) to be ra diom e tric a 1ly reset by heating due to overriding of the Lewis p la te . They in d ic a te that thrusting occurred during the Late Cretaceous to e a rly Eocene. The upper age l i m i t of th ru s tin g is obtained by the deposition of the Kishenehn Formation (la te Paleocene) in the Kishenehn basin (Constenius, 1982). The Kishenehn basin formed on the down-dropped side of the FI athead-B1acktai 1 normal f a u l t system along the west side of Glacier Park. As the F Ia th e a d - B la c k t a i1 normal f a u l t system offsets structures in the Lewis thrust sheet, the B la c kta il f a u l t postdates the Lewis th ru s t f a u l t . Thus, the age of the Kishenehn basin presumably provides an upper l i m i t for the movement along the Lewis th ru s t. Despite zones of complex structures observed near the Lewis thrust f a u l t surface, structures within the Lewis th ru s t plate were t r a d i t i o n a l l y considered to be simple. The plate within Glacier Park is characterized by a broad syncline (Ross, 1959; Mudge, 1977; Gordy et a l . , 1 9 7 7 , 1982). In 197 9, the U.S. Geological Survey began a comprehensive study to update the geology of Glacier National Park. As an important part of this p r o je c t, the Lewis allochthon was system atically mapped in the eastern and southern portions of the park by G.A. Davis and his students at the U niversity of Southern C a l if o r n i a (Davis, 8 unpublished mapping Davis and Jardine, 1984; Jardine, 1985; Kelty, 1986; Hudec, 1986; Winn, unpublished mapping; and this study). Preliminary results of th i s geologic mapping show that structures within the e n t i r e Lewis allochthon are complex and that m u ltip le episodes of deformation occurred during the development of the thrust system (Yin et al . , 1 986), Geography Glacier Park lie s within the Lewis Range and the Livingston Range immediately south of the U.S-Canada in te r n a tio n a l boundary in northwestern Montana. The study area is a E-W s t r i p across a southern portion of the park from Peril Peak to Bison Mountain (Fig. 1-2, Plate 1), covering an area of about 200 square kilometers. Cirques, U-shaped v a lle y s , sharp ridges, and horns of glacial o rigin are dominant geomorphological features in the area. The floors and lower slopes of most valleys and cirques are covered by morainal deposits and ta lu s . The r e l i e f in the study area is about 1,600 m (2,560 f e e t ) . Snow patches are lo c a l ly preserved above 1,062 m (7000 fe e t) throughout most of the year except July and August. G lacier Park is accessible by commercial a irpla n es , t r a i n s , and highways. The eastern end of the study area can be reached by the Two Medicine Lakes road within the Figure 1-2, Location of the study area in G lac ier Park KS, K a l is p e ll; WG, West Glacier; EG, East G lac ier; LM , Lake McDonald; SM, Saint Mary LS, Lake Sherburne; LSK, Lake Saint Mary. Location Map ol Glacier National Park and Map Area Canada a it a I K \ ] IAJI U .S .A . Canada V 1 - M T | | G la c ie r N a t io n a l P a rk LS S M LSM Study Area L H VC KS y N EC ! 3 15 3 3 km Reservation east of the park boundary. Many t r a i l s are maintained in the park during summer season by professional t r a i l crews. F ield Wo rk Field work was carried out during 12 weeks in the summers of 1984 and 1985. Geologic mapping was conducted d i r e c t l y on 7.5' U.S.G.S. Squaw Mountain, Mount Rockwell, and Mount St. Nicholas topographic maps ( 1 : 2 4 , 0 0 0 ) . Mapping was f a c i l i t a t e d by inspection of v e r tic a l aerial photographs and oblique color photographs provided by h e lic o p te r a e ria l surveys. The eastern part of the study area was mapped mainly by one-day hikes. The western part of the study area was mapped during three- to six-day overnight t r i p s with h e lic o p te r and horse packing support. 12 CHAPTER 2: STRATIGRAPHY General Statement The s tra tig r a p h y in the study area can be divided into units that are allochthonous and autochthonous units with respect to the Lewis th ru s t f a u l t , although a ll pre middle T e r t ia r y units in the map area are allochthonous with respect to the North Arne rican craton. The allochthonous units are part of the Precambrian Belt Supergroup. The autochthonous units are Cretaceous sedimentary rocks (Fig. 2 - 1 ). Because exposures of the Cretaceous rocks are very lim ited in the study area, no attempt was made to d i f f e r e n t i a t e them into d i f f e r e n t s tra t i g r a p h ic u n its . Allochthonous Units: the Belt Supergroup The Belt Supergroup in the United States and it s Canadian e q u iv a le n t, the Purcell Supergroup, are thick successions of Middle Proterozo ic sedimentary rocks that are exposed over wide areas of western Montana, eastern Idaho, southwestern A lberta, and southeastern B ritis h Columbia (P r ic e , 196 4; Harrison, 197 2). The thickness and d e ta ile d lith o lo g y of the Belt rocks vary from place to place in the northwestern United States (H arrison , 197 2; Whipple et a l . , 1984; Winston, 198 6). Such v ariations ____________________________________________________________________________ 13 have been a t t r i b u t e d to (1) change of depositional loci with respect to the edge of the Belt basin (H arrison, 1972), (2) tectonism during deposition of the Belt rocks (Winston, 198 6), and (3) la te Mesozoic to e a rly Cenozoic th ru s tin g (Whipple and others, 1984). The estimated maximum thickness of the Belt Supergroup in the United States is about 23 km (75,000 f e e t ; Harrison, 1972). The to ta l thickness of the Belt Supergroup in G lacier National Park is , however, much less, ranging from 4,700 m (14,000 fe e t; C h ilders, 1963) to 5,700 m (17,000 fe e t; Whipple et a l . , 1984). This study provides f i e l d evidence on the role of la te Cretaceous to early T e r tia r y deformation in complicating our understanding of the s tra tig ra p h y of the Belt Supergroup. This section is divided into two parts: (1) regional c o rre la tio n s of the Belt Supergroup, (2) descriptions of the Belt s tra tig ra p h y in the study area. Regional C o rrelations Belt s t r a t a were f i r s t s y ste m a tic ally studied by Walcott (1899) in the L i t t l e Belt and Big Belt Mountains of central Montana. Since then, numerous workers have studied the stra tigrap hy of the Belt Supergroup in the northwestern United States and it s northern e q u iv a le n t, the Purcell Supergroup, in Canada. Nomenclature of the 14 B e lt Supergroup v a rie s a g re a t deal from p la ce to place. I Harrison (1972) presented a general s t r a t i graphic : c o r r e la tio n chart for the Belt Supergroup in the western ; lUnited States (Table l a ) . The names of Belt s tra t a in G la c ie r Park were proposed by W i l l i s (1902), Ross (1959), !and Childers (1963). Mudge (1977) t r i e d to apply the !names of Belt s t r a t i g r a p h ic units defined by Walcott (1899), e.g. Greyson, Spokane, Empire, and Helena, to the formations named by W i l l i s (1902) in the park, although th is e f f o r t has met with l i t t l e success. R ecently, the Belt s tra ta in G lacier Park were studied by Whipple et a l . , I i i ( 1984). Table lb is a s t r a t i g r a p h ic c o r r e la tio n chart of Belt Supergroup in eastern and western areas of Glacier Park taken from Whipple et a l . (1984). j i i S tr a tig r a p h ic Descriptions Rocks of the Belt Supergroup are ex ce p tio n a lly i 'exposed in Glacier National Park. The s tra tig ra p h y of the Belt s tra ta in G la c ie r Park has been studied by W i lli s | (1902), Fenton and Fenton (1931), Ross (1959), Childers j ( 1963), and Whipple et al . ( 1984). The nomenclatures of the Belt s tr a ta used in th is study follow that of Whipple 1 et a l . (1984). Five formations of the Belt Supergroup (A ltyn, Appekunny, G r in n e ll, Empire, and Helena) (Fig. 2- ! 1) are exposed in the study area. The d e scriptio n of the j i 15 ' Table la. Strati graphic correlations of the Belt Supergroup in the northwest United States (after Harrison, 1972). Table lb. Stratigraphic correlations of the Belt Supergroup in Glacier National Park (after Whipple and others, 1984). 16 a. GLACIER NATIONAL PARK West Top not exposed McNamara formation Bonner. Quartzite Top not exposed Mount Shieias Formation Lava Shepard Formation ShepardFotmatior Purcell Lava Purcell Lava c o U _ Helena Formation Helena Formation Empire Formation Empire Formation G r i nne II Grinne II Formation Formation Upper part Al+yn Formation Lower part e i posec b. I WASHINGTON, IDAHO, AND ADJACENT PARTS OF MONTANA VICINITY OF MISSOULA, ALBERTON, AND ST. REGIS, MONTANA 3 GLACIER NA TIONAL PARK AND THE WHITEFISH RANGE, MONTANA SOUTH FROM GLACIER NA TIONAL PARK TO HELENA AND BUTTE, MONTANA CAMBRIAN WINDERMERE SYSTEM OF —v CAN ADA M o n k Fm UNCONFORMITY- ^Huckleberry Fm. Flathead Ouartzite or its equlvolent U N C O N F O R M IT Y -------- 0 . 3 O a : o (t Ui o . D C O o. 3 O o : C D < _J 3 O CO CO Libby Fm -U N C O N F O R M IT Y - Pilcher Otz Garnet Range Fm McNamara Fm x ; Bonner Qtz Miller Peak Fm Garnet Range Fm McNamara Fm Bonner Otz Mount Shields Fm Shepard Fm Purcell Lovo Snowslip Fm Garnet Range Fm McNamara Fm Bonner Otz Mount Shields Fm_____ Shepard Fm Snowslip Fm (Middle Belt carbonote) Wallace Fm Wallace Fm Helena Dolo Helena Dolo St. Regis Fm liJ c o RAVALLI GROUP -\Spokane St. Regis v Fm Fm sxxEmpire Fm* Empire Fm Revett Fm Revett Fm ^> Spokane Fm Spokane Fm Burke Fm Burke Fm Appekunny Fm Greyson Sh (Pre-Ravalli or lower Belt) Altyn Ls Newland Ls Prichord Fm Prichard Fm Waterton Fm of Conoda Chamber- r 0 lain Sh O PRE-BELT CRYSTALLINE ROCKS Neihart Otz Base not — -------— expose d — \UNCONFORM!TY- 18, Figure 2-1. Simplified stratigraphy in the study area 1 9 I 1 STRATI CRAP HI IN THE STUDY AREA Rockwell faulc Crinnell Rn Brave Dog fault Appekunny Ra Altyn Ra Scenic Point Fault Lewis Thrust __ _ Cretaceous Rocks 20 I Table 2. Classification of thickness of stratification units (from Ingram, 1953). 21 too PAYNE 19 4 2 CLOUO, 9 A R N E S .S B R IO G E S 1 9 4 3 MCKELVEY 1 9 4 7 WENGERO l « 4 1 MCKEE ft WEIR 1 9 3 3 INGRAM 19 3 4 Very Thick Very Thick Slrolo 1 Massive Thick Medium Thick Massive 1 Beds 1 Thick Thin m n 9 CD Thick Medium Medium Flaggy Thin Thin Thin Very Thin Very Thin Very Thin o 0 c 1 a Shaly Thin Fissile Shaly 1 Lominoe 1 Lam. 1 Lamina* ] Thick Lam. Thin Lam. Fissile Thin Lorn. 22 Belt s tr a t a w i l l be in ascending order, i . e . , from older to younger. C l a s s i f i c a t i o n of thickness of s t r a t i f i e d units follows the d e f i n i t i o n of Ingram ( Table 2; Ingram, 1954). Altyn Formation i The Altyn Formation was o r i g i n a l l y described by i ■W illis (1902). The type l o c a l i t y of the formation is on i j c l i f f s at the base of Appekunny Mountain near Many G lacier i | in the northeastern corner of G la c ie r Park. S tra tig ra p h y |of the Altyn Formation in Glacier Park was re ce ntly [studied by White (1984), Whipple et a l . (1984), Jardine (1985), Kelty (1986), and Hudec (1987). The Altyn Formation is exposed only along the eastern [side of the study area immediately above the Lewis t h r u s t . I ; I t consists of as much as 100 m of dolomite, dolomitic I Mimestone, sandy dolomite, and quartz a r e n ite . The Altyn j I Formation is overlain discomformably by the Appekunny | i i jFormation (Hudec, 1987). Since the Lewis t h r u s t f a u l t | i cuts out lower portions of the Altyn Formation, the I I s tra t ig r a p h y of the Altyn Formation is not complete in the study area. The s tra tig ra p h y of the Altyn Formation has j also been studied by Kelty (1986) in the area immediately : I south of the study area, and by Hudec (1987) immediately north of the study area. In both areas, the lower part of , 1 i I 23 I the Altyn Formation is cut out by the Lewis th r u s t . A complete Altyn Formation is present in the Yellow Mountain area, 50 km north of the study area, where the Altyn Formation lie s comformably above the Waterton Formation ( J a rd i ne, 1 985). Complete understanding of the s tra tig ra p h y of the Altyn Formation has been complicated by la te Cretaceous to e arly T e r tia r y deformation. For example, a major in tra fo rm a tio n a l low-angle f a u l t , the Scenic Point f a u l t , was mapped in the study area. This f a u l t places r e l a t i v e l y undeformed Altyn rocks in its upper p late over highly fa u lte d Altyn rocks in its lower p l a t e . The s t r a t i graphic throw along th is f a u l t is unknown. A part of the Altyn Formation may be e it h e r omitted or repeated along i t . The Scenic Point f a u l t lie s p a r a l l e l to bedding in its upper p la te but is generally discordant to bedding in its lower p la t e . More d e ta ile d descriptio n of t h is f a u l t is presented in Chapter 3. The general s tra tig ra p h y of the Altyn Formation in the study area is summarized in Fig. 2-2. The Altyn Formation is divided into two p a rts , one above, and the other below, the Scenic Point f a u l t . The Altyn Formation above the Scenic Point f a u l t is l i t t l e deformed on the north c l i f f of Scenic P o in t, 1.5 km south of the middle Two Medicine Lake (P la te 1; Fig. 3 - 3 ) . I t is composed of bu ff-w eatherin g dolomite, yellowish 24 Generalized s tra tig r a p h y of the Altyn Formation in the study area. Generalized Stratigraphic Section of the Altyn Formation Appekunny Fm. (yap) Altyn Fm. A rg il!ite Scenic Point Fault Do! omi te Cherty Dolomite Sandy Dolomite Altyn Fm. Quartz Arenite Lewis Thrust Cretaceous Sed. rocks. Covered arenaceous dolom ite, and gray ish-v/h i t e quartz a r e n ite . The arenaceous dolomite is usually medium- to coarse grained. The s t r a t i f i c a t i o n of the dolomite ranges from medium- (10-50 cm) to thin-bedded (5-10 cm), commonly with cross lam inations. A coarse-grained and t h i n l y bedded quartz a ren ite unit 5-8 m t h i c k , li e s immediately above the Scenic Point f a u l t on the north c l i f f of Scenic Point. The thicknesses of the Altyn Formation above the Scenic Point f a u l t range from 15-20 m at Scenic Point to about 25 m on the east side of Head Mountain. The part of the Altyn Formation above the Scenic Point f a u l t in the study area is l i t h o l o g i c a l l y s im il a r to member 3 of Jardine (1985) at Yellow Mountain. The s t r a t i g r a p h i c section of the Altyn Formation below the Scenic Point f a u l t has been extremely complicated by extensional and c ontractional f a u l t s , p a r t i c u l a r l y at the north c l i f f of Scenic Point (see Chapter 3 ). The s tru c tu ra l thickness (not s t r a t i graphic) of the Altyn Formation below the Scenic Point f a u l t varies from zero near the southeast corner of Head Mountain, where the Lewis thrust f a u l t and the Scenic Point f a u lt j o i n , to about 40 m at the north c l i f f of Scenic Point. The lower Altyn Formation in the study area is composed of fin e - g r a in e d , thick-bedded (>50 cm) grayish dolom ite, grayish dolom itic limestone, l o c a l l y interbedded with _________ 27 medium-, to thin-bedded dolomite and medium-bedded areneceous dolomite. The arenaceous dolomite is a minor component in the lower A ltyn, probably less than 20% based on observations on the north c l i f f of Scenic Point. Blackish-gray chert beds, 3-10 cm t h i c k , are occasionally present in th is s t r a t i graphic i n t e r v a l , and are helpful as markers of bedding in the massive dolomite in the f i e l d . The Altyn Formation below the Scenic Point f a u l t in the study area is 1i t h o 1o g ic a lly s im ila r to what has been described as member 2 of the Altyn Formation by Jardine (1985) at Yellow Mountain. To the north from the study area, a major in tra fo r m a tio n a l f a u l t , the Spot Mountain f a u l t , was mapped by (Hudec, 1987). The Thicknesses and li t h o l o g y of the Altyn Formation above and below the Spot Mountain f a u l t are s im il a r to those above and below the Scenic Point f a u l t in the study a re a. D etailed d e s c rip tio n of the Scenic Point f a u l t and possible c o r r e l a ti o n between the Spot Mountain f a u lt and Scenic Point f a u l t w i l l be presented in Chapter 3. Ap pek un ny Fo rma t i on The Appekunny Formation was f i r s t described by W i l l i s (1 9 02 ). The type l o c a l i t y of the formation is at Appekunny Mountain in northeastern Glacier Park. The 28 Appekunny Formation consists predominantly of grayish- green to gra y is h -b lu e a r g i l l i t e , b1 a c k is h - grey s i l t i t e , and white a r e n it e . The e n tir e formation is exposed on the eastern side of the park in the study area, where the Appekunny Formation o v e rlie s the Altyn Formation discomformab1y and underlies the Grinnell Formation comf ormably. In the eastern study area, f iv e quartz a r e n ite units are present w ith in the formation. These quartz a r e n ite unites are l a t e r a l l y p e r s is te n t and were mapped as marker beds in the study area. They are assigned as marker beds A through E. Four of the f iv e marker beds (A, C, D, and E) can be traced or c o rre la te d to the north in the area mapped by Hudec (1987), and to the south in the area mapped by Kelty (1986). Using the tops of marker bed C, D, and E as boundaries, the Appekunny Formation is divided into four informal members on the east side of the study area (Fig. 2 - 3 ) . This d iv is io n of the Appekunny Formation is consistent with that of Hudec (1987), and is e s s e n t i a lly the same as that of Kelty (1986) except that Kelty used the bases of marker bed C, D, and E as the boundaries of the informal members. Whipple et a l . (1984) divided the Appekunny Formation into f i v e informal members. Their member 1 and 2 approximate member 1 in t h i s study, and member 3, 4, and 5 approximate member 2, 29 3, and 4 of th is study. In the Yellow Mountain area, Jardine (1985) mapped two quartz a r e n ite units as marker beds in the Appekunny Formation. The lower marker bed is whitish quartz a r e n ite and is about 11 m t h i c k . This bed is about 50 m above the A1tyn- Appekunny contact. The upper marker bed varies in thickness from 12 to 30 m, and is dominantly a l i g h t ora n g ish -w h ite, dolom itic s i l t i t e that is about 160-170 m above the Altyn-Appekunny contact. The two marker units mapped by Jardine (1985) can be c o rrelate d with marker bed B and C r e s p e c t iv e ly in th is study area based on t h e i r s i m i l a r lith o l o g y and s t r a t i graphic positions (see discussions below on marker bed B and C), and on t h e i r e s s e n tia l c o n t in u ity between the two areas (G. A. Davis, unpublished mapping). Generalized s tr a tig r a p h y of the Appekunny Formation in the eastern side of the study area is summarized in Fig. 2 -3 . On the western side of the park in the study area, only member 4 and the upper part of the member 3 are exposed. The lower parts of the Appekunny Formation are covered by morainal and a l l u v i a l deposits. A major in t ra f o r m a tio n a l low-angle f a u l t , the Brave Dog f a u l t , l i e s w ith in the Appekunny Formation throughout the e n t i r e study area. Such a complication makes the construction of a complete s t r a t i graphic section of the 30 Figure 2-3. Generalized s tra tig r a p h y of the Appekunny Formation in the study area. i i i i 31 Grinnel Format ionfygl ) Appekunny Format ion (yap) i S I A rg il!ite o m m a> cm 6 o a ) o 6 cm MBE Brave Dog Faul t Quartz Arenite Sandstone < u m m i Fine-grained Sandstone < u o 6 o C M M BD a) m a ) o e o Dolomite M BC {-• O c u m A CM (1 ) O S o C M M BB (Yap) Altyn Formation 32 Appekunny Formation impossible. The li th o lo g y of the Appekunny Formation is described as fo llo w in g . Member 1: Member 1, the basal member of the formation between the Appekunny-Altyn contact and the top of marker bed C ( M B C) , is 2 0 0-250 m thick and consists of orange f i n e grained sandstone, green to b 1 ackish-gray a r g i l l i t e , and brownish- to whitish~gray quartz a r e n it e . The contact between the Appekunny Formation and the Altyn Formation was observed d i r e c t l y at the northwest side of Scenic Point where i t is a sharp planar surface p a r a lle l to bedding in both underlying and o v e rly in g s t r a t a . On the southern flank of Spot Mountain, the contact between the Appekunny Formation and Altyn Formation was examined by Hudec (1987). He reported that the contact is an erosional surface. It was th e r e fo r e in te r p r e t e d as a discornformity by Hudec (1987). Marker bed A (MBA) is the s t r a t i g r a p h i c a l l y lowest quartz a r e n ite unit in the Appekunny Formation. It is exposed on the eastern side of the study area and extends north and southwest into areas mapped by Hudec (1987) and Kelty (1986) r e s p e c t i v e l y . Marker bed A is a 3-5 m th ic k , t h i n - to medium-bedded white coarse-grained (grain size: 0.5 to 1.0 mm) quartz a r e n ite . Quartz grains are well 33 sorted and well rounded. Cross laminations are absent in ] marker bed A. The mineral composition of marker bed A is l i s t e d in Table 3. Immediately above the marker bed is a sequence, 30-40 m th i c k , of predominantly carbonate- i jcemented, brownish-orange fin e - g r a in e d (0.10 to 0.25 mm) j i E I 1 sandstone beds, which are interbedded with minor b la c k is h - gray and brownish-gray a r g i l l i t e . Above th is sequence and below marker bed B, b1 ackish-gray a r g i l l i t e beds, 5 -1 .0 , meters t h i c k , are interbedded with dark-brown f in e -g r a in e d (<0.25 mm) sandstone beds. Marker bed B is a 3-5 m th ic k , w h itis h -g ra y medium- to coarse-grained (0.25 to 1.0 mm) quartz a r e n it e . Its mineral composition is l i s t e d in Table 3. Marker bed B is only present on the eastern side I jo f Head Mountain and Bison Mountain in the northeastern ! part of the study area. It can be traced northward in to areas mapped by Hudec (1987) and Jardine (1985), but is ; not present to the southwest in the area mapped by Kelty ■ i ! (1986). ! The lith o lo g y of member 1 immediately above marker i bed B is s im il a r to that below marker bed B except th a t the b lackish-g ray and brownish-gray a r g i l l i t e beds are j dominant components r e l a t i v e to the f in e - g r a in e d sandstone ; i beds. At about 5-10 m below the marker bed C, this j lith o l o g y changes to wel 1 -1 aminated b lu is h -g ra y a r g i l l i t e . ! Above the b luish-g ray a r g i l l i t e is the marker bed C (MBC). ! 34 Marker bed C, about 12 to 15 meters thick in the I I i study area, is a l a t e r a l l y p e r s is t e n t , t h i n - to medium- ; bedded white coarse-grained quartz a r e n ite . This c l i f f - ■ I {forming unit can be traced in to the area to the south I {mapped by Kelty (1986) and to the north in the area mapped i j I I by Hudec (1987). Its mineral composition is l i s t e d in Table 3. |Membe r 2: j ; Member 2 li e s between the top of MBC and the top of | jmarker bed D (MBD). I t is approximately 200-250 m t h i c k , i :and consists predominant1y of we 11 -1 aminated blu is h -g ra y j a r g i l l i t e . The li th o lo g y of th is member is very s i m i l a r f , Jto that of the uppermost part of member 1 immediately ; | 1 |beneath MBC. Thick t h i n l y bedded red a r g i l l i t e s , 2-4 I * {meters t h i c k , are l o c a l ly present in this s t r a t i g r a p h i c I i n t e r v a 1 . Marker bed D (MBD) is a recessive unit l o c a l ly exposed along the eastern side of Appistoki Peak (P la te j . i 1). It contains t h i n - to medium-bedded, brownish-orange, j coarse-grained a r e n ite beds. The thickness of th is marker bed is about 25-30 meters. Grain size of quartz ranges from 0.4 to 1.0 mm. Besides quartz grains, MBD also 1 contains a r e l a t i v e l y larger percentage of fe ld s p a r grains i i ( 0 . 5 - 1 . 2 mm; Table 3) than the other quartz a r e n it e marker . 35 Table 3. Mineral compositions of arenite beds in the Appekunny Formation. i l i 36 ' M in e r a l c o m p o s itio n o f m a rk e r beds i n A ppekunny F o rm a tio n Marker Bed A B D Quartz Feldspar Lithic Fragments carbonate Opaque Others Matrix Grain size 93% 2 % 4% 1 % mica +opaque +s ilica 0.2 0.4 m m 80% 5% 12% 1 % 2% 0% carbonate +opaque +silica 0.5 1.0 rrnn 91% 7% 1% 1% mica +silica 85% 94% 10% 2% 2% 4% 2% 1 % carbonate lithic +lithic 0.1 0.2 0.4 1.0 0.3 0.5 mm mm mm 37 beds in the study area. Both quartz and feldspar grains are poor 1y so rted . Member 3: Member 3 l i e s between the top of MBD and the top of marker bed E (MBE). This member consists of a 100-150 m t h i c k , interbedded sequence of grayish-green and brownish- green, t h i n l y laminated a r g i l l i t e , and greenish-brown to tan, medium- to coarse-grained sandstone. The high content of coarse c la s tic components (sandstone beds) distinguishes member 3 from member 2. The top 10-15 m of member 3 consists e n t i r e l y of greenish-brown, t h i n l y laminated a r g i l l i t e . The Brave Dog f a u l t li e s within this s tratigrap hic i n t e r v a l . This f a u l t is parallel to the bedding in its upper plate, but l o c a l l y truncates bedding in its lower plate in the study area (see description of the Brave Dog fa u lt in Chapter 3). The Brave Dog f a u l t cuts downsection to the east across its footwall at a very shallow angle (3-5 degrees) to the east in the d ire c tio n of its tectonic transport, omitting part of member 2 and member 3 in the southern edge of the Lewis allochthon mapped by Kelty (1986) and the study area. Marker bed E is the uppermost part of member 3. It is a approximately 6 m thick white quartz arenite (Table 3). Beds in maker bed E are 30-60 cm th ic k . This marker 38 unit is l a t e r a l l y persis te nt, and can be traced into areas mapped by Kelty (1 986) and Hudec ( 1 98 7 ) re s pe ct i ve 1 y . Member 4: Member 4 is the uppermost member of the Appekunny Formation, It extends from the top of MBE ,to the base of the Grinnell Formation, The thickness of member 4 ranges from 200 to 250 m. This member can be divided into two parts. The lower part, approximately 150-200 m thick, consists primarily of th i n ly laminated, brownish- to tanish-gray a r g i l l i t e s . This unit can be traced across the study area from east to west. On the eastern side of the study area, the uppermost 20 to 35 m of this member is a sequence of cl i f f - f o r m i n g , massive bluish-green a r g i l l i t e s . To the west, this sequence changes from dominant red a r g i l l i t e s to a sequence, 30-50 m thick, interbedded red and green a r g i l l i t e s . Each red or green a r g i l l i t e bed is about 5 to 8 m th i c k . The basal Grinnell Formation immediately above this sequence consists e n t i r e l y of red a r g i l l i t e s . Gr i n ne11 F o rma t i on The Grinnell Formation was f i r s t described by W i l l i s (1902) in Glacier Park. The type area of the formation is located at Mount Grinnell in Many Glacier area, 39 northeastern Glacier National Park. This formation lies comformably between the Appekunny and Empire Formations. Continuity of the Grinnell stratigraphy in the study area, as for the underlying units, has been complicated by la t e Cretaceous to early T e r tia r y deformation. Several low-angle f a u lts of regional extent were mapped within the Grinnell Formation (Plate 1). Since the s t ra tig r a p hic throws along these intraformational faults are unknown, i t has not been possible to construct a complete s tra tigrap hic section of the Grinnell Formation in the study area. The Grinnell Formation in the park has been recently studied in detail by Whipple et a l . (1984). The following description of the formation is based mainly on th e ir results. The Grinnell Formation is, in general, a quartzose redbed sequence. The thickness of this formation varies from 7 90 m (2,600 feet; Whipple et a l . , 1984) near the Lake Mcdonald on the west-central part of the park, to 300-550 m (1 ,0 00-1,800 fe e t; W i l l i s , 1902) in the northeast corner of the park. In the study area, the minimum thickness of the Grinnell Formation ranges about 800 m (2,600 fe e t) thick in the west to about 500 m (1,700 feet) thick in the east. The Grinnell Formation in the eastern portion of the park is composed of quartz a renite and interbedded red 40 a r g i l l i t e . The content of quartz arenite ranges from 4 0% to 60%. Beds of quartz a renite are t y p i c a l l y white, medium- (0 .2 5 -0.5 mm) to coarse-grained ( > 0 . 5-1.0 mm). Primary sedimentary structures such as cross-bedding, mud cracks, rip-up clasts of a r g i l l i t e s , and ripple marks, are common. In general, beds of quartz arenite become more common upward in the formation. Interbedded with these arenites are prominent red or purplish-red laminated s i l t i t e , s i l t y a r g i l l i t e , and a r g i l l i t e . Laminae range from even p a r a l l e l to wavy nonparallel, and lo ca lly include cross-laminae. Disrupted bedding due to mud cracking and flu id escaping processes is ubiquitous in these red beds. Beds of greenish-gray s i l t i t e and a r g i l l i t e are rarely present on the east side of the park. Whipple et a l . (1984) report that a marked l i t h o l o g i c change occurs in the Grinnell Formation from east to west across Glacier Park. The intimate in t e r la y e r in g of quartz arenite and red a r g i l l i t e in eastern exposures changes northwestward to a li th o f a c ie s that contains l i t t l e quartz arenite (and most of that is in the upper half of the formation). In contrast, grayish-green and grayish-purple s i l t i t e and a r g i l l i t e are abundant. This change in li t h o f a c i e s between is also observed in the study area. At Peril and Cloudcroft Peaks in the western study area, the Grinnell Formation consists dominantly of grayish- 41 green and red s i l t i t e and a r g i l l i t e . The a r g i l l i t e is j evenly laminated and contains a few thin beds of r i p p l e - | i Jmarked, whitish-red quartz a ren ite . The thicknesses of > \ the grayish-green a r g i l l i t e beds and s i l t i t e beds are from 30 to 50 m. In the lower portion of the formation, 30% of ' beds are grayish-green s i l t i t e and a r g i l l i t e , but the percentage of the grayish-green s i l t i t e and a r g i l l i t e appears to decrease upwards. Emp i re Fo rma t ion The Empire Formation comformably overlies the [Grinnell Formation and underlies the Helena Formation. It | i is a sequence of interbedded a r g i l l i t e , q u a r tz it e and carbonate that was separated from the lower part of the . Siveh Formation in Glacier Park by Mudge (1977). The i | remaining upper limestone sequence of the Siveh Formation [was named the Helena Formation (Mudge, 1977). The I I formation, 150-180 m t h ic k , is well exposed at Finch Peak, Rising Wolf Mountain, and Mount P h i l l i p s in the northern ! part of the study area. Its s tra tigraphy was examined in some detail at Finch Peak in the northeastern part of the i i study area. The lower 20 m of the formation is dominated j by beds of white to orange quartz arenites that range in ! I thickness from 20 to 30 cm. These arenites are j interbedded with minor, thin-bedded gray a r g i l l i t e j beds. The middle part of the formation, about 80 to 100 m thick, consists predo minantly of gray a r g i l l i t e . The number and thickness of the a ren ite beds decrease with a cor responding increase in gray a r g i l l i t e beds and carbonate beds. The upper part of the formation is about 50 to 80 m t h ic k . This s t r a t i graphic interval is dominated by carbonate beds. Gray a r g i l l i t e and quartz arenite are minor components in this s t r a t i graphic i n t e r v a l . Arenite beds are absent in the upper 20 m of the formation. Helena Formation The Helena Formation comformably overlies the Empire Formation, and comformably underlies the Snowslip Formation. Only the lower part of this formation is present in the study area. The following descriptions of the Helena Formation combine the observations at Finch Peak of the northeastern study area with those of Whipple et a l . (1984) in other parts of the park. The Helena Formation is about 850 m thick in Glacier Park, and consists primarily of dolomite, limestone, and quartz a ren ite. This formation was recently divided by Whipple et a l . (1984) into three parts. Only the lower and the lower middle part of the Helena Formation of Whipple et al. (1984) are present in the study area. The 43 lower member is about 200 m thick and consists predominantly of thin-bedded dolomite, lo c a l ly interbedded with minor quartz arenite beds. The middle part of the formation is about 600 meters thick and consists prim arily of dolomitic molar-tooth beds, some as much as 30 m thick. A few thin beds of quartz arenite and stromato1i t i c limestone are present. The upper part of the formation is said to consist predominantly of interbedded s t r o m a t o l i t i c limestone, o o l i t i c limestone, and quartz arenite. An a re a lly extensive d i o r i t e s i l l about 45 m thick intrudes the lower part of the Helena Formation in the study area. This s i l l climbs section from near the base of the Helena Formation in the southeastern park, to the lower part of the Snowslip Formation in northernmost park area (Whipple et a l . , 1984). The d i o r i t e s i l l is well exposed at Finch Peak and Mount P h i l l i p s . Autochthonous Rocks W i l l i s (1902) f i r s t recognized the presence of recessive, s1 ope-forming Cretaceous rocks below the Lewis thrust f a u l t . The Cretaceous rocks in and around Glacier National Park have been mapped by Mudge and Earhart (1980, 1983). According to t h e i r mapping, the Cretaceous rocks at the eastern edge of the study area belong to the Marias River Shale of upper Cretaceous age. 44 The Marias River Shale, which consists mainly of dark-gray, marine mudstone in the study area, is only exposed along streams. The exposures, isolated by Quaternary a l l u v i a l , morainal, and landslide deposits, range from several square meters to several tens of square meters. Due to the limited exposures, no attempt was made to d i f f e r e n t i a t e the Cretaceous rocks in the lower plate of the Lewis t h r u s t . 4 5 CHAPTER 3: GEOMETRY OF THE LEWIS THRUST SYSTEM General Statement This chapter describes the geometry of the Lewis thrust and the structures in its upper plate. The purpose of this description is to establish the geometrical framework of the Lewis thrust system. D efinitions of contractional and extensional fa u lts used in this study follows that of Norris (1958) and Dennis et a l . (1981): a contractional f a u l t is a f a u l t which shortens bedding; an extensional f a u l t is a f a u l t which extends bedding. It is important to point out that use of the term "extensional fa u lt " does not necessarily imply extensional t e c to n ic s . F i r s t , the a t t i t u d e of bedding may not be horizontal when an extensional f a u l t formed. Therefore the extension of bedding may or may not represent a true crustal extension in the horizontal d ire c tio n . Second, a low-angle curviplanar f a u l t may change its dip dire c tio n along its d i f f e r e n t segments, and i t can cut upsection and downsection in the dire ction of its tectonic transport along its d i f f e r e n t segments. This f a u l t is an extensional f a u l t along the segments where the f a u l t cuts down section in the transport d ire c tio n and the length of bedding is extended; i t is a contractional f a u l t along the segments where the f a u l t cuts upsection in its transport 46 dire c tio n and the length of bedding is contracted. Structural elements in the Lewis allochthon in southern Glacier Park include imbricate thrust systems, low- and high-angle extensional and contractional f a u l t s , and broad and t i g h t folds. Major structural components in the Lewis plate are the Scenic Point structural complex, frontal zone, eastern structural b e lt, Elk Mountain imbricate system (Kelty, 1986), Brave Dog f a u l t ( Ke1t y ,1986), Mount Henry imbricate system, Rockwell f a u l t , Two Medicine Pass f a u l t , Akamina syncline or Continental Divide syncline, and Lone Walker f a u lt (Fig. 3-1, Plate 1, Plate 2). These structural components, together with the Lewis thrust f a u l t i t s e l f , define the Lewis thrust system. Fig. 3 -la is a sim pli fied geologic map of the study area, and Fig. 3-lb is a s im p li f ie d geologic cross section through the study area. Detailed structural relationships within the Lewis allochthon are shown in Plates 1 and 2. Note that the cross-section line in Fig. 3-la does not exactly coincide with the cross- section line in Plate 1. Lewis Thrust Fault The Lewis thrust fa u lt is the dominant structure in the study area (Fig. 3 -2). It juxtaposes the mid- Proterozoic Belt rocks in its upper plate with the late ___________________________________________________ 47 Figure 3 -1. a. Simplified geologic map of the Lewis allochthon in the study area. b. Geologic cross-section through line AA' A'' in a. Mo v e rtic a l exaggeration. 48, S im plified Geologic Map of The Study Area a . Explanation l i n d i f f e r e n t i a t e d C r e t a c e o u s K o c 1 . Yhl s/s / / / , V Vap H e l e n a F o r m a t i o n : D o l o r , i c e , L i m e s t o n e . E m p i r e F o r n a c l o m : D o l o r i c e , A r g l . ' . l i t e , a n d Q u a r t z i t u . C r l n n u l l F o r m a t i o n : R e d .1 r e , i 1 1 i t . . Q u a r . z i c e . A p p e . c u n n y F o r m a t i o n : d r a v A r r i l l l t t Q u a r c z i t e . 0 T h r u s t F a u l c . H i c ' n A n c 1 e F a n 1 c . A n t i c l i n e e n d S v n e l i r . i L u k e a n d . - c r e a m . 49 r i I Geologic Cross-section ^ S S fft UW RF CD O t h a S t u d y A r e a > r / t 10060* 7 5 0 0 * M M ^ 4 m ile b . Figure 3-2. View to the north-northwest along the trace of the Lewis thrust f a u l t , southeastern side of Glacier Park. The trace of the f a u l t (LT) follows the break of slope and separates upper-plate Proterozoic Belt rocks ( l e f t ) from lower-plate Cretaceous rocks ( r i g h t ) . In the foreground, a thin orange layer of Altyn carbonate rocks li es d i r e c t l y above the Lewis thrust and brush-covered slopes of lower-p late Cretaceous s tr a t a . Appekunny rocks ov erlie the Altyn and exhibit considerable complexity in the v i c i n i t y of the prominent valley bordered by green vegetated pattern (cf. F i g . 3 -6). 52 Cretaceous Marias River Shale (Mudge and Earhart, 1983) in its lower plate. In the study area, the Lewis thrust f a u l t gradually cuts up-section l a t e r a l l y to the south, thinning the Altyn Formation from about 300 feet (92 m) at the north c l i f f of Scenic Point immediately south of the middle Two Medicine Lake, to about 50 feet (14 m) at Squaw Mountain 5 miles (8 km) fa th e r south. The a t t i t u d e of the Lev;is thrust fa u lt from a direct measurement at the western end of Forty One Mile Creek is N20oW 6oSW. The Lewis thrust f a u l t is located at an elevation of about 6000 feet along the eastern edge of the map area (Plate 1), but li es beneath the Belt strata exposed at elevations less than 4500 feet in western areas. Outcrop information and a geologic cross section of the Lewis plate across the study area (Plate 2) indicate that the Lewis thrust f a u l t dips towards the west at a shallow angle across southern Glacier Park. The Lewis thrust f a u lt is sharply delineated between the c l i f f - f o r m i n g Altyn Formation and slope-forming Cretaceous sedimentary rocks in the study area. Although the fa u lt surface can be generally located within several meters, it is rarely exposed, as i t is usually covered by talus or landslide blocks. The exposure of the fault surface in the study area was only found in the headwater of Forty One Mile Creek at the southeastern corner of 53 Bison Mountain. The Lewis thrust f a u l t at this l o c a l i t y is a sharp planar surface separating the Altyn Formation from the la te Cretaceous Marias River Shale. Intensely sheared shales l i e d i r e c t l y beneath the f a u l t . An apparently s im ila r b r i t t l e shear zone beneath the Lewis thrust f a u l t was described and studied by Wilson (197 0) along the Dry Fork of Two Medicine Creek, 7 km north of the study area, and along Rose Creek near St. Mary Lake. Scenic Point Structural Complex The Scenic Point s tru ctu ra l complex (SPSC) li es e n t i r e l y within the Altyn Formation. This complex, bounded above by the low-angle Scenic Point f a u l t (SPF) and below by the Lewis thrust, is best exposed on the north c l i f f of Scenic Point, 2 km south of the lower Two Medicine Lake (Figs. 3-1 and 3-3, Plate 1). The Scenic Point f a u l t is pa ra lle l to bedding in its upper plate, but is discordant with respect to highly deformed bedding below i t . On the north c l i f f of Scenic Point the trace of the fa u lt li es 30-40 meters below the A lty n - Appekunny contact and follows the base of a 3 meter-thick white qu artzit e bed. On the southeastern side of Head Mountain, the f a u l t joins the west-dipping Lewis thrust f a u l t . Whether or not the Scenic Point f a u l t also joins with the 54 Figure 3-3. Scenic Point Structural Complex as viewed to the south from the north base of Scenic Point. Scenic Point f a u l t (SPF) lies beneath a whitish gray quartz aren ite bed and separates l i t t l e deformed strata in its upper plate from highly deformed rock packages in its lower plate . MBC is marker bed C of a quartz arenite bed. Ya, the Altyn Formation; Yap, the Appekunny Formation. The Lewis thrust is covered by talus deposits at the base of the c l i f f . Figure 3-4 shows detailed structures on the north c l i f f of Scenic Point. 55 56 Lewis thrust westward is not known because the Scenic Point stru ctural complex becomes covered by talus west of Scenic Point (Plate 1). The Scenic Point f a u l t and shallowly dipping s trata in its upper plate have been folded into broad a n tic lin e s and synclines (Figs. 3-3 and 3-4) . Rocks in the lower plate of the Scenic Point fault are complexly faulted and comprise a structural assemblage cut by both contractional and extensional f a u l t s . Offsets along most of these fa u lts are uncertain because good marker beds are rare in the Altyn Formation. The contractional faults generally dip to the west except a few minor backthrusts which dip to the east (Fig. 3 -4 ). The extensional f a u l t s , however, only dip eastward (Figs. 3-3 and 3 -4 ). The o r ie nta tio ns of contractional and extensional f a u lt s in the Scenic Point structural complex are sub-perpendicular to the inferred d i re c tio n of tectonic transport along the Scenic Point f a u l t (see discussion in Chapter 4 ). On the southwestern side of Fig. 3-4, west-dipping contractional faults appear to cut east-dipping extensional f a u l t s ; on the northeast side of Fig. 3-4, however, an east-dipping extensional f a u l t beneath the crest of the eastern a n t i c l i n e appears to cut and offset a west-dipping contractional f a u l t . In general, contractional fa u lts appear to have developed 57 Figure 3-4* Sketch of Scenic Point structural complex on the north side of Scenic Point. View from northeast. 58 L evis Thrust f a u lt p o ih t SCe m c f a u l t 1 Ian 59 immediately beneath the a n t i c l i n e s , whereas the extensional f a u l t s to have developed beneath the synclines (Fig. 3-4). This structural re la tio n s h ip suggests that the development of some f a u l t s in the Scenic Point structural complex and folding of the Scenic Point fa u lt immediately above them may have been g e n e t ic a ll y r e la ted . Possible kinematic processes responsible for the development of the Scenic Point stru c tu ra l complex and broad folds immediately above i t will be discussed in the next chapter. A truncational re la tio ns h ip exists between the Scenic Point fa u lt and some minor contractional f a u l t s in its lower plate. Figs. 3-3 and 3-4 show that the beds of Altyn Formation in the lower plate were also cut by the Scenic Point f a u l t . This truncational rela tio nship suggests that at least some structures in the Scenic Point stru ctural complex predate the Scenic Point f a u l t . Structures in the upper plate of the Scenic Point f a u l t are r e l a t i v e l y simple on the north c l i f f of Scenic Point. Three high-angle (ca. 60-7 0°) normal f a u l t s , all located at the northeastern corner of Scenic Point (P la te 1), are present in the upper plate of the Scenic Point f a u l t (Figs. 3-3 and 3 - 4 ) . Two of them dip to the east, and one to the west. The strikes of these fa u lts are p a r a lle l to the trend of the fold hinges above the Scenic Point 60 structural complex (Plate 1). Offsets along these faults are small, approximately 3-8 meters. The two east- dipping normal fault s are terminated downward at the Scenic Point f a u l t . Whether the two normal f a u lts in the upper plate of the Scenic Point f a u l t are truncated downward by or merge downward with the Scenic Point f a u l t is not c le a r, as offsets along these normal f a u lt s are small (Fig. 3 -4 ). The west-dipping normal f a u l t is , however, o ffs e t by a west-dipping low-angle contractional f a u l t for about 15 meters (Figs. 3-3 and 3-4) which appears to branch off from the Scenic Point f a u l t (Fig. 3 - 3). This re la tio n s h ip suggests that the Scenic Point f a u l t predates the west-dipping normal f a u l t . Minor contractional fa u lts with displacement of 1-2 meters which appear to sole into the Scenic Point f a u l t are also observed on the north c l i f f of Scenic Point west of the structures shown in Figs. 3-3 and 3-4. Those minor contractional fault s dip predominantly to the west except one back thrust which dips to the east. The development of these predominant west-dipping, e a st-dir ected minor contractional fa u lts immediately above the Scenic Point f a u l t suggests that the transport dire c tion along the Scenic Point f a u l t is generally to the east. Hudec (1987) mapped a low-angle f a u l t , the Spot Mountain thrust f a u l t , in the Altyn Formation in an area 2 km north of the 61 study area. This f a u l t li es sub-parallel to bedding in its upper plate 1-20 m below the top of the Altyn Formation, Structures in the lower plate of the Spot Mountain f a u l t consist of complexly faulted and folded structural packages. Like the Scenic Point f a u l t , the Spot Mountain f a u l t is also broadly folded and truncates beds and some of the fa u lts in its lower plate. Steeply- dipping reverse fa u lts in the frontal zone of Hudec's area o ffs e t the Spot Mountain f a u l t and cut structures in its upper and lower plates (Hudec, 198 7). This relationship is, however, not clear in the study area because the relationships between the frontal zone and Scenic Point fa u lt are not d i r e c t l y observed. The Spot Mountain f a u l t may be c o r r e l a t i v e with the Scenic Point f a u l t because (1) both fa ult s l i e at s im ila r s t r a t i graphic levels , (2) deformational styles in t h e i r upper and lower plates of the two f a u lt s are s i m i l a r . Fig. 3-5 shows a tt it u d e s of a few f a u lt s in the Scenic Point f a u l t complex and s t r i a t io n s along the corresponding f a u l t s . The average s tri k e of the f a u lts o determined from this limited data set is N3 0 W. Frontal Zone The frontal zone (FZ, Fig. 3 - 1 ) , which is about 100 to 200 m wide, is located along the eastern edge of Head 62 Figure 3-5. Attitudes of f a u lts in the Scenic Point Structural Complex. o shows a t tit u d e s of s t r i a e measured on f a u l t s . Dashed lines show a ttitude s of extensional f a u l t s , solid lines show a ttitude s of contractional f a u l t s . n=8. 63 64 Mountain and Bison Mountain in the easternmost study area (Plates 1 and 2). S t r u c t u r a l l y , the zone occupies the eastern edge of the Lewis allochthon as presently exposed. To the west, structures in the frontal zone are s im il a r to those in the eastern structural b e l t , both contain east- and west-dipping contractional fa ults and concentric fo ld s . However, structures in the frontal zone are more in tensely deformed ( t i g h t e r folds which are commonly asymmetric and verge to the east, greater displacements along f a u l t s , and more closely spaced fa u lt s ) than those in the eastern structural belt (Fig. 3-1, Plates 1 and 2). In the frontal zone, east-dipping contractional f a u l t s dip more steeply (nearly vertica l and were lo c a l ly rotated to west-dipping apparent normal f a u l t s ) than in the eastern b e l t . Despite changes in structural styles along s t r i k e , this zone of complex structures and the eastern s tru ctu ra l b e lt can be traced to the north through areas mapped by Hudec (198 7) and G.A. Davis (unpublished map). Structures in the frontal zone are extremely complex. They consist of east-vergent open to t i g h t folds, west- dipping (synthetic) and east-dipping ( a n t i t h e t i c ) contractional and extensional f a u l t s , and pop-up structures (contractional fault-bounded horsts; Butler, 198 2). Cross-cutting rela tionships between these structures indic ate that m u ltip le episodes of deformation 65 Figure 3-6. a. View to the northwest of structures in the frontal zone on the east side of Bison Mountain. b . A geologic cross-section based on Figure 3 -6a. See te x t for descriptions of structures in the frontal zone. Kinematic in te r p r e t a tio n s of these frontal zone structures are presented in Chapter 4. 66 67 N50E FA FB MBC FE FF / MBC FG Appek.im.ny Formation MBB FK MBA FH FI MBB MBA AJLtyn Fm. Cretaceous sediments °%**<> marker bed C, MBC : marker bed B, MBB 7 marker bed A, MBA t-- Altyn Formation 68 are responsible for its development. On the northeastern side of Head Mountain, a west-dipping reverse f a u l t with minimum displacement of 100 m juxtaposes Altyn rocks over member 2 of the Appekunny Formation (Plate 1). This f a u l t is cut by an east-dipping a n t i t h e t i c reverse f a u l t . This east-dipping f a u l t is in turn truncated from below by the Lewis th r u s t. The east-dipping reverse f a u l t can be traced southward from Head Mountain to the eastern side of Bison Mountain, which is f a u l t G in Fig. 3-6b. It can be seen in Fig. 3-6b that the dip d i re c t io n of f a u l t G on the eastern side of Bison Mountain is changed to the west. Such a change in dip dire c tio n of reverse f a u l t s along t h e i r s t r i k e were also recorded by Hudec (198 7) in the Spot Mountain area. For example, f a u l t C (Hudec, 1987, Plate 3) dips to the east in Hudec's northern area, but i t dips to the west in his southern area (see cross sections AA1 and DD1 in Plate 3, Hudec, 198 7). Such a change in dip dire c tion of f a u l t G in the study area and f a u l t C in Hudec's area along t h e ir s tr i k e indicates that these faults are not planar and may have been twisted a f t e r t h e i r formation. Structures of the frontal zone on the eastern side of the Bison Mountain are more complicated than those at Head Mountain. A complexly deformed structural package is bounded by a east-dipping reverse f a u l t zone ( f a u lts B, C, 69 and D in Fig, 3-6b) to the west, and by a west-dipping f a u l t ( f a u l t G in Fig, 3-6) with an extensional geometry to the east. Two high-angle f a u lt s within this structural package, fa u lt s E and F (Fig. 3- 6 b ) , e xhib it extensional geometries. A possible "drag" fold in the footwall of f a u l t E (Fig. 3-7) is an uncommon structure in the map area. A possible sequence of deformation responsible for the development of the frontal zone structures at the Bison Mountain area will be discussed in Chapter 4. Displacements along fro n t a l- z o n e fault s vary from several meters to more than 100 meters. Many are truncated sharply below by the Lewis thrust on the eastern side of Bison and Head Mountains. Cross sections through the frontal zone cannot be balanced because some a l 1ochthonous rocks and older structures that a f f e c t them have been cut off from below by the younger Lewis thrus t. The a t tit u d e s of l i m i t e d extensional and contractional fa u lt s in the frontal zone measured on the east side of Bison and Head Mountains are plotted in Fig. 3-8. The west-dipping fa u lts dip considerably shallower (less than 45 degrees) than the east-dipping faults (greater than 65 degrees). The s trik e of these f a u l t s varies from N25°E to N75°W, but the dominant s trike d i re c tio n is about N25oW. 7 0 Figure 3-7. A small-scale drag fold developed in the foot wall of a west-dipping high-angle extensional f a u l t on the east c l i f f of Bison Mountain. Picture is viewed from southeast to northwest. Clark Davis (gray s h i r t ) and the w r i t e r (red s h i r t ) for scale. 71 Figure 3-8, Attitudes of fa u lts in the frontal zone measured on the east side of Bison Mountain and Head Mountain, data including both mappable faults ( o f f s e t > 40 f t or 13 m) and unmappable f a u lt s . Solid lines represent contractional f a u l t s , dashed lines represent extensional f a u l t s . n=8. 73 A / 7 4 Eastern Structural Belt The eastern structural belt ( E S B , Fig. 3-1) li es between the Mount Henry imbricate system and frontal zone (Plate 1). It contains broad synclines and a n t i c l i n e s (wavelength from 30 to 100 m) , southwest-dipping (synthetic) and no rtheast-dipping ( a n t i t h e t i c ) contractional f a u l t s , and minor extensional f a u lt s dipping both to the northeast and southwest. The boundary between the structures in the eastern stru c tu ra l belt and those in the frontal zone is t r a n s i t i o n a l . Structures in this 3 km-wide s tru c tu ra l belt are truncated upward by the overlying Brave Dog f a u l t and downward by the Lewis thrust f a u l t . To the west, contractional f a u l t s in the eastern belt dipping both eastward and westward are o ffs e t by the Mount Henry imbricate system (Pla te 1). Spacing between faults in the eastern belt ranges from 30 m to more than 150 m (Fig. 3-1, Plate 1). Faults in this belt are generally planar, although l i s t r i c thrusts have been observed. The s trik es of f a u lt s in the eastern belt are plotted in Fig. 3-9. The a t t it u d e s of the same set of fa ult s are plotted in Fig. 3-10. The dominant s t r i k e d i r e c tio n of these f a u lt s is N 2 0° W - N 3 0° W . Dip angles of contractional f a u l t s in this belt range from 10 to 70 degrees, but most f a u lt s have dips of 30 to 40 7 5 Figure 3-9. Rose diagram for s trik es of contractional fa u lts in the eastern stru ctu ra l b e l t . n = 3 2 . 76 1 * 2 o o C' o o * * “ « r o I \ J-*** • j £ - \ \ y c- C M V O O SO O G 1 3 0 o m i - j c a ? £ & M a a £ E n jcf G C ^ 77 Figure 3-10. Stereographic projectio n of contractional and extensional f a u lts the eastern s t r u c t u r a l b e l t . Ncon=32, Nex=5. Neon and Nex are sample sizes of contractional f a u l t s and extensional f a u l t s . The data set for the contractional f a u l t s is the same as that plotted in Figure 3-9. 78 Poles of contractional and extensional faults In the eastern structural belt. vest-dipping (synthetic) contractional faults east-dipping (antithetic) contractional faults Nc=32, Ne«*5. *-: extensional fault3, *:contractional faults. 79 degrees (Fig. 3 - 1 0 ) . Displacements along all these faults are small, generally less than 30 m. Symmetrical folds with wavelengths of 30 m to 100 m are well developed in this b e l t . Dip angles of fold limbs are shallow, usually from 5 to 20 degrees ( P la te 1). The o o average trend of the folds is N20W to N30W, an o r i e n t a t i o n p a r a lle l to the average strike of the major steep fa ult s in this b e l t . Many small-scale folds (wavelength from 0.5 to 2.0 m, amplitude from 0.5 to 1.0 m) are present on the limbs of open folds in the east b e l t . Their vergence ( i . e . , d i r e c t io n of te ctonic transport) is towards the hinges of the a n t i c l i n e s , suggesting the folding mechanism is f le x u r a l s l i p . The average trend of these small-scale folds (Fig. 3-11) is p a r a lle l to that of the open folds and the average s t r i k e of the f a u l t s in this structural b e l t . East- and west-dipping reverse fa ult s in the eastern structu ra l belt are truncated upward by the low-angle Brave Dog f a u l t . This r e l a t i o n s h i p is best exposed at Squaw Mountain and on the western c l i f f of Scenic Point. Fig. 3-12 shows that an east-d ipping reverse f a u l t is truncated above by the Brave Dog f a u l t which li es pa r a lle l to the bedding within its upper pla te . The r e la t io n s h ip between the f a u lt s in the eastern stru ctural 80 Figure 3-11. Attitudes of s m all-scale fold hinges in the eastern structural b e l t . n=59. Contouring done by hand. For contouring method see Davis ( p . 82-86, 1 98 5). 81 4 -7 Z 2 -4 1 -2Z 82 Figure 3-12. An east-dipping contractional f a u l t in the eastern structural belt is truncated by the Brave Dog f a u l t (BDF). The p ictu re is viewed from northwest to southeast on the northwest side of Scenic Point. The Brave Dog f a u l t is p a r a l l e l to bedding of its upper plate and truncates a northeast-dipping minor f a u l t . The drag fold in the hanging wall of the minor f a u l t indicates that this f a u l t is a reverse f a u l t . 83 84 belt and the underlying Lewis thrust f a u l t is not clear because offsets along fa ults in the eastern belt are small, and the trace of the Lewis thrus t and lower portion of the eastern belt are covered by Quaternary morainal deposits and ta lu s . However, a l i n e of evidence suggests that structures in the eastern belt are truncated from below by the Lewis thrust. In the area mapped by Hudec (198 7), structures equivalent to these of the eastern belt and frontal zone in the w r i t e r ' s area are reported as being truncated from below by the Lewis thrust (Hudec, 1987). The i n t e n s i t y of deformation in the eastern structural belt decreases westward from the frontal zone. The westernmost structures in the belt are o ffs e t by fa u lts of the Mount Henry imbricate system at the southwestern corner of Scenic Point and northern side of Appistoki Peak (Plates 1 and 2 ). Elk Mountain Imbricate System The Elk Mountain imbricate thrust system (EMIS) was f i r s t mapped by Kelty (1986) at south-central Glacier Park. This imbricate system was o r i g i n a l l y named the Brave Dog Mountain imbricate system (K elty, 1986, p. 130). Yin et a l . (1986) referred to the system as the Elk Mountain imbricate system in order to avoid the confusion 85 with the Brave Dog f a u l t , a major low-angle f a u lt within the Appekunny Formation in southern Glacier Park. The Elk Mountain imbricate system consists of approximately 7 major west-dipping thrust f a u l t s that l i e above the Lewis thrust f a u l t and below the Brave Dog fault (Plate 3B, Kelty, 198 6). Thrusts in the system dip about 10 to 15 degrees at lower elevations and appear to f l a t t e n into the Lewis t h r u s t . At higher elevations these f a u l t s dip more s teep ly , from 40 to 60 degrees. At the highest levels of the imbricate system, the f a u l t s dip less steeply, from 30 to 40 degrees. The angles between thrust f a u lt s and bedding in the Appekunny Formation are about 25 to 30 degrees in the upper portion of the imbricate system. But f a u l t s and bedding are nearly p a r a l l e l to each other in the lower portion of the imbricate system; both dip at shallow angles (ca.20 degrees). This geometrical r e la t io n s h ip requires that the geometry of the thrust f a u lts in the Elk Mountain imbricate system is l i s t r i c . Kelty (1986) reported that some s t r u c t u r a l l y higher imbricate thrusts o f f s e t s t r u c t u r a l l y lower f a u l t s . He used th is stru ctu ra l re la tio ns h ip as evidence for hindward development of f a u l t s in the Elk Mountain imbricate system. Thrust fault s in the Elk Mountain imbricate system and the beds of the Appekunny Formation, are truncated from above by a 86 planar f a u l t , the Brave Dog f a u l t . This r e la t io n s h ip is best exposed at the south side of Brave Dog Mountain mapped by Kelty (see F i g . 39, Kelty, 1986). The Elk Mountain imbricate system can be extended from the area mapped by Kelty (1986) in southeastern Glacier Park to the western side of the study area mapped by the w r i t e r . On the northwestern c l i f f of Cloudcroft Peaks, the upper members (members 3 and 4 ?) of the Appekunny Formation is repeated by several thrust f a u l t s . These thrust fa ult s were mapped from a distance because of t h e i r i n a c c e s s i b i l i t y . Reverse o ffs e ts along the f a u l t s appear to be from 2 0 meters to 100 meters. At the top, these fa u lt s are terminated by a f a u l t that li es s u b - p a r a lle l to the bedding within its upper p l a t e , 100 to 150 m below the Appekunny-Grinnell contact. The minor thrust f a u l t s and the bedding-controlled f a u l t above them are in t e r p r e t e d to be c o r r e l a t i v e with thrust f a u l t s of the Elk Mountain imbricate system and the Brave Dog f a u l t r e s p e c t iv e ly . Brave Dog Fault The Brave Dog f a u l t was f i r s t mapped by Kelty (1986) at Brave Dog Mountain in the southern edge of Glacier National Park. This shallowly dipping f a u l t can be traced throughout at least 900 km2 the southern portion of Glacier National Park as shown in Fig. 3-1 and Plate 1 87 (K elty, 1987; this study). It dips about 2 to 3 degrees to the east along the southern edge of Glacier Park (K e l t y , 1986), and lies within the upper part of the Appekunny Formation. The f a u l t surface is p a r a l l e l to bedding in i t s upper pl ate, but cuts downsect ion eastward through its lower plate in the d i r e c t i o n of tr a n s p o r t. At Brave Dog Mountain, the f a u l t lies 2-3 m below Appekunny marker bed E, and about 150 m above marker bed D. Further to the east, this f a u l t gradually cuts downward through lowe r-pla te member 3 of the Appekunny Formation and cut out marker bed D in it s lower plate (K e lty , 1986). Since the Brave Dog f a u l t cuts downsection s t r a t i g r a p h i c a l ly , this f a u l t is extensional in the sense that the length of beds were extended by displacement along i t . In the study area (Plate 1), the Brave Dog f a u l t li es immediately under Appekunny marker bed E at Squaw Mountain, but f a r t h e r below, about 15 to 20 m, marker bed E at Scenic Point. Marker bed D is only l o c a l l y present on the eastern side of Appistoki Peak and is absent at Scenic Point to the east. The absence of marker bed D at Scenic Point appears to be the consequence of the ju x ta p o s itio n of younger rocks over older rocks along the Brave Dog f a u l t . The Brave Dog f a u l t lies p a r a l l e l to bedding in its upper plate in the study area. I t , however, truncates 88 Figure 3-13. a. Truncational r e la tio n s h ip between the thrust f a u lt s of Mount Henry imbricate system and the Brave Dog f a u l t . This picture is viewed from north on the northern side of Squaw Mountain. About 200 m across the l e f t side to the r igh t side of the p ic t u re , b. Sketch of major structures shown in Figure 13a . 89 90 numerous east- and west-dipping minor contractio nal and extensional f a u l t s of the eastern be lt in i t s lower plate at Scenic Point and Squaw Mountain (Plates 1 and 2). However, the Brave Dog f a u l t is m u lti p ly o ffs e t by f a u l t s of the Mount Henry imbricate system in the eastern side of the study area. This r e l a t i o n s h i p is well exposed at Squaw Mountain. Fig. 3-13 documents that the Brave Dog f a u l t , p a r a lle l to bedding in its upper p l a t e , is displaced by more steeply dipping fa u lt s of the Mount Henry imbricate system. Mount Henry Imbricate System The Mount Henry thrust imbricate system (MHIS) is spectacularly exposed at Mount Henry in the southeastern corner of Glacier Park (Fig. 3 -1 4 ) . Southwest-dipping imbricate thrust fa u lts in t h is system are concentrated in a zone about 2 to 3 km wide. Their average s t r i k e is o o about N30 W (Fig. 3-15) and the dip angles vary from 10 to 6 5°. Displacements along the thrust imbricates range from 50 m to more than 300 m. Displacements are greater along the f a u l t s in the central part of the imbricate system, except the westernmost f a u l t along which the displacement is about 200 m (Fig. 3-14, Plate 2). The d i s t r i b u t i o n of displacement in the MHIS is d i f f e r e n t from the classic imbricate systems as defined by Dahlstrom 91 Figure 3-14. a. Mount Henry imbricate system at Mount Hen ry . b. Sketch of structures shown in Figure 3 -14a. Yap, the Appekunny Formation; Ygl, the Grinneil Formation. 92 93 Figure 3-15. Attitudes of mesoscopic contraction a 1 f a u lt s in the Mount Henry imbricate system. All the measurements shown here were taken within member 4 of the Appekunny Fm. and Grinnell F m. 94 s 95 (1970). In the c l a s s i f i c a t i o n of Dahlstrom, the maximum displacement is e i t h e r along the frontal f a u l t (leading edge imbricate system) or along the rear f a u l t ( t r a i l i n g edge imbricate system). Displacements along at le a s t some of thrust imbricates in the Mount Henry imbricate system appear to decrease upward (greate r displacement along thrust imbricates in t h e i r lower portions than that in the upper p o r t i o n s ) . The lower portions of the MHIS are well exposed at the base of Summit Mountain and southern flank of Eagle Ribs Mountain. Displacement along one of the MHIS thrusts on the southern side of Summit Mountain, determined by the o ffs e t of A1tyn- Appekunny contact, is about 500 m (Pla te 2 of K e lty , 1986). The lower level of this f a u l t was also mapped by Winn (unpublished map, 198 5) on the southern flank of Eagle Ribs Mountain where displacement along this f a u l t is about 400-450 m. The upper portion of this f a u l t is exposed on Bearhead Mountain in the study area where the displacement along the f a u l t is about 200-250 m. The imbricate thrusts in the Mount Henry imbricate system are g enerally p a r a l l e l to one another. The r e l a t i o n s h i p between the MHIS and the Lewis thrus t f a u l t was observed by Kelty (Kelty, 1986, p . 91-92) at Summit Mountain, where the imbricate f a u l t s f l a t t e n downward into 96 the Lewis t h r u s t . The lower portion of the imbricate system is also exposed on the north side of Appistoki Peak (P la te 1), where the dip angles of its c onstituent fa ult s are shallow, ge ne ra lly from 10° to 20°. The angles between the f a u l t s and the bedding are less than 2 0°. Its upper le vels of the imbricate system are well exposed on Mount Henry and Bear Head Mountain. The dip angles of the fa u lt s are steeper (5 0° to 6 5°) in t h e i r upper portions than that in lower ele v a tions , and the angles between the o o fa u lts and bedding increase upward from about 10 to 45 . The above observations indic ate that the geometry of the imbricate th r u s t f a u lts in the Mount Henry system is 1 i s t r i c . The Mount Henry imbricate system can be traced along s tr i k e northward to Rising Wolf Mountain, and southward to Squaw Mountain. Thrust f a u lt s of the system offs e t the Brave Dog f a u l t in the eastern part of the study area as mentioned above. The imbricate thrusts are themselves terminated at Rising Wolf Mountain by the Rockwell f a u l t , a low-angle f a u l t of regional extent that li e s s t r u c t u r a l l y above the Brave Dog f a u l t . This r e la t io n s h ip is described in the next section. Ro ck we 11 Fault The Rockwell f a u l t is a major in tra fo rm a tio n a 1 low- 97 angle f a u l t in the Grinnell Formation. On a regional scale, this f a u l t li e s p a r a lle l or s u b -p a r a lle l to bedding in the Grinnell Formation in the study area (Fig. 3-1, Plate 1 and 2). At Cloudcroft Peaks in the western side of the study area, this f a u l t truncates a broad syncline in it s upper plate (Fig. 3-22) and ramps down across the s t r a t i graphic section of its lower plate to the east in the d i r e c t io n of its te c to n ic t ran s p o rt (Fig. 3 -1 6 ) . The s t r a t i graphic throw across th is shallow east-dipping ramp is about 100 to 150 m. Such a geometry, that the Rockwell f a u l t cuts downsection s t r a t i g r a p h i c a l ly in the d i r e c t i o n of its tec to nic transport, in dicates that the Rockwell f a u l t is an extensional f a u l t along it s segment at Cloudcroft Peaks. At Caper Peak southeast of Cloudcroft Peaks, the Rockwell f a u l t is p a r a l l e l to bedding in both upper and lower plates. Further to the east on the western side of Rockwell Mountain, th is f a u l t l o c a l l y cuts upward across the s t r a t i graphic section of its upper plate to the east (Fig. 3-17). The dip of this west-dipping ramp is a few degrees to the southwest. Its length is about 1.5 km. Farther east, along the eastern side of Rockwell and Sinopah Mountains, the f a u l t surface becomes p a r a l l e l to bedding in Grinnell Formation in both its upper and lower plates ( F i g . 3 -1 7 ). It maintains the same s t r a t i graphic position towards the eastern side of the 98 Figure 3-16. The Rockwell f a u l t (RF) cuts down section to the east on the southern c l i f f of Cloudcroft Peaks. Viewed from southeast to northwest. Note that two minor contractio nal f a u l t s on the eastern side of the pictu re f l a t t e n downward into the Two Medicine Pass f a u l t (TMPF), but they steepen upward and appear to be truncated by the Rockwell f a u l t . On westernmost and central part of the p i c t u r e , south west-dipping c ontractio nal f a u l t s o f fs e t the Two Medicine Pass f a u l t which are terminated above by the Rockwell f a u l t . 99 1 00 Figure 3-17. The Rockwell f a u l t (RF) at Mount Rockwell. Viewed from southeast to northwest. Note th a t the f a u l t ramps up to the east on the southwestern side of Mt. Rockwell. The Rockwell f a u l t is o f f s e t by the Lone Walker f a u l t (LWF) for about 150 m. 1 01 102 w s ! study area. The Rockwell f a u l t surface is well exposed in the study area. Rocks immediately above and below the f a u l t surface are l i t t l e deformed along segments where the f a u l t is p a r a lle l or sub parall el to bedding. Fig. 3-18 shows the f a u l t surface exposed at Caper Peak where i t lies p a r a lle l to bedding in both upper and lower p la te s . The Rockwell f a u l t at t h i s l o c a l i t y is c haracteriz ed by a zone of black gouge about 3 to 5 cm t h i c k . Sedimentary structures d i r e c t l y above and below the f a u l t at this l o c a l i t y are l i t t l e deformed. In c o n tra s t, along the sha llow ly-d ipping f a u l t ramp at Rockwell Mountain, rocks adjacent to the f a u l t surface are highly deformed. Small- scale drag f o l d s , overturned to the northeast, in both hanging wall and footwall are well developed. The Rockwell f a u l t c l e a r l y truncates the westernmost f a u l t of the Mountain Henry imbricate system. Further to the east on the central part of Rising Wolf Mountain, minor splays of the imbricate thrusts in Mount Henry imbricate system are also terminated by the Rockwell f a u l t . The Rockwell f a u l t is o f f s e t at Mount Rockwell by the Lone Walker f a u l t (Fig. 3 - 1 7 ) , a high-angle reverse f a u l t . The Lone Walker f a u l t is described separately in t h i s chapter. 1 03 Figure 3-18. Gouge zone along the Rockwell f a u l t exposed, on the southern c l i f f of Caper Peak. 1 04 1 05 Two Medicine Pass Fault Two Medicine Pass f a u l t (TMPF) is a b e d d in g -p a r a ll e l f a u l t within the lower Grinnell Formation. This fa u lt li e s s t r u c t u r a l l y above the Brave Dog f a u l t and below the Rockwell f a u l t . The Two Medicine Pass f a u l t is best exposed at Two Medicine Pass in the central part of the study area. Many small-scale f o l d s , overturned to the northeast, are present immediately above and below the f a u l t surface. This f a u l t can be c l e a r l y traced westward to Caper Peak and Cloudcroft Peaks and eastward to Sinopah Mountain. On Cloudcroft Peaks, several minor contractional f a u l t s appear to o f f s e t the Two Medicine Pass f a u l t (F ig . 3 - 2 2 ). These minor f a u l t s , however, terminate upwards at the o v e rly in g Rockwell f a u l t . To the east from Two Medicine Pass, t h i s f a u l t appears to be a thrust f a u l t because i t ramps upsection to the east at a very shallow angle (several degrees) and is truncated by the Rockwell f a u l t at Sinopah Mountain. The above s tru c tu ra l r e l a t i o n s h i p indic ates that the Two Medicine Pass f a u l t is older than the Rockwell f a u l t . I t , thus, cannot be of same fa m ily of f a u l t s as the Rockwell f a u l t . The age r e la t io n s h ip between the Two Medicine Pass and Brave Dog f a u lt s is , however, not c le a r because no d i r e c t or in d ir e c t c r o s s -c u t ti n g r e la tio n s h ip s have been observed 106 in the study area. The amount of displacement along the Two Medicine Pass f a u l t is unknown. Akamina Syncline Akamina syncline (Fig. 3-19) (Dahlstrom, 1970) is a doubly plunging syncline that occupies the e n t i r e Lewis s a lie n t from North Kootenay Pass to Marias Pass. This syncline, the Lewis thrust f a u l t , and the Flathead normal f a u l t , were f i r s t recognized by W i l l i s (1902). Reg io n ally , the Akamina syncline is bounded by the Lewis th rus t to the east, and the F l a t h e a d -B la c k ta il normal f a u l t system to the west. The hinge of the syncline is p a r a l l e l to the s t r i k e of the Lewis thrus t f a u l t and other major s tr u c tu ra l trends in the Lewis allochthon. Within Glacier National Park, the hinge of the syncline follows along the Continental Divide and ends at the south-central edge of the park. Rocks in the upper plate of the Lewis t h r u s t , about 10 km south of Glacier Park, are also folded by a major syncline, the Continental Divide syncline (Mudge and Earhart, 1980; 1983). This syncline extends from the Middle Fork of the Flathead River, 10 km south of the southern boundary of G la c ie r Park, to the Dearborn River. The length of th is syncline is about 116 km (Fig. 1 of Mudge and Earhart, 1980). The axis of the Continental 1 07 F ig u re 3-19. General geology of the Flathead area (from D ah lstr o m , 197 0). 1 08 MACDONALD RANGE FOLDS AYLOR RANGE FOLDS NORTH KOOTENAY PASS CATE a HAIG WINDOWS FERNIE BASIN HOWELL CREEK WINDOW T E R TIA R Y (KISHENEHN FM.) | | c r e t a c e o u s JU RASSIC TO T R IA S S IC PALEOZOIC PR O TER O ZO IC TH R U ST NORMAL FAULT PLUNGE OF FOLDS M ARIAS PASS M IL E S Area 109 Divide syncline s t r i k e s southerly through T r i n i t y and S i l v e r t i p Mountains, along the White River, to Junction Mountain. The trac e of fold axis along th is segment is about 20-30 km south of the Continental Divide. From Junction Mountain southward, the axis follows along the Continental Divide. The axis of the Continental Divide syncline, l i k e th a t of Akamina syncline, is also p a r a lle l to the s t r i k e of the Lewis th r u s t f a u l t . The minimum breadth of the Akamina syncline is about 23 km in the study area. Its amplitude is at le a s t 200 m using the Appekunny-Grinnell contact as the reference surface in the study area. The amplitude increase no rt h wa r d . The Brave Dog and Rockwell f a u l t s are folded by the syncline. Whether or not the Lewis th r u s t f a u l t is folded by this broad syncline is unknown in the study area. In the northern portion of the Lewis s a l i e n t , Canada, however, the Lewis thrust f a u l t is c le a r l y warped in the Akamina syncline ( P ric e , 1965; Dahlstrom et a l . , 1962). The Lewis th rus t is also l o c a l l y folded in the area mapped by Kelty in the southern edge of G la c ie r Park (1986). Structures in the Lone Walker Mountain Complicated mesoscopic fold and thrust s tru c tu re s are l o c a l l y present on Lone Walker Peak (Fig. 3 - 2 0 ) . These 110 Figure 3-20. View to the northwest of highly folded Grinnell Formation at Lone Walker Mountain. Robert Hansen provides scale. Beds are mostly overturned. Folds verge northeastward. 1 11 structures are truncated above by the planar Rockwell f a u l t (Fig. 3-21) and below by the Two Medicine Pass f a u l t . They cannot be traced to the north and south, and are i n t e r p r e t e d as local f e a tu r e s . Minor Contractional Faults at Cloudcroft Peaks Several c ontractio nal f a u l t s , dipping from 25 to 75 degrees to the southwest, are exposed on the south c l i f f of Cloudcroft Peaks (Fig. 3 -2 2 ). These f a u lt s were mapped from a distance because of i n a c c e s s i b i l i t y , and displacements along them appear to be small (several meters to several tens of m eters). The easternmost two contractio nal f a u l t s in Fig. 3-14 f l a t t e n downward into the Two Medicine Pass f a u l t and steepen upward. This s tru c tu ra l r e l a t i o n s h i p suggests that the Two Medicine Pass f a u l t is a t h r u s t . These two fa u lt s appear to be truncated from above at the Rockwell f a u l t . On the eastern side of Fig. 3-22, a f a u l t cuts across the Two Medicine Pass f a u l t . Drag folds in the hanging wall of this f a u l t indic ate s that i t is a reverse f a u l t . Offset seems to be very small, probably several tens of meters at most. This reverse f a u l t does not extend above the Rockwell f a u l t (Fig. 3 -2 2 ). This r e l a t i o n s h i p again suggests that the Rockwell f a u l t is younger than the Two Medicine f a u l t . Since displacements along these 1 1 3 Figure 3-21. Highly folded Grin ne ll quartz a r e n ite and a r g i l l i t e appear to be truncated by the plan ar Rockwell f a u l t above. The Rockwell f a u l t surface (RF) is concealed by ta lu s and v e g e ta tio n . Viewed from southwest to northeast at Lone Walker Mountain. 1 14 Figure 3-2?. A minor reverse f a u l t cuts across the Two Medicine Pass f a u l t (TMPF) and te rm inates by the Rockwell f a u l t (RF) on the southeastern side of Cloudcroft Peaks. Note that a white quartz a r e n ite bed is truncated from below by the Rockwell f a u l t . View is to the northwest. 116 117 contractio nal f a u l t s appear to be small and the surface of the Rockwell f a u l t is covered by t a l u s , i t is d i f f i c u l t to determine whether or not these minor c ontr actio nal f a u lt s are truncated by or merge with the Rockwell f a u l t . The r e l a t i o n s h i p between these contra c tio na l f a u l t s and the Brave Dog f a u l t s is also not c le a r because the Brave Dog f a u l t is mostly covered by ta lu a and morainal deposits. Lone Walker Fault The Lone Walker f a u l t is a high-angle contractio nal f a u l t (Fig. 3 - 2 3 ) . The average s t r i k e of the f a u l t , N7 0°W, is anomalously west of most s tru c tu ra l trends in the Lewis allochthon (N25°W). It dips from 4 5° to 7 0° to the southwest. Displacement along this f a u l t increases northwestward from about 30 m on the southwestern side of Squaw Mountain, to at least 500 m in the pass between Eaglehead Mountain and Mount Pinchot. The Lone Walker f a u l t o f f s e t s a ll s tru ctu res in the upper plate of the Lev;is th r u s t f a u l t (Plates 1, and 2). Whether or not the f a u l t offs ets the Lewis th ru s t f a u l t is not c le a r . I f the Lone Walker f a u l t maintains its planar geometry as seen at the surface near Eaglehead Mountain, i t must cut the Lewis thrus t f a u l t at depth as shown in Plate 2. However, i t is conceivable that t h i s f a u l t could f l a t t e n downward and merge with the Lewis t h r u s t f a u l t at 1 1 8 Figure 3-23. View of the Lone Walker f a u l t (LWF) to the eastsoutheast on the northwestern side of Mount Rockwell. Note that the Lone Walker f a u l t o ffs e ts the contact between the Grin nell Formation (Ygl) and Empire Formation (Yem). It also o f f s e t s the Rockwell f a u l t ( R F). The Lone Walker f a u l t dips steeply (ca 75 degrees) to the southwest. 1 1 9 1 20 depth. 121 CHAPTER 4: DEFORM AT I ONAL HISTORY OF THE LEWIS THRUST SYSTEM General Statement In t h i s chapter, I w i ll f i r s t discuss the developmental processes responsible for the formation of major s t r u c tu r a l components in the Lewis thrus t system, I w i l l then present a reconstru ction of s tru c tu ra l evolution of the Lewis allochthon in the study area based on c ro s s c u ttin g r e l a t i o n s h i p s . F i n a l l y , s tr u c tu r a l im p lic a tio n s of the deformational h isto ry of the Lewis t h r u s t system w i ll be discussed. Secondary structures are used to determine d i re c tio n s of displacement along the Lewis th r u s t and f a u l t s within the Lewis allochthon, and c r o s s - c u t t i n g r e l a t i o n s h i p s are used to determine the r e l a t i v e timing of each deformational event. S t r i a t i o n s on f a u l t surfaces in the study area are t y p i c a l l y defined by slickensided grooves or mineral f i b e r s . S t r i a t i o n s on f a u l t surfaces are the results of f r i c t i o n a l s l i d i n g along f a u l t s at low temperatures (Suppe, 198 5). S t r i a t i o n s usually do not provide unique information about s lip d i r e c t io n s along f a u l t s . In order to determine the displacement vectors along f a u l t s in the Lewis a llo c h t h o n , s t r i a t i o n s on f a u l t surfaces in conjunction with o f f s e t s of marker beds are 122 used. For the f a u lt s having prolonged h isto ry of movement, the d i r e c t i o n of te c t o n i c tran sport along these f a u l t s may have changed with time during t h e i r development. Later movements along those f a u l t s may have wiped out s t r i a t i o n s on the f a u l t surfaces resulted from e a r l i e r movement, and l e f t s t r i a t i o n s only r e f l e c t i n g the l a t e s t movement. "Drag" f o l d s , which formed e i t h e r as the r e s u lt of f r i c t i o n a l drag across f a u l t s , or by bending of s t r a t a p r i o r to f a u l t rupture, are also re la ted to the kinematics of f a u l t s . The r e l a t i o n s h i p between t h e i r o r i e n t a t i o n s and the t r a n s p o r t d ire c tio n s of associated f a u l t s can be v a r i a b l e . In the study area, however, hinges of small- scale drag folds (wavelength less than 3 m) are g enerally p a r a l l e l to the hinges of mappable folds (wavelength greater than 30 m). The hinges of mappable folds are l i t t l e curved and p a r a l l e l to the average s t r i k e of fa u lts in the upper plate of the Lewis th ru s t (P la te 1). Fold hinges and s t r i k e s of f a u l t s are in turn e it h e r o perpendicular to or l i e at a high angle (> 75 ) to , the i n f e r r e d tran s port d i r e c t i o n of the Lewis thrust determined by s t r i a t i o n s on the f a u l t surface. Lewis Thrust Fault As described in Chapter 3, the surface of the Lewis 123 th ru s t is mostly covered by ta lu s which makes i t d i f f i c u l t to measure s t r i a t i o n s d i r e c t l y on the Lewis th r u s t f a u l t . The only exposure of the Lewis t h r u s t surface in the study area is found at the ju nc tio n between the western headwater of Forty One Mile Creek and the eastern side of Bison Mountain in the eastern map area (P la te 1). The a t t i t u d e of the s t r i a t i o n measured d i r e c t l y on the fa u lt surface at th i s l o c a l i t y is S56°W 8° (Fig. 4 - 1 ) . A few s t r i a t i o n s on shear surfaces immediately below the Lewis th r u s t surface in the 1ower-plate Cretaceous sedimentary rocks were also measured at the same l o c a l i t y . The o o average trend of these s t r i a t i o n s is about N75 E-S75 W, S t r i a t i o n s on shear surfaces w ith in the Altyn Formation approximately 10 to 20 m above the Lewis t h r u s t f a u l t , were measured at the northeastern corner of Scenic Point (P la t e 1). Trends of these data are more scattered than those on the eastern side of Bison Mountain. This may be p a r t i a l l y a t t r i b u t e d to t h e i r g r e a ter distance from the surface of the Lewis t h r u s t . The average trend of s t r i a t i o n s measured in the Altyn Formation is about N45oE- S45oW. Neverthele ss, all s t r i a t i o n measurements f a l l in the NE and S W quadrants (Fig. 4 - 1 ) . The l i m i t e d measurements of s t r i a t i o n s on and adjacent to the Lewis thrust surface suggest that the average tran s port d i r e c t i o n of the Lewis thrust at the eastern edge of the 124 Figure 4-1. Stereographic p r o j e c t i o n of s t r i a t i o n s (n=17) measured on the Lewis th r u s t surface and on shear surfaces above and below the f a u l t along the eastern side of Bison Mountain and northwestern side of Scenic Point (see d e t a i l e d d escriptio ns in the t e x t ) . Note t h a t the trend of s t r i a e measured from the upper plate is about 20-30 degrees more n o r t h e r ly than those measured from lower p l a t e . The trend of s t r i a e measured from the upper plate is more-or-1ess perpendicula r to the average trend of fold hinges measured from the fr o n t a l zone (Cf. Fig. 4 - 2 ) . 1 25 u s m : Measurements from upper pLate, northern Scenic Point. a : Measurements from lower plate, eastern Bison Mountain. 0 : Measurement directly taken from the surface of the Lewis thrust. 1 26 map area is approximately N65oE. This d i r e c t i o n is sub perpendicular ( g r e a te r than 80 degrees) to the average trend of s tru c tu re s in the Lewis allochthon in the study area (Pla te 1). Displacement along the Lewis t h r u s t f a u l t in southern Glacier Park has not been p r e c is e ly determined. The minimum displacement along the Lewis thrus t f a u l t is 24 km. This is obtained by e s tim atin g the distance between the northe asternmost exposure of a l 1ochthonous rocks at Head Mountain ( P la t e 1) and the southwesternmost exposure of autochthonous rocks at Elk Mountain ( K e l t y , 1986) in the tr ansport d i r e c t i o n (N65°E) of the Lewis t h r u s t . Frontal Zone Hinges of f a u l t - a d j a c e n t folds ("drag" folds ) in the fr o n ta l zone have been measured for kinematic a n a l y s i s . These data are p lo tte d in Fig. 4-2. The trends of the hinges f a l l between N45°W-S45°E and N-S. Their average o trend is about N20 W, which is consistent with the average s t r i k e of f a u l t s in the fr o n t a l zone (Figs. 3-9 and 3 - 1 0 ) . Displacements along f a u l t s in the fr o n ta l zone range from several meters to more than 150 m. Structu res in the frontal zone are very complex (see de s c rip tio n in Chapter 3). At Head Mountain, a west- dipping, e a s t - d i r e c t e d c o n tra c tio n a l f a u l t is cut by a 127 Figure 4 2. Stereographic p r o je c t ion ( n = 17) measured from the of drag fold hinges fr o n ta l zone. 1 28 N 1 29 east dipping, w e s t-d ir e c te d contr act i onal f a u l t . Towards south, this e a s t -d ip p i n g f a u l t , f a u l t 6 in Fig. 3-6b, changes i t s dip d i r e c t i o n from east to west at Bison Mountain. Fault G at Bison Mountain cuts a w e s t -d ir e c t e d f a u l t zone and a east-ve rgent f o l d . But f a u l t G i t s e l f is truncated from below by the Lewis t h r u s t . The complexity of the f r o n t a l - z o n e s tru c tu re s is evident in a photo-based cross section on the southeastern side of Bison Mountain (Fig. 3 - 6 ) . This section li e s in o o the d i r e c t i o n of N 5 0 E-S50 W , s u b - p a r a l l e l to the d i r e c t i o n of t e c to n i c transport of the Lewis th ru s t f a u l t . S t r a ti g r a p h y at th i s l o c a l i t y is well understood. The c1i f f - fo r m in g , w h itis h - y e l 1 ow un it in the lower part of Fig. 3-6a is the Altyn Formation, and the slope-forming rock unit beneath the Lewis t h r u s t is the la te Cretaceous Marias River Shale of Mudge and Earhart (1983). The Lewis thrust is very planar at this l o c a l i t y . Marker bed A of the Appekunny Formation d i r e c t l y ov e r lie s the Altyn Formation. Marker bed B of the Appekunny Formation is not c l e a r l y shown on the p ic tu re due to the poor contrast of l i g h t . It is, however, e a s ily located in the f i e l d (Fig. 3-6b). The t h i c k c l i f f - f o r m i n g unit at the upper part of Fig. 3-6a is marker bed C. In the northeastern h a l f of Fig. 3-6a, the Altyn Formation and the lower part of the Appekunny Formation 1 30 are folded and overturned to the northeast. Folds are cut by two west- d i p p i n g , high-angle f a u l t s ( f a u l t s E and F) which show normal-sense of displacement along them. These high-angle f a u l t s are term inated by an e a s t - d i p p i n g , c o n tr a c tio n a l f a u l t zone conta in in g f a u l t s B, C, and D. Faults E and F can be e i t h e r splays of f a u l t D, or f a u l t s truncated and displaced by the e a s t e r ly dipping f a u lt zone, f a u l t s B, C, and D, c o l l e c t i v e l y . Because the amount of displacement along the f a u l t zone is p r e c is e ly determined by the o f f s e t of marker bed C, the po sitio ns of the two f a u l t s in the footwall of the f a u l t zone can be determined i f they were indeed truncated and disp la ced. The predicted f a u l t s were, however, not f o u n d in the fo o tw a ll of this east-d ip ping f a u l t zone. Since no other s t r u c t u r a l com plexities tha t can explain the absence of the two w e s t-dip pin g, high-angle normal f a u l t s in the footwall of the e a s t e r ly dipping f a u l t zone, i t is l i k e l y t h a t the two f a u l t s , f a u l t s E and F, are splays of the e a s t-d ip p in g f a u l t zone. This i n t e r p r e t a t i o n appears at f i r s t problematic since the e a s t e r l y dipping f a u l t zone shows r e ve rs e - sense of displacement, whereas the two west- dipping f a u l t s show a normal-sense. However, as the geometry of the e a st-d ipping f a u l t zone is l i s t r i c , movement along the zone would cause r o t a t io n of beds, as well as f a u l t surfaces, in i t s hanging w a ll . Thus, the 131 two westerly dipping apparent normal f a u l t s , f a u l t s E and F, may have formed by the r o t a t i o n of primary reverse f a u l t s along l i s t r i c reverse f a u l t s of the e a s te r ly dipping f a u l t zone. It is also possible tha t the r o t a t i o n of f a u l t s E and F were caused in a dditio n by a l a t e r top- t o - t h e - e a s t simple shear (see discussion below). The e a s t-d ip p in g f a u l t zone consists of three major f a u l t s : f a u l t s B (FB), C (FC), and D (FD) (Fig. 3 -6 b ). FB and FD bounds marker bed A (MBA) at its bottom and top. FC li e s between MBA and marker bed B (MBB), Only the upper part of MBA is present in t h i s f a u l t zone. Between FC and FD is a complexly fa ult e d s t ru c tu ra l package. The e a s te r ly dipping c ontra c tions ! f a u l t zone truncates a f a u l t , f a u l t H (FH), and MBA in its f o o t w a l l . FH and MBA in the footwall of f a u l t B are t i g h t l y folded. A reverse f a u l t , f a u l t A (FA), is present in the hanging wall of the e a s t-d ip p in g , high -angle f a u l t zone (FB, FC, and FD). This f a u l t can be e i t h e r the upper part of FH displaced by the high-angle f a u l t zone, or a splay of the high-angle f a u l t zone. As the amounts of displacement along FH and FA are s i m i l a r (about 80 m), and FH is truncated by fa u lt B, C, and D, i t is l i k e l y that the FA and FH were the same f a u l t and were o f f s e t l a t e r by the east-d ip ping high-angle f a u l t zone. This i n t e r p r e t a t i o n implies th a t FA and FH are older than the e a st-d ipping high-angle f a u l t zone. 132 The f a u l t zone (FB, FC, FD) is cut by a westerly dipping high-angle f a u l t , Fault G (FG), which juxtaposes the Altyn Formation in it s footwall with the Appekunny Formation in i t s hanging wall at Bison Mountain. This f a u l t and the units in the Altyn Formation in it s footwall are cut o f f from below by the Lewis t h r u s t . Several minor s y n th e tic th r u s t f a u l t s , f a u l t s I , J, K, and L, appear to branch o ff from the Lewis t h r u s t s ur fa c e . C ro s s -c u ttin g r e l a t i o n s h i p s , kinematic data, and o ff s e ts of marker beds provide foundations f o r the p a l i n s p a s t i c re constru ction of the fronta l zone. Fig. 4-3 is an attempt to restore deformation of the fr o n t a l zone at Bison Mountain. Constant thickness of s t r a t i graphic un its and f l e x u r a l - s l i p fo l d in g mechanism are assumed during the development of the fronta l zone. Field observations in the fronta l zone in dic ate th a t these assumptions are a p p r o p ria te . Stage 1: A b e d d in g -c o n tro lle d f a u l t developed from the base of marker bed A ( f a u l t A). This f a u l t cuts upsect ion a b r u p tly through the lower part of the Appekunny Formation to the southwest ( F i g . 4 -3a ). Stage 2: New thrus t f a u l t s ( f a u l t s B, C, and D) were developed at the middle and top of MBA (Fig. 4-3b) which cut upsection through the Appekunny Formation to the southeast. These f a u l t s , dipping shallower than f a u l t A, 1 33 o f f s e t this older f a u l t (Fig, 4 - 3 c ) . Stage 3: Faults B, C, and D merge upwards with each other and produced a complexly fa u lte d s t r u c tu r a l package between f a u l t s B and D (Fig. 4 -3 d ). The order of deformational sequence for the formation of the three f a u l t s (B, C and D) can be in terc h a n g e a b le. Folding of the segment of f a u l t A and MBA in the footwall of f a u l t B (Fig. 4-3d) might have been caused by the movement along this f a u l t . It is also possible that f a u l t A was t i g h t l y folded f i r s t and then truncated and displaced by f a u l t B. This a l t e r n a t i v e i n t e r p r e t a t i o n is not favored because displacement along the f a u l t zone is known, but no t i g h t l y folded s t r u c t u r a l package was found at the predicted positio n of the hanging wall of the f a u l t zone (FB, FC, and FD in Fig. 3- 6 b ) . The e a st-v e rg e n t a n t i c l i n e in the hanging wall of the f a u l t zone ( f a u l t s B, C, and D) is the r e s u lt of movement along the l i s t r i c f a u l t surfaces of f a u l t s B, C, and D. Stage 4, Small splays of reverse f a u l t s , f a u l t s E and F, were branched o f f from f a u l t D. As the reverse f a u l t s were transported along the l i s t r i c f a u l t surface of f a u lt D, these f a u l t s (E and F) were ro t a t e d . Such a r o t a t i o n in addition with a possible top--to -t h e-e a s t simple shear (see below) may have caused transforming the primary reverse f a u l t s (E and F) to apparent normal f a u l t s . This 134 i n t e r p r e t a t i o n also helps to explain why "drag" folds present in the footwall of west-dipping high-angle f a u l t s ( f a u l t s E and F in Figs. 3-6b) and changes in dip d i r e c t io n s along some reverse f a u l t s in the fr o n t a l zone. Stage 5: Two kinematic processes are possible to have produced the observed s t r u c t u r e s . These two possible processes are shown in Figs. 4-3e and 4 - 3 f , and Figs. 4- 3 g , 4 - 3 h , 4 - 3 i and 4 - 3 j , r e s p e c t i v e l y . (1) The stru ctures developed in the previous stages were folded (Fig. 4-3e) and then cut by a westerly dipping high-angle extensional f a u l t , f a u l t G (Fig. 4 — 3 f ). The westerly dipping extensional f a u l t s was truncated and o f f s e t from below by the present Lewis thrust f a u l t . (2) A southwest-directed contra c tio na l f a u l t , f a u l t G, was developed (Fig. 4 — 3 g ) . This c ontra c tiona l f a u l t cut stru ctu re s developed e a r l i e r ( F i g . 4 -3h) , and was then folded and rotate d by an e a s t - d i r e c t e d , b e d d i n g -p a r a l l e l simple shear. The simple shear event caused f a u l t G to be rotated to the east, e x h i b i t i n g an apparent extensional geometry at Bison Mountain (Fig. 4 - 3 i ) . The younger Lewis th r u s t f a u l t cut th i s folded westerly dipping c ontra c tiona l f a u l t , f a u l t G, and displaced i t s upper- p l a t e portion to the northeast ( F i g . 4 - 3 j ) . I n t e r p r e t a t i o n (1) requires major e a s t - d i r e c t e d f o l d in g of the e n t i r e fronta l zone followed by development 1 35 of a high-angle extensional f a u l t . I n t e r p r e t a t i o n (2) implies that the development of f a u l t G in Fig. 3 - 6 b , and th a t the e n t i r e a n t i t h e t i c t h r u s t / r e v e r s e f a u l t sequences is l a t e r ro ta t e d by a t o p - t o - t h e - e a s t shear. I n t e r p r e t a t i o n 2 is favored because i t explains changes in dip d ire c tio n s along s t r i k e s of some f a u l t s in the f r o n t a l zone. The reasons for the required t o p - t o - t h e - e a s t shear are not known. I t was possibly r e la t e d to the emplacement of the Brave Dog f a u l t . No matter which i n t e r p r e t a t i o n in stage 5 is c o r r e c t, the tr u n c a tio n a l r e l a t i o n s h i p between (1) the westerly dipping planar f a u l t ( f a u l t G) and (2) bedding of the Altyn Formation in the hanging wall of f a u l t G and the Lewis th ru s t f a u l t requires that the Lewis f a u l t be a younger s t r u c tu r e which o ffs e ts e a r l i e r s t ru c tu re s in the overly in g fr o n t a l zone (Fig, 4 - 3 ,j ) . This s tru c tu ra l r e l a t io n s h ip th a t s tru ctu res in the fr o n ta l zone are truncated from below by the present Lewis t h r u s t was also observed by Davis (Davis, unpublished map) and Hudec (Hudec, 198 7) in the areas f a r t h e r to the no rth. Stage 6: Minor w e s te rly dipping c ontra c tiona l f a u l t s , f a u l t s I, J, K, and L in Fig. 3-6b, were developed a f t e r the formation of the Lewis t h r u s t . They were probably a r e s u lt of motion along the Lewis t h r u s t f a u l t . It is important to point out that the photo-based 1 36 Figure 4-3. P a li n s p a s ti c recons tru ctions of the f r o n ta l zone. Figure 4-3a to 4-3j shows possible deformational episodes responsible for the development of s t r u c tu r e s in the f r o n t a l zone. MBA, MBB, and MBC are marker beds in the Appekunny Formation. 1 37 \ % Fault A \ \------------- MFC ^----- \ \ \ \ mv, --------------------------------------------------------------~ MBA Altyn Fm. Fault A Fault B, C, D. MBC Fault D MBB Fault C M B A Fault B Altyn Fault A Fault E MBC Fault F Fault D Fault A Fault C MBA Fault B Altyn Fm. o Fault A Fault E Fault B, C, D. , Fault F MBC shear zone N between Fault C and Fault D. Fault D MBB Fault C folded Fault A MBA FaultB Altyn Fm. Fault A Fault B ,C,D Fault E Fault MBC Fault /l Fault B , C ,D and shear zone folded Fault MBA Altyn- Fm. e . 1 42 143 Fault A ult E Fault ault G ault F\ MBC Fault B,C,D and shear zone MBB folded Fault A MBA — Fault G Altyn Fm. f. 144 iFault G Fault A Fault B,C,D Fault Fault B,C,D nn c \ sIipoi" zone folded Fault A MBA Fault G Altyn Fm. I'mi It A M B C MBB MBA A1 tyn Fm/ Fn u l t n.C.D vv Fault B,C,D and shear zone folded Fault A ✓ / /? minor thrusts ault G Fault E Fault — — Lewis thrust Fault G h. I-1 U 1 917 L Fault G Fault A F a u lt BjCjD Fault E 'Fault MBC Fault B,C,D and j shear zone M B B folded Fault A MBA Lewis thrust ^ tyu minor thrusts Fm. i . cross section in Fig. 3-6 only shows a portion of the fr o n ta l zone. A complete deformational h i s t o r y of the f r o n t a l zone may be more complex. The high i n t e n s i t y of deformation and a v a r i e t y of deformational featu re s in the f r o n t a l zone contrast strongly to other s t r u c t u r a l elements in the Lewis t h r u s t system (Plates 1 and 2) . Its o r ig in is , however, enigmatic. As the formation of the fr o n ta l zone predates the present Lewis t h r u s t , it s development may have been associated with a th r u s t f a u l t which was responsible for an e arly emplacement of the Lewis a llo c hthon . Hudec (1987) po stulated t h a t the development of the f r o n t a l zone was associated with a west-dipping th r u s t ramp along a major t h r u s t , the Lewis th r u s t I , tha t predates the Lewis t h r u s t . This i n t e r p r e t a t i o n is consis tent with a mechanical model proposed by Wiltschko (1979). Wiltschko showed th a t a t h r u s t ramp is a c i t e of stress c o n c e n tr a tio n . Its existence can cause development of extensional and c o n tr a c tio n a l f a u l t s . Hudec's proposal is, however, very d i f f i c u l t to be te s te d by the a v a i l a b l e data. As mentioned in Chapter 3, the boundary between the fr o n ta l zone and eastern s t r u c t u r a l b e lt are t r a n s i t i o n a l and somewhat a r b i t r a r i l y drawn. The deformati onal styles in the two s t r u c t u r a l belts are s i m i l a r except th a t 147 primary e a st-d ipping c o n tr a c tio n a l f a u l t s in the fronta l zone commonly dip more s teep ly than those in the eastern s t r u c t u r a l b e lt (see Chapter 3). As s tru c tu re s in the east belt are not rotated by the t o p - t o - t h e - east simple shear which caused r o t a t i o n of s tru ctu re s in the f r o n t a l zone, the eastern belt must postdate the fr o n ta l zone. No d i r e c t f i e l d r e l a t i o n s h i p s between the Brave Dog f a u l t and s tru c tu re s in the f r o n t a l zone are preserved. However, s tru c tu re s in the eastern belt are t ru n c a te d above by the Brave Dog f a u l t (see de scriptions in Chapter 3 and discussions in the next s e c t i o n ) , i n d i c a t i n g th a t the Brave Dog f a u l t postdates the eastern b e l t . As the eastern b e lt and f r o n ta l zone may have developed synchronously before the formation of the Lewis t h r u s t , the Brave Dog f a u l t should also postdate the f r o n t a l zone. Eastern S t r u c t u r a l Belt Three sets of kinematic data have been c o l l e c t e d from the Appekunny rocks of the eastern s t ru c tu ra l b e l t : hinges of folds th a t are not s p a t i a l l y associated with f a u l t s , s t r i a t i o n a l l i n e a t ions along bedding surfaces, and hinges of f a u l t - a s s o c i a t e d (drag) f o l d s . Folds th a t are not s p a t i a l l y associated with f a u l t s ( h e r e a f t e r r e fe r r e d to as t y p e -1 folds ) include small- scale folds (wavelength less than 2 m), and the mesoscopic 1 48 folds (wavelength g reater than 30 m). All mesoscopic type-1 folds are conc e ntric , and e x h i b i t symmetric geometries. Limbs of these mesoscopic folds dip at 2 shallow angles, us ua ll y less than 25 degrees. Sm a ll-s c a le type-1 f o l d s , however, are asymmetric in c h a r a c t e r . Their geometries range from open to t i g h t . Some type-1 small- scale folds verge toward hinges of mesoscopic a n t i c l i n e s . This is evident on the northern side of Scenic Point. Fig. 4-5 is a stereographic p r o j e c t i o n of axial planes measured from these s m all-scale f o l d s . Sampling positions and the vergence of sm all- s c ale folds are shown in Fig. 4- 4. The geometric r e l a t i o n s h i p shown in Figs. 4-4 and 4-5 suggests that the formation of some type-1 s m a ll- s c a le and mesoscopic folds were g e n e t i c a l l y r e la t e d ; i t also suggests tha t the f o l d in g mechanism for the formation of type-1 mesoscopic folds is f l e x u r a l - s l i p (Davis, 1984). Sm all-scale folds ge n e r a lly verge to the northeast where mesoscopic folds are absent. Those folds may have been r e la ted to the t o p - t o - t h e - e a s t , bedding p a r a l l e l simple shear. Hinges of mesoscopic folds were determined by the pi- diagram method (see Davis, 198 4). Fig. 3-9 shows o r i e n t a t i o n s of type-1 f o l d s . The pre fe rre d trend of the fold hinges is N20oW-S20oE which is p a r a l l e l to the average s t r i k e of minor f a u l t s in t h i s s t r u c tu r a l belt 149 Figure 4-4. Sketch of the r e l a t i o n s h i p between some s m a ll- s c a le folds and mesoscopic f o l d s . Measurements taken on the western slope of Scenic Point. Note that s m all-s c ale folds verge to the hinge of the a n t i c l i n e . Numbers shown on the sketch represent approximate po s itio n s of the measured small folds (Cf. Fig. 4 - 4 ) . 1 50 Figure 4-5. Stereographic p r o j e c t i o n of hinges and axial planes of the small folds shown in Figure 4 -3* Note that hinges and the axial planes were measured independently in the f i e l d . This has introduced errors and causes hinges to depart s l i g h t l y from the corresponding a x i a l - p l a n e great c i r c l e s . Solid li ne s represent measurements from e a s t - d ip p i n g fold limbs, the dashed lin e s represent measurements from west-dipping fold limbs. 152 w \ \ w H » o s • 1 53 (see Fig. 3 - 8 ) . In terbedding s t r i a t i o n a l ! i n e a t ions are common in the eastern b e l t . These l i n e a t ions are e i t h e r defined by grooved and l i n e a te d morphology, or by elongated mineral f i b e r s of quartz or c a l c i t e on bedding surfaces. These s t r i a t i o n a l l i n e a t ions are g e n e r a lly pe rp e n d ic u la r to the hinge of a s m a l l- s c a l e fold where seen near such folds ( F i g . 4 - 6 ) . This spatial r e l a t i o n s h i p supports t h a t the formation of some t y p e - 1 fo l d s , i f not a l l , was r e l a t e d to f l e x u r a l s l i p . O r ie n ta t io n s of these l i n e a t ions are p lo tte d in Fig. 4-7. They c l u s t e r in two average O o r i e n t a t i o n s : (1) N15 E trend and plunge n o rth e a s t, (2) S70oW trend and plunge southwest. Trends of the two sets of data are quite d i f f e r e n t , i . e . , about 55 degrees a part. O r ie n ta t io n s of the s t r i a t i o n a l l i n e a t ions from set (1) are more dispersed than those of set (2). This may be a t t r i b u t e d to a l a r g e r sampling area of set ( 1 ) than t h a t of set ( 2 ) . Data of set (1) were c olle cte d on the eastern side of Appistoki Peak and in the pass between Mount Henry and Appistoki Peak where mesoscopic folds are b a s i c a l l y absent or poorly developed. These measurements were mostly taken w i t h in member 4 of the Appekunny Formation which commonly contains along-bedding s lip surfaces. The average trend of set (1) is about 50 degrees from the average tr a n s p o r t d i r e c t i o n of the Lewis t h r u s t f a u l t 154 Figure 4-6, Li neat ions of mineral f ib e r s along bedding of the Appekunny Formation at Scenic P oint. Steps facing to the u p p e r - l e f t corner on the bedding surface in d ic a te s that the sense of shear is top to the r i g h t . Such steps are present across the fold hinge. This suggests t h a t fold is l a t e and has warped a s t r i a t e d bedding surface. 1 55 156 Figure 4-7. Stereographic p r o j e c t i o n of s t r i ations al bedding planes in the Appekunny Formation the eastern s t r u c tu r a l b e lt (n=18). ong i n 1 5 7 i s r s 1 r , p . L v..' ) (N65°E). Data from set (2) were a ll c o ll e c t e d from one l o c a l i t y on the northwestern corner of Scenic Point w i t h in member 2 of the Appekunny Formation. Mesoscopic folds are well developed near t h i s sample l o c a l i t y . The average trend of set 2 (N7 0°E-S7 0°W) is very close to the average tran s port d i r e c t i o n of the Lewis th r u s t f a u l t . The discrepancy between the two sets of data could be the consequence of e i t h e r two episodes of deformation in the eastern s t r u c t u r a l b e l t . However, no superpositions of s t r i a e of the two o r i e n t a t i o n s have been observed in the study area. Hinge measurements of f a u l t - a s s o c i a t e d f o l d s , drag folds ( h e r e a f t e r r e fe r r e d to as t y p e - 2 f o l d s ) , were also c o l l e c t e d from the eastern s t r u c t u r a l b e l t . Their o r i e n t a t i o n s are plotted in Fig. 4-8. The trends of 95% of type-2 fo ld hinges f a l l between N-S and N45oW-S45oE. o The average trend is about N 2 5 W, an o r i e n t a t i o n c o n s is t e n t with the predominant s t r i k e of f a u l t s in the eastern belt (Fig. 3 - 8 ) . Mesoscopic folds in the eastern s t r u c tu r a l b e lt must predate f a u l t i n g because folds in th i s b e lt are cut by f a u l t s in a nonsystemati c way. Structures in the eastern belt are trunc a te d above by the Brave Dog f a u l t as described in Chapter 3 (see Fig. 3 - 1 0 ) . They are t h e r e f o r e older than the Brave Dog f a u l t . 159 Figure 4-8- Stereographic p r o j e c t i o n of "drag" fold hinges along f a u l t s in the eastern s t r u c t u r a l b e l t ( n = 3 0 ) . 1 60 1 61 The r e l a t i o n s h i p between the Lewis t h r u s t f a u l t and s tru ctu res of the eastern belt is not c le a r in the study area. But s t ru c tu re s e q u i v a l e n t to those in both the f r o n t a l zone and eastern s t r u c t u r a l b e l t were mapped north of the study area by Hudec (1987) and Davis (unpublished map). Their mapping in d ic a te s th a t s tr u c tu r e s in both zones are truncated from below by the Lewis t h r u s t . Therefore, they are older than the Lewis t h r u s t f a u l t . Structures in the eastern belt are truncated and o f f s e t by the Mount Henry im bric ate th r u s t system at Appistoki Peak ( P la te 1), suggesting t h a t the eastern b e lt predates the Mount Henry im b r ic a te system. Scenic Point S t r u c tu r a l Complex At the n o rthe a s te rn corner of Scenic Point, several minor f a u l t surfaces in the Scenic Point s tr u c tu r a l complex are well exposed. Displacements along these f a u l t surfaces are d i f f i c u l t to estimate due to the lack of good marker beds in the Altyn Formation, but they appear to be small. A t t i t u d e s of measured f a u l t surfaces and s t r i ations on them are plotted in Fig. 3-5. Angles between the dip d i r e c t i o n s and s l i p d i r e c t i o n s on these f a u l t surfaces are less than 2 5 degrees f o r a l l eight measurements (F i g . 3 - 5 ) , i n d i c a t i n g that dip s l i p is the dominant s l i p component along these f a u l t s . S t r ik e s of 1 62 the f a u l t s are, with but one exception, in the NW quadrant. The trends of the s t r i a t i o n s shown in Fig. 3-5 range from N50°E-S5Q°W to E-W. The average trend is about N7 0°E-S70°W, which is about 10 degrees more e a s t e r l y than the in f e r r e d tr a n s p o r t d i r e c t i o n of the Lewis t h r u s t f a u l t ( N 6 5°E ) . The r e l a t i o n s h i p between the Lewis th r u s t f a u l t and s tru c tu re s in the Scenic Point s t r u c t u r a l complex is not c l e a r in the study area because the lowermost portion of the Scenic Point s t r u c t u r a l complex and the Lewis thrust f a u l t beneath i t are covered by t a l u s . However, s t ru c tu re s e q u iva len t to those in the Scenic Point s t r u c tu r a l complex were mapped by Hudec (1987) and Davis (unpublished map) in the areas north of the study area. Their mapping in d ic a te s t h a t the e quivalen t s t r u c tu r e s are trunc a te d below by the Lewis t h r u s t f a u l t . Based on t h i s , the Scenic Point s t r u c t u r a l complex predates the p r e s e n t ly exposed Lewis t h r u s t . On the western side of Fig. 3 -4 , t i l t e d beds of the Altyn Formation in the lower plate of the Scenic Point f a u l t are cut by the Scenic Point f a u l t . As the o f fs e t Altyn beds are not present in the preserved outcrops of upper plate Altyn rocks (see F i g . 3 - 4 ) , the minimum displacement of the Scenic Point f a u l t is at l e a s t 3 km i f a northeast tr a n s p o r t d i r e c t i o n is assumed (see Fig. 3 - 4 ) . 1 63 Some of the minor cont ract ional f a u l t s in the lower plate of the Scenic Point f a u l t are truncated above by the Scenic Point f a u l t . This is p a r t i c u l a r l y evident on the western side of Figure 3-4. Such r e l a t i o n s h i p i n d i c a t e s th a t the Scenic Point f a u l t postdates at le a s t some of the c o n t r a c t io n a l f a u l t s in i t s lower p l a t e . Because the Scenic Point f a u l t is fo l d e d , a foldin g event must postdate the formation of the Scenic Point f a u l t . However, the f oldin g event may be synchronous with the formation of some minor c o n tr a c t i onal and extensi onal f a u l t s w i t h in the Scenic Point s t r u c t u r a l complex, as c o n t r a c t i o n a l f a u l t s are us uall y present beneath hinges of a n t i c l i n e s whereas extensional f a u l t s are us uall y present beneath hinges of synclines. Faults e q u iv a le n t to those in the f r o n t a l zone and eastern b e lt cut the f a u l t e q u i v a l e n t to the Scenic Point f a u l t , the Spot Mountain f a u l t , in the area mapped by Hudec (198 7). This r e l a t i o n s h i p suggests th a t the Scenic Point f a u l t predates s tru c tu re s in the fr o n ta l zone and eastern b e l t . It t h e r e f o r e predates the Lewis t h r u s t as the f r o n t a l zone is older than the present Lewis t h r u s t . The Scenic Point f a u l t probably postdates the minor normal f a u l t s above them as discussed e a r l i e r in Chapter 3. 1 64 Brave Dog Fault The general d i r e c t i o n of movement along the Brave Dog f a u l t is eastward. This was determined by Kelty (1986) on the south side of Brave Dog Mountain where cleavage and mesoscopic f a u l t s immediately below the f a u l t feather upward into i t . This was i n t e r p r e t e d by Kelty (19861 as the r e s u lt of drag along the Brave Dog f a u l t which gives a top - t o - t h e - e a s t sense of displacement along the f a u l t . S t r i a t i o n s on a bedding surface d i r e c t l y above the Brave Dog f a u l t were measured by Kelty (19861 at the south O O O o side of Brave Dog Mountain, as N 7 5 E 6 and N 6 5 E 9 . Since the bedding plane is p a r a l l e l to and close to the Brave Dog f a u l t , the s t r i a t i o n s were i n t e r p r e t e d as due to movement along the Brave Dog f a u l t ( K e l t y , 1986). I f this i n t e r p r e t a t i o n is c o r r e c t , the t r a n s p o r t d i r e c t i o n of the Brave Dog f a u l t is about N75oE. In the study area, the Brave Dog f a u l t is mostly covered by t a l u s . Several s m a ll- s c a le fold hinges were measured approximately 1-3 m above the f a u l t on the western side of Scenic Point. The fold hinges ( F i g . 4-9) appear to be d i s t r i b u t e d in two sets: the average trend of o one is about N15 W, whereas t h a t of the second is about o S45 E. The dispersio n of the fo l d - h in g e o r i e n t a t i o n s may or may not be the r e s u l t of a complex h i s t o r y of movement along the Brave Dog f a u l t . 165 Figure 4 - 9. Stereographic p r o j e c t i o n of hinges of mesoscopic drag folds along the Brave Dog f a u l t c o ll e c te d at the west side of Scenic Point ( n = 8 ) . 1 66 167 The Elk Mountain im b ric a te system is truncated above by the Brave Dog f a u l t at Brave Dog Mountain on the southwestern corner of G l a c i e r Park in the area mapped by Kelty (198 6 ). This r e l a t i o n s h i p in d ic a t e s th a t the Brave Dog f a u l t postdates the Elk Mountain im bric ate thrust system. Exposure of the upper plate of the Brave Dog f a u l t can be traced 23 km continuously northeastwa rd from Brave Dog Mountain (in the d i r e c t i o n of t r a n s p o r t ) to Squaw Mountain fo r 23 km (PI. 1, K e l t y , 1986; PI. 1, this study). Because the truncated part of the Elk Mountain imbricate system is not found in the upper p l a te of the Brave Dog f a u l t in the present Lewis a llo c h th o n , the minimum displacement along t h i s f a u l t is i n t e r p r e t e d to be 23 km (also see K e l t y , 1986). The Brave Dog f a u l t is m u l t i p l y o f f s e t in the eastern part of the study area by the Mount Henry im bric ate system (Chapter 3 ). Thus, the Brave Dog f a u l t predates the development of the Mount Henry im bric ate t h r u s t system. Mount Henry Im bric a te System Sm all-scale folds present along f a u l t s of the Mount Henry im bric ate system were measured on the southwestern side of Mount Henry with in the Grinnell Formation (see Fig. 3 - 1 2 ) . These folds are only developed w i t h in 1-2 m of the f a u l t s . They are g e n e r a lly small with wavelengths 1 68 being less than 2 m. The o r i e n t a t i o n s of the fold hinges range from N-S to N6 O0 W-S6 O0 E (Fig. 4 - 1 0 V The average trend of the fold hinges is about N3 OoW •• S 3 Go E , an o r i e n t a t i o n compatible with the s t r i k e s of f a u l t s in the Mount Henry imbricate system which range from N25oW to N35oW (PI- 1)* A single measurement of s t r i a e from a f a u l t in the central part of the Mount Henry system is N7 5 0 W 4 3o . Displacements along the im b r ic a t e thrusts vary (see Chapter 3). The shortening r a t i o (present length v.s. o r i g i n a l length) across the Mount Henry im bricate system is approximately 5 0%. The sequence of f a u l t i n g for the development of the Mount Henry imbric ate th r u s t system is poorly known because no c r o s s - c u t t i n g r e l a t i o n s h i p s between imbricate thrusts in the Mount Henry im bric ate system were observed in the f i e l d . T r a d i t i o n a l l y , forward and hindward development were proposed as two possible sequences of im bric ate f a u l t i n g [Dahlstrom, 1970). Dahlstrom (1970) proposed that geometry of im bric ate f a u l t s and the d i s t r i b u t i o n of displacement along them can provide di agnostic in fo rm a tio n on the sequence of f a u l t i n g during the development of im bricate t h r u s t systems. For the development of a leading edge imbricate system, the sequence of f a u l t i n g is hindward. Whereas for the 1 69 F i g u r e 4-10. S t e r e o g r a p h i c p r o j e c t i o n of hinges of " dra g " f o l d s measured from th e Mount Henry i m b r i c a t e system (n = 2 3). 1 70 1 7 1 development of a t r a i l i n g edge im bricate system, the sequence of f a u l t i n g is forward. Figs. 4-11 and 4-12 are t e s ts of possible kinematic processes responsib le for the formation of the Mount Henry im bric a te system. The constructio ns were accomplished by hand drawing assuming th a t the thicknesses of s t r a t i graphic units remained constant during the formation of the im bric ate system. Only two possible sequences of f a u l t i n g , forward and hindward, were examined. The i n i t i a l geometry of each im bric a te t h r u s t in the Mount Henry imbricate is assumed to be s i m i l a r to each othe r. The imbricate system is s i m p l i f i e d into 6 major imbricate t h r u s ts . Geometry of the im bric ate t h r u s t s , spacing between the t h r u s t f a u l t s , and displacements along the imbricate thrus ts shown in Figs. 4-11 and 4-12 are a p p r o p r i a t e l y scaled from the cross section in P la te 2. The f in a l f a u l t patterns produced by the two f a u l t i n g sequences are s l i g h t l y d i f f e r e n t . The model of forward development shown in Fig. 4-11 p redic ts systematic steepening of dip angles of the im bricate th r u s t assuming a l l the im b r ic a te thrusts were i n i t i a t e d in a s im il a r geometry. Fig. 4-12 shows kinematic processes of hindward development of im b r ic a te f a u l t i n g . This model f i t s f i e l d observations (Fig. 3-12) b e t t e r because no systematic steepening of each formed im bric a te thrusts is required 1 72 F i g u r e 4-11. A k i n e m a t i c model f o r a fo rw a rd development of the Mount Henry i m b r i c a t e system. 1 73 Q < r > ® © 1 74 F i g u r e 4 -12. A k i n e m a t i c model f o r a hindward development of th e Mount Henry i m b r i c a t e system. 1 75 C P (?) 1 76 during th i s process. I t is important to point out tha t forward and hindward development of imbric ate systems are only two of many possible end-member sequences of im b ric a te t h r u s t i n g . Mechanically, th e re is no reason to preclude other possible sequences of f a u l t i n g . Another problem is that geometry and d i s t r i b u t i o n of displacement in an im bric ate system is not n e c e ss a r ily d iag n o s tic for a unique determination of sequence of im bric ate t h r u s t i n g . This has been demonstrated by using computer-synthesized s t r u c tu r a l cross sections (John, 1984). However, the geometric constructio ns do help to exclude impossible sequences of f a u l t i n g . In a f u t u r e study of t h r u s t kinematics of the Mount Henry im b r ic a t e system planned by the w r i t e r , computer-synthesized s t r u c t u r a l cross sections w i l l be prepared. Because the Mount Henry im b ric a te t h r u s t system offsets the Brave Dog f a u l t and s t r u c tu r e s in the eastern b e lt (Chapter 3 ) , i t predates these s t r u c tu r a l elements. As described in Chapter 3, t h r u s t f a u l t s in the Mount Henry im bric a te system f l a t t e n downward and merge at depth with the Lewis t h r u s t f a u l t , a geometric r e l a t i o n s h i p which suggests th a t the Mount Henry im bric a te system formed during movement along the Lewis t h r u s t f a u l t . Thrusts in the Mount Henry im b r ic a te system are truncated 1 7 7 above by the Rockwell f a u l t (Chapter 3 ) , but this r e l a t i o n s h i p does not enable de te rm ina tio n of the r e l a t i v e age of la s t movements on the Rockwell and Lewis f a u l t s . Elk Mountain Im bricate System The t r a n s p o r t d i r e c t i o n for the t h r u s t f a u l t s in the Elk Mountain im bric a te system is poorly constrained ( K e l t y , 1986). The general trend of the im b r ic a te system is in the d i r e c t i o n of N30°W, which is approximately perpendicular to the t r a n s p o r t d i r e c t i o n of the Lewis t h r u s t (N66°E) as determined by Kelty (1986) in the area near Marias Pass. S t r u c t u r a l l y higher im b ric a te f a u l t s l o c a l l y cut and o f f s e t s t r u c t u r a l l y lower f a u l t s in the Elk Mountain im bric a te system. This r e l a t i o n s h i p was i n t e r p r e t e d by K e lty as evidence th a t the im b r ic a t e system must have, at l e a s t l o c a l l y , developed hindward ( K e l t y , 1 986) . Rock we 11 Fault S t r i a t i o n s on the Rockwell f a u l t surface were measured at one l o c a l i t y on the south c l i f f of Caper Peak. Their average o r i e n t a t i o n is N45°E 5° (Fig. 4 - 1 3 ) . Small- scale folds immediately beneath the Rockwell f a u l t surface are present on the southwestern side of Mount Rockwell 1 78 Figure 4 -1 3. Stereograph i c p r o j e c t i o n of fol s t r i a e along the Rockwell f a u l t sampie l o c a l i t i e s ) . hinges and (see t e x t f o r 1 79 1_ striae fold h i n g e fold hinges 1 80 where the Rockwell f a u l t l o c a l l y ramps up to the east (F i g . 3 - 1 7 ) . These s m a ll-s c a l e folds are overturned to the east which in d ic a te s that the tr a n s p o r t d i r e c t i o n of the Rockwell p late was eastward. The predominant trend of the fold hinges is about S50oE, which is high angle to the tran s port d i r e c t i o n of the Rockwell f a u l t in d ic a t e d by s t r i a e (Fig. 4 - 1 3 ) . The t r a n s p o r t d i r e c t i o n of the Rockwell f a u l t in d ic a te d by these s t r i a e and fold hinge data is about N40°E, a d i r e c t i o n consid era bly more n o rth e r ly (20 degrees) than the l i k e l y t r a n s p o r t d i r e c t i o n of the Lewis t h r u s t . This possible kinematic i n c o m p a t i b i l i t y between the Lewis th r u s t and Rockwell f a u l t may suggest t h a t they developed at d i f f e r e n t tim es. Although the Rockwell f a u l t can be traced through the e n t i r e study area, the amount of displacement of Grin nell rocks along i t is unknown. Several c o n t r a c t i o n a l fa u lt s with displacement of a few tens of meters, and a highly folded rock package on the south side of Lone Walker Mountain (Fig. 3-20 and 3-21) are truncated by the planar Rockwell f a u l t . The r e l a t i o n s h i p between the minor c o n tr a c tio n a l f a u l t s and the Rockwell f a u l t appears to be t r u n c a t i o n a l . The amount of l o c a l i z e d shortening represented by the folded package is very high (30-4 0%; see Fig. 3-20 and 3 - 2 1 ) . I f such shortening occurred in the lower p l a t e of the Rockwell f a u l t a f t e r the 181 development of the Rockwell f a u l t , the Rockwell f a u l t surface would be warped across t h i s folded s t r u c t u r a l package. This is, however, not observed. In stead, the Rockwell f a u l t above the folded rock package is planar (F i g . 3 - 2 1 ) . Based on t h i s ob servation, the folded s t r u c t u r a l package is i n t e r p r e t e d here to be tr u n c a t e d by the Rockwell f a u l t , and di splaced eastward along i t . However, an o f f s e t e q u iva len t of the folded s t r u c t u r a l package in the upper p l a t e of the Rockwell f a u l t has not been found. Offset along the Rockwell f a u l t is unknown. Two Medicine Pass Fault Numerous s m a l l - s c a l e folds (wavelengths less than 1 m) are present along the trac e of the Two Medicine Pass f a u l t (TMPF) at Caper Peak and Two Medicine Pass. O r ie n t a t io n s of these folds are p l o tte d in Fig. 4 -1 4 . The vergence of these folds is predominantly to the east except those l o c a l l y developed along u p p e r - p la t e e a s t - dipping minor th r u s t f a u l t s (s plays o f f the TMPF) (Fig. 4- 12). The average trend of the shallow plunging fold hinges is N60°W. This o r i e n t a t i o n is about 30 to 40 degrees o f f from the general o r i e n t a t i o n of folds in the Lewis allochth on ( P l a t e 1). I f the tr a n s p o r t d i r e c t i o n of the Two Medicine Pass p late is pe rpendicula r to the average o r i e n t a t i o n of the f a u l t - a d j a c e n t fold hinges, the 1 82 F i g u re 4-14. S t e r e o g r a p h i c p r o j e c t i o n of drag f o l d \ along the Two M e d ic in e Pass f a u l t , n = 2 '- i n g e s 1 83 1 84 i n f e r r e d t r a n s p o r t d i r e c t i o n of the Rockwell f a u l t is approximately N3 0°E. This is a d i r e c t i o n 35 degrees more no r th e r ly than the l i k e l y t r a n s p o r t d i r e c t i o n of the Lewis thrus t f a u l t s . I t may i n d i c a t e t h a t the Two Medicine f a u l t and the Lewis thrus t f a u l t developed at d i f f e r e n t times, or th a t the two f a u l t s were k i n e m a t i c a l l y independent of each other. As discussed in Chapter 3, the Two Medicine Pass f a u l t is o f f s e t by a few minor c o n t r a c t io n a l f a u l t s (both thrus ts and reverse f a u l t s ) seen on the southern c l i f f of Cloudcroft Peaks. As the amount of displacement along these c o n t r a c t io n f a u l t s appears to be small (several meters to several tens of meters ?), i t is d i f f i c u l t to determine whether or not these f a u l t s are truncated by or merge to the Rockwell f a u l t . In e i t h e r case, the Two Medicine Pass f a u l t predates the Rockwell f a u l t . The amount of displacement along the Two Medicine Pass f a u l t is unknown. Lone Walker Fault No kinematic data were measured along the Lone Walker f a u l t . Its s t r i k e is about N7 0°W, a d i r e c t i o n s i g n i f i c a n t l y more westerly than the average trend of folds and th r u s t s in the Lewis allo c h th o n . The Lone Walker f a u l t cuts all s t ru c tu re s in the 1 85 upper p l a t e of the Lewis t h r u s t ( P la te 1), It may also cut the Lewis t h r u s t f a u l t i t s e l f (see discussion in Chapter 3). It is thus a s t r u c t u r e postdating all the s tru c tu re s in the Lewis p la te and possibly the Lewis t h r u s t i t s e l f . Deformational History of the Lewis Allochthon C r o s s -c u tt in g r e l a t i o n s h i p s observed in the study area permit a re co ns tru ction of the deformational h is to ry of the Lewis allochthon- The sequence of deformational events from older to younger is summarized below: 1. Minor normal f a u l t s on the north c l i f f of Scenic Point. 2. Minor c o n tr a c tio n a l f a u l t s which are truncated by the Scenic Point f a u l t on the north c l i f f of Scenic Point. The order of events 1 and 2 can be in terc ha nge a ble. 3. Scenic Point f a u l t . 4. Open f o ld in g event which a f f e c t s eastern part of the st udy area. 5. Formation of the f r o n t a l zone. 6. A t o p - t o - t h e - e a s t simple shear which caused r o t a t i o n of s tr u c tu r e s in the f r o n t a l zone. 7. Formation of the eastern b e l t . 8. Formation of the present Lewis t h r u s t . 9. Two Medicine Pass f a u l t ? 1 86 10. Elk Mountain im bric ate system. 11. Brave Dog f a u l t . 12. Mount Henry imbricate system. 13. Rockwell f a u l t . 14. Formation of Akamina/Continental Divide s y nc lin e. 15. Lone Walker f a u l t . S tructures formed in stages 1 to 7 predate the formation of the present Lewis t h r u s t . Why normal f a u l t s formed in stage 1 is not c l e a r . As the trends of those s tru c tu re s formed in stage 1 to 7 are p a r a l l e l to the trends of s t r u c t u r e s formed a f t e r the formation of the Lewis t h r u s t , those may have been r ela ted to movement along a major t h r u s t which predates the form ation of the present Lewis t h r u s t . This i n t e r p r e t a t i o n was f i r s t proposed by G. A. Davis (personal communication, 1984; unpublished map) based on t r u n c a t i o n a l r e l a t i o n s h i p s between the Lewis th ru s t and i t s u p p e r - p la t e s t ru c tu re s observed at D iv id e Mountain, and was l a t e r confirmed by Hudec (1987) in the Spot Mountain area. The i n f e r r e d deformational h i s t o r y of the Lewis allochthon is diag ra m m atic ally shown in Fig. 4-13. The enigmatic minor normal f a u l t s developed during stage 1, and the development of the Akamina syncline and Lone Walker f a u l t , are not shown in the f i g u r e : 1. An i n f e r r e d t h r u s t , the i n f e r r e d Lewis t h r u s t I 1 87 shown in Fig. 4 -13a, was developed which li e s s t r u c t u r a l l y below the present Lewis t h r u s t . Some c o n t r a c t i o n a l f a u l t s , which are part of Scenic Point s t r u c t u r a l complex, formed in it s upper plate during t h i s stage. This t h r u s t truncated the normal f a u l t s ( F i g . 4 - 1 3 a ) . ?., Scenic Point f a u l t tru n c a te d and displa ced these minor c o n tr a c tio n a l f a u l t s ( F i g . 4 -13 b ). 3. Scenic Point f a u l t was fo ld e d . This f o l d i n g event may be re la te d to the development of some extens i onal and c o n t r a c t io n a l f a u l t s in it s lower p l a t e ( F i g . 4 - 13c). Those minor c o n t r a c t i o n a l and extensional f a u l t s , t o g e th e r with the Scenic Point f a u l t and older minor c o n t r a c t io n a l f a u l t s , defin e the Scenic Point s t r u c t u r a l complex. 4. S truc ture s in the f r o n t a l zone developed (see Fig. 4-3 for hypothesized developmental stages of the f r o n t a l zone) which cut the Scenic Point f a u l t (F i g . 4 -13 d ). 5. A t o p - t o - t h e - e a s t simple shear caused r o t a t i o n of s tr u c tu r e s in the f r o n t a l zone ( F i g . 4 - 1 3e). 6. The eastern s t r u c t u r a l b e lt formed which is c h a r a c t e r iz e d by pop-up s t r u c t u r e s (F ig . 4 - 1 3 f ) . 7. The present Lewis t h r u s t sta rte d developing. The Lewis t h r u s t t ru n c a te d s tr u c tu r e s in the eastern b e lt and f r o n t a l zone (Fig. 4-13g) and displaced them to the no rtheast . 8. The Elk Mountain im b r ic a t e system was developed. 1 88 Figure 4-15. Possible kinematic processes responsible for the formation of the Lewis t h r u s t system. Explanations see t e x t . 189 * NE SPSC Inferred Lew is thrust I a 1 90 * NE In ferred Lew is thrust I In fe rre d Lew is thrust I * NE SPF In ferred Lewis thrust I 4 NE 0 0 SPSC T----- Inferred Lewis thrust I * NE \ SPF \ 1 94 1 NE Inferred Lewis thrust I 1 95 Lewis thrust NE * NE EMIS s s % > \— r / A 7 U \ / / Lewis thrust y \ h 1 97 * NE BDF EMIS Lewis thrust / i 1 98 * NE MHIS Lewis thrust j 1 99 Lewis thrust EMIS MHIS k 200 The Lewis t h r u s t may have been a c tiv e synchronous during the development of the Elk Mountain imbricate system ( F i g . 4 - 1 3h ) . 9. The Brave Dog f a u l t was i n i t i a t e d which cuts down s t r a t i graphic section to the east at a shallow angle. The Brave Dog f a u l t truncated the Elk Mountain im bric ate system, eastern s t r u c t u r a l b e l t , and f r o n t a l zone ( F i g . 4- 1 3i ) . 10. Mount Henry im b r ic a te system o f f s e t the Brave Dog f a u l t . The Rockwell f a u l t began to develop (Fig. 4 - 1 3 j ). 11. The Rockwell f a u l t trunc a te d and displaced p o rti o n of the Mount Henry im bric a te system to the no rtheast ( F i g . 4-13 k ) . Discussions on the Sequence of Thrust Fault Development The sequence in time and space of t h r u s t f a u l t development has been a major subject in studies of the c o r d i 1 leran forela nd fold and t h r u s t belt ( e . g . Armstrong and O r i e l , 196 5; Bally et a l . , 196 6; Dahlstrom, 197 0; Price and Mountjoy, 1970 ; P r i c e , 1981; Royse et a l . , 1975; Wiltschko and Dorr, 198 3). Armstrong and Oriel (196 5) in t h e i r c l a s s ic paper c l e a r l y lin k the events in the Idaho- Wyoming-Utah t h r u s t b e lt to de po sition al events in sedimentary basins to the e a s t. The axes of these basins, 201 which received coarse c l a s t i c deposits eroded from the th r u s t b e l t , migrated eastward as the t h r u s t b e lt moved eastward in the time period from Late J u ra ss ic to Early Eocene. Later s tudies ( e . g . , Royse et a l . , 1975; Dixon, 1982; Wiltschko and Dorr, 1983) confirmed t h a t the major th r u s t f a u l t s are younger to the east in the Idaho- Wyoming-Utah t h r u s t b e l t . Wiltschko and Dorr (1983) showed t h a t there is a progression in t h r u s t di spla cem ent, apparent du ra tio n of motion, and p a l i n s p a s t i c p o s i t io n of t h r u s t traces from west to east. Those in the west moved f a r t h e r , for an a p p a r e n t ly longer period of tim e, and were more widely spaced in t h e i r restored po sitio n s than those tov/ard the e a s t. In the southern Canadian Rocky Mountains, such an eastward progression in t h r u s t development during Late J urassic to Early Eocene time was also observed from the m igration of d e p o s itio n a l centers of fore la nd basins and c r o s s - c u t t i n g r e l a t i o n s h i p s between t h r u s t f a u l t s and p l u to n i c rocks ( B a l ly et a l . , 1966; Price and Mountjoy, 197 0; Pr ice, 1981). The i n i t i a t i o n of t h r u s t f a u l t development in Canadian Rockies was f i r s t recorded in the Upper Jurassic Kootenay Formation. It is marked by an abrupt reversal in the provenance of Mesozoic c l a s t i c sediment from the NE c r a t o n i c province to the SW C o r d i l l e r a n provenance th a t c h a r a c t e r i z e d the exogeocli nal 202 assemblage ( P r i c e , 1981), Pric e (1981) showed that thrus t f a u l t s in the western Canadian Rocky Mountains ( i . e . , Hall Lake f a u l t and St. Mary f a u l t s ) developed during la te Jurassic to mid-Cretaceous tim e , whereas major t h r u s t f a u l t s (Lewis t h r u s t , McConnell th r u s t ) in the eastern Canadian Rocky Mountains developed during l a t e Cretaceous to e arly Eocene time. The sequence of th r u s t f a u l t development has not only been examined on a regional scale (hundreds of k i l o m e t e r s ) as discussed above, but also on a local scale (tens of k ilo m e te r s ) between d i f f e r e n t geometrical elements w i t h in a sin gle allo c h th o n (Dahlstrom, 1970). Using the s t r u c t u r a l a s s o c i a t io n in the Lewis plate of the Waterton area, Canada, as an example, Dahlstrom (1970) defined a duplex f a u l t zone as a s u i t e of more s teeply dipping minor t h r u s t f a u l t s bounded by low-angle f a u l t s at the top (the roof t h r u s t ) and at the bottom (the sole t h r u s t ) . Dahlstrom (1970) proposed a kinematic model f o r the formation of a duplex zone (F i g . 4 - 1 6 ) . In his model, the sequence of minor th r u s t development is forward, i . e . , in the d i r e c t i o n of te c to n ic t r a n s p o r t . This model was r e c e n t l y modified by Boyer and E l l i o t t (1982; Fig. 4 - 1 7 ) . They proposed t h a t the roof and sole th r u s t f a u l t s are g e n e t i c a l l y r e l a t e d during the development of a duplex f a u l t system. The roof t h r u s t and sole th r u s t jo in 203 F i g u r e 4 -1 6. a. Geometry of s t r a i l i n g edge i m b r i c a t e system or duplex f a u l t zone as defin ed by Dahlstrom (197 0). b. A kinematic model fo r the development of a duplex f a u l t zone (Dahlstrom, 1970 ). 204 o IM B R IC A T E s t a c k b SEQ UENCE or IM B R IC A T E D E VE LO PM EN T 205 to g e t h e r both at the back and the fr o n t of the more s teep ly dipping zone of minor t h r u s t s . During the development of some duplex systems, motion is t r a n s f e r r e d from the sole t h r u s t f a u l t to the roof t h r u s t f a u l t (F ig . 4 - 1 7 ) . The Boyer and E l l i o t t model, modified from th a t of Dahlstrom, provides a possible process for the development of some t h r u s t duplex systems. S tr u c tu r a l associatio ns mapped in t h i s study area by t h i s w r i t e r and mapped in the southwestern park area mapped by Kelty (1986) are g e o m e t r i c a l l y s i m i l a r to the duplex f a u l t zones of Dahlstrom (1 9 7 0 ). The Elk Mountain im bric a te t h r u s t s are bounded by the lo w-angle Brave Dog f a u l t and the Lewis t h r u s t f a u l t , but no forward j o i n i n g of the Brave Dog f a u l t and the Lewis t h r u s t is seen. The Mount Henry im b r ic a t e th r u s t s are s i m i l a r l y bounded by the upper 1 o w - angle f a u l t , the Rockwell f a u l t , and the lower lo w-angle f a u l t , the Lewis t h r u s t f a u l t . Truncational r e l a t i o n s h i p between the im b r ic a te systems (Elk Mountain and Mount Henry im bric ate system) w i t h in the Lewis allo c h th o n and the upper bounding low-angle f a u l t s (Brave Dog and Rock we 11 f a u l t s ) ( K e l t y , 1986), preclude the geometric model as well as the kinematic process, proposed by Boyer and E l l i o t t (1982) for the development of t h r u s t duplexes. As the Brave Dog f a u l t s and Rockwell f a u l t s developed at a 206 F i g u r e 4 -17. A k i n e m a t i c model f o r th e development of a d u ple x system (Boyer and E l l i o t t , 1982). 207 Boyer and Elliott C I 9 8 2 ) initial Stage So M A JO R T H R U S T S H E E T So --------— ------------- — T B \ FOOTw ALL i n c i p i e n t RAMP FRACTURE U P P ER I LOW ER G LIDE H O R T O N S Stage I S. 208 higher level with respect to the two im b r ic a te systems, they are out--of-sequence f a u l t s . Boyer and E l l i o t t ' s duplex model require s t h a t the roof f a u l t be of compressi onal o r i g i n . I f the Brave Dog f a u l t and Rockwell f a u l t are roof th r u s t f a u l t s and the Lewis t h r u s t f a u l t the sole t h r u s t f a u l t , then the Brave Dog and Rockwell f a u l t s should cut down-sect ion westward and ca rry o ld e r rocks over younger rocks. These r e l a t i o n s h i p s are, however, not seen. Instead, the Brave Dog f a u l t and Rockwell f a u l t cut g e n e r a lly down-sect ion eastward and juxtapo se younger rocks over olde r rocks in t h i s study area and the area mapped by Kelty (1986) (see Chapter 3 ) . 209 CHAPTER 5: A MECHANICAL ANALYSIS OF THE LEWIS THRUST SYSTEM General Statement The geometric framework of the Lewis t h r u s t system and the sequence of i t s development have been described, and ra is e questions regarding mechanical c onditio ns f o r t h r u s t p la t e development. The Lewis th r u s t allo c h th o n was displaced more than 65 km in the central part of the Lewis s a l i e n t ( P r i c e , 1965). What mechanical c ond it io ns are required to make such a great displacement possible ? As discussed in Chapter 4, the Elk Mountain and Mount Henry imbricate systems developed at an unknown d istance from the leading edge of the Lewis t h r u s t f a u l t . Why did these systems develop ? What c o n t r o l l e d t h e i r l i s t r i c geometry ? S t r u c tu r a l h i s t o r y of the Lewis th r u s t system suggests the a l t e r n a t i n g development of low-angle f a u l t s and im b r ic a te systems. What mechanical c o n d itio n s might control such s equentia l development ? F i n a l l y , how did low-angle f a u l t s of probably extensi onal geometry develop w i t h i n the a llo c h t h o n during i t s t h r u s t r ela te d emp1ac ement ? Mechanical Paradox of Overthrusts The mechanics of th r u s t f a u l t i n g have been one of the 21 0 most I n t e r e s t i n g subjects in s t r u c t u r a l geology since the beginning of t h i s century. Soon a f t e r the e x is te n c e of l a r g e - s c a l e t h r u s t sheets with great displacement was e s t a b l i s h e d , the mechanical u n l i k e l i h o o d of t r a n s p o r t i n g rocks with t h e i r l i m i t e d strengths was proposed by Reade (190 8 ). He wrote: In attempts to unravel some of the w e i g h t i e r problems of geology i t has l a t e l y been assumed t h a t c e r t a i n discordances of s t r a t i f i c a t i o n are due to the t h r u s t i n g of old rocks over those of a l a t e r geological age. Without in any way suggesting that the geology has in any p a r t i c u l a r instance been misread, I should l i k e to point out the d i f f i c u l t i e s in accepting the e x p la n a tio n looked at from a dynamic point of view when app lie d on a scale t h a t seems to ignore mechanical p r o b a b i l i t i e s . Some of the enormous o v e r th r u s ts postu la ted are estimated at f i g u r e s approaching 100 m ile s . If such a movement has ever taken place, would i t not r e q u ir e an i n c a l c u l a b l e fo rc e to t h r u s t the upper over the l o w e r . . . ? I ventur e to t h in k tha t no force applied in any of the mechanical ways known to us in Nature would move such a mass, be i t ever so adjusted in thickness to the purpose, even i f supplemented with a l u b r i c a n t generously applied to the t h r u s t - p1 ane. These are the thoughts tha t n a t u r a l l y occur to me, but as my mind is q u ite open to receive new ideas I shall be glad to know in what way the reasoning can be met by others. In response to the c h a ll e n g e , Smoluchowski (1909) discussed t h i s problem by c onsid ering a r e c t a n g u la r block s l i d i n g along a h o rizo n ta l plane s urfa ce . Taking b as the length of the block p a r a l l e l to the motion, a i t s breadth, c i t s th i c k n e s s , w i t s weight per un it volume, and e the c o e f f i c i e n t of f r i c t i o n , he showed th a t the fo r c e required to s lid e the block would be F = ( a b c ) w e . 21 1 Hence the pre ssure applied at the end ac would be F/a c =wb e . Assuming e=0.15 (note that e is much lower than what is normally accepted [ 0 . 6 0 - 0 . 8 5 ] f o r the c o e f f i c i e n t of f r i c t i o n ) , b = 1 0 0 m ile s , and a maximum force per unit (F/ac) less than the crushing s tre n gth of g r a n i t e , the required thickness (c) of the block would be 15 m iles (2 4 km). This appeared to be too t h i c k for a t h r u s t sheet. He then concluded that under the mechanical conditions assumed, i t is impossible to move such a block. He suggested, i n s t e a d , that th r u s ts may have occurred down an i n c l i n e d plane, or th a t the rocks involved were p l a s t i c with a s t i l l lower c o e f f i c i e n t of f r i c t i o n assumed. A problem, among the others, in his c a l c u l a t i o n is the assumption of the r e c t a n g u la r t h r u s t block which is not a p p r o p r i a t e to the geometry of a t h r u s t sheet in the real world. Assuming th a t the geometry of a t h r u s t sheet in c r o s s - s e c tio n p a r a l l e l to t h r u s t p l a t e t r a n s p o r t is t r i a n g u l a r (or wedge-shaped), Lawson (192 2) r e c a l c u l a t e d the maximum length of a t h r u s t sheet th a t could move during t h r u s t i n g . He found t h a t the maximum length is in v e r s e ly dependent on the dip angle of the t h r u s t f a u l t . A major problem in his c a l c u l a t i o n s is t h a t the basal f r i c t i o n along a t h r u s t f a u l t surface is assumed to be 2 1 2 constant- This is not v a lid because the normal stress on a t h r u s t f a u l t increases in the down dip d i r e c t i o n due to the th ickening of rock column in the hanging w a l l . The f r i c t i o n a l r e s i s t a n c e in turn increases in the downdip d i r e c t i o n along the t h r u s t f a u l t because i t is prop ortiona l to the normal stress across the f a u l t . In order to resolve the mechanical paradox of thrust plates e x h i b i t i n g large displacem ents, Hubbert and Rubey (1959) proposed t h a t abnormal p o r e - f l u i d pressures along t h r u s t f a u l t s w i l l reduce normal stresses across the f a u l t s . The reduced normal stress would in turn reduce the f r i c t i o n a l r e sista nc e along the f a u l t surface. Considering the e f f e c t of p o r e - f l u i d pressure, Hubbert and Rubey (1959) r e c a lc u la t e d the maximum length of o v e rthrus t blocks (Figure 5 - 1 ) . They found th a t by assuming an abnormally high p o r e - f l u id pressure along a f a u l t , the c a l c u l a t e d maximum length of t h r u s t blocks is c o n s is te n t with what observed in the f i e l d . The fundamental problem in Hubbert and Rubey's c a l c u l a t i o n s , s i m i l a r to t h a t of Smoluchowski (190 9), is the assumption of r e c t a n g u la r geometry fo r the o v e r th r u s t blocks. The geometry of the Lewis t h r u s t sheet may be best approximated by a wedge because the Lewis t h r u s t , in order to have c a r r i e d P r o t e r o z o ic rocks to the s u r fa c e , dips to the east. A balanced cross section through the study area 21 3 Rectang ular model fo r c a l c u l a t i n g the maximum length of the o v e r t h r u s t blocks (Hubbert and R u b ey , 1 95 9) . Maximum Length of a Rectangular Block (Hubbert and Rubey, 1959 ) 21 5 F i g u r e 5 - 2. G e o l o g i c cross s e c t i o n s and p a l i n s p a s t i c r e c o n s tr u c ti o n of the Belt basin ( F r i t t s and K 1 i pp i ng 9 1 98 7a 5 b ) a. Regional geologic map of the B elt Basin and lo c a t io n s of geologic cross sections in Figure 5 - 2 b . b . Geologic cross sections across the Belt basin. Locations of the c r o s s - s e c t i o n s see 5 - 2 b . c. Tectonic model f o r the development of the Lewis t h r u s t sheet on a regional s c a l e . 21 6 KEY :A * a a H O A i.c r ic ^ D * N c a s t u i * T v r » n c L D ww « e c T A tH C M ca c h e u c n c v D U tO O L C k l t LOCKOUT B U T T E F rC L O -t o X 25 M ILES (a) 2 1 7 T h ru s t s y m b o ls : P t-P in k h a m ; Dt-Ounsjre; T it-T e n L a n e s . W t-W h ite tis h ; H t-H e lty : L t-L a w is (»•) (b) 2 1 8 I. BEFORE TRANSPORT OF LEMS THRUST SHEET h t l T __________ 2. EMPLACEMENT OF LEWIS THRUST SHEET (UPPER PLATE) rr ot wt mt 1 O W E R 1 FORMATION OF WESTERNMOST -QUPLEX pt e r r w t h t 4 CULMINATION OF THRUSTING AND DUPLEX FORMATION AND EASTWARD DISPLACEMENT OF EARLIER FOPMED DUPLEXES 5. LISTRIC NORMAL FAULTING AND EROSION PURCELL A N T IC L IN O R U U K1SHENEHN BASIN H T P T PlNKHAJJ T H R U S T O T DUN SIRE TH R U S T W T W W T E F iS M TH R U S T H T H E F T Y T H R U S T L T L E W IS T H R U S T • . T E R T IA R T -O U A T E P N A R Y m e s o z o ic [H r p a l x c z o c I UPPER B E L T M IO O LE B E L T I LO W ER B E L T (c) 2 1 9 in d ic a te s th a t the Lewis t h r u s t f a u l t dips from 4 to 7 degrees ( P la t e 2). Regional p a l i n s p a s t i c r e c o n s t r u c t i o n s of the Belt basin based on surface mapping, deep d r i l l i n g , and seismic r e f l e c t i o n p r o f i l e s , also i n d i c a t e t h a t the Lewis t h r u s t f a u l t is a s h a llo w ly we st-dip pin g f a u l t of regional e xte nt (Fig. 5-2; F r i t t s and K l i p p i n g , 1987a, 1987b). The dip angle of the Lewis t h r u s t at its in ce ption is about 7 degrees according to a re store d cross se ction ( F ig . 5-2c; F r i t t s and K l i p p i n g , 198 7a, 1987b). As was mentioned in Chapter 1, the minimum displacement along the Lewis t h r u s t f a u l t is 65 km, as determined in an area immediately north of G la c ie r National Park ( P r i c e , 1965). This displacement could be p a r t i t i o n e d by shortening w i t h i n the Lewis t h r u s t sheet and a pure t r a n s l a t i o n of the Lev;is th r u s t sheet along the Lewis t h r u s t f a u l t . The t o t a l shortening w i t h i n the Lewis t h r u s t sheet in southern G l a c i e r National Park is less than 25%. which could only compensate several kilo m e te rs displacement along the Lewis t h r u s t f a u l t . Thus, an eastward t r a n s l a t i o n of the Lewis allochthon near G l a c i e r Park area on the magnitude of several tens of kilo m e te rs along the Lewis t h r u s t f a u l t is r e quire d. Such a la rge t r a n s l a t i o n along the Lewis t h r u s t would l i k e l y require th a t the Lewis t h r u s t reached the surface during i t s development. A l i n e of evidence that supports this 2 2 0 argument is th a t the Lev/is t h r u s t f a u l t juxtaposes Belt rocks atop l a t e Cretaceous non-marine deposits south of G l a c i e r Park (Mudge, 1982) and the area immediately north of the I n t e r n a t i o n a l Boundary in Canada (Gordy et a ! . , 1977; P r ic e , 1981). These non-marine deposits c onsis t of c l a s t i c sediments shed from t e c t o n i c a l l y u p l i f t e d areas in the west. As the upper part of t h i s succession of non- mar i an deposits is a l l u v i a l p l a i n o r i g i n (McLean and J e r z y k i w i c z , 1978), the Lewis t h r u s t f a u l t must have reached the s ur fa c e during i t s development. In the mechanical model presented in the next s e c tio n , I w i l l examine the role of the assumed wedge- shaped geometry in c o n t r o l l i n g the maximum length of the o v e r th r u s t b lo c k . I w i l l show th a t the maximum length of a o v e r t h r u s t sheet is not ju s t dependent on the c o e f f i c i e n t of basal f r i c t i o n , p o r e - f l u i d pressure r a t i o , and f r a c t u r e stre n gth of t h r u s t sheets, i t is also dependent on the dip angle of the t h r u s t f a u l t . Controls on Thrust F a u l t Geometry By applying Coulomb f r a c t u r e c r i t e r i o n and assuming t h a t the p r i n c i p a l stresses in the upper crust are e i t h e r ho rizo n ta l or v e r t i c a l , Anderson (194 2) proposed a simple model to e x p la in the mechanical o r i g i n for the geometry of the three fundamental types of f a u l t s , lo w-angle t h r u s t 22 1 f a u l t s (<30 degrees) , high angle normal f a u l t s ( c a . 60 degr ees), and sub v e r t i c a l s t r i k e - s l i p f a u l t s . Anderson’ s f a u l t theory p r e d i c t s a plan ar geometry for f a u l t s ra th er than a c u r v i p l a n a r geometry as with l i s t r i c t h r u s t f a u l t s . L a t e r studies on f a u l t mechanics suggest th a t the geometry of l i s t r i c f a u l t s can be c o n t r o l l e d e i t h e r by change in rheology with depth (Jackson and McKenzie, 1983; Bradshaw and Zoback, 198 8) or a change in stress o r i e n t a t i o n s w i t h i n the crust ( H a f n e r , 1951). Since the two im bric ate systems in the Lewis t h r u s t p l a t e , Elk Mountain and Mount Henry, occurred w i t h i n s i m i l a r l i t h o l o g i e s ( p r i m a r i l y a r g i l l i t e s in the Appekunny and G r i n n e l l Formations) and no change in deformation mechanism with depth across these i m b r i c a t e t h r u s t f a u l t s is observed. Th eir c u r v i p l a n a r geometries must be c o n t r o l l e d by v a r i a t i o n in the s t a t e of s tre s s w it h in the t h r u s t wedge at the time they formed. In order to c a l c u l a t e possible s tre ss c o n d itio n s responsible f o r the formation of l i s t r i c t h r u s t s with in the Lewis t h r u s t a l l o c h t h o n , a kinematic model f o r the development of the Elk Mountain and Mount Henry im b r ic a te systems must be assumed. In Chapter 4, we have discussed r e l a t i v e t im in g between the Lewis t h r u s t f a u l t and the two im b r ic a t e systems. Kinematic c o m p a t ib i1i t y between the two im bricate systems and the Lewis t h r u s t suggests t h a t they may have formed synchronously. Two possible 2 2 2 kinematic models. out of ma ny , are c ons is te nt with the s t r u c t u r a l r e l a t i o n s h i p between the Lewis t h r u s t f a u l t and the two im b r ic a t e th ru s t systems* These two models are s h own in Fig. 5 - 3 . Model 1 shows tha t the Elk Mountain im b r ic a t e system was developed during the upward propagation of the Lewis t h r u s t s u r fa c e . Model 2 shows th a t the Elk Mountain im bric a te system had not been developed u n t i l the Lewis t h r u s t f a u l t reached the s urfa ce. The a v a i l a b l e data are unable to determined uniquely which one is c o r r e c t . However, the two d i f f e r e n t processes for the i n i t i a t i o n of l i s t r i c t h r u s t f a u l t s w i l l r e q u i r e completely d i f f e r e n t boundary c o n d i t i o n s . For model 1, the surface of the sole t h r u s t f a u l t can be approximated as a s e m i - i n f i n i t e i n plane shear crack (Kanninen and Pop elar, 198 5, p . 139; Fig. 5 -4 a ). The stress d i s t r i b u t i o n around the f a u l t t i p can be determined i f the rupture v e l o c i t y V( x , t ) as a f u n c t i o n of space and time along a f a u l t is prescribed (Aki and Richard, 1980, p . 8 5 1 - 8 5 2 ) . For earthquake f a u l t i n g , the rupture v e l o c i t y of f a u l t i n g can be determined from observed sei smograms. This in f o r m a tio n is , however, not a v a i l a b l e fo r the f a u l t s t h a t were a c t i v e in the past. T h e r e fo r e , i t is not a p p l ic a b le to the problems we are i n t e r e s t e d in here. Further mechanical studies on t h i s sub ject need to be done in order to t e s t the possible 223 Fi gure 5-3. Two po ss ib le kin em atic processes f o r the formation of l i s t r i c t h r u s t s in the im b r ic a t e systems in the Lewis a llo c h th o n . a. Im b r ic a te t h r u s t s began to develop before the Lewis t h r u s t f a u l t reach the s urface. b. Im bric ate t h r u s t s began to develop a f t e r the Lewis t h r u s t reach the s u r fa c e . 224 Model 1 stage 2 The ground surface thrus ewi Elk Mountain imbricate system b. 226 Figure 5-4. Mechanical models corresponding to the two kinematic models proposed in Figure 5 - 3 . a. Mode I I of a cracked body, s l i d i n g mode ( Kanni nen and Pop elar, 1 98 5 ). b. A wedge-shaped body pushing from the rear of the wedge. 227 (a) (b) 228 process of model 1 for the form ation o f l i s t r i c thrus t f a u l t s - For model 2, i t r e q u ir e s th a t f r i c t i o n a l s l i d i n g occurs along the f a u l t (Fig 5-4 b )• The stress d i s t r i b u t i o n w i t h i n the wedge-shaped t h r u s t sheet is determined by the f r i c t i o n a l shearing along the f a u l t and the force which pushes from the rear of the t h r u s t sheet and is able to overcome the 2 f r i c t i o n a l r e s i s t a n c e . In the fo l l o w i n g c a l c u l a t i o n s , only model 2 is assumed f o r the development of the two im b r i c a t e systems. 229 M e c h a n ic a l M o d e l F o rm u la tio n s F ig . 5-5 re p re se n ts th e id e a l g e o m e try o f an e la s tic th r u s t w edge a n d th e fra m e w o rk o f re fe re n ce used in th e c a lc u la tio n s w h e re a is th e s u rfa ce slope a n d is th e d ip an gle o f th e th r u s t fa u lt. S ig n c o n v e n tio n used in th is p a p e r fo llo w s th a t o f e la s tic ity w h ic h is s h o w n in F ig . 5-6. T h e b a sic e q u a tio n s g o v e rn in g th e d e fo rm a tio n o f a c o n tin u o u s m e d iu m are ^ + ^ + * = 0, (1) ox oy ^ C J 4i ^ TV ii / • . — + - £ * - + Y = 0. ( 2 ) oy ox ( 1 ) a n d ( 2 ) are e q u a tio n s o f s ta tic e q u ilib riu m in th e x a n d y d i re c tio n s a n d X a n d Y are b o d y fo rce c o m p o n e n ts in th e x a n d y d ire c tio n s . X = —prgs in u ; Y = pr gcosa w h e re pr is th e average d e n s ity o f th e ro c k s c o m p o s in g th e th r u s t w edge a n d g is th e accel e ra tio n o f g ra v ity . S ince ro c k s are n o t a b s o lu te ly d ry , th e re m a y e x is t p o re flu id p re ssu re s w it h in th e ro cks. C o n s id e rin g th e e ffe c tiv e stresses (Je a g e r a n d C o o k , 19 7 9 ), 230 F igure 5 - Geometry of a t r i a n g u l a r wedge and framework of r e fe r e n c e used in the c a l c u l a t i o n . the 231 232 F i g u r e 5 -6 . Sign co n ve n t io n f o r s o l v i n g e l a s t i c prob l ems 233 ( + ) (-)19 <-*-) l 'j 7 (0 '0 ) 2 34 (Tx = crx + p f , (3) a v ~ a y + P f ’ ( 4 ) w h e re ox a n d ay are e ffe c tiv e stresses a n d p / is p o re - flu id p re s s u re . S u b s titu tin g (3 ) a n d (4 ) in to (1 ) a n d (2 ), we ha ve dox drxv dpf , . i r - + i r - + x + - j t = 0 ’ 5 ox oy ox dcrv drxv dp* , . + * y + T 7 ~ ° - (6 ) T h e p o re - flu id p re s s u re ca n b e w r itte n as p / = ApTgyh, (7 ) w h e re h is th e d e p th a n d A is th e ra tio b e tw e e n th e p o re -flu id p re s s u re a n d th e lith o s ta tic p re s s u re , w h ic h is k n o w n as th e p o re -flu id p re s s u re r a tio (H u b b e r t a n d R u b e y , 19 59). S ince dh/dx — — s in a, ( 8 ) dhjdy = cosct, (9) 235 in s e rtin g (T ), (8 ) a n d (9 ) in to (5 ) a n d (6 ), we ha ve <7~ , + X . = 0, (1 0 ) ox oy ^ ^ ^ 7 *2 T / / x - 5 ^ - + - 5^ + y. = 0 , ( 1 1 ) eft/ arc w h e re X e = —(1 — A) prg siria = peg sina, ( 12 ) Y c = (1 — X ) p Tg coscx. — p e c o s o l . (1 3 ) a n d p e = (1 — A) p r is th e e ffe c tiv e d e n s ity . I t ca n be seen fro m ( 1 2 ) a n d (1 3 ) th a t th e ro le o f th e p o r e - flu id p re s s u re is to re d uce th e m a g n itu d e s o f th e b o d y fo rce c o m p o n e n ts . T h is is th e b o u y a n c y e ffe ct d iscu sse d b y H u b b e rt a n d R u b e y (1 9 5 9 ). B ased on th e re s u lts o f e x p e rim e n ta l ro c k m e ch a n ics (Je a g e r a n d C o o k , 1 9 7 9 ), it has be en a ssu m ed th a t th e b r it t le u p p e r c ru s t d e fo rm s e la s tic a lly (fo llo w in g H o o k e ’s la w ) b e fo re th e in itia t io n o f fa u ltin g - U s in g H o o k e ’s la w an d th e c o m p a tib ility c o n d itio n , th e stress c o m p a tib ility e q u a tio n ca n be d e riv e d (F u n g , 1965), w h ic h is + &„). (1 4 ) N o w we have th re e e q u a tio n s , (1 0 ), (1 1 ), a n d (1 4 ) w ith th re e u n k n o w n s ox1 Oy, a n d rxy. In o rd e r to solve (1 0 ), (1 1 ), an d (1 4 ), we need to k n o w th e b o u n d a ry c o n d itio n s a ro u n d th e th r u s t w edge. 236 F o r b o u n d a ry c o n d itio n s , it is a ssu m e d th a t th e e a r th ’s s u rfa ce is stre ss-fre e , i.e ., Oy (x, 0) = rxy(z, 0) = 0. (1 5 ) I t is also a ssum ed th a t fr ic tio n a l s lid in g o c c u rs a lo n g th e fa u lt p la n e o f th e L e w is th r u s t th a t is g o ve rn e d b y A m o n t o n ’s la w (J e a g e r a n d C o o k , 19 78), Tb(x , y — x tanS) = -M & (1 — ^& ) &b(&, V — x tanO), (1 6 ) w h e re fib is c o e ffic ie n t o f fr ic tio n a lo n g th e fa u lt p la n e , ob is th e n o rm a l stress across th e fa u lt p la n e , 77, is th e sh e a r stre ss a lo n g th e fa u lt p la n e , a n d pb is th e p o re -flu id p re s s u re ra tio a lo n g th e fa u lt p la n e . S ta tic e q u ilib r iu m o f th e fo rce s y s te m a ro u n d th e b o u n d a ry o f th e e la s itic w edge in F ig . 5-5 re q u ire s r x o t a n O E F x = I ox {x,y)dy Jo - f I rb{x,y = xtan9)cos9ds J S :y — x ta n O — I a b(x^y = xtan9)sin9ds J S :y — x t a n 0 — l j 2 p egsinatan9xQ2 — 0, (1 7 ) 237 r x o t a n d 'SFtj = / rI]t(x, y)dy Jo + I 7 5 (x , y = x£arc0 ) sindds J S :y — x tan.0 + I &b{x j y — xtanO)cosdds J 5:wmt<in9 5 :y — xta.nO -\-l/2 p egcosatan8xQ2 = 0, (IS ) r x q t a . n 0 E M = / &x(xo,y)ydy Jo rxQta.n$ - I rxy(x 0, y)x Qdy Jo I &b{x - > y = xtan$)sds v S ;« — r tn.n/? 5 '.y ~ x t a .n 0 + j X eydx dy J D:Q<x<XQ,Q<y<.xcjtari9 — I Yexdx dy = 0, (1 9 ) Z?:0<ac <xo ,0<y < xo w h e re Y1F x a n d E F y are s u m m a tio n s o f th e fo rc e c o m p o n e n ts in th e x a n d y d ire c tio n s , E A f is th e s u m m a tio n o f th e r o ta tio n a l m o m e n t, a n d 0 = (a fi). In o rd e r to o b ta in th e stress d is tr ib u tio n w ith in an e la s tic th r u s t w edge u n d e r b o u n d a ry c o n d itio n s (1 5 ), (1 6 ), a n d th e c o n d itio n s o f 238 s ta tic e q u ilib riu m (1 7 ), (1 8 ) a n d (1 9 ), th e A ir y stress fu n c tio n m e th o d is a p p lie d (T im o s h e n k o , 1951; H a fn e r, 1 959). T h e A ir y stre ss fu n c tio n se le cte d fo r th is c a lc u la tio n is = k\x^ -b k2x2 y k$xy2 -h k^y^ , ( 2 0 ) w h ic h s a tis fie s th e b ih a rm o n ic e q u a tio n V 4 $ = 0. (2 1 ) T h e stress c o m p o n e n ts can th e re fo re be d e riv e d fro m (1 8 ) (H a fn e r, 1959) d 2 $ crz - — - X e x = 2k$x + 6 ^ 4 y ~h pegsina x, ( 2 2 ) d 2® d x 2 = § k ix -+ - 2k2y — pgcosa y, (2 3 ) - d 2 ® „ S = — - Y e x Tx y — dxdy = — 2k2x — 2 k$y, (2 4 ) w h e re ki,k2, &3, a n d k± are a r b itr a r y c o n s ta n ts to be d e te rm in e d b y th e b o u n d a ry c o n d itio n s a n d th e c o n d itio n s o f s ta tic e q u ilib r iu m . 239 Equation (15) requires k i — &2 — 0, (25) T h e stre ss c o m p o n e n ts in (2 2 ), (2 3 ), a n d (2 4 ) s a tis fy (1 9 ). F ro m (1 6 ), (1 7 ), a n d (1 8 ), we can d e te rm in e k3 a n d k ± , k 3 = (2 6 ) ( a n a 22 — ^ 1 2 ^ 2 1 ) = ( a H » 8 - a » » l ) (27) (fit 1 1 0 2 2 “ ^ 1 2 ^ 2 1 ) w h e re a n = 2tan9 H- Zsin2 6{^ii — tanO), (2 8 ) a 12 = Ztand sin2d[iii> — tanO) + Ztan26, (2 9 ) a2i — Z{iLitand + 1 )s in20 — ta n2d, (3 0 ) & 2 2 = ZtanO sin2 B(iLitanO + 1 ), (3 1 ) b 1 = — pg[sinatan$ — l/2sin/3tan9 + 1 /2 sin0(fii — tand) 240 (sinoisind — cosacos *)), (3 2 ) 6 2 = ~ 1 / 2 pg[(iibtan$ + 1 ) (s in as in 2 $ — cosasinOcosB) -b cosfitanO)], (3 3 ) a n d Ub is th e e ffe c tiv e c o e ffic ie n t o f b a s a l fr ic tio n , Jib = —(1 — (3 4) T h e s o lu tio n o f e q u a tio n s (9 ), (1 0 ), a n d (1 1 ) u n d e r th e b o u n d a ry c o n d itio n s a n d th e c o n d itio n s o f s ta tic e q u ilib r iu m lis te d above is &x = 2k^x + 6 &4 y - f pes in a x , (3 5 ) (Ty = —pegcosay , (3 6 ) = —2kzy, (3 7 ) w h e re £ 3 a n d £ 4 are d e te rm in e d b y (2 6 ) a n d (2 7 ). 241 D ire c tio n s o f P r in c ip a l Stresses a n d G e o m e try o f P o te n tia l F a u lts T ra je c to rie s o f p r in c ip a l stresses fo r th e stress s y s te m re p re s e n te d in e q u a tio n s (3 5 ), (3 6 ), a n d (3 7 ) w ere d e te rm in e d b y (H a fn e r, 1959) x nJ. 2 r*» - i k 3y ta n 2(p — -------------- = —---------- —-------------------;------------------------------, (3 8 ) <y^ — cry 2/C 3X 6 ^ 4 y + pegstnay -f- pegcosay w h e re 4> is th e a n g le b e tw e e n th e m a x im u m p r in c ip a l stress a n d th e x -a x is (F ig . 5 -7 ). A s th e th re e stress c o m p o n e n ts in (3 5 ), (3 6 ), a n d (3 7 ) are lin e a r fu n c tio n s o f x a n d y,<j> is c o n s ta n t a lo n g a s tra ig h t lin e y = x t a n a 0 . T h is p r o p e r ty o f th e s o lu tio n p ro v id e s a c o n v e n ie n t w a y o f c o n s tru c tin g th e tra je c to rie s o f p r in c ip a l stresses. B y u s in g th e C o u lo m b fra c tu re c r ite r io n a n d a s s u m in g th a t th e a n gle o f in te rn a l fr ic tio n is 3 0 ° (i.e ., = ran,3 0 °, pL^ is c o e ffic ie n t o f in te rn a l f r ic tio n ) , p o te n tia l fa u lt p a tte r n s c a n be p lo tte d . In o rd e r to s im u la te th e stress d is tr ib u tio n w ith in th e L e w is th r u s t p la te , a p p ro p ria te p a ra m e te rs need to be chosen. T h e c o e f fic ie n t o f fr ic tio n pti v a rie s fro m 0.65 to 0.85 a c c o rd in g to B y e rle e ’s la w (B y e rle e , 1978). I choose pi = 0.70 as an averge v a lu e . T h e p o re - flu id p re ssu re w ith in th e w ed ge A is chosen to be 0.5. T h is v a lu e is s lig h tly h ig h e r th a n th e h y d ro s ta tic p re ssu re r a tio (A — 0.43 fo r pr = 2 .3 ). T h e d iffe re n c e b e tw e e n 0.50 a n d 0.43 is n o t s ig n ific a n t 242 F i g u r e 5-7. D i r e c t i o n of th e p r i n c i p a l s t r e s s e s p o s i t i v e in the d i r e c t i o n shown in f i g u re. ^ is the 243 y (o. o) 244 a n d i t w ill n o t in flu e n c e th e fin a l co n c lu s io n s re a c h e d in th is c h a p te r. H ig h p o re -flu id p re s s u re ra tio ( > 0.9 0) is n o t a s s u m e d in th e c a lc u la tio n because th e L e w is th r u s t fa u lt o c c u rre d in th e P ro te ro z o ic B e lt s tra ta . T h e B e lt s u p e rg ro u p b e g a n to be d e p o s ite d a b o u t 1,400 M a . ago (H a rris o n , 19 72), w h e re a s th e L e w is th r u s t fa u lt fo rm e d d u rin g L a te C re ta ce o u s to e a rly E ocen e tim e (M u d g e a n d E a r h a rt, 1980). T h e tim e in te rv a l b e tw e e n th e tw o events, m o re th a n 1000 M a ., is to o lo n g to p re s e rv e d a n y in it ia l a b n o rm a l h ig h p o r e - flu id pressures w it h in B e lt ro c k s . H o w e v e r, once a fa u lt s u rfa c e is fo rm e d , p o re -flu id p re s s u re a lo n g th e fa u lt s u rfa ce m a y b e co m e h ig h e r th a n th a t in its a d ja c e n t ro c k s b e cause d e fo rm a tio n o f ro c k s a lo n g th e fa u lt su rfa ce te n d s to re duce th e p e r m e a b ility d u rin g g ra in size re d u c tio n . F o r a s te a d y s ta te (c o n s ta n t ra te o f flo w o f flu id ), flu id p re s s u re is in v e rs e ly p r o p o r tio n a l to th e p e rm e a b ility o f ro cks a c c o rd in g to D a r c y ’s la w (Ja e g e r a n d C o o k , 1979, p . 2 1 0 ). A s p e r m e a b ility o f a fa u lt zone (su ch as fa u lt go uge ) is lo w e r th a n its a d ja c e n t ro c k s , th e p o re -flu id p re ssu re w o u ld be h ig h e r a lo n g a fa u lt th a n th a t in its a d ja c e n t ro cks. F o r th is reason, th e p o re - flu id p re ssu re r a tio a t th e base o f th e fa u lt, Xi = 0 .6 , is chosen to be s lig h tly h ig h e r th a n th e h y d ro s ta tic p re s sure. T h e su rfa ce slop e a is a d iffic u lt p a ra m e te r to choose. Since we are s im u la tin g th e s ta te o f stress b e fo re fa u ltin g o c c u rre d w ith in th e L e w is th r u s t she et, a s s u m in g th a t m o d e l 2 is a p p ro p ria te fo r th e 24 5 fo r m a tio n o f th e m a jo r im b ric a te system s, th e s u rfa c e slop e, w h ic h is p re s u m a b ly re la te d to th e d e fo rm a tio n o f th e th r u s t w edge, s h o u ld its e lf be s m a ll a t th is tim e . T h u s , o l is a s s u m e d 1 °. F o r th e d ip an gle o f th e L e v/is fa u lt, we choose /? = 7 °. T h is v a lu e is b a se d o n re g io n a l p a lin s a s tic re c o n s tru c tio n s o f th e L e w is th r u s t sheet as d is cussed e a rlie r in th is c h a p te r. F ig u re 6-8 sh o w s th e tra je c to rie s o f p r in c ip a l stresses a n d th e p o te n tia l fa u lt p a tte r n s fo r th e p a ra m e te rs se le cte d a b o ve . W e ca n see th a t th e stress d is t r ib u t io n w it h in th e e la s tic w ed ge is c o m p a tib le w ith th e fo r m a tio n o f lis tr ic th r u s t fa u lts . I t is n o te w o r th y th a t in th e u p p e r h a lf o f th e w edge, th e o r i e n ta tio n o f th e p r in c ip a l stresses fa vo rs th e fo r m a tio n o f e x te n s io n a l fa u lts . T h is re s u lt p re d ic ts th e p o ssib le c o n te m p o ra n e o u s d e v e lo p m e n t o f e x te n s io n a l a n d c o n tra c tio n a l fa u lts w it h in th e sam e th r u s t sheet. A s im ila r re s u lt was reached in a m e c h a n ic a l m o d e l d e v e lo p e d b y R o y d e n a n d B u rc h fie l (1 9 8 7 ), w h o sh o w e d th a t to p o g ra p h ic lo a d ca n cause lo c a lly d e v e lo p e d e x te n s io n a l fa u lts in a n c o n v e rg e n t p la te s e ttin g . A n e x a m p le o f such c o n te m p o ra n e o u s fa u lts w it h in th e sam e th r u s t sheet w ill be d iscu ssed in th e la s t s e c tio n o f th is c h a p te r. F ig . 5-9 show s th e tra je c to rie s o f p r in c ip a l stresses a n d th e p o te n tia l fa u lt p a tte r n fo r A = 0.5, fib = 0 .7 , a =■ 5 °, f3 = 1 0 °, A*, = 0.6. B o th th e d ip angle a n d th e surface slo p e h a ve b e e n in c re a s e d fro m th e p re v io u s case. T h e d iffe re n ce b e tw e e n th e p o te n tia l fa u lt 246 F i g Tr aj oct ori gs of p r i n c ’’ p a p o t e n t i a"! ^ ; S u i / £ f = t c n 3 0 p a 11 e o o r c ssss a A.t = 0 A . = 1 J • Q - 1 Ql = 247 Trajectories of Principal Stresses = 0 . 7 , \ = 0 . 6 , oi = l" ; p ==7° CT, <^3 Potential Fault Surfaces 248 p a tte r n in F ig . 5-8 a n d th a t in F ig . 5-9 is th a t th e u p p e r p o r tio n c o n ta in in g e x te n s io n a l fa u lts in F ig . 5-9 is la rg e r th a n th a t in fig u re 6-10. P o te n tia l lis tr ic th r u s t fa u lts are also p ro d u c e d in F ig , 5-9. F ig . 5-1 0 sho w s th e tra je c to rie s o f p r in c ip a l stresses a n d th e p o te n tia l fa u lt p a tte r n fo r A = 0.6, = 0.7. A& = 0.6, a = 1 2 °, ft = 1 0 °. W e h a ve chosen th e sam e p a ra m e te rs as th o se in F ig . 5-9 e x c e p t th e su rfa c e s lo p e a. H e re th e su rfa c e slo p e is in c re a s e d fro m 5° to 1 2 °. C o m p a rin g th e p o te n tia l fa u lt p a tte rn s in F ig s . 5-9 a n d 5-10, I fo u n d th a t th e s te e p e r s u rfa ce slo p e in F ig . 5-10 p ro d u c e s a stre ss d is tr ib u tio n c o m p a tib le w ith th e fo r m a tio n o f fo r w a rd d ip p in g lo w -a n g le e x te n s io n a l fa u lts . T h is re s u lt show s th a t th e in te rn a l stress d is tr ib u tio n is n o t o n ly c o n tro lle d b y th e b a s a l fr ic tio n , it is also c o n tro lle d b y th e g e o m e try o f th e th r u s t w ed ge, p a r tic u la r ly th e s lo p e a n g le . T h e p h y s ic a l m e a n in g o f th is s o lu tio n is th a t i f th e su rfa c e s lo p e is in c re a s e d , th e c o m p o n e n t o f g r a v ita tio n a l fo rc e in th e d ir e c tio n p a r a lle l to th e s u rfa c e slo p e ( X = — pgsina) w ill also be in c re a s e d . T h e e x te n s io n a l fa u lts s h o w n in F ig . 5-10 are in fa c t th e re s u lt o f s u c h g r a v ita tio n a l b o d y fo rce w h ic h causes g r a v ity s p re a d in g o r e a s t-d ire c te d e x te n s io n a l fa u lts . P o ssib le g e o lo g ic a l im p lic a tio n s a n d p ro b le m s w it h th e a p p lic a b ility o f th is m o d e l to e x p la in th e e v o lu tio n o f th e L e w is th r u s t s y s te m w ill b e d iscu sse d la te r in th is c h a p te r. 249 Figure 5-9. T r a j e c t o r i e s of p r i n c i p a l stresses and the p o t e n t i a l f a u l t p a t t e r n f o r Ab = 0 .6 , X. = 0 . 5 , y ^ t a n S O 0 , p = 10® &=5° . 2 50 Trajectories of Principal Stresses 0.6 0.7 = 10' Potential Fault S u r fa c e s 251 Figure 5-1 0. T r a j e c t o r i e s of p r i n c i p a l stresses and the p o t e n t i a l f a u l t p a t t e r n fo r X{, = 0 . 6 , A. = 0 , 5 , yu h = 0 . 7 , JX + =t an3 0 , = 1 0 ° , ^ = 12° . 252 C O lO C M rfl , ' Ip V O O "\\ cO— tn < 1 ) U i c n c v ) H to r-A *rA o 5 V< £ ■ > O •H V O *J u a ) • r - » C D M O r- o JD M a x im u m L e n g th o f Q v e r th ru s t B lo c k s D u r in g m o v e m e n t o f a th r u s t sh e e t, tw o m e c h a n ic a l c o n d itio n s s h o u ld b e s a tis fie d : (1 ) th e fo rce p u s h in g a t th e re a r o f a th ru s t sheet s h o u ld b e e q u a l to o r g re a te r th a n th e fr ic tio n a l re s is ta n c e a lo n g th e fa u lt s u rfa c e ; a n d (2 ) th e m a g n itu d e o f stre ss w it h in th is th ru s t sheet ca n be a t m o s t e q u a l to th e s tre n g th o f ro c k s be ca u se once th e stress reaches c r itic a l va lu e s o f fra c tu re fa ilu re , fa u ltin g w it h in th e th r u s t sheet w ill n o t a llo w th e m a g n itu d e o f stresses to in cre a se any h ig h e r. F o r p o rtio n s o f th e th r u s t sheet w h e re n o fa u ltin g o c c u rs , d e fo rm a tio n o f ro c k s is p re s u m a b ly e la s tic as su g g e ste d b y e x p e rim e n ta l ro c k m e c h a n ic s (J a e g e r a n d C o o k , 19 79 ). T h e d is ta n c e b e tw e e n th e lo c a lity o f fa u ltin g w it h in a th r u s t sheet a n d th e le a d in g edge o f th e th r u s t p la te d u r in g its m o v e m e n t a lo n g a th r u s t fa u lt, defines th e p o s s ib le m a x im u m le n g th o f o v e rth ru s t b lo c k s . B ecause i t is a ssu m e d th a t th e L e w is a llo c h th o n d e fo rm s ela s tic a lly b e fo re b r it t le fa ilu re o c c u rs , th e e la s tic w edge m o d e l d e riv e d b e fo re ca n also be u se d to c a lc u la te th e m a x im u m le n g th o f th r u s t p la te s be cause th e b o u n d a ry c o n d itio n s a ssu m e d in th is m o d e l (fo rc e b a la n c e s in th e x a n d y d ire c tio n s ) s a tis fy re q u ire m e n t ( l ) fo r m o v e m e n t o f a th r u s t sheet a lo n g a th r u s t fa u lt. In o rd e r to s a tis fy re q u ire m e n t (2 ) th a t th e m a g n itu d e o f stress ca n b e a t m o s t e q u a l to c r itic a l v a lu e s o f fra c tu re fa ilu re , I assum e th a t im b ric a te fa u ltin g 2 54 w it h in th e L e w is a ilo c h th o n was in itia te d fro m its base a n d th e n p ro p a g a te d u p w a rd th r o u g h th e L e w is a ilo c h th o n . T h is a s s u m p tio n is based on th e o b s e rv a tio n th a t d is p la c e m e n t a lo n g th e im b ric a te th r u s ts decreases u p w a rd , in d ic a tin g th a t th e m o tio n a lo n g these fa u lts s ta rte d fro m th e b o tto m a n d th e n p ro p a g a te d u p w a rd th ro u g h th e L e w is a ilo c h th o n . In th is s e c tio n , I fir s t c a lc u la te w h e re fa u ltin g w ill o c c u r w ith in a n e la s tic w edge d u r in g th r u s tin g . I th e n c o m p a re th e c a lc u la te d re s u lt to w h a t has b e e n o b s e rv e d in th e fie ld to in fe r th e ro c k s tre n g th a n d th e b o u n d a ry c o n d itio n s fo r th e L e w is th r u s t s y s te m . L e t L b e th e m a x im u m le n g th o f th e o v e rth r u s t b lo c k (F ig . 5- 1 1 ). A t th e p o in t (L. L tan O ) , th e th re e stre ss c o m p o n e n ts can be d e riv e d fro m e q u a tio n s (3 5 ), (3 6 ), (3 7 ) <rx {L. LtanO ) = 2k$L 4- 6k^Ltan9 -f- pegsinctL, (39) a-y (L, LtanO) = — pegtanOcosaL, (4 0 ) rxy(L, LtanO) = — 2k^tan9 L. (41) A s s u m in g th a t th e C o u lo m b fr a c tu r e c rite r io n h o ld s fo r th e in itia t io n o f fa u ltin g w ith in th e L e w is p la te , th e p r in c ip a l stresses aq a n d a 3 a t th is p o in t s h o u ld s a tis fy 255 Figure 5-11. Maximum length of t h r u s t wedge i L. At the point (L, Lta n 0 ), the s tr e s s reaches the c r i t i c a l valu Coulomb f r a c t u r e c r i t e r i o n . de fin ed by state of of the 256 257 Maximum Length of Thrust Wedge &1 = C 0 + qaz (J e a g e r a n d C o o k , 1 9 7 9 ), w h e re (42) Co — 2So[(}i<f> 2 4- l ) 1^" 4- (43) 1 = [(m * 2 + 1 ) 1/2 + ^ ] 2 - (4 4 ) Cq is th e u n ia x ia l co m p re s s iv e s tre n g th , So is th e co h e sive s tre n g th , a n d ii$ is th e c o e ffic ie n t o f in te r n a l fr ic tio n . (4 2 ) is a g e n e ra l fo r m o f th e C o u lo m b fra c tu re c rite r io n . T h e m a g n itu d e f p r in c ip a l stresses a t th e p o in t ( L , LtanO) ca n b e c a lc u la te d fro m (3 9 ),(4 0 ), a n d (4 1 ) (H a fn e r, 1 9 5 9 ), <71,3 = W x + ° y ) / 1 ± ^/(0-x “ crj/)2/4 + T*y , (45) P u ttin g a i a n d a z o b ta in e d fro m (4 5 ) in to th e e q u a tio n (4 4 ), we ha ve L (A + B ) + q(A — B ) ’ ^ w h e re A = 1 / 2 \J{2kz 4- 6k^tan0 4- pgcosatand)2 4- 4k^tan20, (4 7 ) 258 B = l / 2(2&3 -f § k±ta nd -H p g s in a — p g c o s a ta n O ). (48) F ig . 5-12 is a p lo t o f L as a fu n c tio n o f th e dip: a n g le o f th e fa u lt, p. F o r th e p a ra m e te rs , A = 0.5 , = ta n30 ° , fib — 0-6, A& = 0.6, a n d a = 1 °. T h e p a ra m e te rs ch o sen h e re are s im ila r to th o se chosen fo r th e p lo t in F ig . 5-8 a n d are b e lie v e d to b e s t f it th e c o n d itio n s fo r th e L e w is th r u s t fa u lt. We ca n see th a t th e le n g th o f th e o v e rth r u s t b lo c k s is s tro n g ly d e p e n d e n t o n th e d ip an gle o f th e th r u s t fa u lt j3. E x c e p t fo r /? < 4°, th e le n g th o f th e th r u s t w ed ge decreases w it h in cre a se o f th e d ip an gle. T h e th in n e r th e w edge, th e lo n g e r i t c o u ld be . T h is re s u lt is s im ila r to th a t o b ta in e d b y L a w s o n (1 9 2 2 ) m o re th a n a h a lf c e n tu ry ago. F ig . 5-12 also show s th e r e la tio n s h ip b e tw e e n th e le n g th o f th e th r u s t p la te s a n d th e va lu e o f th e ir in te r n a l co h e sio n , Sq. R e s u lts o f e x p e rim e n ta l ro c k m e ch a n ics in d ic a te th a t S q ca n v a ry w id e ly , fro m s e v e ra l te n s o f ba rs u p to 1.5 k b a rs (M o g i, 1966; B y e rle e , 1 9 7 8 ). F ig . 5-12 show s th a t a n in c re a s e o f S q w ill in cre a se th e le n g th o f th r u s t p la te s . L is p a r tic u la r ly s e n s itiv e to th e coh e sive s tre n g th fo r 4° < (3 < 1 0 °. A s s h o w n b y the b a la n c e d cross s e c tio n in s o u th e rn G la c ie r N a tio n a l P a rk (P la te 2) a n d th e re g io n a l p a lin s p a s tic re c o n s tru c tio n s d iscu sse d p re v io u s ly , th e d ip a n g le o f th e L e w is th r u s t fa u lt is 7 ° ± 2 ° , 2 59 Figure 5 - 1 2 . Plot of the maximum length of the o v e r t h r u s t wedge L as a f u n c t i o n of the dip angle and cohesive s t r e n g th . See t e x t for e x p l a n a t i o n s . 260 J \ ~ 0 - 6 A i = T a n 3 0 DIP ANGLE (DEGREE) ro cn a n d th e d is ta n c e b e tw e e n th e tra c e o f th e p re s e n t L e w is th r u s t fa u lt a n d th e E lk M o u n ta in im b ric a te s y s te m is a b o u t 23 k m . a m in im u n v a lu e fo r th e d is ta n c e b e tw e e n th e le a d in g edge o f th e L e w is th r u s t fa u lt a n d th e lo c i o f th e E lk M o u n ta in im b r ic a te s y s te m a t th e tim e th e s y s te m fo rm e d . I f th e d ip a n g le o f th e L e w is th r u s t fa u lt at th e tim e th e E lk M o u n ta in im b r ic a te s y s te m fo rm e d was th e sam e as w h a t w as su g g e ste d b y th e re g io n a l p a lin s p a s tic re c o n s tru c tio n s , a n d i f th e d is ta n c e b e tw e e n th e p re s e n t tra c e o f th e L e w is th r u s t fa u lt a n d th e E lk M o u n ta in im b r ic a te s y s te m is ta k e n as th e m in im u m le n g th o f th e u n fra c tu re d th r u s t w edge a t th e tim e th e E lk M o u u n ta in im b r ic a te syste m d e v e lo p e d , th e n th e in te rn a l s tre n g th o f th e L e w is a ilo c h th o n was a t le a st 350 zt 50 b a rs . I f we k n o w th e c o h e s iv e s tr e n g th o f th e L e w is a ilo c h th o n , th e p o re - flu id p re s s u re r a t io a lo n g th e L e w is th r u s t fa u lt d u r in g th e d e v e lo p m e n t o f th e E lk M o u n ta in im b r ic a te s y s te m c a n be p r e d ic te d . F ig . 5 -1 3 sho w s th e m a x im u m le n g th o f th e o v e rth ru s t b lo c k s w it h d iffe re n t b a s a l p o re - flu id p re s s u re s as a fu n c tio n o f th e d ip a n g le . F o r th e p a ra m e te rs use d in th is p lo t, A = 0.5, = 0.6, = tanZO0 , a n d Sq = 40 0 b a rs . T h e fig u re show s th a t th e m a x im u m le n g th o f th e th r u s t b lo c k , L , increases w it h in c re a s e o f b a s a l p o re - flu id p re s s u re r a tio A&. L is p a r tic u la r ly s e n s itiv e to A& fo r A& > 0.7. T h is re s u lt is c o n s is te n t w it h th e H u b b e rt a n d R u b e v (1 9 5 9 ) p o re - flu id p re s s u re 2 6 2 th e o ry th a t a b n o rm a lly h ig h p o re - flu id p re s s u re ca n re d u c e th e b a sa l f r ic t io n a n d a llo w th e t h r u s t b lo c k to s u s ta in a p u s h fro m th e re a r. T h e re la tio n s h ip b e tw e e n th e su rface slop e a a n d th e m a x im u m le n g th o f th e o v e rth r u s t b lo c k s L is p lo tte d in F ig . 5-14. I t show s th a t th e le n g th o f o v e rth r u s t b lo c k s L decreases w it h incre a se in slo p e a n g le a . T h is re s u lt m a y be o f som e in te re s t to p ro b le m s o n th e slo p e s ta b ility in e n g in e e rin g geology. 263 F i g u r e 5-1 3. P l o t of the maximum l e n g t h o f th e o v e r t h r u s t wedge L as a f u n c t i o n of the dip angle and the p o r e - f l u id pressure r a t i o along the t h r u s t f a u l t . See t e x t fo r e x p l a n a t i o n s . 264 O o w 1 ► — 4 D i l 0 o P i D W t > r -®* Q W K D o b o £ 2 D & H o o s o X H O * 7 " p < D ^ j *—} "j T 1 --------1 --------i----~J 'I" " " ' "I*....." T .."'" *1" T 1 T = 0.6 J ii — T a n 3 0 O 4 DIP ANGLE (DEGREE) ro cri cn F i g u r e 5-14* P l o t of the maximum l e n g t h of th e o v e r t h r u s t wedge L as a f u n c t i o n of the surfa ce slope and cohesive s t r e n g t h . See te x t for e x p l a n a t i o n s . 266 2 0 0 0 b a r s 1 6 0 0 b a r s 1 2 0 0 b a r s 8 0 0 b a r s 4 0 0 b a r s Ph O Z ID SLOPE ANGLE (DEGREE) I V ) cn 'vj Di scuss ions Maximum Length of O v erthrusts Hubbert and Rubey f 1 9 5 91 showed t h a t pore f l u i d pressure can f a c i l i t a t e movement along l a r g e t h r u s t f a u l t s . The model developed here shows a s i m i l a r r e s u lt th a t in c r e a s i n g p o r e - f l u i d pressure along a t h r u s t f a u l t w i l l r e s u l t in in c r e a s in g the length of an u n fr a c t u r e d wedge-shaped o v e r t h r u s t blocks. However, t h i s model suggests t h a t p o r e - f l u i d pressure can be s i g n i f i c a n t in f a c i l i t a t i n g movement of a wedge-shaped t h r u s t block along an i n c l i n e d t h r u s t f a u l t only i f the p o r e - f l u i d pressure r a t i o is higher than 0.8 ( F i g . 5 - 1 3 ) . Such a high basal p o r e - f l u i d pressure r a t i o is d i f f i c u l t to defend f o r the Lewis a i l o c h t h o n . As discussed b e fo re , the Belt supergroup began at about 1,400 Ma. ( H a r r i s o n , 1 9 72 ), whereas the development of the Lewis t h r u s t system occurred between the Late Cretaceous to e a r l y Eocene (Mudge and E a r h a r t , 1980). The time d u ra tio n between the two events is more than 1,000 Ma.. Abnormal high f l u i d pressure w i t h i n the Belt s t r a t a could not have been preserved f o r such a long t im e . This suggests t h a t the abnormally high p o r e - f l u i d pressure could not be an im po rtant mechanism f o r i n i t i a t i n g movement along the Lewis t h r u s t . I f high p o r e - f l u i d pressure along the Lewis t h r u s t cannot be c a l l e d upon to f a c i l i t a t e t h r u s t i n g , then 268 how can movement along the Lev/is t h r u s t be p o s s ib le ? The r e s u lt s of my c a l c u l a t i o n provides an a l t e r n a t i v e e x p l a n a t i o n f o r t h i s mechanical paradox- As shown in Fig. 5-10, for a. pore f l u i d pressure r a t i o equal to 0 . 6 , a dip angle of the Lewis t h r u s t f a u l t equal to 7 degrees, and a cohesive s t r e n g t h of the Lewis a ilo c h t h o n equal to 380 b ars, the minimum dis ta n c e between the leading edge of the Lewis t h r u s t f a u l t during i t s movement and the place where i n t e r n a l f a u l t i n g would occur is 23 km. For a cohesive s tre n g th equal to 800 bars which is a reasonable value for the lo w-grade metamorphosed Belt rocks, and a dip angle of a t h r u s t f a u l t equal to 5 degrees (an p l a u s i b l e lower l i m i t of the dip angle fo r the Lewis t h r u s t , P la te 2), the length of the u n fr a c t u r e d o v e r t h r u s t block can be as long as 80 km. This r e s u l t suggests t h a t the mechanical paradox of o v e r t h r u s t s may not need to appeal the abnormal p o r e - f l u i d pressure as mechanism fo r i t s development. The geometry of a t h r u s t sheet should also be consid ere d, which is c l e a r l y an important f a c t o r in c o n t r o l l i n g the maximum leng th of an o v e r t h r u s t b l o c k . A l t e r n a t i o n of Low-angle F a u lts and Im b r ic a t e Thrusts The s t r u c t u r a l h i s t o r y of the Lewis t h r u s t system suggests the a l t e r n a t i n g development of lo w-angle f a u l t s and im b r ic a te systems. F i e l d evidence suggests t h a t these 269 lo w-angle f a u l t s are e xte n sional (see Chapter 3 ) . The younger im b r ic a te system was developed east of the o l d e r im b r i c a t e system, whereas the younger lo w-angle f a u l t was developed at a higher s t r u c t u r a l le v e l than the o l d e r one. As the o r i e n t a t i o n s of f a u l t s are c o n t r o l l e d by o r i e n t a t i o n s of p r i n c i p a l stre s s e s w i t h in the Lewis t h r u s t s heet, such an a l t e r n a t i o n of lo w-angle f a u l t i n g and im b r ic a te t h r u s t i n g i n d i c a t e s a change in s tre s s d i s t r i b u t i o n w i t h i n the Lewis ailo c h th o n during its emplacement. What mechanical c o n d itio n s might control such a change in s tre s s d i s t r i b u t i o n ? Several possib le models are proposed below and problems with each model are discussed. 1. The f i r s t e x p la n a tio n is t h a t the a l t e r n a t i o n of low-angle f a u l t s and im b r ic a te t h r u s t s in the Lewis t h r u s t system r e s u lte d from changes in geometry of the Lewis t h r u s t wedge or the geometry of the e n t i r e f o r e l a n d t h r u s t wedge. P l a t t (1986) suggested th a t the i n t e r n a l stress d i s t r i b u t i o n in an orogenic wedge can be r e l a t e d to the geometry of the wedge, a geometry defined by the dip angles of the bounding t h r u s t f a u l t below and the bounding surface slope above. By assuming viscous rheology fo r the de form ation of a t h r u s t wedge, and by assuming a f i x e d dip angle of the basal t h r u s t , he showed th a t once the s urface slope is g r e a t e r than a c e r t a i n v a lu e , e x ten sio n a l f a u l t s 270 w i l l be dominant in the t h r u s t wedge i f , however, the surface slope is lower than t h i s valu e, c o n t r a c t i o n a l f a u l t s w i l l be dominant. As the surfa ce slope of orogenic wedges is a consequence of t h r u s t dynamics, and i t changes during the development of the orogenic wedge, he proposed th a t temporal i n t e r a c t i o n s between the geometry of t h r u s t wedges and the i n t e r n a l s tre s s d i s t r i b u t i o n could lead to e i t h e r s h o r t e n in g or extension w i t h i n the t h r u s t wedge. C o n t r o l l i n g the s t a t e of i n t e r n a l stress d i s t r i b u t i o n by the geometry of a th r u s t wedge is also shown by the e l a s t i c wedge model presented in t h i s paper. The e l a s t i c wedge model suggests t h a t a t h i c k e r wedge contain s a l a r g e r p o r t i o n of p o t e n t i a l e x t e n s io n a l f a u l t s ( F i g . 5- 1 0 ) , whereas a t h i n n e r wedge c ontain s a s m a lle r p o r t i o n of p o t e n t i a l e x te n s i o n a l f a u l t s ( F i g . 5 - 8 ) . The model t h a t the changes in geometry of a t h r u s t wedge r e s u l t in changes in stre s s d i s t r i b u t i o n s w i t h i n the t h r u s t wedge may help e x p la in the a l t e r n a t i o n of low-angle f a u l t i n g and i m b r i c a t e t h r u s t i n g . The lo w -a ngle f a u l t s (Brave Dog and Rockwell f a u l t s ) have the geometry of e x t e n s io n a l f a u l t s , and t h e i r r o le is to t h i n the t h r u s t wedge. However, the role of the im b r ic a te t h r u s t systems is to th i c k e n the t h r u s t wedge. Thinning the t h r u s t wedge by e x te n s i o n a l f a u l t i n g w i l l r e s u l t in a change in s tre s s 27 1 d i s t r i b u t i o n w i t h i n th e t h i n n e r wedge w i l l f a v o r compressions! f a u l t i n g as p r e d ic te d by P l a t t s model (1986) and the e l a s t i c model of t h i s study ( c f . Fig- 5-8 and 5 - 1 0 ) . Once compressional f a u l t i n g becomes dominant, the t h r u s t wedge w i l l again begin to t h i c k e n . The t h i c k e n i n g of the t h r u s t wedge w i l l lead to a f u r t h e r change in s tre s s regime from one fa v o r in g predom in an tly compressional f a u l t i n g to e x ten siona l f a u l t i n g . R e p e t i t i o n of these processes could lead to observed a l t e r n a t i o n in G l a c i e r Park, (1) f o r w a r d - d i p p i n g , low- angle e x te n s io n a l f a u l t s , and (2) l i s t r i c im b r ic a t e t h r u s t s . One aspect of t h i s i n t e r p r e t a t i o n is t h a t t h i s model re quires a g e n e t ic r e l a t i o n s h i p between c o n t r a c t i o n a l f a u l t s and e x te n s io n a l f a u l t s . The development of one type of f a u l t s would r e s u l t in the i n i t i a t i o n of the o t h e r . As the e n t i r e t h r u s t wedge is considered as one dynamic un it in these models ( P l a t t , 1986; t h i s s t u d y ) , both compressional f a u l t i n g and e x ten sio n a l f a u l t i n g are expected to occur p e r v a s i v e l y through the t h r u s t wedge. This p r e d i c t i o n c o n t r a d i c t s with the o b s e r v a t i o n that s hortenin g a s s o c ia te d with the Mount Henry im b r ic a t e system is l o c a l i z e d . The width of the Mount Henry im b r ic a t e system is about 3 to 4 km- This s h o r te n in g event only a f f e c t e d the preserved eastern edge of the 272 d i s t r i b u t i o n w i t h i n the t h i n n e r wedge w i l l f a v o r compressional f a u l t i n g as p r e d i c t e d by P l a t t ' s model (198 6) and the e l a s t i c model of t h i s study ( c f . Fig. 5-8 and 5 - 1 0 ) . Once compressional f a u l t i n g becomes dominant, the t h r u s t wedge w i l l again begin to t h i c k e n . The t h i c k e n i n g of the t h r u s t wedge w i l l lead to a f u r t h e r change in s tre s s regime from one fa v o r in g predom in an tly compressional f a u l t i n g to extensional f a u l t i n g . R e p e t i t i o n of these processes could lead to observed a l t e r n a t i o n in G l a c i e r Park, (1) f o r w a r d - d i p p i n g , low- angle e x t e n s i o n a l f a u l t s , and (2) l i s t r i c i m b r i c a t e thrusts. One aspect of t h i s i n t e r p r e t a t i o n is t h a t t h i s model r equir es a g e n e tic r e l a t i o n s h i p between c o n t r a c t i o n a l f a u l t s and exten siona l f a u l t s . The development of one type of f a u l t s would r e s u l t in the i n i t i a t i o n of the o t h e r . As the e n t i r e t h r u s t wedge is considered as one dynamic u n it in these models ( P l a t t , 1986; t h i s s t u d y ) , both compressional f a u l t i n g and extensional f a u l t i n g are expected to occur p e r v a s i v e l y through the t h r u s t wedge. This p r e d i c t i o n c o n t r a d i c t s w ith the ob s e r v a tio n that s horte nin g a s so cia ted with the Mount Henry im b r i c a t e system is l o c a l i z e d . The width of the Mount Henry im b r ic a te system is about 3 to 4 km. This s h o r t e n in g event only a f f e c t e d the preserved eastern edge of the 273 Lewis a ilo c h th o n because the o l d e r , higher Brave Dog f a u l t is e s s e n t i a l l y plan ar and unbroken by younger t h r u s t f a u l t s west of the Mount Henry i m b r i c a t e system. The surface of the younger lo w -a ngle Rockwell f a u l t , however, extends more than 2 0 km to the west from the Mount Henry im b r ic a te system. I t is d i f f i c u l t to e x p l a i n why s hortenin g occurred w i t h i n the Mount Henry im b ric a te system could i n i t i a t e or t r i g g e r development of a low- angle e x te n s io n a l f a u l t more than 20 km towards the west. One could argue, however, t h a t the i n i t i a t i o n and development of the Rockwell f a u l t is a consequence of sh ortenin g through the e n t i r e f o r e la n d fo ld and th r u s t b e l t r a th e r than t h a t through the Lewis a ilo c h t h o n i t s e l f . Shortening accommodated by t h r u s t f a u l t s beneath the Lewis t h r u s t could also c o n t r i b u t e to the change in to p o gra phic slopes. The s hortenin g event associated with the development of the Mount Henry im b r ic a te system could have occurred s im u lta n e o u s ly with t h r u s t t h i c k e n i n g of rocks in the fo o tw a ll of the Lewis t h r u s t . The width of the Elk Mountain i m b r i c a t e system is unknown because i t s western p a r t is cut and o f f s e t by the younger B l a c k t a i l normal f a u l t . I f t h i s i m b r ic a t e system is s i g n i f i c a n t l y wide, say, several tens of k i l o m e t e r s , i t s development might have been a cause f o r the i n i t i a t i o n of the Brave Dog f a u 1 t . 2 74 The exact thick ne s s e s of sections omitted by the Brave Dog and Rockwell f a u l t s are not known. Since the two f a u l t s ju x ta p o s e same s t r a t i graphic u n its in t h e i r upper and lower p lates (Brave Dog f a u l t w i t h i n the Appekunny Fm., and Rockwell f a u l t with the G r i n n e l l Fm.}, one may argue t h a t s t r a t i g r a p h i c omissions by the two f a u l t s are not s i g n i f i c a n t . T h e r e f o r e , t h e i r development might not s i g n i f i c a n t l y a f f e c t e d geometry of the t h r u s t wedge . The e f f e c t of topography on c o n t r o l l i n g stress d i s t r i b u t i o n w i t h i n an orogenic b e l t was i n v e s t i g a t e d by Royden and B u r c h f ie l ( 1 9 8 7 ) . By assuming t h a t the l i t h o s p h e r e deforms e l a s t i c a l l y , they developed a mechanical model showing po ss ib le stress d i s t r i b u t i o n under i n f l u e n c e of a t o p o gra phic lo ad. T h e ir model i n d i c a t e s t h a t the presence of to po graphic load in an orogenic b e l t could produce e x te n s io n a l f a u l t s di pping to the f o r e l a n d . This r e s u l t suggests again t h a t topography could play an impo rtant role in producing f o r e l a n d - d i p p i n g , lo w -a ngle e x te n s io n a l f a u l t s . 2. R e c e n t l y , a mechanical model fo r the development of fo r e la n d f o l d and t h r u s t b e l t s and a c c r e t i o n a r y wedges based on noncohesive Coulomb f r a c t u r e rheology was proposed by Davis et a l . ( 1 9 8 3 ) , Dahlen et a l . (198 4 ) , Dahlen (198 4 ) . Davis et a l . (1983) considered the o v e r a l l 275 f o r e l a n d fo ld and t h r u s t b e l t s and a c c r e t i o n a r y wedges along compressive p l a t e boundaries to be analogous to t h a t of a wedge of soil or snow in f r o n t of moving b u l l d o z e r . The m a te r ia l w i t h i n the wedge deforms u n t i l a c r i t i c a l t a p e r is a t t a i n e d . A f t e r the c r i t i c a l t a p e r is a t t a i n e d , the wedge w i l l s l i d e s t a b l y . The wedge w i l l c ontinue to grow s e l f - s i m i l a r l y as a d d i t i o n a l m a te r ia l is encountered at the t o e . The i n t e r n a l s t a t e of stre s s w i t h i n a c r i t i c a l t a p e r is at the verge of Coulomb f r a c t u r e f a i l u r e everywhere. The geometry of a c r i t i c a l t a p e r is determined by the c o e f f i c i e n t of f r i c t i o n on the basal decoll ement and the stre n gth of the rocks composing the wedge. I f the wedge becomes t h i c k e r than i t s c r i t i c a l t a p e r geometry, the wedge is s u p e r c r i t i c a l . The s u p e r c r i t i c a l wedge can s l i d e along the t h r u s t s u r fa c e w i t h o u t i n t e r n a l de fo r m a tio n . In c o n t r a s t , i f the wedge is t h i n n e r than i t s c r i t i c a l t a p e r geometry, s l i d i n g w i l l not occur along the t h r u s t f a u l t . In s te a d , i n t e r n a l s hortenin g w i l l occur, leading the wedge to be thickened u n t i l i t again a t t a i n s the c r i t i c a l t a p e r geometry. One s o l u t i o n of the noncohesive Coulomb wedge model was presented by Dahlen ( 1 9 8 4 ) . His s o l u t i o n p r e d i c t s p l a n a r f a u l t s w i t h i n a t h r u s t wedge during its c o n s t r u c t i o n . He showed t h a t the o r i e n t a t i o n of f a u l t s w i t h i n the t h r u s t wedge is c o n t r o l l e d by the basal 276 f r i c t i o n along a d e coll em ent. An increase in p o r e - f l u i d pressure r a t i o from 0 .9 to 0 .9 8 along a decollement would r e s u l t in a change in s tr e s s d i s t r i b u t i o n w i t h i n the t h r u s t wedge. Such a change would produce changes in d e fo r m a tio n a l s t y l e from p l a n a r c o n t r a c t i o n a l f a u l t s to normal f a u l t s . In order to e x p l a i n the a l t e r n a t i o n of c o n t r a c t i o n a l and e xtensional f a u l t i n g w i t h i n the Lewis a i l o c h t h o n , p e r i o d i c a l changes in p o r e - f l u i d pressure along the Lewis t h r u s t and an extre m ely high basal pore- f l u i d pressure r a t i o (0 .9 8 ) is r e q u ir e d . Such a high p o r e - f l u i d pressure r a t i o , as discussed e a r l i e r , is u n l i k e l y to have occurred along the Lewis t h r u s t during i t s form ation and development. Ex tensio nal Faults v . s . C o n t r a c t i o n a l Faults An i n t e r e s t i n g r e s u l t of the mechanical model developed in t h i s d i s s e r t a t i o n is the p r e d i c t i o n of e x te n s i o n a l f a u l t s w i t h in a wedge-shaped t h r u s t sheet ( F i g . 5 - 8 , 5 - 9 ) . Such r e s u l t appears s u r p r i s i n g , but it is not one in c o m p a ti b le with g e o lo g ic a l o b s e r v a t i o n s . As shown in Fig. 5 - 8 , the extensional f a u l t s and c o n t r a c t i o n a l f a u l t s could occur s im u lt a n e o u s ly . Recent o b s e r v a tio n s on the surface de form ation asso cia ted with the El Asnam Earthquake in A l g e r i a in 1980 show t h a t e x te n s io n a l f a u l t s developed on the s u r f a c e while 2 7 7 t h r u s t i n g w i t h i n the t h r u s t sheet ( i n d i c a t e d by f a u l t - plane s o lu t io n s of a f t e r s h o c k s ) was oc c u rrin g (King and Y i e l d i n g , 1 9 8 4 ). King and V i t a - F i n z i (1981) i n t e r p r e t e d t h i s as r e s u l t i n g from a n t i c l i n a l u p l i f t caused by s t r a i n s a ssociated with motion on the un d e rlyin g t h r u s t f a u l t s . This i n t e r p r e t a t i o n is c o n s is t e n t with the r e s u l t of the mechanical model developed in t h i s paper which i n d i c a t e s th a t devi a t o r i c str e s s can change from exten siona l in the upper part of the th r u s t wedge to compressional in the lower part of the t h r u s t wedge (see F ig . 5 - 8 ) . It is not impossible t h a t e xten sional f a u l t s occurred in the upper p o r t i o n of the Lewis t h r u s t f a u l t . Since part of the Lewis a ilo c h t h o n has been eroded, t h i s p r e d i c t i o n is d i f f i c u l t to t e s t . 278 CHAPTER 6 SUMMARY An area of a p p r o x im a t e ly 200 km2 was mapped across an E-W s t r i p of the Lewis a ilo c hthon from Bison Mountain to P e r il Peak in southern G l a c i e r National Park, Montana. Along the e astern edge of the study area, the Lewis th ru s t juxtaposes the Altyn Formation of the P r o t e r o z o i c Belt Supergroup over the l a t e Cretaceous Marias R iv e r Shale. S t r a t i graphic un its of the Belt Supergroup mapped in the study area are A lt y n , Appekunny, G r i n n e l l , Empire, and Helena fo r m a t i o n s . The Appekunny Formation is f u r t h e r divided in t o f o u r informal members in order to f a c i l i t a t e mapping d e t a i l e d s t r u c t u r e s . The exposures of the lower- p l a t e Cretaceous sedimentary rocks are very poor. No e f f o r t was made to subdivide the Cretaceous Marias River Shale. Geologic mapping has shown t h a t the a r c h i t e c t u r e of the Lewis t h r u s t system is extremely complex. Major s t r u c t u r a l elements in t h i s system are the present Lewis t h r u s t , Scenic Point s t r u c t u r a l complex, f r o n t a l zone, eastern s t r u c t u r a l b e l t , Elk Mountain i m b r i c a t e system, Brave Dog f a u l t , Mount Henry im b r ic a t e system, Rockwell f a u l t , Two Medicine Pass f a u l t , Lone Walker f a u l t , and A ka m in a/Contin enta l Div ide sync 1i ne. Both the Brave Dog and Rockwell f a u l t s dip g e n e r a l l y to the east at shallow angles, and cut down s t r a t i graphic s ections in the d i r e c t i o n of t h e i r t r a n s p o r t . They are t h e r e f o r e 279 i n t e r p r e t e d as lo w-a ngle e x ten sio n a l f a u l t s in the sense t h a t the lengths of beds were extended. However, these two f a u l t s could be of t h r u s t o r i g i n . The extensional geometry could be ex pla in ed by e i t h e r an i r r e g u l a r f a u l t geometry ( o r i g i n a l f a u l t surface dips in both east and west d i r e c t i o n along d i f f e r e n t segments of the f a u l t s ) or by planar f a u l t s c u t t i n g broadly folded s t r a t a . Trun ca tio na l r e l a t i o n s h i p s between the Lewis t h r u s t and s t r u c t u r e s in the f r o n t a l zone, eastern s t r u c t u r a l b e l t , and Scenic Point s t r u c t u r a l complex suggest that those s t r u c t u r e s were developed p r i o r to the f o r m a tio n of the present Lewis t h r u s t f a u l t . Th e ir development may have been r e l a t e d to the emplacement of the Lewis a l l o c h t h o n along an o ld e r t h r u s t f a u l t , the Lewis t h r u s t I, which l i e s s t r u c t u r a l l y below the present Lewis t h r u s t f au1 t . The d e fo rm a tio n a l h i s t o r y of the Lewis a l l o c h t h o n in the study area has been c onstr ucted based on c r o s s - c u t t i n g r e l a t i o n s h i p s between major s t r u c t u r a l elements in the Lewis t h r u s t system. The sequence of deformation is 1) development of minor normal f a u l t s dipping both to the east and west, 2) development of minor c o n t r a c t i o n a l f a u l t s associated with the form ati on of the Lewis t h r u s t I , 3) formation of the Scenic Point f a u l t which t r u n c a t e d the minor c o n t r a c t i o n a l and e xten sional f a u l t s , 4) open 280 f o l d i n g which a f f e c t e d th e e a s t e r n m o s t p a r t of th e study a r e a , 5) f o r m a tio n of the f r o n t a l zone, 6) occurrence of t o p - t o - t h e - e a s t simple shear which caused r o t a t i o n of s t r u c t u r e s in the f r o n t a l zone: some primary, e a s t - d i p p i n g t h r u s t s were rota ted to w e s t - d i p p i n g apparent normal f a u l t s , 7) fo r m a tio n of the e a s te r n s t r u c t u r a l b e l t , 8) f o r m a tio n of the present Lewis t h r u s t which l i e s s t r u c t u r a l l y above the o l d e r t h r u s t f a u l t , the Lewis t h r u s t I , 9) (?) development of the Two Medicine Pass f a u l t , 10) development of the Elk Mountain im b r ic a te system, 11) form ation of the Brave Dog f a u l t , 12) development of Mount Henry i m b r i c a t e system, 13) fo rm atio n of the Rockwell f a u l t , 14) a broad f o l d i n g event which a f f e c t e d the e n t i r e study area and formed the Akami na/Conti nental Divide s y n c l i n e , 15) fo r m a tio n of the Lone Walker f a u l t which cut a l l e a r l y s t r u c t u r e s in the Lewis a l l o c h t h o n and p o ssib ly o f f s e t the Lewis t h r u s t . St ri at ions measured from the Lewis t h r u s t and shear surfaces a d ja c e n t to i t i n d i c a t e th a t the t r a n s p o r t d i r e c t i o n of the present Lewis t h r u s t is about N 5 5 o E - N75oE. The minimum displacement along the Lewis t h r u s t in i t s t r a n s p o r t d i r e c t i o n is 24 km. The o r i g i n of complex s t r u c t u r e s in the f r o n t a l zone, Scenic Point s t r u c t u r a l complex, and eastern s t r u c t u r a l b e l t is not well understood. T h e i r development may have 281 been r e la t e d t o a t h r u s t ramp along an old e r t h r u s t , the Lewis t h r u s t I . Thrust f a u l t s w i t h i n the i m b r i c a t e systems ( t h e Mount Henry im b r ic a t e system and Elk Mount im b r ic a te system) appear to have developed hindward. The im b r ic a t e systems themselves w i t h i n the Lewis t h r u s t system developed forward, i . e . , Elk Mountain i m b r i c a t e systems f i r s t , then Mount Henry im b r ic a t e system. However, 1 ow-angle e x ten siona l f a u l t s (Brave Dog f a u l t and Rockwell f a u l t ) developed hi ndward with the younger of the two f a u l t s developed at a hig h e r le v e l than the older one. Two major i m b r i c a t e t h r u s t systems, the Elk Mountain im b r ic a te system and Mount Henry im b r ic a te system, and two major lo w-angle e x te n s io n a l f a u l t s , the Brave Dog f a u l t and the Rockwell f a u l t , were developed during the emplacement of the Lewis a l l o c h t h o n along the present Lewis t h r u s t . The development of two im b r ic a te systems and two 1 ow-angl e extensional f a u l t s w i t h i n the Lewis a llo c h t h o n were ^alternated. The a l t e r n a t i o n of 1 ow-angl e extensional p a u l t i n g and t h r u s t i m b r i c a t i o n s w i t h i n the Lewis a!,l ochthon can be best ex pla in ed by the temporal i n t e r a c t i o n s between the geometry of t h r u s t wedges and i n t e r n a l s tre s s d i s t r i b u t i o n s ( P l a t t , 1986; t h i s s t u d y ) . A simple e l a s t i c wedge model was developed in t h i s study in order to i n v e s t i g a t e p o s s ib le mechanical controls 28 2 on the geometry of thrust faults and location of major imbricate systems in the Lewis allochthon. It shows that the maximum length of a thrust sheet is not only controlled by the strength of the thrust sheet and the coefficient of basal f r ic t io n , but also by the dip angle and surface slope of the thrust wedge. 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Creator Yin, An (author)

Core Title Geometry, kinematics, and a mechanical analysis of a strip of the Lewis allochthon from Peril Peak to Bison Mountain, Glacier National Park, Montana

Degree Doctor of Philosophy

Degree Program Geological Sciences

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c29-352116

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