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Structural geology of the Chiwaukum schist, Mount Stuart region, central Cascades, Washington

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Structural geology of the Chiwaukum schist, Mount Stuart region, central Cascades, Washington

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Content STRUCTURAL GEOLOGY OF THE CHIWAUKUM SCHIST MOUNT STUART REGION CENTRAL CASCADES WASHINGTON by Nicholas W. Taylor A Thesis Presented to the FACULTY OF Tl IE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (Geology) December 1994 Copyright 1994 Nicholas W. Taylor UNIVERSITY O F SO U TH ER N CALIFORNIA THE GRADUATE SCHOOL UNIVERSITV PARK LOS ANOELES. CALIFORNIA SOOOT This thesis, written by NICHOLAS WILLIAM TAYLOR under the direction of hAs Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE D ** m Date October 6, 1994 THESIS COMMITTEE TABLE OF CONTENTS ACKNOWLEDGEMENTS...................................................................................v i ii ABSTRACT........................................................................................................... 1 INTRODUCTION..................................................................................................3 GEOLOGIC SETTING..........................................................................................1 0 Description of Rocks in the Field Area........................................... 1 3 Chiwaukum Schist..................................................................13 Pelitic Schist..................................................................1 4 Sandy-Biotite Schist........................................................1 5 A m p h ib o lites................................................................1 5 U ltram afics....................................................................1 6 Intrusive Bodies...................................................................... I 7 Mount Stuart batholith.................................................. 1 7 H ornblendites...............................................................1 8 Garnet Bearing Dikes................................................ I 8 Structures in the Batholith.............................................................19 Contact Relations..................................................................... 2 1 STRUCTURAL ELEMENTS IN COUNTRY ROCKS....................................2 2 In tro d u ctio n ...........................................................................................2 2 Transposition Cycles and Polydeformed Terranes...........................................................................................2 2 Tranposition Cycles.............................................................................2 7 Transposition Cycle 1............................................................2 7 Transposition Cycle 2 ............................................................3 0 Transposition Cycle 3 ............................................................4 1 Transposition Cycle 4 ............................................................4 3 Transposition Cycle 5 ............................................................5 5 Jo in ts............................................................................................. 5 5 Slikensides..................................................................................5 6 C -Surfaces...................................................................................5 9 Summary of Country Rock Deformation.................................. 6 0 STRAIN.................................................................................................................. 6 I In tro d u ctio n ........................................................................................... 6 1 Strain Analysis Methods......................................................................... 6 2 ii Rf/<t>...............................................................................................6 2 Strain from Folds...................................................................6 7 Strain from Boudins....................................................................7 1 Discussion............................................................................................... 7 4 Strain Data.................................................................................7 4 Strain Distribution..................................................................7 7 Strain History...........................................................................7 9 METAMORPHISM............................................................................................ 8 0 Porphyroblast/Matrix Relationships........................................ 8 0 Biotite............................................................................................. 8 0 Cordierite....................................................................................... 8 1 G a rn e t.......................................................................................... 8 4 H orn blend e................................................................................8 6 Andalusite......................................................................................8 7 Sillim anite..................................................................................8 9 M uscovite...................................................................................90 Summary of Porphyroblast Matrix Relationships..............9 2 Summary of Metamorphism......................................................... 9 3 Pelitic Schists............................................................................9 4 A m p h ib o lites............................................................................9 8 U ltram afics................................................................................ 9 9 SUMMARY OF DEFORMATION....................................................................1 0 1 DISCUSSION........................................................................................................ 1 0 7 Timing and Nature of Deformation............................................107 Pluton Emplacement....................................................................1 1 2 Emplacement Mechanisms........................................................1 I 4 Regional Deformation............................................... 114 Wall Rock Flow................................................................1 1 4 Stoping................................................................................1 1 6 Dike/Sill Emplacement............................................ 116 Summaryof Pluton Emplacement...................................119 Translation vs T ilt.............................................................................. 119 Regional Tectonics.............................................................................. 122 CONCLUSIONS AND SUMMARY..................................................................12 8 REFERENCES.........................................................................................................134 iii FIGURE INDEX Figure 1. Locations of tectonostratigraphic terranes of Tabor et al., (1987b) in the Cascades Crystalline Core...................4 Figure 2. Generalized geologic map of the Nason Terrane...........6 Figure 3. Locations of Icicle Canyon domains.......................................... 8 Figure 4. Map showing variation of uranium-lead, hornblende, and biotite cooling ages in the Nason terrane 1 2 Figure 5. Photomicrograph of hand sample of garnet bearing dike cut parallel to cleavage 2 0 Figure 6. Photomicrograph and line drawing of cordierite porphyroblast 2 9 Figure 7. Geologic map showing highest known transposition cycle achieved in different domains in the Icicle Canyon area 3 1 Figure 8. Geologic map showing the orientation of foliation in all units in the Icicle Canyon area 3 2 Figure 9. Stereoplots showing cleavage orientations in domains 1, 5, 6 and 7 3 3 Figure 10. Cross section A-A‘ showing attitudes of contacts of the Ingalls Complex and the Mount Stuart batholith with the Chiwaukum Schist 3 5 Figure 11. Geologic map showing the orientation of fold axes in the Icicle Canyon area 3 6 Figure 12. Stereoplots showing fold axes in domains 1, 4, 5, and 6 3 7 i v Figure 13. Geologic map showing the orientation of stretching lineations in the Icicle Canyon area 3 9 Figure 14. Stereoplots showing stretching lineation orientation in domains 1, 2, 3 and 6 4 0 Figure 15. Stereoplots showing poles to cleavage in domains 2, 3 and 4 4 4 Figure 16. Geologic map showing detail of cleavage in the Chiwaukum Schist in domain 2 4 5 Figure 17. Geologic map showing contact region in domain 3 4 6 Figure 18. Stereoplots showing mesoscopic fold axes in domains 2 and 3 4 8 Figure 19. Geologic map showing selected sample locations in the Icicle Canyon area discussed in text 4 9 Figure 20. Photomicrographs of crenulations of cycle 3 cleavage 5 2 Figure 21. Geologic map showing the orientation if SCr. SCc and St * in the Chiwaukum Schist and amphibolitic lenses in the Ingalls Complex 5 4 Figure 22. Stereoplots showing poles to joint surfaces in the Icicle Canyon area and slickensides from domain 7 5 6 Figure 23. Geologic map showing the orientation of joints in the Icicle Canyon area 5 7 Figure 24. Log-Flinn plot of strain markers y/z vs. x/y ratios.......................................................................................................................6 3 Figure 25. Geologic map of the Nason Terrane showing the locations of various strain samples analyzed 7 3 v Figure 26. Line drawing of thin section from sample 140B-T90 of cordierite boudins 7 7 Figure 27. Photomicrograph and line drawing of cordierite porphyroblast from sample 141B-T90 8 3 Figure 28. Photomicrographs of garnet porphyroblast from sample 217-T90 8 5 Figure 29. Photomicrograph and line drawing of chiastolite porphyroblast from sample 239-T90 8 8 Figure 30. Photomicrographs showing fibrous aggregates of fibrolite crystals from sample 239-T90 9 1 Figure 31. Photomicrographs from sample 239-T90 showing muscovite porphyroblasts cross-cutting the external schistosity 9 2 Figure 32. Geologic map showing the overall structural makeup of the Mount Stuart batholith..................................................117 Figure 33. Cross section B-B’ showing contact relations and foliation patterns in the Chiwaukum Schist and the eastern and western bodies of the Mount Stuart batholith................................................................................................................ 1 1 8 Map of field area.............................................................................................. pocket vi TABLE INDEX Table 1. Attitudes of selected contacts 2 1 Table 2. General characteristics of domains in the Icicle Canyon region 2 3 Table 3. Mount Stuart fabric ellipsoids 6 4 vii ACKNOWLEDGEMENTS This study benefited greatly from the generous assistance and input of many individuals. First of all I would like to thank my advisor Dr. Scott R. Paterson for his valuable guidance and assistance in the field and classroom, and the remaining members of my thesis committee Profs. Gregory A. Davis, and J. Lawford Anderson for their input into my education and this manuscript. Secondly, I would like to thank the following individuals for their useful help in realizing this goal: Prof. Bob Miller, Yu Hao, Tom Brudos, John Bendixen, Dave Mayo, Ke-Sheng Bao, Paul Godin, Richard J. Lisle and others too numerous to m ention. Finally I would like to thank Scott Paterson and the U. S. C. graduate student research fund for their generous financial assistance without which, none of this could have been possible. ABSTRACT Mid-Cretaceous deformation in northwestern Washington and British Columbia is manifested by upper greenschist to upper-amphibolite facies metamorphism, thrust faulting and plutonism. Several different models have been proposed to explain this period of orogenesis. In one, the Cascades crystalline core is footwall to west-directed nappes that comprise the Northwest Cascades thrust system (NCWS). In another model orogenesis is a result of transpression, with the core being part of a large dextral shear zone. A third model, suggests that high pressure metamorphism and some ductile deformation is related to pluton loading. The study area (Icicle Canyon) includes the only known juxtaposition of the NCWS and the Cascade crystalline core not modified by Cenozoic deformation. Here, the ophiolitic Ingalls Complex is separated from the underlying Chiwaukum Schist by the Windy Pass thrust. This contact was subsequently intruded by the tonalitic 96-93 Ma Mount Stuart batholith. In the study area the batholith has a moderately well developed magmatic foliation with a minor solid-state overprint. The Chiwaukum Schist in the Icicle Canyon area, has experienced extensive polydeformation with anywhere from two to four recognizable cycles of transposition. Field and microstructural features in the schist show that there is a 1 composite, northwest-striking schistosity ( S j) and at least two subsequent generations of foliation and stretching lineation transpositions associated with domainal folding. Transposition cycles 2, 3 and probably 1 occurred during top-to-the-SW shearing, NW-SE extension and SW-NE contraction. Continued contraction probably caused lop-to-the- SW Windy Pass thrusting and transposition of older structures parallel to the thrust. Growth of biotite, cordierite, hornblende and possibly garnet occurred during this period. Transposition cycle 4 occurred during the emplacement of the Mount Stuart batholith. During emplacement, domainal syn-emplacement deformation occurred where cycle 3 and 4 structures were folded macroscopically along axes parallel to the batholith margin. Transposition cycle 4 was coeval with the growth of andalusite and probably garnet, but appears to pre-date the growth of sillimanite and muscovite in the study area. Post emplacement deformation is predominantly brittle in nature. Fabric analyses indicates that strain was probably plane in nature with extensional axes and xy planes sub-parallel to stretching lineations and cleavage respectively. The transposition of country rock structures in the contact aureole and presence of sloped blocks indicates that ductile flow of country rocks and stoping were material 2 transfer processes (MTP's) during emplacement of the Mount Stuart batholith. Anomalous paleomagnetic pole positions in the Mount Stuart batholith coupled with a SE-NW decrease in cooling ages, and a SE-NW increase of metamorphic grade indicate some in- situ NW-side-up lilt occurred in the Nason Terrane. However, some northward translation is also needed to explain the observed pole discordance. Further work is needed to quantify the roles of translation vs. tilt in the deformation of the Nason Terrane. INTRODUCTION Mid-Cretaceous orogenesis in northwestern Washington and southwestern British Columbia is manifested by upper- greenshist to upper-amphibolite facies metamorphism, plutonism and deformation in the Cascades crystalline core and large scale thrusting in the lower grade rocks of the Northwest Cascades thrust system (NCWS) west of the core (fig. 1). Several different models have been proposed to explain this period of orogenesis. In one, the Cascades Crystalline Core is footwall to west-directed thrust sheets that comprise the NCWS (Brandon and Cowan, 1985; Brandon 1989; McGroder, 1989). Thrusting may be a result of the collision of the Insular and Intermontane superterranes (Monger et al., 1982), or perhaps 3 1 2 2® n o ® WASHINGTON A r*fl o f FlgtOt I C A N A D A _ U N IT E D S T A T E S » Chiwaukum Grabcn J Tertiary Rocks Leavenworth I'aull Figure 1. Locations of lecionosiraligraphic lerranes o f Tabor et al. (1987b) in the Cascades Crystalline Core. Contacts between tenanes are faults. Barbed contacts arc thrust faults. 4 intra-arc contraction (Rubin et al., 1990. In another model, orogenesis results primarily from transpression, and the NWCS and the Cascades core are part of a broad dextral shear zone (Brown, 1987, 1989; Brown and Talbot, 1989). In a third model Brown and Walker (1991) suggest that high pressure metamorphism and some ductile structures are the result of loading by Cretaceous plutons. The biggest problem with evaluating these hypotheses is the unknown nature of the timing and kinematics of deformation in the NCWS relative to the core. The core and NCWS are separated along most of their length by the dextral Eocene Straight Creek Fault (fig. 1) which has major (80-190 km) displacement (e.g. Misch, 1977; Klienspehn, 1985; Monger, 1985; Brandon 1989). Metamorphism and deformation in the Paleocene further complicated the situation by overprinting the earlier Cretaceous orogenesis. The only juxtaposition of crystalline core and NCWS rocks, not modified by Cenozoic faulting or Paleogene metamorphism, occurrs near where the Late Jurassic (Southwick, 1974) Ophiolitic Ingalls Complex (Pratt, 1958) has been emplaced over rocks of the core (fig. 2) along the Windy Pass thrust (Miller, 1980; 1985). Most workers consider the Windy Pass thrust to be part of the NWCS (e. g. Miller, 1985; Brown, 1987; Brandon et al., 5 NASON TERRANE ;ht-Creek ault Leavenworth Fault Windy-PassVjpl •V T hrust i l l 10 Km. Figure 2. Generalized geologic m ap of the Nason Terrane. Ch = Chiwaukum Schist, Mst = M ount Stuart Batholith, Is = Ingalls Complex St = staurolite isograd Ky = kyanitc isograd. The enclosed area is the Icicle Canyon region. 6 1988). However, Tabor et. at (1987a; 1987b) considers the Ingalls Complex and the Nason terrane to have been assembled before their arrival at the North American margin. The Windy Pass thrust is intruded by the composite mid-Cretaceous Mount Stuart batholith (Pongsapich, 1974; Erickson, 1977) which provides an important temporal marker for thrusting. The structural and tectonic evolution of the rocks adjacent to the Windy Pass thrust/Mount Stuart batholith contact is crucial in understanding orogenesis in the Cascade Mountains and is the focus of this investigation. The study area lies in Icicle Canyon located in the east-central Cascade Mountains in Wenatchee County, Washington approximately 125 miles west of Seattle. The field area comprises ~ 100 km^ surrounding Icicle Canyon in which exposed rocks are primarily Chiwaukum Schist. It is surrounded on the north and east by the eastern body of the Mount Stuart batholith and to the south by the western body. To the south it is overlain by the Ingalls Complex along the Windy Pass thrust (fig. 3). There is considerable debate over the relative timing of metamorphism recorded by the Chiwaukum Schist (Page, 1938), the predominant rock type of the Nason Terrane. In one interpretation, regional metamorphism predated, at least in part the emplacement of the Windy Pass thrust, and is followed 7 tB S B fl! ‘ 'Suteliglu Pluion (Msi) 1 MILE Big Jim Figure 3. Locations of la c k Canyon domains. Characteristics of these domains are summarized in table 1 .Units shown here are the Chiwaukum Schist (Ch), Mount Stuart batholith, Ingalls Complex and the Big Jim Intrusive Complex by contact metamorphism associated with the emplacement of the Mount Stuart Batholith (Plumber, 1980; Miller, 1985; Taylor et al., 1991). In another interpretation low pressure contact metamorphism resulting from the emplacement of the batholith predates high pressure regional metamorphism and later local low pressure static assemblages (Evans and Berti, 1986). Yet another interpretation is that pluton emplacement occurred during regional metamorphism which continued after emplacement (Miller and Paterson, 1991; Taylor et al., 1992; Paterson et al., ms. 1993). The Mount Stuart batholith has also been the focus of a paleomagnetic controversy in the Cordillera. Beck (1976; 1980) has interpreted paleo-inclination data to indicate that the batholith, and its older units originated far to the south and has been an important element in the Baja-British Columbia model (Irving, 1985; Umhoefer, 1987). Butler et al. (1989) interpret the anomalous paleomagnetic poles to indicate a 35° tilt with little or no northward translation of the batholith. Structural and metamorphic considerations indicate that the batholith and its wallrocks have been tilted at least 15° about a NE-SW axis with its NE side up (Miller and Paterson, 1990) assuming relatively uniform paleogeothermal gradients. Some controversy exists regarding the means of emplacement of the Mount Stuart Batholith as well. Plummer 9 (1969; 1980), Pongsapich (1974) and Evans and Berti (1986) state that emplacement was in large part forceful. However, Paterson et al. (1993) point out that a maximum of 20-30% of the space now occupied by the batholith can be accommodated by ductile shortening of the plutons wall rocks indicating the need for multiple emplacement mechanisms. The objectives of this study are to determine the structural and metumorphic history associated with the study area and the north Cascades and use this information to evaluate the controversies listed above. GEOLOGIC SETTING In the area examined (fig 2), the Nason Terrane consists of three main units: 1) the predominantly pelitic Chiwaukum Schist (Page, 1933); 2) the structurally overlying Ingalls Complex, a late Jurassic ophiolite (Southwick, 1974; Miller, 1985) and 3) the Cretaceous Mount Stuart batholith (Pongsapich, 1974; Erickson, 1977). These units are truncated to the west and east by the Tertiary Straight Creek and Leavenworth faults, respectively. The polydeformed Chiwaukum Schist consists of metamorphosed pelitic and lesser amphibolitic schists and gneisses that locally include blocks and lenses of ultramafic materials, amphibolites and marbles (Plummer, 1969, 1 0 Getzinger, 1978). It typically has a moderate to steep WNW- striking cleavage, with WNW-trending shallowly plunging Hneations and fold axes. Little is known about protolith ages although Magloughlan (1993, p. comm.) reported a whole rock Rb/Sr age of 209 ± 19 Ma. Peak metamorphic grade increases towards the northeast corner with staurolite and kyanite isograds identified (fig. 4). The Ingalls Complex (Pratt, 1958, Southwick, 1974, Miller, 1985) consists primarily of ultramafic tectonites with less abundant gabbros, diabases, basalts, cherts and fine grained greywackes. It outcrops in a wide belt near the southern part of the Mount Stuart batholith. Near the Windy Pass thrust the Ingalls Complex is imbricated with meta-sedimentary, and meta-plutonic slices that may be part of the crystalline core not exposed in the lower plate (Miller, 1985). Imbrication was synchronous with amphibolite facies metamorphism in the Nason Terrane. Thrusting was interpreted to be north-directed (Miller, 1985; Vance, 1980), but is poorly constrained. The composite Mount Stuart batholith consists of a 95.5 Ma mafic phase, locally called the Big Jim complex, and a 93 Ma main phase that consists mostly of biotite-hornblende tonalite, but ranges in composition between two-pyroxene diorite and granodiorite (Anderson, 1992).The main phase forms two large 1 1 NASON TERRANE ;ht Creek ault tS ffS U/Pb 93-95 ma HBL 88 ma BIO 82 ma Leavenworth Fault UlPb 93-951 HBL 89 ma J I O 84 ma Windy Pass' ^ T h ru st 10 Km Figure 4. Map showing variation uf uranium-lead (U/Pb), hornblende (HBL), and bioiite (BIO) cooling ages in the Nason Terrane (Tabor et al., 1987a; Haugerud, 1991) 1 2 bodies; a smaller western sill-like body and a larger eastern body. The eastern body consists of a hook-shaped northwestern end which extends into a sill-like central region which becomes a mushroom shaped body in the southeast (fig. 2). The 95.5 Ma phase is everywhere moderately deformed. However the 93 Ma phase is only deformed along its northeast margins. Elsewhere magmatic foliations and lineations are weakly to moderately developed. During its emplacement, the Mount Stuart Batholith formed a narrow (<1 - 3 km) structural and thermal aureole that overprints the Chiwaukum Schist, Ingalls Complex and Windy Pass Thrust. Description of Rocks in the Field Area Chiwaukum Schist The Chiwaukum Schist was named by Page (1939). The predominant rocks of the Chiwaukum Schist of Icicle Canyon are middle to upper amphibolite facies pelitic schists, with subordinate amphibolitic, and weakly foliated ultramafic rocks. Most workers (e.g., Plummer, 1980, Getzinger, 1978) believe the Chiwaukum protolith to be a, sandy to argillaceous deposit with lesser mafic igneous rocks and carbonate rocks. These ultramafics and amphibolites are thought to be imbricate slices 1 3 related to the Windy-Pass thrusting (Miller, 1985), or of serpentine-slide origin (Tabor et al., 1978). Pelitic Schist In the study area the pelitic schist is generally a medium-fine grained, well-laminated graphitic plagioclase- quartz-biotite-schist. Locally, it has abundant cordierite and garnet, with rare hornblende or muscovite. Common accessory minerals are tourmaline, apatite, sphene, opaque oxides, very fine-grained graphite, and zircon inclusions in biotite. Common secondary minerals are chlorite, epidote and hematite. Chlorite and hematite often occur in conjunction with secondary opaque oxides. K-Ar dating done on biotite grains in pelites yields cooling ages of 88 Ma in the vicinity of Icicle Canyon with a decrease to the northeast (Tabor el al., 1987a; Haugerud, 1991). Quartz-plagioclase-rich layers in the pelitic schists generally occur as polygonal mosaics. Plagioclase and quartz are medium-to fine-grained and display a granoblastic texture. Plagioclase grains are generally untwinned and less common than quartz. These quartz-plagioclase layers are often discontinuous, showing frequent pinch and swell and boudinaged structures. Spatially associated with the contact of the Mount Stuart batholith are fine-grained, massive pelitic rocks that owe their 1 4 appearance to contact metamorphism. Occurring in these rocks are porphyroblasts of andalusite, sillimanite and muscovite. Sandy-Biotite Schist The sandy-biotite schist is a medium-grained well- laminated plagioclase-quurtz-biotite-schisl. It is differentiated from the pelitic schist by its higher quartz and plagioclase content responsible for its coarser texture, and by its tendency to develop stretching lineations which are rare in pelitic schists in the study area. A m p h ib o lites Amphibolites occur in gneissic and schistose varieties as imbricate slices related to Windy Pass thrusting (Miller, 1985). They have been observed throughout the Chiwaukum Schist, and Ingalls Complex, but are most common near the thrust contact. These rocks occur as concordant, discontinuous, dark green to greenish-black bands varying from a meter to tens of meters in size. K-Ar ages from hornblende indicate amphibolites cooled below 500° C by 93 to 88 Ma in the Icicle Canyon area. Mineral assemblages in the amphibolites consist of blue- green to olive hornblende as a dominant phase, plagioclase, and minor amounts of quartz. Plagioclase and quartz are commonly I 5 anhedral and possess polygonal grain boundaries and granoblastic textures. Plagioclase is generally untwinned. Common accessory minerals are epidote, sphene, zircon, opaque minerals, biotite, apatite, and retrogressive chlorite; opaques, apatite, and zircon are common as hornblende poikoblasts. Ultramafic Rocks Weakly serpentinized hartzburgites and dunites make up most of the ultramafic rocks in the Ingalls Complex, in the southern part of the field area, with widespread but minor pyroxenites and chromites comprising the remainder (Miller, 1985). The hartzburgites consist of forsterite (70-75%), and enstatite (25%), and minor diopside in some samples. Hartzburgite is distinguished from dunite in the field by the presence of compositional layering manifested by knobby, resistant clinopyroxene-rich layers while dunites are massive, and weather to a smooth appearance (R. Miller, p. comm.). Ultramafic rocks in both upper-and lower-plates of the Windy- Pass thrust display a weak foliation which weathers to a grayish-white color. The massive rocks weather to a rusty-red- orange color and typically form blocky-jointed outcrops. In the lower-plate of the Windy-Pass thrust ultramafic rocks occur either as discontinuous, concordant lenses or as large nearly flat-lying bodies. These bodies occur with increasing frequency 1 6 and size as the Windy-Pass thrust contact is approached. The size of these ultramafic lenses varies from meters to hundreds of meters in length (fig. 3). In tru siv e Bodies Mount Stuart batholith The tonalitic Mount Stuart batholith surrounds the study area, on the north, south, and west. Minerals present in the part of the batholith exposed in the study area are plagioclase, quartz, hornblende, biotite with smaller amounts of secondary sphene, apatite, and ilmenite (J. L. Anderson, p. comm.). The tonalites are phaneritic, medium grained, hypidiomorphic and granular in texture. The color index for these rocks falls between 15 and 40 (Tabor et at., 1987a) Near its contacts, the batholith generally exhibits a moderately well-defined magmatic foliation, manifested by aligned mafic minerals such as hornblende and biotite, aligned plagioclase laths, and oriented enclaves. Away from the intrusive contact the foliation becomes less well defined, but is still discernible. A satellite pluton of the Mount Stuart batholith is exposed in the western part of the field area (fig. 3). It is compositionally identical to the tonalites of the Mount Stuart, except that it has a lower percentage of mafics (C.I.=3-30, 1 7 Tabor et al., 1987a). The pluton is structureless, except for a weakly developed foliation near the eastern margin of body. H ornblendites Hornblendites, which represent part of the Big Jim Complex, occur throughout the study area but are most common near the contact of the Mount Stuart batholith, especially in domains 2 and 3 (fig 3). Contacts between the hornblendites and Mount Stuart tonalites are generally abrupt, but can be compositionally gradational. Hornblendite occurs in a variety of grain sizes, from fine grained sills and dikes to, extremely coarse grained bodies. The fine grained variety of hornblendite is often lineated and consists primarily of elongate hornblende and plagioclase crystals. The coarse-grained variety is composed primarily of hornblende, and weathers to a reddish brown. This hornblende is a brown poikiolitic variety with numerous orthopyroxene and clinopyroxene inclusions (Getzinger, 1978). Other minerals include, biotite, interstitial plagioclase and retrogressive talc. Garnet-Bearing Dikes Garnet-bearing intrusive bodies occur locally as sills or dikes from centimeters to 2 meters in width. In thin section, foliation is defined by aligned, fine grained, pale varieties of 1 8 hornblende and biotite. Reddish-pink garnets are surrounded by leucocratic depletion coronas that consist primarily of feldspar. and quartz and appear to have been formed by a solid-state reaction of garnets with their mafic matrix. These coronas are elongate, and define a stretching lineation in the plane of the foliation (fig. 5). Structures in the Batholith Magmatic foliation and lineation in the batholith is best defined by the alignment of euhedral biotite, hornblende, plagioclase, and elongate microgranitoid enclaves. Evidence for plastic deformation is lacking in the study area, with the exception undulose extinction, minor subgrain development in quartz. Near the batholith’s contacts foliation is concordant, moderately well developed, steeply-dipping and parallel to sub-parallel with cleavage in the wall rock. In the interior portions of the batholith, foliation consists of a faint planar alignment of minerals, locally parallel with the contacts but often dipping more shallowly. Flow segregation or schlieran layering is fairly common away from the pluton contacts. This layering is manifested by higher concentrations of mafic and felsic minerals in adjacent layers. 1 9 Figure 5. Photograph of hand sample of gamet bearing dike cut parallel to cleavage. A stretching lineation is defined by elongate depletion coronas consisting of plagioclase and quartz fibers. These coronas are caused by the reaction of garnets with their mafic matrix. 20 C on tact Relations The contacts between the Mount Stuart batholith and country rock are generally well exposed, and steeply dipping. They vary from being knife-sharp and planar to quite irregular on map scale. Contact attitudes are summarized in Table 1. The attitudes of contacts were obtained through either direct measurements or solving three-point problems. Table 1 Attitudes of selected pluton Contacts C o n ta c t A ttitude (strike, dip) Domain 2/pluton 5, 84 Domain 3/pluton 315, 68 Domain 4/pluton 278, 80 Domain 5/pluton 280, 66 Southern body @ B {figs. 7, 15) 336, 60 Within around 500 meters the batholith contacts, foliations in the Chiwaukum Schist, amphibolitic slices of the upper plate of the Windy-Pass thrust, and foliation in the pluton show a large degree of parallelism with the contact. Intensity of deformation increases as you approach the contact from either side. However, ultramafic rocks of the upper plate show no increase in ductile behavior near the pluton. The only 2 1 observable effects are some limited contact-parallel open- folding and some metasomatic alteration. An increase in the number of aplite and pegmatite dikes, and largely concordant hornblendite bodies occurs in the contact aureole. In domains 2 and 3, this is especially noticeable where numerous small (meter scale) and a few mappable size concordant sill-like hornblendite bodies exist (fig. 3). STRUCTURAL ELEMENTS IN COUNTRY ROCKS I n t r o d u c t io n The purpose of this section is to describe the structures in the Icicle Canyon area's country rocks, to use these structures to define transposition cycles, and to relate these cycles to the area's deformational history. In the following analysis, the study area is divided into 7 domains with the purpose of isolating the characteristics of different generations of structures (fig. 3). Domain 1 typifies regional structure i.e., steeply-to moderately dipping WNW striking cleavage, shallowly plunging WNW trending fold axes and stretching lineations. Domains 2 through 7 were chosen to facilitate the examination of structures adjacent to the batholith and thrust contacts. The general characteristics of these domains are summarized below in table 2. 22 Transposition Cycles and Polydeformed Terranes It is now well established that during progressive deformation multiple generations of folds, cleavages and lineations can form and be continuously transposed into new orientations (e.g. Williams, 1985; Lagarde and Michard, 1985; Table 2 General characteristics of domains in Icicle Canyon Region Domain 1 Typical Nason Terrane structure {WNW striking, moderately dipping foliation, WNW plunging stretching lineations and fold axes.} Domain 2 North-south structural grain steep down-dip lineations and fold axes. Domain 3 Steep down dip lineations and fold axes. Dom ain 4 Contact-parallel antiformal structure defined by cleavage Domain 5 Contact-parallel cleavage Domain 6 Flat lying thrust-parallel cleavages Domain 7 Upper plate of Windy Pass thrust Tobisch and Paterson, 1988). Recognizing and correlating different generations of structures can be very difficult in 23 ductiley deformed terranes (e.g., Williams and Campgnoni, 1983). Traditional chronological notations of structural elements (e.g. D i, D2, D3) can give erroneous impressions of how large regions evolve geologically. For example, it is conceivable in a deforming rock mass that a continuous cleavage may develop in one domain, while in a contiguous domain, or in a localized zone of high strain crenulations of that foliation may be forming simultaneously. Therefore, the nomenclature of Tobisch and Paterson (1988), who considered the concepts of a composite schistosity and transposition cycles as vehicles to objectively evaluate different sets of structures, will be used. A composite foliation can be defined as a planar surface, or group of planar surfaces whose components share similar morphological features (e. g. fold axes stretching lineations) and orientations. A transposition cycle can involve various mechanisms such as: a) simple rotation of an earlier foliation b) complex rotation of an earlier foliation by passing through crenulation cleavage phases during progressive superimposed folding accompanied by superposition of pre existing cleavages, and c) recrystallization and formation of new grains, porphyroblasts and stretching lineations. As stated above, an original foliation is likely to go through distinct changes in developing a new continuous cleavage. Tobisch and Paterson (1988) suggest that three well 24 defined stages of new foliation development are commonly preserved in multiply deformed rocks. During the earliest transposition cycle (Cycle 1), imposed on rocks that have not been deformed ductilely, a continuous cleavage (e. g., where cleavage planes are pervasive and continuous rather than spaced) will develop. Cycle 1 may, or may not involve transposition of a sedimentary fissility; which is a general term for the roughly planar, parallel surfaces existing along bedding planes (Weber, 1981). Listed below are Tobisch and Paterson's (1988) three stages of new foliation development: Cycle . I Sb: bedding fissility and compositional layering Sc : continuous cleavage. Once a rock contains a continuous cleavage, subsequent cycles will go through the following stages: C \cle 2. 3 etc. Stage 1: Scr, the crenulation a continuous cleavage (Sc), into a zonal crenulation cleavage, (Scr); Stage 2: Sec, the formation of a new discrete crenulation cleavage (Sec), with a new continuous cleavage forming locally in thin domains; 25 Stage 3: St , the formation of a new composite foliation where the new foliation, ST is a composite of Sb, Sc. Scr. and Sec forming a new composite foliation. As previously mentioned, other processes accompany transposition cycles are folding, lineation development, grain rotation and recrystallization. The recognition and interpretation of stretching lineations is especially problematic, and the types of information these features include needs careful consideration. In the study area, they consist of mineral, and elongate-pebble stretching lineations, and slickensides. Stretching lineations exist on cleavage surfaces, and in pre-to syn emplacement dikes and sills. Evidence of stretching parallel to lineation direction consists of numerous boudinage structures. Boudinaged felsic layers in pelitic and amphibolitic rocks layer stretching is parallel to lineations. In garnet-bearing dikes (see rock description section) mineral growth is parallel to stretching lineation direction as well. It should be noted that it is not necessary for a cleavage to pass through every stage or to transform completely to stage 3. Application of the concepts of a composite foliation (St , Williams and Campagnoni, 1983) and transposition cycles help to depict the way in which large rock masses deform. With a 26 heterogeneous rock mass undergoing progressive deformation the following is likely to hold true: each portion of the mass is likely to have a unique deformation path; their is likely to be contemporaneous development of foliations of different generations and morphologies, e. g. the simultaneous crenulation and transposition of continuous and crenulation cleavages respectively. Therefore transposition cycles can give you the advantage of: 1) grouping various structures indicative of different stages of a single cycle; 2) temporally relating cycles instead of trying to relate individual structures. Four cycles have been recognized in the Chiwaukum Schist, and 2 in the Ingalls Complex in the Icicle Canyon area. To the north, Miller and Paterson (1991) have documented a fifth cycle in the Chiwaukum Schist along the northeast margins of the Mount Stuart batholith. Below, I will describe the characteristics locations and geometries associated with transposition cycles in the Icicle Canyon area. Tranposition Cycles Transposition Cycle 1 A relict cleavage Sci (i = internal to porphyroblasts) associated with transposition cycle 1 is represented as inclusion trails within porphyroblasts. Sci, the oldest demonstrable 27 metamorphic element in the Chiwaukum Schist is a moderately to intensely developed continuous cleavage formed during transposition cycle 1. Whether this cleavage actually represents the culmination of the "first" cycle is unknown. However, there must have been at le a st one cycle as bedding in the schist protolith would have had to been transposed into a metamorphic cleavage. This cleavage consists of aligned biotite, quartz, plagioclase, and locally graphite plus other opaque and trace minerals. Sci is ubiquitous in cordierite, and is locally present in biotite, and hornblende porphyroblasts. It is not clear whether the internal foliations of cordierite, biotite and hornblende are coeval as the porphyroblasts that preserve Sci rarely occur together. Crenulations of Sci have only been observed in cordierite grains as a zonal Seri, or differentiated crenulation cleavages (Scci, fig- 6). In thin section Scci, can be distinguished from Seri in that the inter-limb angle of the crenulations decreases. Sc i is usually sub-parallel to, but generally not continuous with St (the external schistosity) except near irregular margins of cordierite grains where strain shadows exist. In general, Sci and St are roughly coplanar. Crenulation axes internal to cordierite have been observed only in samples cut perpendicular to St , thus hinge-lines, and 28 « c - W V ^ V ” ^ < ?'/7 '" ” ^ Y ' '?£ / ° o f S i S * -tf'-Vf? f\ t> a* JT ^ k4^nw , - Figure 6. Photomicrograph and line drawing of cordierite porphyroblast (6.3x). Thin section was cut ~ parallel to lineation direction and perpendicular to external schistosity. The external foliation is cycle 2 cleavage ST, the included cleavage in cordierite is cycle 1 cleavage Sci, and the crenulation cleavage running - east-west across the porhyroblst is Scci. Sci, Seri, and Scci were transposed during transposition cycle 2 to form ST. 29 stretching lineations appear to be colinear with similar features in ST- Transposition Cycle 2 Transposition cycles 1 as well as cycle 2 (fig. 7) are believed to be associated with regional deformation in the Cascades crystalline core and occur throughout the study area. The following summary pertains to the structures found in domain 1 as the highest cycle achieved there was cycle 2. ST, the most pervasive structure in the Icicle Canyon area, is a penetrative, composite schistosity composed of the transposed elements: Sc i, Seri, and SCci- It can be found in metamorphic rocks in the lower plate and in amphibolitic rocks in the upper plate of the Windy Pass Thrust. St is manifested by alternating pelitic and quartz and-plagioclase-rich layers. The pelitic layers are comprised of aligned-flattened biotite, cordierite, garnet and rare hornblende. In amphibolitic rocks, S t is defined by either gneissic layering, or discrete folia of hornblende-rich and quartz-and plagioclase-rich domains. In meta-peridotitic lenses in the Chiwaukum Schist, St is generally poorly developed and defined by light colored talc- tremolite rich layers. In domain 1, St strikes northwest, and dips moderately to the northeast (300, 40, figs. 8, 9) and is remarkably 30 1 MILE Figure 7. Geologic map shows highest known tranposition cycles achieved in different domains of the Icicle Canyon area. These cycles are explained in detail in the texL \ 64 '—* . i l l i l l l l | ; slteplpswi fflfl^pafeas^gg ■ : S £ $ : Figure 8. Geologic map showing the orientation of foliation in the all units in the Icicle Canyon area. Included arc the locations of cross-section A-A‘ (fig. 10) and cross-section B-B’ (fig. 33). PO I.K S T O DOM A IN I CI.KAVAGK G reil Circle = 300. 4(1 POI.K S T O DOM AIN S CLKAVAGK Great Circle ° 305, 74 liquil Are* C.I ± 2 1 1 sigma C l a 2 0 sigma PO I.K S T O DOM AIN t C l.KAVAGK Greet Circle * 305, 11 POI.K S T O DOM AIN T CLKAV ACK Greet Circle « 293.49 l-'igure 9 Slcreoplots show ing cleav ag e orientations in dom ains 1,5,6 anti 7. N ote that dom ain 6 cleavage is shallow er than the regional average orientation o f cleavage (dom ain 1) and the orientation o f the W indy P ass thrust (2157, 24) w hile dom ain 7 cleavage is steeper. D om ain 5 cleavage attitudes are typical of rocks in the M ount S tu an hatholith s contact aureole in that they are sleep, and contact parallel. consistent in orientation throughout the domain. This orientation is roughly parallel to the regional orientation of cleavage in the Nason Terrane. S t was folded asymmetrically along shallowly plunging WNW axes (8/287, figs. 11, 12) in domain 1. A general increase in degree of limb attenuation and a decrease in inter-limb angle is observed as the thrust contact is approached. Mesoscopic folds in cleavage are present nearly everywhere in the Nason Terrane while parasitic (Ramsay, 1987b) microscopic folds are locally associated with mesoscopic folds. Post cycle 2 folds are asymmetric, close to isoclinal in geometry and typically have wavelengths between 1-50 cm. Folds deform cleavage in pelitic and amphibolitic schists, and compositional banding in gneissic amphibolites. Fold type varies from class 1C (Ramsay, 1987b) - flattened buckle folds (Lisle, 1992) to class 3 folds locally with strongly attenuated limbs. It is difficult to establish kinematics associated with transposition cycles 1 and 2. Asymmetric folding likely occurred during the development of Sci; and was part of the process which formed and subsequently transposed Sci, Seri, and Scci to form S j (fig- 6). However, folds preserved in cordierite a sense of shear could not be obtained because not enough of the folds were preserved. These crenulation axes are colinear with fold axes and stretching lineations in St perhaps 34 CROSS-SECTION A-A' Windy-Pass Thrust 1 MILE Vertical Exageration = I Figure 10. Cross section through A-A' (fig. 7b) showing attitudes of contacts of the Ingalls Complex and Mount Stuart Batholith with the Chiwaukum Schist. Lines projecting downwards represent the average orientations of foliation planes in schistose ana granitoid rocks. Figure 11 Geologic map showing the orientation of fold axes in the Icicle Canyon area. DOM AIN I K O U ) AXKS A v e n g e OricniAlxon 8/287 DOM AIN 4 K O IJ) AXKS Average O neiU A lK in: 8/103 Hqual Arc* b q u a l A re a mhll|!!IH!|:i I i 11 i 11 C l s 2.0 tig m i N = Kl N = 42 C l s 2 0 ligma D OM AIN 5 KOl l) AXKS Average OrienUlion 3/114 DOM AIN b KOI-D AXKS Average O rw nuttocr 7/301 Figure 12. Slcreoplots showing fold axes orientations in dom ains 1 ,4 ,5 and 6. indicating similar kinematics for transposition cycle 2. Meso- scale folds in cycle 2 cleavage suggest NNE-SSW contraction and probable top-to-the SSW shear indicative of post cycle 2 deform ation. Sub-parallel to the hinge-lines of these folds lies a locally well developed stretching lineation. In domain 1, stretching lineations occur in amphibolites, sandy-biotite schists, and garnet-bearing dikes and sills (fig. 5), and less commonly in pelitic schists. Lineations related to cycle 2 deformation formed sub-parallel to WNW-ESE fold axes (figs. 13, 14). In amphibolites, mineral lineations are defined by aligned columnar hornblende crystals. In sandy-biotite schists, stretching lineations occur as alternating stretched biotite and quartz-rich trains. In pelitic schists aligned, and stretched cordierite porphyroblasts locally define stretching lineations. Lineations in garnet-bearing dikes and sills are defined by depletion coronas consisting of aligned felsic mineral surrounding garnet porphyroblasts (fig. 5). These dikes cut folds in cycle 2 structures but their relation to cycle 3 or 4 deformation is unclear. Kinematic indicators, coupled with cleavage and stretching lineation orientations implies WNW-ESE extension simultaneous with top-to-the SSW shear during post cycle 2 38 1 MILE Figure 13. Geologic map showing the orientation of stretching lineations in the Icicle Canyon area. Lqual Atm DOM AIN I ST R K T t'H IN O LIN KA TIO N S Average Orientation 8/307 C l. s 2 0 s ig m a DOM AIN 6 ST R K T C IIIN O LIN KA TIO N S Average Orientation- S/300 C.I. = 2 0 sigma DOMALN 2 S T R tT ( l l l\< ; LINKATIONS Average OnciUalion 76/71 DOM AIN 3 ST R E T C IIIN O LINKATIONS Average Orienlalion: 50/57 Figure 14. Stcrcoplots showing stretching lincation orientation in dom ains 1. 2, 3 and 6. Note the difference from the shallowly plunging "regional" orientation o f dom ains 1 and 6 and the down- dip plunges of stretching lincations in dom ains 2 and 3. 40 deformation. It is likely that this deformational style was occurring during cycle 2 and perhaps cycle 1 as well. Transposition Cycle 3 The influence of transposition cycle 3, is seen throughout the study area although most rocks were weakly effected. Only in a mile-wide panel, adjacent to the Windy Pass thrust, did this cycle go through to completion (fig. 7). Here, St was rotated, recrystallized, folded and sheared intensely, during top-to-the-SSW displacement as indicated by mesoscopic fold asymmetry, while a new continuous cleavage developed. Note that this foliation was transposed without any intervening steps in cleavage development (i.e., Sen Sc c )- The new cleavage is defined by metamorphic minerals of a finer grain than cycle 2 cleavage in schists and amphibolites. Cycle 2 structures were folded more intensely near the thrust contact during cycle 3 as WNW-ESE trending stretching lineations developed in amphibolites and sandy-biotite schists in domain 6 (5/300; figs. 13, 14). Amphibolites in the Ingalls Complex have 2 generations of structures overprinted by cycle 3 and cycle 4 deformation (Miller, 1985; Miller and Mogk, 1987). The second generation consists of cleavages and local folds that could correlate with transposition 2 structures in the Chiwaukum Schist. However, if there are large displacements 4 1 on the Windy Pass thrust, pre-cycle 3 structures in upper and lower plates could be unrelated (Paterson et al., 1993). The contact between cycle 2 and 3 rocks is gradational because the deformation (NE-SW contraction-NW-SE extension) that caused cycle 2, was in all likelihood the cause of Windy Pass thrusting and cycle 3 deformation. Cycle 2 and 3 fold hinge-lines, stretching lineation orientations and strike of cleavage are subparallel, but dip of cleavage gradually decreases as folding intensity increases near the contact between cycle 2 and 3 rocks. Hence the boundary is somewhat arbitrary. The thrust contact is nowhere exposed, but appears to be generally non-planar on a km scale as observed from map patterns. It has a regional orientation of 287, 24 (fig. 10) obtained from three point problems. Structures on either side of the thrust show a similar effect to that observed near the pluton contact; in that the intensity of deformation increases as the thrust is approached. Cleavage in amphibolite imbricates in the upper plate of the Windy Pass thrust (domain 7) is of steeper dip (293, 49, fig. 9) than the thrust itself (287, 24; fig. 10), Miller (1985) interprets this to mean that the emplacement of the sole thrust (Windy Pass thrust) postdates imbrication. 42 The kinematics extrapolated from mesoscopic- asymmeteric folds indicate that shear sense stayed the same during cycle 3, and stretching directions were nearly identical judging from lineation orientations. Transposition Cycle 4 Transposition cycle 4 accompanied emplacement of the Mount Stuart batholith and is localized in the contact aureole of the intrusive and along Windy Pass thrust (fig. 7). Domainal folding and contact metamorphism were coeval with the transposition of older structures. Near the batholith, cleavage was rotated into parallelism with the intrusive contact during emplacement. As a result, a new continuous cleavage was generated through processes of grain recrystallization and rotation. Cleavage is defined by intensely recrystallized pelitic and felsic layers associated with contact metamorphism. Felsic layers in are often discontinuous, exhibiting an increase in pinch and swell and boudinaged structures from domain 1. Cycle 4 cleavage is concordant with the batholith contact, steeply dipping and strikes roughly N-S in domain 2 (figs. 15, 16), WNW-ESE in domains 3 (fig 15, 17) and 5 (fig. 15), and roughly E-W in domain 4 (fig. 15). 43 Contact-parallel, emplacement-related mesoscopic folds occurred during cycle 4 in domains 2-5. Where batholith wall rock contacts strike WNW-ESE (domains 4 and 5, fig. 12) cycle 4 folding was about shallow axes; where the contact is oblique to the country rock structural grain, folding was about moderate to steep axes. Cycle 4 folds are tight-to-isoclinal class 2 to class 3 in domains 2 (81/26, fig. 18) and 3 {59/69 fig. 18) with some severe attenuation of limbs. Coeval stretching lineations are sub-parallel to fold axes in domains 2 and 3 (fig. 14). In domains 2 and 3 amphibolitic as well as pelitic rocks possess stretching lineations as do numerous dikes and sills of Big Jim composition with mineral lineations defined by aligned columnar hornblende crystals. In domains 4 and 5, the only stretching lineations seen are aligned andalusite porphyroblasts whose long axes plunge shallowly, trend parallel to the batholith contact and are aligned parallel to boudinaged layer stretching direction in domains 4 and 5 (samples 239-T90, 41-T91, fig. 19). In sample 239-T90, a very fine-grained relict cleavage is preserved in andalusite cores and rims. Also within this sample, a weak axial-planar crenulation cleavage has been observed associated with ~ 0.5 mm. microscopic folds in cycle 4 cleavage. It is defined by biotite and opaque-rich selvage 44 N - 2)2 C l. > 2 0 sigma PO LES T O DOM AIN 2C L E A V A C K Greal Circle = 350,76 PO L ES T O DOM AIN 3 CLK A V A CE Great Circle = 307, 34 f ilia l Are* N - SO C l * 2 0 tig m a Stereo plot showing poles lo domain 4 cleavage and contact parallel macroscopic fold in cleavage, Fold Am i 8/103 F igure 15. Siercoplots show ing poles to cleavage in dom ains 2, 3, and 4. 4 5 : |g |l ||||! |i |: liiiiiiiiiiiiiliifeiss w i l « ‘ I MtLB Figure 16. Geologic map showing detail of cleavage in ihc Chiwaukum Schist in domain 2. Also shown arc cleavage in the Ingalls Complex and Magmatic foliation of the Mount Stuart batholith. Big Jim Figure 17. Geologic map showing contact region in domain 3. Cleavage in country rocks is shown as well as the magmatic foliation in the Mount Stuart batholith. 47 C I = 2 0 sigma DOM AIN 2 KOI.D AXKS DOM AIN 3 K )L D AXES Average Orieniaiion S1/26 Average O rienuiion: JV/69 H g u rc )H. S tcteo p lu ts show ing m esoscopic fold axes in dom ains 2 an d 3. N ote lhal these folds are steeply piunging as com pared to fold axes in dom ains 1, 4, 5, and 6. 48 X41-T91 X 317-T91 1 MILE w.v.' .yft m X 217-T90 X 239-T90 XS-C X 140B-T90 X 111 T91 X 141B X 396-T91 X 147-T90 X 462-T91 Figure 19. Geoloic map showing selected sample locations in the Icicle Canyon area discussed in text. X = sample location. planes whose hinge lines are subparallel with andalusite long axes. Shear sense of cycle 4 asymmetric folds is erratic in domains 2 and 3 although, statistically a top-to-the SW shear sense was obtained in domain 3. In domain 2, no preferred shear sense was detected. Cycle 4 macroscopic folds occur near the batholith and thrust contacts and are defined by kilometer scale folds of cleavage. The nature of macroscopic folding (i.e., anticlinal- synclinal) was extrapolated from associated mesoscopic fold- asymmetry. In domain 2, a moderately plunging fold exists; where one limb parallels domain 2 cleavage and the other is sub-parallel to the Windy Pass thrust contact. In both domains 4 and 3, parasitic fold asymmetry yields anticlinal geometries for the macroscopic folds in cleavage indicating a top-to-the- SSW sense of shear for rocks nearest the contact (i e., the nothern limb) and a top-to-the-NNE sense rocks of the other limb. The domain 4 macroscopic fold is a shallowly plunging structure (8/103, fig. 15) whose axis is coincident with the mean vector of mesoscopic folding (fig. 12). Some syn-to-post emplacement aplite and pegmatite dikes near the pluton contact are openly-folded with axes roughly colinear with folds in the country rocks. This indicates 50 folding intensity decreased, but strain axes were roughly colinear during and after emplacement. Adjacent to the Windy Pass thrust, lies a zone where syn- emplacement deformation has resulted in a zone of considerable structural complexity. Here crenulations (SCr) and discrete differentiated crenulation cleavages (Sec) of transposition cycle 3 cleavage have formed. Locally in domain 6, all earlier structures, Scr, and SCc become completely transposed to form a new composite schistosity (cycle 4 St ) essentially identical to earlier cleavages (fig. 20). In domain 7, structures identical in appearance to Scr, and Sec exist in aniphibolite lenses. However a complete transposition of older structures during cycle 4 structures has not been observed. In Chiwaukum Schist pelites, Scr and Sec are defined by selvage planes consisting of biotite-and opaque-rich zones. In amphibolites, Scr and SCc consist of selvage planes defined by hornblende-and opaque-rich zones. Cycle 4 ST, is defined by compositional layering consisting of alternating quartz- plagioclase-rich layers containing numerous rootless fold hinges, and biotite-opaque-rich zones in pelitic units, or hornblende-opaque rich layers in and amphibolites. As the thrust is approached from either side, the inter limb angle of the crenulations (Scr) becomes smaller leading to 5 1 the development of SCc in thin domains. Orientations of cycle 3 cleavage preserved in intervening quartz-rich layers are essentially parallel to cycle 4 structures there. In domains 6 and 7, cycles 2 - 4 cleavage attitudes are 305, 11 and 293, 49 (fig. 9) respectively. From domain 1 to 6, a gradual shallowing of cleavage is seen towards the thrust (fig. 10). Both domains are similar to domain I (300, 40, fig. 8) and the thrust contact in strike (287, 24). However, dips in domain 6 are more shallow than both the regional trend and the thrust orientation; while the opposite is true for domain 7. In the eastern- and western-most portions of domains 6 and 7, planar elements dip steeply, parallel to the intrusive contact and are highly oblique to the thrust (figs. 15, 16, 21). Here, thrust parallel cleavages were recrystallized and grade into the aureole foliation. This continuity of cycle 4 cleavages proximal to the thrust and pluton contacts implies a coeval developm ent. Fold asymmetry associated with this deformation defines an antiform with a top-to-the-NNE sense of displacement next to the thrust (i e., the southern limb) which changes to a top- to-the-SSW sense 100-200 meters from the contact. Fold/crenulation axes and coeval stretching lineations are roughly colinear with the regional trends although greater scatter is seen near the thrust. 52 4 V Y • Figure 20. Photomicrographs (2.5x) showing crenulations of cycle 3 cleavage. The top photomicrograph was taken from an amphibolitic sample (147-T90) and exhibits the cleavage Scr running diagonally across the photograph. The bottom photomicrograph was taken from pelitic sample 462-T91 showing the cleavage See running lengthwise across the photograph. Both samples were cut perpendicular to cleavage and ~ parallel to stretching lineation direction. These crenulation cleavages formed during transposition cyle 4. 5 3 Figure 21. Geologic map shows orientation of Scr, See and ST' in the Chiwaukum Schist and amphibolitc lenses in the Ingalls Complex. Note the obliquity of cleavages to the thrust contact in both upper and lower plates. 5 4 Transposition Cycle 5 To the north of the study area, contact metamorphism achieved sillimanite grade and was followed by the growth of kyanite, staurolite and garnet (Evans and Berti, 1986; S. R. Paterson p. comm.). Transposition cycle number 5, related to syn- to post-emplacement reverse faulting is penecontemporaneous with the growth of these minerals (Miller and Paterson, 1991). Kinematics of this deformation as indicated by near-solidus and high-temperature, solid-state S-C surfaces and ductile shear zones in batholith and wall rocks are top-to-the-SW . Joints Joints are the latest widespread structure occurring in the Icicle Canyon area; they postdate all structures except some late apiite and pegmatite dikes and slickensides. The joints tend to be oriented perpendicular to the central sill-like body (fig. 2) of the Mount Stuart batholith (figs. 22, 23). Therefore, in domains 2, 3, and 4 and the eastern parts of domains 1 and 6 where the contact is east-west and steeply dipping in orientation, joints are roughly north south and steeply dipping. In domain 5 and the western part of domains 1 and 6 where contact strikes northwest and dips steeply to the northeast, joint surfaces strike southwest-northeast and dip steeply. 5 5 These joints appear to be extensional as evidence for lateral displacement is quite rare. So an origin as tension cracks is likely. Locally along joint surfaces, aplite and pegmatitic dikes were emplaced and occur with greater frequency towards the pluton contact. North of the study area many of these dikes can be linked directly to Mount Stuart magmatism. Paterson et el., (ms., 1993) suggest that joints formed late during emplacement as a brittle response to NW-SE extension. S likensides Slickensides have been observed in plulonic, schistose and amphibolitic rocks of the lower plate, and meta-peridotites of the upper plate of the Windy Pass thrust. In the plutonic and schistose rocks, they are defined by polished, and/or striated linear features on joints and less frequently on cleavage surfaces. In the Ingalls Complex, they occur on joint surfaces in altered meta-peridotites as aligned fibrous antigorite crystals near the Windy Pass Thrust. Slickensides front two locations were taken from joint surfaces of ultramafic rocks of the Ingalls Complex. They have a northeast-southwest orientation (fig. 22) and step-up to the southwest. This pattern of stepping implies left-lateral or top-to-the southwest sense of shear. The relationship of these slickensides have to the overall 5 6 gni* Figure 22. S tcreo p lu ls show ing pu les to joint surfaces in the Icicle C anyon area (great circle = 29, 87) an d slckensides from dom ain 7. '['he S tepping P attern o f slicken sid es im plies a lop-lo the N N li sense o f shear. 5 7 Figure 23. Geologic map showing the orientation of joints in the Icicle Canyon area. kinematics of deformation is unclear. However, since the slickensides postdate jointing, they are probably related to late extension, uplift and/or cooling and not significant in the area's older deformational history. C-Surfaces C-surfaces, or shear surfaces exist in only one known locale in the Icicle Canyon area in the middle of domain 1 (fig. 19). Here, two distinct planar structures exist, S j , and the C- surface. The C-surfaces are roughly planar and are comprised of concentrations of pelitic minerals on the slip surfaces in a sandy-biotite schist. The S-C relationships yield a right-lateral, or top-to-the-north to northwest sense of shear and cut earlier mesoscopic folds of cycle 2 cleavage. This yields an opposite sense of shear than do asymmetric folds there. The meaning of these features is equivocal. However, they likely occur late in the area’s deformational history, and represent a transition from pervasive ductile structures to brittle behavior. To the north of the study area, Miller and Paterson (1991) have documented syn-emplacement S-C surfaces which yield top-to-the SW displacement in the wall, and granitoid rocks along the northeast margins of the batholith. Here, folds boudinage and stretching lineations in country rocks, dikes and the batholith record strong sub-horizontal WNW-SSE stretching, 59 and SSW-ENE contraction. These features and coeval reverse faults are consistent with the style of deformation and kinematics evident throughout the study area. This may suggest that the stretching lineations have no clear-cut correlation to the area's kinematic history and are features more clearly related to tectonic strain. Summary of Country Rock Deformation The evidence and observations presented above points to a complicated deformational history which indicates that the Chiwaukum Schist was complexly deformed during WSW-ENE contraction and WNW-ESE extension. Deformation occurred under prograde metaniorphic conditions culminating in the emplacement of the Mt. Stuart batholith and, continued to some extent after emplacement (Miller and Paterson, 1991; Taylor et al. 1992). Transposition cycles 1 and 2 took place during regional deformation. This is supported by the subparallet nature of these structures to Nason terrane structures and the overprint of these structures by deformation associated with thrust and batholith emplacement. Transposition cycle 3 was localized to a kilometer wide zone proximal to the thrust. Cycle 3 deformation can be explained by thrusting during WNW-ESE extension and NW-SE contraction. The progressive flattening of 60 cleavage in the lower plate, and increased shearing and folding intensity in domains 6 and 7 is consistent with top-to-SSW thrusting. Cycles 2 and 3 appear to have been part of a prograde continuation of cycle 1 in that cleavage, hinge lines, stretching lineations and displacement directions are roughly coincident, although kinematics for cycle 1 are unclear. Cycles 4 and 5 were a result of continued regional deformation + emplacement related deformation. Pluton emplacement was coeval with the transposition of all older structures parallel to the thrust and batholith contacts. This deformation is a product of displacement and high strain along the pluton and thrust contacts during pluton emplacement and simultaneous regional deformation. ST R A IN I n t r o d u c t io n Strain analysis was used to try to quantify: 1) the magnitude of ductile flow; 2) the relationship between strain axes and orogen wide features (e.g., cleavage, fold axes, stretching lineations); 3) the magnitude of ductile flow which occurred during emplacement of the Mount Stuart batholith. It is important to recognize that strain is only one component of 6 1 the total displacement field that consists of rigid-body translation, rigid-body rotation, and distortion or strain. Rocks analyzed for strain include greywacke meta conglomerate, pelitic schists with reduction spots, andalusite- bearing pelitic schists, hornblende-bearing amphibolites, enclaves and plagioclase crystals in the pluton and garnet depletion coronas in garnet-bearing dikes. Results and data for each location are summarized in Table 3 and plotted on a Flinn diagram (fig. 24). This is a graph of the ratios of strain axes y/z vs. x/y (Flinn, 1965). Strain Analysis Methods Samples were prepared by making three perpendicular cuts. Long and short axes as well as the angle between long axes and an arbitrary reference angle were measured in a right-handed coordinate frame for markers visible on each face. Strain ellipses are calculated for each face, using the techniques of Shimamota and Ikeda (1976) and Miller and Oertel (1979) yielding the three dimensional strain ellipsoids listed in Table 3. Assumptions made in utilizing the Rf/tJ> technique include: 1) no viscosity contrast between markers and matrix; 2) strain homogeneity at the specimen scale; 3) no volume 62 o GT X HBL • AND □ CONGL + FiNC.'L O RF.D SP A PL.AG STRAIN DATA 10 10 Y/Z Figure 24. Log-Flinn plot of strain markers y/z vs. x/y ratios Ik = (X/Y)/(X/Z)| Suain markers: GT = garnet coronas, CONGL = mctaconglomcralc clasts, RED SP = reduction spots, HBL = hornblende crystals. ENCL = enclaves, PLAG = plagioclasc crystals, and AND = andalusite crystals. 63 TABLE 3 MOUNT STUART FABRIC ELLIPSOIDS P itm m 's Data IIiU aioBi Khinaduii S am p le M a r t e n K X Y z SI L F X XY M S -I5 A G T -Corona5 - 0 9 78 3 -16 0.84 0.08 101/49 M S-54 GT -C orona^ I 1 56 I •35 0 62 -0 05 9 0 * 7 M S-77 E o c l a ^ 1 1 222 . } 68 1 63 4 )0 5 15/103 291/62 M S -115 Enclave^ 1.3 82 -9 -39 0 79 -0.26 30/285 102/48 M S-295 Enclaves 1 8 93 -20 -36 0 8 2 -0.59 11/134 3t5/S8 M S-299 Enclaves. 0.8 94 6 -51 0 9 8 0 12 9/206 203/70 M S-338 Enclaves 2 0 59 21 -21 0.57 1 123076 327/80 M S-389 Enclave*. 0 7 8 8 13 -53 0 9 9 0.26 5/110 2 8 8 * 8 M S-*09 Reduction SplHs 0 3 268 4' -82 2.17 0 39 19/296 2 8 9 * 9 M S -424 Enclaves, 0.5 323 29 -82 2 24 0 24 .V144 13 5 0 0 MS-6BA GT-Corotu*. 1 5 65 10 3 -33 0 65 0.61 Miller's Data C A 421 H B L iam phi 0 6 ~ 7] 19 51 0 9 0 4 2 CA-21 Flag ^ S tu a rt' 0 6 29 17 -34 0 51 0 7 1 12/069 269/40 Taylor's Data 317-T9I Cong] Claris 1 0 38 -0 5 0 45 0.33 22/313 258/27 396-T91 GT Corona-. 0 9 38 5 -*t 0 4 9 0.27 4/290 288/31 4 I-T 9 I Andalusite Ptirpti' 0 9 63 5 42 0 729 0 (4 1 1 /122 3 0 9 * 0 2 39-TO Andalusite Porph- 1.5 55 13 25 0 545 0.593 V 3 0 I 300/79 111-T O B u c k le ' Folds 1 40 0 ■30 0 5 -0 25 2 t & 4 2 U 1 B -T 9 0 Buckle* Fold*. 1 40 0 .30 0.5 -0.25 264/32 (V264 462 T 9 1 Buckle* Folds I 40 0 • 30 0.5 0.25 228/28 10/340 change, and 4) randomly-oriented or initially spherical m arkers. In the samples that 1 analyzed (317-T91. 396-T91, 41- T91, 239-T90, fig 4), rocks had as few as 23 markers per face, but in general at least 30 markers were used. In the case of microgranitoid enclaves (Table 3) data was obtained from individual markers and aspect ratios were measured in the field. Some minor viscosity contrasts were visible between garnet coronas and matrix in garnet bearing dikes. Garnet porphyroblasts deformed brittley within their depletion coronas and foliation shows some deflection around garnet depletion coronas. However, the actual strain markers consist of felsic coronas which completely enclose the porphyroblasts and show no evidence of brittle deformation. The biotite- hornblende-rich matrix has a pervasive L-S metamorphic fabric with no evidence of brittle deformation or viscosity contrasts on the scale of the hand sample. The computed strain intensities are a good estimate of matrix strain after garnet growth. Strains from the clast-supported meta-conglomerate are minimums since the pelitic layers adjacent to the conglomeratic lens likely accommodated more strain. Data from reduction spots pose the opposite problem in that they formed in a biotite-rich layer. Since such a layer would deform more 6 5 readily, a higher whole rock strain intensity is obtained than would be recorded for a typical pelitic schist. Strain associated with magmatic flow in the Mount Stuart batholith was found by using plagioclase grains and microgranitoid enclaves as strain markers. In these rocks quartz has deformed more readily than feldspar (undulose extinction, minor subgrain development). However, this solid- state overprint is insignificant compared to the magmatic component. Enclaves as well as plagioclase grains show little evidence for viscosity contrast with their matrix, as foliation deflection is minor to non-existent around their edges (S. R. Paterson pers. comm.). Assumption 3, regarding volume change, is impossible to evaluate with the available data, and will be discussed no further here. The effect of primary fabrics, that is original objects shapes and orientations, is probably small for meta conglomerate samples. This is supported by low strain with final x:z ratios as low as 1.2:1 and by the x-axis, and xy plane directions matches well with cleavage and stretching lineation measured in the field. This assumption is probably valid for reduction spots as well as they are generally spherical in undeformed rocks. 66 With andalusite crystals, all the above assumptions are invalid if porphyroblasts do not behave as passive markers. If they fail to rotate freely with their matrix, strain data from those samples is difficult to evaluate. Even if they do rotate, assumption 1 is invalid as porphyroblasts are more viscous than their matrix as cleavage does deflect around the crystals. The andalusite crystals grew in the contact aureole of a syntectonic pluton (Miller and Paterson, 1991) in an area which was already undergoing regional deformation. Therefore a strain field existed at the time of their growth and probably partially aligned their long axes which invalidates assumption 4. Thus, the Rf/(|> data from andalusite, are better viewed as fabric ellipsoids defined as strain + primary fabric ellipsoids. Strain from Folds Folds from three locations in the Icicle Canyon area (stations 11I-T90, 141B-T90, 462-T91, fig. 20) were measured to obtain estimates of bulk shortening associated with folding. Buckle folds are those in which the orthogonal thickness of the folded layer is constant throughout the fold (class 1 - class IB, Ramsay, 1967). The folds analyzed were not buckle folds as limbs in all cases showed attenuation relative to hinges, that is a component of flattening occurred in addition to buckling. Therefore an amount of 30% shortening was assumed for the 67 buckling component as these folds can only accommodate that much shortening (Ramsay, 1967; 1987). To attempt to quantify the flattening component the inverse thickness technique of R. J, Lisle (1992) was utilized. This technique assumes that the fold had a class IB (parallel) geometry before flattening and the stretch (new length/old length) at any position on the flattened layer is inversely proportional to its orthogonal thickness. Therefore, a polar graph showing inverse thickness as a function of layer orientation will yield directly the shape and orientation of the flattening component for the strain ellipse. Measurements of initial and final lengths, thickness, and orientation (dip isogons) were made in the lab from slide photographs and photomicrographs. To quantify the post- buckle flattening component of strain, inverse thickness was plotted as a function of layer orientation on a polar graph yielding a strain ellipse that represents the post-buckle flattening. Assumptions inherent in the technique(s) are: 1) no area change in the section examined (cuts are made parallel to y-z plane), or no volume loss in three dimensional strain estimates; 2) plane-strain for three dimensional estimates; 3) a pre-flattening class IB (buckle) fold geometry; 4) the stretch of the fold being inversely proportional to the orthogonal thickness of the flattened fold; 5) coaxial shortening in the 68 section analyzed for buckling and flattening for combined calculations. For the buckling component of folds, a k value of 1 was assumed in calculating three dimensional values, which may be a reasonable assumption given the average values (fig. 24). The highest shortening strains from folds which includes both buckling (30%) and post-buckle flattening were seen near the thrust (63% and 67%) with the lower strain occurring in the middle of domain 1 (42%). Unfortunately, no folds were analyzed near the pluton contact. Details of these calculations are given below. Strain from Folds 2 Dimensional Strain A s s u m p t i o n s Assum e initial sphericity: R(radius) = 1. M eaning that a unbuckled layer is analogous to a strain ellipsoid o f radius 1 where: x = long axis, y = intermediate axis. z=short axis. Strain Ellipsoid: 4 /3 rrR 3 = 4/3rt(xyz) R = I; y = ! ;* /.= 0.7; 1 = 0.7x; so x = 1.4 * z must equal 0.7 because a m axim um of 30% shortening can be accom m odated during buckling (R am say, 1967). 69 A ssum ption o f . plane strain: x /y /y /z = 1.4 /1 /!/ 0.7 = k - 1 L inear Straini c = (1-loVlo where: lo = initial length, I = final length, e 1 = sh o rten ing strain e2 = in le n n e d ia te strain C3 = cx tcn sio n al strain Assum ing all initial axes lengths = 1; l0 =0 1 T h e n : c 1=0.4, c2=0, c3=-0.3 Strain Intensity ULs)l E s = l/(3 )l/2 [( C i-e 2 )2 + (e 2 -e 3 )2 + ( c 3 - e i ) 2 | 1/ 2 E s = 1/(3) 1/2|(.4 -0 )2 + O H - .3 ) ) 2 + (-.3-.4)2 !1/ 2 = 0.50 Past buckle Flattening strain: Equation for area of a strain ellipse: n R 2 = n(ab) where: a = (inverse thickness) short axis o f strain ellipse (1/la) b = (inverse thickness) long axis of strain ellipse (1/tb) 70 Solving for R, the initial radius of unde formed ellipse radius: R = [(a)(b)]1/2 Sam ples (111-T90) R = [{2.0)(5.6)]l/2 = 3.4 (141B-T90} R = H 4.6)(6.8)|1/ 2 = 5.6 (4 6 2 -T 9 1 } R = |(].2 )(5 .4 )|l/2 = 2.5 P ost-B u ckle F lattening ; (R - a)/R = C2 C2 = (3.4 - 2.0)/3.4 = 0.47 c 2 = (5.6 - 4.6)/5.6 = 0.17 C2 = (2.5 - 1.2)/2.5 = 0.53 Final Length; i2 = 11 - * 1 (C2) 12 = 0.7 - 0.7(.47) = 0.37 12 = 0.7 - 0.7(. 17) = 0.58 12 = 0.7 - 0.7(.53) = 033 Total Sh ortening; d o - l2)/lo = c t : 1 - -37 = 0.63. I- .58 = 0.42. 1- . strain 33 = 0.67 7 I Strain from Boudins A shortening estimate was obtained from cordierite boudins in sample (140B-T90, fig. 25). Figure 26 shows a sketch of a photomicrograph cut parallel to lineation and fold axes and perpendicular to cleavage. A calculation has shown that a minimum of 73% layer-normal shortening was involved in cycle 2 deformation in this locale. The cleavages seen are Sci (cycle 1 cleavage), interior to the boudins and S'f (cycle 2 cleavage) the external cleavage. The assumption here is that the boudins have preserved the relict distance between cleavage planes during transposition cycle 1. By finding the difference in distance between the internal cleavage and St cleavage planes and dividing this by the distance between internal cleavage planes one can get an estimate of the amount of shortening involved in cycle 2 deformation. This estimate represents a minimum because it does not take into account any additional contraction enjoyed by the cordierite boudin during cycle 2 deformation. D is c u ss io n Strain Data In the Icicle Canyon area, k values range from .9 to 1.5 with an average of 1.1. Strain intensities Es, were calculated from the equation Es = l/(3)l/“ {(e ] - e2)^ + (e2 - Q3)~ + (e3 - 72 e 1)2 ] 1/2 where e l, e2, and e3 are the principle natural strains (Hossack, 1968). Strain intensity values range from 0.45 midway between the northern and southern bodies of the batholith to 0.73 near the contact with an average of 0.53 (fig. 25, table 3). To the north of the study area k values range from 0.9 - 1.5 with an average of 1.2 while strain intensity varies from 0.62 - 2.17 with an average of 1.1. The available data indicates that deformation in the region approximates plane strain (k = 1, fig. 24) with a possible strain gradient that increases to the north in the wall rocks as seen from samples 396-T91, MS-45A, MS-54, and MS-68A (garnet bearing dikes, table 3, figs. 24, 25). However, given the small data base one should not put much credence in this observation. Using the computed strain ellipsoids to separate total strain quantitatively into components that can be assigned to pre-emplacement emplacement-related, and post-emplacement deformation is next to impossible with the samples available. However, in the following paragraphs an attempt will be made to assess the timing of the strain accommodated in the rocks exam ined. Strain intensities calculated for samples 317-T90 (Es = .45; meta-greywacke conglomerate) and MS-409 (Es = 2.17, reduction spots) are total strains because the strain markers are primary sedimentary structures. Although, for the reasons 73 NASON TERRANE Straight-Creek Fault Leavenworth Fault ’ S-424 1 JLMS-4\ MS-68 A M S. Figure 25. Geologic m ap o f the Nason Tcrrane showing the locations of the various strain samples analyzed. Results o f these analysis arc in table 3. 7 4 outlined above these numbers are probably not representative of average whole rock strains. In samples MS-45A, MS-54, MS-68A, and 396-T91 (garnet coronas) computed intensities represent minimum strains (see above) accrued during and/or after the growth of garnet (Es = 0.65). This brackets their timing to late-syn St (post transposition cycle 2) to post-pluton emplacement (see porphyroblast-matrix section). The strains associated with buckle folds, and flattened- buckle folds (samples 141B-T90, 111-T90, 462-T91) represent strain accommodated in transposition cycles 3 and 4; although, sample 141B-T90 was not affected profoundly by cycle 4 nor transposed during cycle 3. Post-buckle (2D) flattening- shortening associated with this fold was 12%, as opposed to 32% and 37% in samples 111-T90 and 462-T91 located near the thrust contact (fig. 6). When the buckling component of strain is added to these estimates, shortening is 42%, 62% and 67% respectively. Samples I11-T90 and 462-T91 were transposed during cycle 3 and possess crenulation cleavages associated with cycle 4. The computed strains in all samples are minimums as measurements were taken from felsic layers in pelitic rocks and therefore represent values lower than those of the whole rock. In samples 111-T90 and 462, evidence for substantial volume loss exists in the form of well-defined 75 selvage planes representing the crenulation cleavage SCc- Here the percentage of micaceous to felsic minerals (insolubles/soluables) is much higher in these pelitic layers than in pelitic layers in cycle 2 cleavage. Three dimensional estimates can not be made of the post-buckling component since extension is assumed to be perpendicular to contraction in the section examined, and cuts are made parallel to the y-z plane. The only estimate for strain involved in transposition cycle 2 appears to be a good one as the cleavages representing cycles 1 and 2 are present, and continuous in a protected zone (strain shadow, fig. 26). If we assume plane strain, which may be a reasonable approximation given the distribution of k values (fig. 26), strain intensity (Es) = 1.12. Fabric ellipsoids calculated for samples 239-T90 and 41- T91 includes the syn-emplacement strain which they have accommodated. The meaning of these numbers is unclear if andalusite crystals do not behave as passive markers, and/or are more viscous than their matrix, and/or if they were aligned by a previously existing strain field. Strain Distribution Given the limited number of analyzed strain markers from the study area, qualitative estimates of strain have been 76 lo = "original" d istance betw een cleavage planes 1 = "final d istan ce betw een cleavage planes Percent shortening equation: (1 -lo)/lo x 100% lo = 38m m , 1 = 10.3m m (38m m - 10.3m m )/38m m x 300% - 73% Figure 26. Line draw in g o f thin section from sam ple 140B -T90 (6.3x) of cordierite boudins. C leavage internal to boudins is cycle 1 cleavage Sci, and external cleavage is cycle 2 cleavage, ST. C leavage is continuous betw een boudins. C alcu clatio n o f sh o rten in g involved in th etransposition of Sci to S T indicates a 73% layer-norm al shortening involved in transposition cycle 2. 77 made. A strain gradient exits with two maxima, one on the pluton contact and one on the Windy Pass Thrust contact, with a strain decrease in both north and south of the contacts.. This conclusion is based upon the fold tightness, foliation intensity, and the succeeding generations of structures which exhibit themselves in the schists and amphibolites (transposition cycles 3, and 4). Within the batholith a gradual increase in magmatic foliation intensity occurs across domains 2, 3, 4 and 5 towards the contact. This is manifested in the increased degree of alignment of hornblende, biotite and plagioclase laths as the foliation steepens towards the contact. Actual quantitative evaluation of the strain gradient in the wall rocks and pluton is prevented by the lack of strain markers within all units. However, this hypothesis is consistent with buckling + post-buckle flattening-shortening estimates: 32% (141B-T9Q) in domain 1 as opposed to 62% and 67% in samples 111-T90 and 462-T9I located near the thrust contact (fig. 20). The latter two were transposed during cycle 3 and possess crenulation cleavages associated with cycle 4. If we assume a shortening value of 32% for transposition cycle 2 (fig. 7) and 62% for cycles 3 and 4 bulk shortening is estimated at ~ 1.5 km for cycle 2, 3 and 4 structures (~3 km total). Using the 6 kilometers across Icicle Canyon from thrust to pluton contact 78 (at its widest point), avalue 50% orogen-normal shortening is determ ined. Strain History The above observations suggest a strain history as follows: 1) general foliation, folding, and lineation development as the result of regional deformation during WNW - ESE extension, NNE - SSW contraction with strains of cycle 2 Es > 1.12 (sample 140B-T90) cycles 2 and 3 Es > .5 (buckle folds) followed by; 2) foliation, folding, and lineation development during transposition cycle 4, regional deformation emplacement with a concentration of strain near the batholith and thrust contacts Es > .55 (sample 239-T90) during which regional strain axes stayed the same; 3) brittle deformation including the development of joints and slickensides. 79 METAMORPHISM Porphyro blast/M atrix R elationships Biotit e Biotite occurs mainly in pelitic rocks and tends to concentrate in layers separated by quartz-rich laminae. It varies in color from greenish to dark-brown to reddish-brown. The size of biotite grains ranges 0.5-2 mm, but near the batholith recrystallized grains, are generally ~ 0.25 mm in length. Biotite tends to be elongate parallel to cleavage, with x, y/z ratios ofO 2-4:1 with x > y except in contact areas where recrystallized grains are nearly equant. Some biotite grains have inclusion trails defined by quartz, zircon halos, tourmaline, graphite and other opaque minerals. This internal S surface (Sci) is generally straight in biotite, and is both parallel to oblique to St * Biotite is the most common constituent of Scj (see Structural Elements section) included in cordierite, so it must have grown during transposition cycle 1. These xenoblasts vary from being equant, to x/z ratios of 3:1. Biotite and other minerals in Sci w ere folded and crenulated (Seri. Scci, tig- 6) during the growth of cordierite. Growth and recrystallization of biotite continued during transposition cycles 2 and 3 in pelitic layers comprised 80 mainly of aligned-flattened biotite. Recrystallization continued during transposition cycle 4 as earlier structures were transposed parallel to the margins of the Mount Stuart batholith giving rocks there a hornfelsic texture. Recrystallization could have continued during post- emplacement thermal metamorphism as siliimanite and muscovite (see below) were growing. Biotite appears to have been a stable phase throughout the deformation history of the Chiwaukum Schist in the Icicle Canyon area from the first transposition cycle to post Mount Stuart emplacement static recrystallization. Cordierite Cordierite is common in Icicle Canyon area but less common near the Mount Stuart batholith and occurs in pelitic and sandy-biotite schists. It is poikioblastic and xenoblastic and includes fine-grained biotite, plus lesser amounts of quartz, and "whisps" of graphite and blebs of other opaque minerals. Cordierite crystals can be unzoned or exhibit patchy zoning. In many samples, large, elongate, ovoidal, poikioblastic cordierite grains occupy a large volume of the pelitic layers, with x >. y/z ratios of ~ 3-5:1. It occurs rarely as pseudohexagonal crystals of short prismatic habit. 8 1 Cordierite grains are aligned parallel to sub-parallel to cycle 2 cleavage, St . It is sometimes folded (fig. 27), microboudinaged and/or ruptured by shearing with quartz, opaques and biotite occurring in the intervening spaces. The poikioblasts within the cordierite grains define an internal foliation (Sc i) which is composed of biotite, quartz opaque minerals and tourmaline. Sci 'n cordierite can be straight but often is folded and displays crenulation cleavages in varying degrees of development. This internal foliation is usually discontinuous, but often parallel with the external schistosity, Sj* Where strain shadows exist, or in some microboudinaged cordierite grains inclusion trails can be followed through porphyroblasts boudins into the intervening necked area. This pattern of alignment through boudinaged grains would indicate pre- to- early-syn-ST growth, and syn- to post-kinematic deformation of cordierite (fig. 27). Cycle 2 cleavage St abuts against cordierite showing some deflection around the edges of pophyroblasts in most samples. Growth was coeval with the crenulation of Sci (fig- 6) as indicated by relict inclusion trails seen in some samples. This indicates that cordierite was growing during transposition cycle 2 and was a pre-to early-syn kinematic phase with respect to ST. 82 % > & V ° i ° % it f t A P S % \ i v V * * > V ? \ ' * a ^ o < ^ S Figure 27. Photomicrograph and line drawing of cordierite porphyroblast from sample 141B-T90 (2,5x). Thin section was cut - parallel to lineation direction and perpendicular to external schistosity (ST). ST abuts against cordierite porphyroblast (which includes internal cleavage Sci), showing some deflection around the porphyroblast indicating a pre- to early-syn ST origin for cordierite. 83 Cordierite would best be characterized as a pre-Mount Stuart batholith-emplacement phase. This assertion is supported by cordierite growth beginning during transposition cycle 2 which in turn predates the final emplacement of the Windy Pass thrust and the batholith. Garnet Garnet is a common constituent of pelitic schists and also occurs in early-syn-post emplacement mafic dikes. In contrast to cordierite, garnet is for the most part inclusion free, making the interpretation of its timing difficult to interpret. Garnets appear unzoned and are generally equant and sub-idioblastic to idioblastic. S t generally wraps around garnet porphyroblasts sometimes leaving strain shadows which could imply a syn-to late syn- ST origin for garnet (fig. 28). However, garnets also grew in mafic dikes that locally postdate the development of S j , and later mesoscopic folds of St . The lineation defined by depletion coronas surrounding garnets in these dikes are roughly coincident with the regional WNW-ESE stretching lineation on cleavage surfaces. In pelitic rocks, St locally deflects around garnet grains and it occurs in dikes and sills which cut asymmetric folds in 84 Figure 28. Photomicrographs of garnet porphyroblast from sample 217-T90 (2.5x). Thin section was cut - parallel to lineation direction and perpendicular to external schistosity (cycle 2 cleavage) ST. ST abuts slightly against the garnets strain shadow but primarily deflects around the porphyroblast. This porphyroblast-matrix relationship indicates an syn to late-syn-ST origin for garnet. Garnet is also present in mafic dikes which cut folds in ST indicating that it remained stable as a post-cycle 2 phase as well. 85 ST- None of these folds however, can be positively linked with intrusion related deformation. Therefore timing is bracketed between late-syn St (transposition cycle 2) development to possible post-pluton emplacement growth. H ornblende Hornblende occurs throughout the Icicle Canyon area as a trace mineral in pelitic schists, but most frequently in amphibolitic gneisses or schists near the Windy-Pass thrust. It generally displays brownish-green-to olive pleochroism with lath shaped grains typically of axial ratio - 3:2:1. Amphibolitic rocks are generally more fine grained than pelitic rocks, with hornblende typically around 0.5 mm in length. Grain size decreases in amphibolitic rocks near the thrust and the pluton contacts presumably due to deformation and recrystallization related to transposition cycles 3 and 4. Locally hornblende has an internal foliation defined primarily by opaque minerals usually parallel with the external cleavage. Hornblende usually occurs in nearly monomineralic layers accompanied by oxides, and locally by epidote. These layers are often folded, and crenulated, and are separated by quartz-plagioclase-rich layers. Hornblende growth began before, since it locally preserves a cleavage, and probably during the development of 86 cycle 2 cleavage St , as hornblende rich layers separated by plagioclase quartz layers locally define ST- Hornblende growth, as well as amphibolite imbrication related to Windy Pass thrusting can best be described as a pre-to at the latest syn-Sx phase. Near the pluton contacts, these rocks have a sugary- hornfelsic texture, it appears as if recrystallization of hornblende continued during thrusting and pluton emplacement as well. A n dalu site Andalusite, fibrolite, and muscovite occur as porphyroblasts spatially associated with the batholith. Andalusite crystals are colorless to faint yellow in thin section, generally are sub-idioblastic and in most samples contain chiastolite crosses. Crosses are inclusion rich with very fine grained opaque minerals and graphite, quartz and tourmaline. Andalusite grains generally are elongate, with axial ratios of x:y:z of 3-5:1:1 Andalusite is found locally in domains 4 and 5 in the study area. In sample 239-T90, a relict cleavage is preserved in chiastolite crosses (fig. 29). Inclusion trails consist of quartz, opaque minerals, and tourmaline and are moderately to strongly oriented. The external (cycle 4) cleavage in andalusite- bearing samples exhibits a hornfelsic texture, and is usually 87 j C4 * 7 y Figure 29. Photomicrograph and line drawing of chiastolite porphyroblast from sample 239-T90 (6.3x). Thin section was cut ~ perpendicular to lineation direction and perpendicular to cycle 4 cleavage. The external (cycle 4 cleavage) abuts against chiastolite interior margins but primarily deflects around its exterior margins. This relationship indicates pre- to early-syn cycle 4 cleavage origin for these porphyroblasts. 88 deflected around the edges of the porphyroblasts while abutting against their interior margins. This could indicate an early synkinematic origin with respect to cycle 4 cleavage for these porphyroblasts. In some grains, inclusion rich rims are continuous with the external cleavage indicating a syn to late synkinematic origin with respect to matrix (cycle 4) cleavage. A weak axial planar crenulation cleavage has been observed in thin section with ~ 0.5 mm. scale wavelength folds associated with andalusite. The long axes of andalusite, microscopic fold axes, crenulation cleavages, and mesoscopic fold axes are approximately colinear; shallowly plunging and parallel the batholith contact. Andalusite would best be described as an early-syn to syn-emplacement phase and grew during transposition cycle 4. This is supported by its variable relationship with its recrystallized matrix, and its colinearity with pluton contact related structures. SilHmanite Sillimanite has been observed in domain 4 (sample 239-T90, fig. 30) where it occurs as colorless transparent needle-like aggregates of fibrolite. Individual crystals are ~ .25 mm in length. 89 It postdates andalusite as it pseudomorphs andalusite grains. It also grows as a result of a reaction with matrix biotite. It overgrows all previous fabric elements and shows no preferred orientation except for where its growth is controlled by its reactants grain boundaries. It appears to be a late syn- to post-emplacement phase as it postdates andalusite and crosscuts all foliations. M uscovite Muscovite is colorless in thin section, and has only been observed in sample 239-T90 which also contains andalusite and sillimanite. It is, irregular to lath shaped and includes the hornfelsic external (cycle 4) cleavage. It is oblique to, and grows across the external fabric. Muscovite as well as sillimanite are post-emplacement phases related to static thermal metamorphism. This assertion is supported by the fact both minerals overgrow, and are oblique to all earlier foliations (fig. 31). Summary of Porphyroblast Matrix Relationships Microstructural evidence derived from samples in the study area are consistent with at least three episodes of cleavage formation, and porphyroblast growth. The first recognized generation of included growth of biotite, quartz and 90 Figure 30. Photomicrographs showing fibrous aggregates of fibolite crystals from sample 239-T90 (6.3x). Thin section was cut ~ perpendicular to lineation direction and external (cycle 4) schistosity. Fibrolite is shown growing on andalusite and biotite grains and cross-cuts the cycle 4 contact aureole cleavage. This indicates a post-cycle 4 cleavage origin for these minerals. 9 1 Figure 31. Photomicrograph from sample 239-T90 (6.3x) showing muscovite porhyroblasts cross-cutting the external (cycle 4) schistosity. Thin section was cut - perpendicular to schitosity and lineation direction This indicates a late-syn to post-cycle 4 origin for these minerals. Both muscovite and fibrolite are interpreted to have formed during post- emplacement static-thermal recrystallization, 92 opaque minerals which formed the first recognizable generation of cleavage, Sci. Cordierite porphyroblasts grew during the crenulation of Sci (see structures section, fig. 6), as their growth predates the formation, at least in part of cycle 2 cleavage, S j- Garnet appears as a later phase which is late-syn St development. Andalusite, fibrolite, and muscovite occur as later porphyroblasts. Andalusite is an early-syn-to syn contact metamorphic phase. The recrystallized matrix foliation in samples containing andalusite and fibrolite, is spatially associated with the batholith, and represents transposition cycle 4 cleavage there. Fibrolite and muscovite completely overgrow cycle 4 cleavages and are therefore late-syn-to-post- emplacement phases. Field-work, and microstructural inclusion trail observations consistently define WNW-striking cleavage throughout the history of the study area. The following section describes individual mineral characteristics and their porphyroblast/matrix relationships. Summary of M etamorphism Unfortunately, the assemblages seen in the wall rocks of the Mount Stuart batholith do not tightly constrain the pressure and temperature of metamorphism in the absence of 93 mineral modal data. However, petrographic analysis and field relationships indicate that metamorphism was prograde in nature, and consisted of at least four stages related to regional metamorphism, thrusting and pluton emplacement. Assemblages were divided into three categories based on mineralogy, and are summarized below. Each category will be dealt with individually. Pelitic Schists Metamorphism in the pelitic schist in a can be looked upon as four progressive stages based upon porphyroblast matrix relationships observed in thin section: 1) Regional Metamorphism; Transposition Cycle I biotite + quartz + plagioclase + opaque oxides; 2) Regional Metamorphism & thrusting; Transposition Cycles 2 and 3 biotite + quartz + plagioclase + opaque+oxides ± cordierite + garnet; 3) Contact + Regional Metamorphism; Transposition Cycle 4 biotite + quartz + plagioclase +. andalusite + garnet; 94 4) Post-Contact Static Thermal Metamorphism biotite + quartz +. sillimanite ± muscovite ± ? Transposition cycles 1, 2, and 3 are associated with regional metamorphism and in the study area. The mineral assemblage of transposition cycle 1 is not very informative; however, biotite is stable indicative of temperatures > 400°C and pressures > 1.5 Kb. (Ferry, 1986; Yardley, 1989). Metamorphic phases associated with transposition cycle 1 are biotite, quartz, and plagioclase + opaque minerals. These crystals are aligned in cordierite porphyroblasts as a relict cleavage (Sci). These strongly-oriented inclusion trails (Sci) in "early" porphyroblasts (especially cordierite) are consistent with a strong fabric due to transposition cycle 1 prior to final emplacement of the thrust. This indicates that the "early" (cycle 1) regional fabric was strong. Metamorphism associated with transposition cycles 2 and 3 includes the minerals of biotite, cordierite and later garnet in pelitic rocks. This assemblage constrains temperatures to a range of 500° C and 650° C (Winkler, 1979). Transposition cycle 2 occurred during the growth of cordierite as Sci was transformed into a new composite schistosity, (St ) with 9 5 cordierite preserving intervening stages in cleavage morphology (Seri and SCci). Garnet growth was coeval with asymmetric folding of St and transposition cycle 3. Also associated with transposition cycle 3 is an additional episode of recrystallization within 1 km of the Windy Pass thrust. In cordierite-bearing schists, St is often oblique to, and only sometimes continuous with Sci (as in fig. 25) thus obscuring the exact relationship between Sci and St at porphyroblast boundaries. Rotation of porphyroblasts followed by dissolution of the edges could accomplish the same discontinuous pattern as cleavage transposition alone. However, one would expect to see a concentration of selvage material at these edges and this has not been observed (Bell et al., 1986; Vernon, 1987) Therefore two generations of regional cleavage (Sc i and ST) development are indicated by; 1) truncations of Sci against porphyroblast edges, 2) St wrapping around cordierite and 3) higher grade composition of constituents of St . Metamorphism associated with transposition cycle 4 in the batholith's contact aureole is characterized by fine grained recrystallized biotite, quartz, plagioclase +. garnet ± andalusite. Rocks of the contact aureole exhibit a hornfelsic texture and at 96 least locally are of a higher grade than regional assemblages. The presence of andalusite and continued stability of garnet constrains pressure to < 3 kb and temperatures to a range of 450°C - 650°C (Yardley, 1989). However, geothermometry and barometry in nearby coeval structures constrain metamorphic conditions between 3-4 kb, and 550° and 685° C in transposition cycle 4 structures (Evans and Berti, 1976; Bendixen et al., 1991; Brown and Walker, 1993). It is not clear whether cordierite was stable in the contact aureole rocks in Icicle Canyon although cordierite growth was coeval with emplacement of the batholith to the north (Paterson et al., ms. 1993). Near the Windy Pass thrust coeval metamorphism was manifested by recrystallization and dissolution of biotite quartz and plagioclase grains during the crenulation of cycle 3 cleavage (Scr and SCc) arid the generation of a new (cycle 4) composite schistosity near the thrust. Post-pluton-emplacement metamorphism consists of growth of muscovite and needle-like aggregates of fibrolite and was probably accompanied by recrystallization of all matrix minerals (i e., quartz, biotite and opaque minerals). Although fibrolite is a fine-grained fibrous polymorph of sillimanite it does not appear to have the same P-T stability field as sillimanite (Kerrick, 1990). Therefore, P-T conditions are difficult to constrain, Assuming that sillimanite to 97 andalusite - fibrolite to andalusite reactions to be colinear, the P-T conditions are constrained to 1-3 kb. and 500°C and 750°C (Yardley, 1989). However, there is considerable evidence that fibrolite can crystallize outside the sillimanite P-T stability field (e.g., Speer, 1982, Berg and Docka, 1983). Kerrick (1990) examined the question of fibrolite metastability in two contact aureoles in Donegal, Ireland. Gamet-biotite geothermometry suggests that the maximum temperature at the fibrolite isograd was approximately 100°C below the sillimanite isograd. Garnet-biotite thermometric data from an Icicle Canyon contact aureole sample (with no Al-silicates) yields a temperature of 685° C (Bendixen et al., 1991). If this is typical of temperatures in the aureole, pressure of metamorphism during fibrolite growth is around 2 kb. assuming sillimanite/andalusite univariant conditions (Yardley, 1989). If the fibrolite isograd is assumed to exist 100°C below the sillmanite isograd then the pressure would be around 1.5 kb (Kerrick, 1990). A m p h ib o lite s Amphibolites occur in gneissic and schistose varieties in both the upper-and lower-plates of the Windy Pass Thrust. They have been observed throughout the Nason Terrane, but are most common near the thrust contact. The rocks tend to be 98 finer grained and exhibit a recrystallized texture near the Windy Pass thrust and Mount Stuart batholith contacts. Pressures and temperatures for this assemblage are difficult to constrain independently. However they can be broadly grouped into the amphibolite facies classification (Yardley, 1989) and occur in the assemblage: hornblende + plagioclase + quartz + epidote + _ biotite. Amphibolites were metamorphosed and transposed during transposition cycle 2. Amphibolites, like rocks of pelitic composition, experienced new mineral growth and recrystailization during transposition cycle 3 due to Windy Pass thrusting and during transposition cycle 4 due to the intrusion of the Mount Stuart batholith. However, no change in mineralogy and/or modal proportions has been observed in the am phibolites Ultramafics The dominant assemblage of the metamorphosed ultramafic rocks in the study area is: fosterite + talc + tremolite + chlorite, which can be classified upper amphibolite facies metamorphism (Miller, 1985). Where observed, in the lower- 99 plate ultramafic rocks display a weakly developed metamorphic cleavage defined by light-colored talc-tremolite rich layers. This cleavage is roughly concordant with cycle 2 cleavage (St ) and thus is interpreted to have resulted from deformation associated with transposition cycle 2. Temperature estimates for metamorphism and deformation in the study area are consistent with the following model: 1) Prograde regional metamorphism at temperatures > 400°C and pressures > 1.5 kb (transposition cycle 1) increasing to cordierite-garnet (T=500° - 650°, P > 2 kb) grade (transposition cycles 2 and 3) followed by; 2) Prograde contact metamorphism with pluton intrusion at >650oC; 3 - 4 kb (transposition cycle 4), during simultaneous regional deformation culminating in sillimanite grade static metamorphism; 3) Rapid cooling of the pluton and the wall rock towards the ambient regional temperature. Conclusion number 3 is supported by blocking temperatures and closure ages where in the study area U/Pb 100 (750° C) closure at 93 Ma is followed by K-Ar in biotite (300° C) at 88 Ma with ages decreasing to the northeast (Tabor et al., 1987a, Haugerud, 1991, fig. 4). This implies extremely rapid rates of cooling and uplift. E t should be noted that there is evidence for regional metamorphism following the intrusion to the north of the Icicle Canyon area. Miller and Paterson (1991) have documented syn-post-intrusion SW-directed reverse faults, while Evans and Berti (1986), S. R. Paterson (pers. comm.), J. E. Bendixen (pers. comm.) have demonstrated that kyanite pseudomorphs andalusite in the contact aureole of the batholith. This implies burial metamorphism after emplacement and a rapid uplift such that nearly all andalusite porphyroblasts have not transformed to the high pressure polymorph. No evidence for this later episode of metamorphism exists in the study area how ever. Although distinct regional episodes of cleavage development are implied from porphyroblasts/matrix, microstructural and field relationships; the coincident nature of succeeding generations of fold axes and lineations, and shallowing of cleavages (see structures section) implies a prograde continuum of metamorphism during non-coaxial simple shear and pluton emplacement. 101 SUMMARY OF DEFORMATION Transposition cycle 1 as well as cycle 2 (fig. 9) are believed to be associated with regional deformation and metamorphism in the Cascades Crystalline Core. Cycle 1 cleavage consists of aligned biotite, quartz, plagioclase, and locally graphite plus other opaque and trace minerals included in cordierite, and locally in biotite and hornblende. In general, Sci and external cycle 2 cleavage, S t are roughly coplanar. Fold and crenulation axes in Sci, fold hinge-lines in (cycle 2 cleavage) S t and cycle 2 stretching lineations appear to be colinear as indicated from petrographic analysis. Mineral assemblages associated with transposition cycle 1 are not very informative; however, biotite is stable indicative of temperatures > 400°C. (Ferry, 1986; Yardley, 1989). Cycle 2 cleavage ST. is the most pervasive structure in the Icicle Canyon area. It is composed of transposed elements Sci. Seri, and Scci- It can be found in metamorphic rocks in the lower plate and in amphibolitic rocks in the upper plate of the Windy Pass thrust, and as a weakly developed foliation in ultramafic rocks. In domain 1, St strikes northwest, and dips moderately to the northeast roughly parallel to the regional orientation of 1 02 cleavage in the Nason Terrane. Frequently on these cleavage surfaces lies well developed WNW-ESE trending gently plunging stretching lineations. Post cycle 2 deformation in domain 1 includes asymmetric-mesoscopic folding of S t along upright to gently plunging WNW-ESE trending axes. Folding intensity increases towards both pluton and thrust contacts. Although structures in domain 1 were not transposed beyond cycle 2, some folding is probably related to transposition of structures due to thrusting (cycle 3) and pluton emplacement (cycle 4). The strain enjoyed by rocks during transposition cycle 2 indicates intensities (Es) were > 1.12 (sample 140B-T90, Table 3) while cordierite was stable throughout transposition cycle 2. This constrains metamorphism to T > 500° C; P > 1.5Kb (Winkler, 1979) Cleavage and stretching lineation attitudes, as well as the available strain and petrographic data indicates that during cycles 1 and 2 the area was undergoing pervasive regional deformation due to WNW-ESE extension and simultaneous ENE- WSW contraction during cycles 1 and 2 during prograde metamorphism. Post cycle 2 kinematic indicators in domain 1 indicate horizontal shear with top-to-the-SW displacement. In a 1 mile wide panel adjacent to the Windy Pass thrust older structures were transposed parallel to the fault contact 103 during transposition cycle 3 due to top-to-the-SSW thrusting. Associated with this deformation are a new continuous cleavage and WNW-ESE trending shallowly plunging folds and stretching lineations. Strain data taken from buckle folds formed during post cycle 2 and cycle 3 deformation indicate that Es > .5 (buckle folds, Table 3). Metamorphism associated with this event involved the growth and recrystallization of hornblende in amphibolitic rocks and biotite in pelites; with cordierite seemingly remaining as a stable phase during the growth of garnet. This phases constrain metamorphism to T > 500° C and P > 2.5 Kb (Yardley, 1989). Kinematics indicate that shear sense was the same during cycle 3, as they were during folding of cycle 2 structures. Stretching and contraction directions were nearly identical judging from cleavage and lineation orientations as well as results from strain analysis. Although cleavage does gradually flatten, as fold tightness increases near the thrust contact. This is probably indicative of increased shearing near the contact during thrusting. This pattern of structural transposition indicates that regional deformation was the cause of thrusting, cycle 3 deformation and increasing metamorphic grade. Transposition cycle 4 accompanied emplacement of the Mount Stuart batholith. It is localized in the contact regions of 104 the pluton and Windy Pass thrust (fig. 7). Domainal macroscopic folding and contact metamorphism were coeval with the transposition of older structures. Cleavage was rotated into parallelism with the intrusive contact during emplacement resulting in the development of a new continuous cleavage. Contact-parallel macroscopic and parasitic mesoscopic folds occurred in domains 2-5 as did coeval stretching Iineations sub-parallel to fold axes in domains 2 and 3. In a narrow zone next to the Windy Pass thrust, cycle 4 structures consist of crenulation cleavages (i. e.„ Scr. See), a new composite schistosity, folds and stretching Iineations. All of these features are essentially parallel to cycle 3 structures. Fold asymmetry associated with this deformation defines an antiform with a top-to-the NNE sense of displacement next to the thrust (i e., the southern limb) which changes to a top-to- the-SSW sense 100-200 meters from the contact. In the eastern and western portions of domains 6 and 7 thrust parallel structures grade into coeval intrusive parallel cleavages. Shear-sense of cycle 4 asymmetric folds is erratic in domains 2 and 3. Although adjacent in domains 4 and 5 a prevalent top-to-the-SSW sense of shear is manifest. Some syn-to-post emplacement aplite and pegmatite dikes are 1 05 openly-folded with axes roughly colinear with folds in the country rocks indicating folding intensity decreased, but strain axes were roughly colinear during and after emplacement. Transposition cycle 3 and 4 deformation near the thrust and pluton and thrust contacts appears to have caused created a strain gradient. Two maxima exist: one on the pluton contact and one on the Windy Pass Thrust contact with a strain decrease in both directions. Fold tightness, foliation intensity increase succeeding generations of structures which exhibit themselves indicate this to be the case. This hypothesis is consistent with buckling + post-buckle flattening-shortening estimates: 32% (141B-T90) in folded cycle 2 cleavage as opposed to 62% and 67%> in samples 111-T90 and 462-T91 in folded and crenulated cycle 4 structures. The only shortening estimate relevant to cycle 4 structures near the batholith indicates shortening in the contact aureole was > 25% (samples 239-T90, 41-T91). However, this shortening was imposed upon rocks that had experienced at least 1 additional transposition cycle over folded cycle 2 structures related to syn- emplacement strain near the batholith. These estimates are broadly consistent with a strain gradient in the study area although the limited number of strain markers in all units does not allow for a quantitative evaluation of this gradient. 1 06 Evans and Berti (1976), Bendixen et al., (1991) and Biown and Walker (1993) constrain metamorphic conditions as lying between 3-4 kb, and 550° and 685° C in transposition cycle 4 structures (Paterson et al., 1993) Transposition 5 structures formed during syn- to post emplacement reverse faulting (Miller and Paterson, 1991). These ductile shear zones are located to the north of the study area at the northeast margins of the batholith. Here, contact metamorphism reached sillimanite grade, and was followed by the growth of kyanite, staurolite and garnet (Evans and Berti, 1986; S. R. Paterson p. comm.). DISCUSSION Timing and Nature of Deformation Active subduction zones have long been noted to be associated with plutonic activity. So it is often the case that plutons are intruded syn-tectonically, with deformation occurring before, during and after emplacement. Field and microstructural evidence suggests the following deformational history in the Icicle Canyon area. 1) Prograde regional metamorphism during NE-SW contraction and NW-SE extension prior to 93 Ma. Conditions varied from > 400° C, > 1,5 kb (transposition cycle 1) and 500° - 650°C, > 2kb 1 07 (transposition cycles 2 and 3). 2) Pluton emplacement and continued regional deformation during transposition cycle 4. Prograde contact metamorphism culminated at > 650°C and 3- 4 kb at 93 Ma with strain being most intense near the batholith and thrust contacts. 3) brittle deformation (development of joints, minor faults and slickensides) during cooling of the pluton between 90 - 80 Ma. Regional strain axes during transposition cycles 1 and 2 were orientated approximately as follows: direction of greatest contraction (e3 ) and direction of greatest extension ( e i ) horizontal with intermediate strain axis (e2) vertical, as indicated by cleavage and lineation patterns and calculated strain axes. Strain could have been a combination of pure and simple shear during cycles I - 3. However, evidence for local simple shear is seen as the thrust contact is approached. Here, cleavage flattens due to the clockwise rotation of total strain axes, which occurred during transposition cycle 3 (e2 = horizontal, e3 = vertical, el = horizontal) due to thrusting. Where pluton contacts cross-cut regional structure, (domains 2 and 3) cleavage, fold axes and stretching Iineations steepened. Total strain axes were rotated (e3 = horizontal, e 2=horizontal(?), ei=vertical) indicated by cleavage and lineation patterns. These structures formed in part by syn- 1 08 emplacement rotation of older ones during wall rock flow (see pluton emplacement below). Where pluton contacts are concordant with regional structure (domains 4 and 5), transposition cycle 4 structures consist of steep cleavage, and shallowly plunging fold axes and stretching Iineations. Fold asymmetry indicates a top-to-the-SSW shear sense for this deformation in domains 4 and 5. In domains 4 and 5 (fig. 7) some aplite and pegmatite dikes are openly-folded with axes colinear (WNW-ESE) with transposition cycle 4 and earlier structures in the Chiwaukum Schist. In some cases these dikes can be traced up to the contact and infrequently into the batholith itself. A gradual increase in magmatic foliation intensity occurs across from domains 2, 3, 4 and 5 towards the contact and are parallel to country rock structures. Fold tightness, foliation intensity increase towards the country rocks in transposition cycle 4 structures. A syn-emplacement deformation occurred near the Windy Pass thrust as well. Here, during transposition cycle 4 asymmetrically folded cleavages and Iineations developed during top-to-the-NNE shear. However, it is not clear how these cleavages, far from the pluton are related to the intrusion, and why they have an opposite sense of shear from coeval (cycle 4) aureole cleavages in domains 4 and 5. Two possible explanations exist that explain the origin of these cycle 4 1 09 cleavages near th thrust: they were formed during top-to-the- NNE translation of upper and lower plates due to pluton body forces (during pluton emplacement) which reactivated the thrust, or; an episode of top-to-the-NNE thrusting which continued during pluton emplacement. In scenario 1 the western body (of the Mount Stuart Batholith) initiates NNE displacement of the upper plate and/or the northern body initiates SSW displacement of the lower plate. The resulting deformation would cause a-top-to-the-NNE sense of shear with deformation most intense near the thrust. In scenario 2 top-to-the-NNE thrusting forms cycle 4 structures in domains 6 and 7 and is accompanied by pluton emplacement. Deformation associated with emplacement transposes structures concordantly with the pluton contact where the pluton intrudes the thrust. Deformation in either scenario is enhanced due to the basement effect (Ayerton, 1980). This effect has been linked to strain softening effects in pre-tectonic plutons. In essence, the ultramafic rocks of the Ingalls complex are a rigid body in a less viscous medium (as would be a pre-tectonic pluton in pelitic country rocks), which serves as a better thermal conductor and a barrier against fluid flow being less permeable than the schists. Therefore, deformation is enhanced in the 1 10 strain softened zone, and is most intense at the thrusting interface where competency contrasts are greatest. Although post-emplacement deformation was minor in the Icicle Canyon area, regional deformation continued northeast of the batholith (Miller and Paterson, 1991) during prograde metamorphism. Evans and Berti (1986) have demonstrated that kyanite, staurolite and biotite among other "regional" minerals pseudomorph andalusite to the north of the study area in the contact aureole of the batholith. However, no clear-cut evidence for this last static period of metamorphism has been observed in the Icicle Canyon area. The only deformation which is clearly post-pluton emplacement is brittle (i e., joint, brittle fault and slickenside development). Regional metamorphism in the study area predated, at least in part the emplacement of the Windy Pass Thrust, and is followed by contact metamorphism associated with batholith emplacement. Post-emplacement low pressure static metamorphism followed (e. g. fibrolite, muscovite growth) in the contact aureole. The evidence seen in the field area is consistent with the interpretations of Plummer (1980), Miller (1985) Taylor et al. (1991), and not those of Evans and Berti (1986) in that regional metamorphism clearly pre-dates contact metamorphism. However, these observations do not rule out the interpretations 1 1 i of Miller and Paterson (1991) Taylor et al. (1992) Paterson et al. (in review) which conclude that regional metamorphism was occurring before and after pluton emplacement. Since static regional metamorphism appears to have occurred after batholith emplacement to the north, the study area could have remained shallow enough in depth such that the overprint on regional and contact assemblages was minor. Pluton E m p lac e m e n t A major part of the Nason Terrane consists of the Mount Stuart batholith. Therefore an important step in understanding the evolution of this terrane and the north Cascades is to determine the role of emplacement of the batholith. The major source of controversy concerns how space is made for large volumes of magma. Some common emplacement hypotheses include ballooning (Holder, 1979, Ramsay, 1981; 1989; Bateman, 1984; 1985), sloping (Buddington, 1959; Pitcher, 1979), assimilation (Ahren et al., 1980) diapirism (Cruden and Aaro, 1989; 1990), fault-related transport and opening (Pitcher and Berger, 1972; Shaw 1980), and emplacement during extension (Tobisch et al., 1986; Hutton, 1989). However, these mechanisms do no more than displace or assimilate wall rocks during emplacement. Since they do not increase the volume of the crust the only true "space making" mechanisms in the crust 1 12 are (1) lowering the Moho or (2) outward displacement of the earth’s surface (Paterson and Fowler, ms.). These emplacement mechanisms are better viewed as combinations of near and f a r field material transfer processes (MTPs), (Paterson and Fowler, 1993). Near field MTPs operate in the structural aureole of plutons (e.g. stoping, wall rock flow); far field MTP's are those which remove material from the plutons aureole towards the surface of the earth or back towards the region of magma generation (Paterson and Fowler, 1993) Previous workers suggested that the Mount Stuart batholith was forcefully emplaced (Plummer, 1969; 1980; Pongsapich, 1974; Evans and Berti, 1986). In this model, space is made for the pluton by ductile shortening of the wall rock and early intrusive phases. One problem with this model is that it doesn't eliminate the space problem, since rocks have limited compressibility rarely exceeding 5% (Paterson and Fowler, 1993). Material may be removed by shortening of the pluton’s wall rocks but the attenuated volume must be transferred elsewhere by ductile flow or other MTPs. In this section I will review the MTPs involved in emplacement of the Mount Stuart batholith by examining structural field relationships, microstructures, metamorphic mineral characteristics, strain patterns near the batholith. 1 13 E m placem ent M echanism s Wall Rock Flow During simultaneous intrusion and regional deformation plutons introduce excess heat, and concentrate fluids around the pluton. This leads to strain-softening of the surrounding wall rocks often causing ductile flow in the aureole. So during a tectonic event, wall rock foliations in general, and especially in schistose rocks will tend to wrap into concordance with pluton contacts. Although, where pluton contacts are at high angles to regional cleavages foliations will often tend to be continuous on either side of the pluton margin (Paterson and Tobisch, 1989). In the Icicle Canyon area, in domains 2 and 3, where pluton contacts are highly oblique to regional structures cleavage, fold axes and stretching Iineations are steep, with mesoscopic folds showing little evidence for a consistent sense of shear (see structures section), unlike the rest of the study area. This pattern is probably a transposition of earlier structures parallel to the contact due to vertical wall rock return-flow . In domains 4 and 5, older structures were transposed due to syn-emplacement wall rock flow, and parallel the regional structures, in a narrow contact aureole (50m - 100m). In order to accurately assess the magnitude of wall rock flow in these domains, one would have to measure strains in the 1 1 4 aureole. However, useful objects for measuring strains are rare near the Mount Stuart batholith. Two samples (239-T90, 54- T9I; fig. 6, Table 3) from domains 4 and 5 were analyzed. Estimates for shortening were 42% and 25%. A maximum bulk shortening of 72m is obtained assuming a 100 meter wide aureole (If = 100m) {x = -.42, lc = lf/(x +1), lo = 172m). Assumming symmetrical wall rock flow on both margins of the sill-like body the total space made is 144m (e. g. 72m + 72m). Since the sill is 1600m wide at its narrowest point, roughly 9% (144m/1600m x 100%) of the sills volume can be accounted for by this MTP. Sloping During stoping, xenoliths of country rock break off at the margin of an intrusion and sink through the magma (Daly, 1903; 1933). Rocks in the space formerly occupied by the pluton must be assimilated into the magma or exist as blocks within or at the base of the magma chamber (Paterson et al., 1991). Discordant contact relations in which the pluton truncates country rock structural trends, are believed to be associated with plutons emplaced by stoping (Buddington, 1959; Pitcher, 1979). These features are sometimes associated with the 1 1 5 preservation of thin roofs. (Barrell, 1907; Bussel et al., 1976, Paterson, et al., 1991). Large and small stoped blocks are preserved near the roof of the Mount Stuart batholith located near the northern margin of the central sill-like body (figs. 32, 33). The country rock/pluton contact is flat here as is cleavage, and is laterally continuous with thin protrusions of schist which lie over the batholith. In map view, the contact here is highly discordant with the rest of the northern margin of the central sill. Numerous xenoliths and enclaves of various sizes, as well as several large pendants are present here as well. The discordant pluton-wall rock contacts on a local scale as well as the presence of numerous stoped blocks in the roof indicate that some material transfer was accomplished by stoping. Further work is necessary in order to assess the magnitude of material displaced by this MTP. Dike!Sill Emplacement Dike and sill emplacement at shallow crustal levels is sometimes associated with dilation and/or structural anisotropies. At depth, dikes and sills are believed to be emplaced in zones of local extension with the caveat that rate of wall rock translation equals the rate of magma emplacement (Paterson et al., 1991). 1 1 6 NASON TERRANE Mushroom Figure 32. G eologic m ap (after Paterson cl al., in review) showing overall structural m akeup o f the Mount Stuart batholith. Also shown are specific areas discussed in the text: the m ushroom , hook, roof area and central sill regions. 1 1 7 CROSS SECTION B-B' Ch Vertical Exageration = I 1 M IL E F igure 33. C ro ss sectio n B -B ‘ (from fig. 8) sh o w in g co n tact re la tio n s and fo liatio n patterns in the C h iw au k u m S chist and ea stern and w estern bodies o f the M o u n t S tu art B atholith. N ote the flat-ly in g co n tact and foliation in both the C h iw au k u m S chist and pluton in "R o o f A rea". 1 18 Magmatic foliation in the western body appears to be generally margin parallel near its contacts, and concordant with regional structures. Emplacement as a large sill in a regional plane of structural anisotropy could be a possibility. Space could have been made by NW-SE extension during regional deformation. This assertion is supported by the conformity of this body to the general WNW-HSE trend, typical of structures in the Nason Terrane and the parallelism of magmatic foliation with contacts. However, further work is necessary to assess the means of emplacement and the complexity of structures in this body. Summary of Pluton Emplacement Pluton and wall rock structures indicate the following: 1) syn-tectonic emplacement with field evidence consistent with stoping, and wall rock flow as dominant MTPs; 2) a transposition of all earlier structures in a narrow contact aureole adjacent to the batholith and along the Windy Pass thrust during emplacement. Translation vs Tilt Pre-Eocene clockwise rotation and/or considerable northward translation of the Nason Terrane and the Ingalls Complex has been suggested by Beck and Noson (1972), and 1 1 9 Beck et. al. (1981), on the basis of anomalous late Cretaceous magnetic pole positions in the Mount Stuart batholith. Models formulated by Irving (1985) and Umhoefer (1987) attempt to explain the alleged northward translation of the Nason Terrane as part of "Baja British Columbia" an amalgamation of Superterranes I and II of Monger el. al. (1982). In contrast, Butler et al. (1989) propose tilting (NE side up) of the Nason Terrane by as much as 35° and little to no translation to explain the anomalous paleomagnctic poles. According to Beck (1980) and Tabor et al. (1987a), post-intrusion in situ tilt can eliminate some of the pole discordance, but not all of it. The strongest evidence for tilting is a supposed tilted depositional contact between the Eocene Swauk Formation and the Mount Stuart batholith. Restoration of 35° Eocene tilt brings the observed paleomagnetic pole of the batholith into concordance with the North American reference pole (Butler et al., 1989). Decreasing cooling ages and increases in metaniorphic grade from SW to NE indicates a probable minimum of 15° of NE side up tilt (N.S.F. proposal, Miller and Paterson, 1990). Butler et al. (1989) uses similar arguments to constrain tilt in the Coast Plutonic Complex and implies that pole discordances in the Mount Stuart Batholith as well as the Coast Plutonic Complex can be resolved by a clockwise rotation on the order of 35°. 1 20 The location of Windy Pass thrusting depends on which on how much translation or tilt of the Nason Terrane has taken place In short, it is difficult to correlate Windy Pass thrusting with structures now in the vicinity of the Nason Terrane unless the Mount Stuart Batholith was emplaced near its current location. The hypothesis that the Ingalls "Terrane" (Tabor et al., 1987b) was part of the Northwest Cascade Thrust System, with provenance east of the Methow Basin (Whetten et al., 1980) would be incorrect, unless its source area (NCWS) was also translated along with the Nason Terrane. Although the evidence for tilt is compelling, it is unlikely that the Nason Terrane has been tilted through an angle of '-35°. Beck (1980; 1981) has advanced effective arguments against coherent tilting of the Mount Stuart and large batholiths in general. Supporting the translation argument is the sense of paleomagnetic discordance which is similar to that found in nearly all assumed allochthonous terranes in the North American Cordillera (Beck et al., 1981). Given the above evidence, the most likely scenario would be one where the Nason, and perhaps other terranes of the crystalline core were amalgamated far to the south before being intruded by Late Cretaceous plutons. During its journey, the Nason Terrane underwent deformation which continued after arrival at the North American margin, and during NE-SW 1 2 1 tilt. Assuming the Nason terrane enjoyed 15° of in situ tilt (Miller and Paterson, 1990), it would have to had been translated about 1000 miles to the north, with provenence somewhere in central California. Regional Tectonics As mentioned previously, several different models have been proposed to explain Mid-Cretaceous to early Tertiary deformation in Northwestern Washington and British Columbia. The models explain orogenesis by contraction, transpression or pluton loading. Country rock structures associated with thrust loading and dextral strike-slip deformation are characterized by moderate-to-steep cleavages, shallowly plunging asymmetric folds that are sub-parallel to stretching lineations. However, with pluton loading one would expect to see steep cleavages with down-dip stretching lineations and shallowly plunging fold axes. The structures in the Cascades crystalline core are characterized by moderately dipping cleavages, and shallowly plunging sub-parallel fold axes and stretching lineations. The structures in the Chiwaukum Schist, as well as Windy Pass and syn-to-post-emplacement thrusting (Miller and Paterson, 1991) could occur in response to orogen-normal shortening (Brandon and Cowan, 1985; McGroder, 1989), or transpression 122 along terrane bounding faults (Brown, 1989; Brown and Talbot, 1989, Maekawa and Brown, 1991). However, it is not clear how Brown and Walker's, (1991) pluton loading model could cause the horizontal shear and thrusting observed in the field. More information than that gathered in this study is needed for these interpretations to be evaluated in detail. However, a brief examination of regional history and structures, in light of my new data, will be undertaken in order assess the feasibility these interpretations. The Cascades crystalline core consists of the 5 tectonostratigraphic terranes of Tabor et al. (1987b, fig. 1). Knowledge of these terranes provenance and amalgamation prior to their arrival to the North American margin is sparse. The probable thrust contact between the Mad River and underlying Swakane terrane and between the Ingalls Complex and the Nason Terrane (fig 1) are tentatively interpreted by Tabor et al. (1987b) to indicate that these terranes were assembled far to the south before arrival at the North American margin. However, Whetten et al. (1980) have interpreted the Ingalls Complex to be part of a regional thrust sheet, the Haystack Thrust, part of the Northwest Cascades Thrust System, exposed in the San Juan Islands. In their view, the Haystack thrust was emplaced in the middle Cretaceous over the already assembled terranes. This would give the 1 23 thrust a top-to-the-W, or SW sense of displacement assuming that NCWS structures originated to the east of the Skagit Gneiss (Brandon and Cowan; McGroder, 1991). Vance et al. (1980) and Miller (1985) envisioned northward thrusting (obduction) of the Ingalls Complex during middle Cretaceous times. Field evidence indicates that a top-to-the-SSW sense of shear is associated with thrusting in the study area. This is broadly consistent with the Skagit Gneiss (Brandon and Cowan; McGroder, 1991) and Haystack Thrust (Whetten et al., 1980) scenarios and opposed to Miller s (1985) interpretation of possible provenance somewhere in the Columbia embayment. A possible implication of the SSW displacement is that the ophiolite formed in a small marginal basin as interpreted by Davis et al. (1978) to the NNF. However, as previously stated if there was any substantial translation of the Nason terrane correlation of the Windy Pass thrust with any North American marginal structures is difficult. If the Nason Terrane is in fact part of the Insular Superterrane and did experience substantial margin-parallel translation (Beck, 1981; Irving, 1985; Umhoefer, 1987; Irving and Thorkelson, 1990), then it likely would have experienced pervasive transpressional deformation during its journey. Brown and Talbot (1989) infer right-lateral transpression from stretching lineation orientations and microstructural kinematic 1 24 indicators throughout the crystalline core. However, the geologic evidence for a Cretaceous fault that could accommodate transpression is circumstantial at best. Miller and Bowering (1990) have documented pre-65 Ma dextral slip on the Ross Lake Fault Zone, located between the Skagit Gniess and Hozameen Group (fig. 1). This fault is a possible candidate. However, magnitude of pre-65 Ma motion is unresolved despite many interpretations of large pre-Tertiary movements (e.g., Misch, 1966; Davis et al., 1978). The structural style (e.g. steep cleavage, horizontal- parallel fold hinges and stretching lineations), as is present in the study area might reflect deformation in a transpressional setting (Sanderson and Marchini, 1984; Ridley, 1986), although they are permissive of deformation in other settings as well. If regional strain axes were such that e3 and e| ~ horizontal and e 2 ~ vertical, a steep cleavage and parallel, horizontal fold axes and stretching lineations could result. This would be a plausible configuration with both thrusting and pluton loading scenarios, but only at considerable depth. With thrusting near the earth’s surface you would expect to see nearly-flat cleavges, horizontal fold axes and down-dip stretching lineations, provided that ductile structures did form ( e [ , e 2 ~ horizontal, e3 = vertical). With pluton loading, steep cleavages, horizontal fold axes and down-dip stretching lineations should develop ( e i , e 3 = 1 25 vertical, e2 = horizontal). Confining pressure (03) in both thrusting and pluton loading, is likely parallel with stretching (e 1). However, confining pressure will increase with depth and cause a "switch" in strain axes configuration at the depth where confining pressure becomes greater than the (former) "intermediate" stress. Therefore, vertical, confining pressure will become the intermediate stress (0 2 ) and 03 will be horizontal and perhaps parallel to stretching (el). Stretching lineations would likely go through the same transition starting as down dip features, at shallow crustal levels and become horizontal at mid-crustal levels. This axes configuration is plausible in both thrusting and pluton loading scenarios however, only thrusting is compatible with the progressive flattening of cleavage, and horizontal sense of shear seen in the Icicle Canyon area. If the pre-syn Mount Stuart emplacement deformation of the Nason Terrane was in-situ the Nason Terrane, or at least the batholith would have had to have been tilted through an angle of about 35°. (Butler et al., 1989; Beck 1980, 1981; Irving, 1985; Irving and Thorkelson, 1990). However, structural evidence for a tectonic environment that could coherently tilt such a large region is absent. Butler et al. (1989) speculates that transpression along the Straight Creek fault, or 1 26 perhaps post-Eocene crustal extension is the mechanism. These hypotheses have not been thoroughly investigated. CONCLUSIONS AND SUMMARY The evidence presented above points to a complicated history of deformation due to thrusting, translation, tilting, and pluton emplacement during regional tectonism. Three phases of deformation, in order of decreasing age, are recognized: 1) NE-SW contraction and NW-SE extension during prograde regional metamorphism and thrusting (> 93 Ma). 2) Continued NE-SW contraction and NW-SE extension with augmented syn-emplacement strain along pluton and thrust contacts. 3) Brittle deformation (< 88 Ma). Regional ductile strain and metamorphism occurred as a result of NW-SE extension and NE-SW contraction during thrust loading. Transposition cycle I as well as cycle 2 (fig. 9) appear to be associated with regional deformation and in the Cascades Crystalline Core. Cycle 1 cleavage consists of aligned biotite, quartz, plagioclase, and locally graphite plus other opaque and trace 1 27 minerals found most commonly in cordierite, and locally in biotite and hornblende. Sci and external cycle 2 cleavage, S t are approximately coplanar. Fold and crenulation axes in Sc i, fold hinge-lines in cycle 2 cleavage (S j) and cycle 2 stretching lineations are collinear. Mineral assemblages associated with transposition cycle 1 are indicative of temperatures > 400°C. Cycle 2 cleavage S t , is composed of transposed elements Sci, Seri, ant* ^ec* *ou,lc^ cordierite. It is found in metamorphic rocks in the lower plate and in amphibolitic rocks in the upper plate of the Windy Pass thrust. S t strikes northwest, and dips moderately to the northeast roughly parallel to the regional orientation of cleavage in the Nason Terrane. These cleavage surfaces are host to NW-SE trending gently plunging stretching lineations. Post cycle 2 deformation in domain 1 includes asymmetric- mesoscopic folding of along axes which parallel stretching lineations. Some of this deformation in domain 1 is probably related to transposition of structures due to thrusting (cycle 3) and pluton emplacement (cycle 4). In a I mile wide panel adjacent to the Windy Pass thrust older structures were transposed parallel to the fault contact during transposition cycle 3 due to top-to-the-SW thrusting. Associated with this deformation are a new continuous 1 28 cleavage and NW-SE trending shallowly plunging folds and stretching lineations. Intensity of cycle 4 (syn-emplacement) deformation increases towards both pluton and thrust contacts. This is manifested by an intensely folded and recrystallized continuous cleavage, and the appearance of andalusite, followed by fibrolite and muscovite. The strain enjoyed by rocks during transposition cycle 2 indicates intensities {Es) were > 1.12 (sample 140B-T90, Table 3) while cordierite was stable throughout transposition cycle 2. This constrains metamorphism to T > 500° C; P > 1.5 CB (Kb, 1979) as well as the available strain and petrographic data i. Post cycle 2 kinematic indicators in domain I indicate horizontal shear with top-to-the-SW displacement. Strain data taken from buckle folds formed during post cycle 2 and cycle 3 deformation indicate that Es ^ .5 (buckle folds, Table 3). Metamorphism associated with this event involved the growth and recrystallization of hornblende in amphibolitic rocks and biotite in pelites; with cordierite seemingly remaining as a stable phase during the growth of garnet. This phases constrain metamorphism to T > 500° C and P > 2.5 Kb (Yardley, 1989). 1 29 Kinematics indicate that shear sense was the same during cycle 3, as they were during folding of cycle 2 structures. Stretching and contraction directions were nearly identical judging from cleavage and lineation orientations as well as results from strain analysis. Although cleavage does gradually flatten, as fold tightness increases near the thrust contact. This is probably indicative of increased shearing near the contact during thrusting. This pattern of structural transposition indicates that regional deformation was the cause of thrusting, cycle 3 deformation and increasing metamorphic grade. Transposition cycle 4 accompanied emplacement of the Mount Stuart batholith. It is localized in the contact regions of the pluton and Windy Pass thrust (fig. 7). Domainal macroscopic folding and contact metamorphism were coeval with the transposition of older structures. Cleavage was rotated into parallelism with the intrusive contact during emplacement resulting in the development of a new continuous cleavage. Contact-parallel macroscopic and parasitic mesoscopic folds occurred in domains 2-5 as did coeval stretching lineations sub-parallel to fold axes in domains 2 and 3. In a narrow zone next to the Windy Pass thrust, cycle 4 structures consist of crenulation cleavages (i. e., Scr. SccX a new composite schistosity, folds and stretching lineations. All of 1 30 these features are essentially parallel to cycle 3 structures. Fold asymmetry associated with this deformation defines an antiform with a top-to-the NE sense of displacement next to the thrust (i e., the southern limb) which changes to a top-to-the- SW sense 100-200 meters from the contact. In the eastern and western portions of domains 6 and 7 thrust parallel structures grade into coeval intrusive parallel cleavages. Shear-sense of cycle 4 asymmetric folds is erratic in domains 2 and 3. Although adjacent in domains 4 and 5 a prevalent top-to-the-SW sense of shear is manifest. Some syn- to-post emplacement aplite and pegmatite dikes are openly- folded with axes roughly collinear with folds in the country rocks indicating folding intensity decreased, but strain axes were roughly collinear during and after emplacement. Transposition cycle 3 and 4 deformation near the thrust and pluton and thrust contacts appears to have caused created a strain gradient. Two maxima exist: one on the pluton contact and one on the Windy Pass Thrust contact with a strain decrease in both directions. Fold tightness, a foliation intensity increase and succeeding generations of structures which exhibit themselves indicate this to be the case. This hypothesis is consistent with buckling + post-buckle flattening-shortening estimates: 32% (141B-T90) in folded cycle 2 cleavage as opposed to 62% and 67% in samples 111 -T90 and 462-T91 in 1 3 1 folded and crenulated cycle 4 structures. The only shortening estimate relevant to cycle 4 structures near the batholith indicates shortening in the contact aureole was ;> 25% (samples 239-T90, 41-T91). However, this shortening was imposed upon rocks that had experienced at least 1 additional transposition cycle over folded cycle 2 structures related to syn- emplacement strain near the batholith. These estimates are broadly consistent with a strain gradient in the study area although the limited number of strain markers in all units does not allow for a quantitative evaluation of this gradient. Evans and Berti (1976), Bendixen et al., (1991) and Brown and Walker (1993) constrain metamorphic conditions as lying between 3-4 Kb, and 550° and 685° C in transposition cycle 4 structures (Paterson et al., in review) Transposition 5 structures formed during syn- to post emplacement reverse faulting (Miller and Paterson, 1991). These ductile shear zones are located to the north of the study area at the northeast margins of the batholith. Here, contact metamorphism reached sillimanite grade, and was followed by the growth of kyanite, staurolite and garnet (Evans and Berti, 1986; S. R. Paterson p. comm.). Syn-emplacement deformation was coeval with the reactivation of the Windy Pass thrust in a top-to-the-NE sense. 132 while an additional generation of structures were transposed into parallelism with the pluton and thrust. Ductile deformation was strongest around bodies which were hotter and/or fluid-rich locations during regional deformation. With enhanced ductility of the wall rocks, rheological contrasts between the pluton and Ingalls Complex, and the Chiwaukum Schist became a more significant factor and localized the transposition of structures. Pluton emplacement was syn-tectonic and involved stoping and ductile flow of wall rocks. The bearing of this study on regional deformation during pluton emplacement is clear. An understanding of the relative timing of large scale tectonic events, associated structures and metamorphism is necessary to evaluate these data without ambiguity. The correct model must explain the structural characteristics observed in the study urea. Although, pluton loading is not inconsistent with the small scale structures, either orogen-normal contraction or thrusting due to a transpresssional step-over are consistent with the large and small scale structures observed. Most deformation seen in the Nason terrane occurred far to the south. After arrival at its current location the Nason terrane was tilted along a NW-SE axis ~ 15° northwest side up. However, further detailed field work coupled with the cautious use of models is necessary for a 133 coherent explanation of the observed structural characteristics in the Nason terrane to emerge. REFERENCES Anderson, J. L., Compositional variation within the high-Mg tonalitic Mount Stuart batholith, north Cascades Washington: Geol. Soc. Am. Abstracts with Programs, v. 24, p. 3. Armstrong, R.L., 1988, Mesozoic and early Cenozoic magmatic evolution of the Canadian Cordillera, eds. Clark, S.P., Burchfiel, B.C., and Suppe, J., Processes in Continental Lithospheric Deformation: Geol. Soc. Am. Special Paper 218, 55-91. Arzi, A. A., 1978. Critical phenomena in the rheology of partially melted rocks: Tectonophysics v. 44, p. 173-184. Ave’ Lallemant, H.G., and Oldow, J.S., 1988, Early Mesozoic Southward Migration of Cordilleran Transpressional Terranes: Tectonics, v. 7, p. 1057-1075. Ayerton, S., 1980, High fluid pressure, isothermal surfaces and the initiation of nappe movement: Geology, v. 8, p. 172 - 174. Babcock, R. S., and Misch, P., 1988, Evolution of the crystalline core of the North Cascades range, in Metamorphism and crustal evolution of the western United states: Rubey v. 7, ed. W.G. Ernst, Prentice Hall, p. 214-232. Barrell, J., 1907, Geology of the Marysville mining district, Montana. A study of Igneous intrusion and contact metamorphism. U. S. Geological Survey Profesional Paper 57, 178 p. Bateman, R., 1985, Aureole deformation by flattening around a diapir during in situ ballooning: the Cannibal Creek granite: Journal of Geology, v. 93, p. 293-310. Bateman, R., 1984. On the role of diapirism in the segregation, ascent and final emplacement of granitoid magmas.: Tectonophysics, v. 110, p. 311-321. Bell, T.H., 1985, Deformation partitioning and porphyroblast rotation in nietamorphic rocks: a radical re interpretation: Journal of Metamorphic Geology, v. 3, p. 109-118. Bell, T. H., Rubenbach, M. J., and Fleming, P. D,, 1986. Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development: Journal of Metamorphic Geology, v. 4, p. 37-67. Beck, M. E„ Jr. Burmester, R. F. and Schoonover, R., 1981, Paleomagnetism and tectonics of the Cretaceous Mount 135 Stuart Batholith of Washington: translation or tilt?: Earth and Planetary Science Letters, v. 567, p. 336-342. Beck, M. E., 1976, Discordant paleomagnetic pole positions as evidence of regional shear in the western cordillera of North America: American Journal of Science, v. 276, p. 694. Beck M.E. Jr., 19S4, Introduction to the special Issue between Plate Motions and Cordilleran Tectonics: Tectonics, v. 288, p. 103-105. Beck, M. E., Jr., 1980, Paleomagnetic record of plate margin tectonic processes along the western edge of North America: Journal of Geophysical Research, v. 85, p. 7115- 7 13 1. Berg, J. 1L, and Dokka, J. A., 1983, Geothermometry in the Kiglapait contact aureole, Labrador: American Journal of Science, v. 283, p. 414-434. Boyer, S. E. and Elliot, D., 1982, Thrust systems. American Association of Petroleum Geologists Bulletin, v. 6, no. 9, 1196-1230. Brandon, M. T., and Cowan, D. S. 1985, The late Cretaceous San Juan Islands-Northwest Cascades thrust system: Geological society of America, Abstracts with Programs, v. 17, no. 6. 1 3 6 Brandon M. T., Cowan D. S. and Vance, J. A., 1988, The late Cretaceous San Juan thrust system, San Juan Islands, Washington: A case history of terrane in the western cordillera: Geological Society of America Special Paper 221, 81 p. Brown, E. II., 1988, Metamorphic and structural history of the northwest Cascades; Washington and British Columbia, in Metamorphism and crustal evolution of the western United Slates: Rubey v. 7, ed. W.G. Ernst, , Prentice Mall, I 96-213. Brown, E, II., 1991, Plutonism is ihe cause of Barrovian metamorphism in the southeast Coast Plutonic Complex, British Columbia and Washington: Geological Society of America Bulletin, Abstracts with Programs, v. 23, no. 2. Brown, E. H., and Talbot, J. L., 1989, Orogen parallel extension in the north Cascades crystalline core, Washington: Tectonics v. 7, no. 8, p. 1 105-1115. Brown, M., Earle, E. M., 1983. Cordierite Bearing Schists and Gniesses from Timor, eastern Indonesia: P-T conditions and metamorphism and tectonic implications: Journal of Metamorphic Geology, v. I, p. 183-203. Butler, R. F., Gehrels, G. E., McClelland, W. C., May, S. R., and Klepacki, D., 1989, Discordant paleomagnetic poles from I 3 7 the Canadian Coast Plutonic Complex: regional tilt rather than large scale displacement? Geology, v. 17, p. 691-694. Castro, A., 1987. On granitoid emplacement and related structures. A review. Geol. Rudsch. v. 76, p. 101-124. Coney P. J., Jones, D. L. and Monger, J. W. H., 1980, Cordilleran suspect terranes: Nature, v. 288, p. 329-333. Cruden, A.R., 1988. Deformation around a rising diapir modeled by creeping flow past a sphere: Tectonics v. 7, p. 1091- 1101 . Cruden, A. R., Flow and fabric development during the diapiric rise of magma, J. Geol., v. 98, p. 681-698. Daily, R. A., 1933, Igneous rocks and the depths of the earth. McGraw-Hill: New York, London, 598 p. Daily, R. A., 1903, The mechanics of igneous intrusion, American Journal of Science v. 5, p. 107-126. Davis, G.A., Monger, G.W.H., and Burchfiel, B.C., 1978, Mesozoic construction of the Cordilleran "Collage", Central British Columbia to Central California, eds., Howell, D.G., and McDougall K.A., Mesozoic Paleogeography of the Western United States: Society of Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleogeography Symposium, v. 2, p. 1-32. 138 Engelbertson, D.C., Cox, A,, and Thompson, G.A., 1985, Relative motions between oceanic and continental plates in the Pacific Basin: Special Paper Geological Society of America, v. 206, p. 1-59. Erickson, E. H., 1977, Petrology and petrogenesis of the Mount Stuart Batholith-plutonic equivalent of the high alumina basalt association?: Contributions to Mineralogy and Petrology,, v. 60, p. 183-207. Etheridge, M. A., 1983, Differential stress magnitudes during regional deformation and metamorphism: Upper bound imposed by tensile fracturing: Geology, v. 11, p. 231-234. Etheridge, M. A., Wall, V. J., and Vernon, R. H„ 1983, The role of the fluid phase during regional metamorphism and deformation: Journal of Metamorphic Geology, v. I , p. 205-226. Essene, E. J., 1982, Geologic thermometry and barometry: Ferry, J. M. (ed.) Characterization of Metamorphism through Mineral Equilibria: Reviews in Mineralogy, v. 10, p. 153- 206. Evans, B. W, and Berti, J. W. 1983, Revised metamorphic history for the Chiwaukum schist. North Cascades, Washington: Geology, v. 14, p. 183-297. 139 Ferry, J. M., and Spear, F.S., 1978, Experimental calibration of the partitioning of Fe and Mg between biotite and garnet: Contributions to Mineralogy and Petrology, v. 66, p. 113- 1 17. Flinn, D., 1963, On the symmetry principle of the deformation ellipsoid: Geolgical Magazine, v. 102, p. 293. Getzinger, J. S., 1978, A structural and petrologic study of the Chiwaukum Schist on Nason Ridge, northeast of Stevens Pass, North Cascades , M. S. thesis, University of W ashinton. Ghosh, S. K., 1982, The problem of shearing along axial plane foliations: Journal of Structural Geology, v. 4, no. 1, p. 63- 67. Haugerud, R. A., Tabor, R. W., Stacey J. and Van der Hayden, P., 1988, What is the core of the North Cascades, Washington? A new look at the structure and metamorphic history of the Skagit Gniess of Misch (1966) Geological Society of america, Abstracts with Programs, v. 20, no. 3. Haugerud, R. A., Van Der Heyden, P., Tabor, R. W„ Stacey, J. S., Zartman, R. E., 1991, Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gniess Complex, North Cascade Range, Washington and British Columbia: Geological Society of America Bulletin, v. 103, p. 1297- 1307. Holdaway, M.J., 1971, Stability of andalusite and the aluminum silicate phase diagram: American Journal of Science v. 271, p. 97-131. Holder, M, T., 1979, An emplacement mechanism for post- tectonic granites and its implications tor their geochemical features: Atherton, M. P. and J. Tarney, eds. Origin of Granitic Balholiths: Geochemical Evidence, 116- I 28. Hopson, C. A., 1973, Mattinson, J. M., 1973, Ordovician and late Jurassic ophiolitic assemblages in the in the Pacific Northwest: Geological Society of America Bulletin, Abstracts with Programs, v, 5 no. I. Hossack, J. R , 1968. Pebble deformation and thrusting in the Bygdin area (S. Norway): Teetonophysics, v. 3, 313-339. Irving, E., and Thorkelson, D. J.. 1989, On determining paleohorizontal and latitudnal shifts: paleomagnetism of Spences Bridge Group, British Columbia: Journal of Geophysical Research, v. 93, no. BI2, p. 19, 213-19,234. Irving, E., Woodsworth, G. J., Wynne, P. J., and Morrison A., 1983, Paleomagnetic evidence of displacement from the south of the Coast Plutonic Complex: Canadian Journal of Earth Sciences,, v. 22, p. 384-398. 1 4 I Kelemen, P. B., and Ghiorso, M. S., 1986, Assimilation of peridolite in zoned calc-alkaline plutonic complexes: evidence from the Big Jim complex, Washington Cascades: Contributions to Mineralogy and Perlrology, v. 94, p. 12- 28. Kerrick, D. M,, The A bSi05 polymorphs, 199], in Ribbe, P H. ed., Mineralogical Society of America, Reviews in Mineralogy v. 22, p. 312-320. Klienspehn, K. L., 1985, Cretaceous sedimentation and tectonics, Tyaughton Methow Basin, southwestern British Columbia: Canadian Journal of Larth Science, v. 22, p. 154-174. Kriens, B., Wernicke, B., 1990, Nature of the contact zone between the North Cascades Crystalline Core and the Methow sequence in the Ross Lake area: Implications for Cordilleran tectonics. Tectonics, v. 9, p. 953-981. Largarde, J. L., and Miehard, A., 1985, Stretching Normal to regional thrust displacement in a thrust wrench shear zone, Rehamna Massif, Morocco: Journal of Structural Geology, v. 5, p. 483-492. Lisle, R. J., Brevia, 1992, Short note: Strain estimation from flattened buckle folds: Journal of Structural Geology, v. 14, no. 3, p. 369-371. I 42 Longtine, M. W., Walker, N. W., 1990, Lute Cretaceous N-S dextral shear in the west-central crystalline core, North Cascades , Washington: Geological Society of America Bulletin, Abstracts with Programs, v. 22, no. 3. Maekawa, H., and Brown, E.H., 1991, Kinematic analysis of the San Juan thrust system, Washington: Geological Society of America Bulletin, v. 103, p. 1007-1016. Marsh, B. D., 1982. On the mechanics of igneous diapirism, stoping and zone melting: American Journal of Science, v. 282, 808-833. McGroder M. P., 1989, Reconciliation of two-sided thrusting, burial metamorphism, and diachronous uplift in the Cascades of Washington and British Columbia: Geological Society of America Bulletin, v. 103, p. 189-209. McGroder, M. F,, Miller, R. B,, 1989, Geology of the Eastern North Cascades. Ed. Joseph, N. L., Geologic guidebook for Washington and Adjacent areas: Washington Division of Geology and Earth Resources Information Circular 86. McShane, D. P., Brown E. H., 1991, Age of loading of the Skagit Gniess and implications for orogeny in the North Cascades Crystalline Core: Geological. Society of America, Abstracts with Programs, v. 23, no. 2. 1 43 Menzies, M., Blanchard D., and Xenophontos, C., 1980, Genesis of the Smartville arc-ophiolite, Sierra Nevada foothills, California: Rare earth element evidence: American Journal of. Science., v. 280-A, p. 329-344. Miller, R. B., 1989, Evolution of the northeastern margin of the Cascades crystalline core of the North Cascades: Geological Society of America Bulletin, Abstracts with Programs, v. 21, no. 3. Miller, R. B. , 1985, The ophiolitic Ingalls Complex, North Cascades Washington: Geo. Soc Am. Bull., v. 96, p. 27-42. Miller, R. B., 1980, The structure, petrology and emplacement of the ophiolitic Ingalls Complex north-central Cascades, Washington: Ph. D. thesis, University of Washington. 422 P- Miller R. B., Bowering S. A., and Hoppe, W. J., 1988, New evidence for extensive Paleocene plutonism and metamorphism in the crystalline core of the North Cascades Washington: Geological Society of America Bulletin, Abstracts with Programs, v. 20, no. 6, p. 432- 433. Miller, R. B., and Bowering, S. A., 1990, Structure and chronology of the Oval Peak Batholith and adjacent rocks: Implications for the Ross Lake fault zone, North Cascades, Washington: Geological Society of America Bulletin, v. 102, p. 1361-1377. 1 44 Miller R. B., and Paterson S. R., 1992, Tectonic implications of syn- and post-emplacement deformation of the Mount Stuart batholith for mid-Cretaceous orogenesis in the North Cascades, Washington and British Columbia: Canadian Journal of Earth Sciences, v. 29, p. 255-258. Miller D. M. and Oertel G, 1979, Strain determination from the measurement of pebble shapes: a modification: Tectonophysics, v. 55, p. T il - T13. Misch, P., 1966, Tectonic evolution of the Northern Cascades of Washington- a west Cordilleran case history: Canadian Institute of Mining and Metalurgy special, v. 8, p. 101- 148. Monger, J. W. H., 1977, Upper Paleozoic Rocks of the Western Cordillera and their Bearing on Cordilleran Evolution: Canadian Journal of Earth Sciences, v. 14, p. 1832-1859. Monger, J. W. H., and Davis, G. A., 1982, Evolving Concepts of the Tectonics of the North American Cordillera: Pacific Division, A. A. A. S., p. 215-247. Monger, J. W. H., and Irving, E., 1980, Northward Displacement of North-Central British Columbia: Nature, v. 285, p. 289- 294. 145 Monger, J. W. H. and Price, R. A., 1979, Geodynamic Evolution of the Canadian Cordillera-progress and problems: Canadian Journal of Earth Sciences, v.16, p. 770-791. Monger, J. W. H., Price, R. A., and Templeman-Kluit, D. J., 1982, Tectonic accretion and the origin of two major metamorphic and plutonic welts in the Canadian Cordillera: Geology, v. 10, p. 70-75. Monger, J. W. H. and Ross, C. A., 1971, Distribution of Fusulinaceans in the Western Canadian Cordillera: Canadian Journal of Earth Sciences, v. 8, 259-277. Oldow, J. S., Bally, A. W., Ave’ Lallenient, H. G., and Leeman, W. P., 1989, Phanerozoic evolution of the North American Cordillera; United States and Canada, eds.. Bally, A.,W., and Palmer A.R., The Geology of North America: D. N. A. G., v. A, p. 139-232. Oldow, J. S., Ave Lallemant, H. G., and Schmidt, W. J., 1984, Kinematics of plate convergence deduced from Mesozoic structures in the western Cordillera: Tectonics v. 3, p. 201-227. Page, B. M., 1939, The geology of the southeast quarter of the Chiwaukum Quadrangle: Ph. D. dissertation,. Stanford California, Stanford University, 203 p. 1 46 Page, R.W. and Bell, T.H., 1986, Isotopic and structural responses of granite to successive deformation and metamorphism: Journal of Geology, v. 94, p. 365-379. Paterson, S.R., and Fowler Jr., T.K., 1993. Extensional pluton- emplacement models: do they work for large plutonic complexes?: Geology, v. 21, p. 781-784. Paterson S. R., and Fowler, T. K., Jr., 1993, Re-examining pluton emplacement mechanisms: Journal of Structural Geology, v. 15, p. 191-206. Paterson, S. R., Miller, R. B., Anderson, J. L., Lund, S., Bendixen, J. E., Taylor, N. W., Fink, T,, (in prep). Emplacement and evolution of the Mount Stuart batholith, in Swanson, D. A., Haugerud, ed., Guides to field trips: Geological Society of America field trip guide, 1994, Geological Society of America annual meeting, Seattle. Paterson, S. R., Tobisch, O.T., Bhattacharyya, T., 1989, Regional, structural, and strain analysis of terranes in the Western Metamorphic Belt, Central Sierra Nevada, California: Journal of Structural Geology, v. I 1 no. 3, p. 255-273. Paterson, S. R., and Tobisch, O. T., 1992, Rates of geological processes in magmatic arcs: implication for the timing and nature of pluton-emplacement and wall-rock deformation: Journal of Structural Geology, v. 14, 291- 300. 1 47 Paterson, S. R., and Tobisch, O. T., 1988, Using pluton ages to date regional deformations: Problems with commonly used criteria: Geological Society of America Bulletin, v. 16, p. 1108-1111. Paterson, S. R., Vernon, R. H., Fowler Jr., T. K., 1991, Aureole Tectonics, in Kerrick, D. M., ed.. Contact Metamorphism: Mineralogical Society of America Reviews in Mineralogy, v. 26, p. 673-722. Paterson, S. R., Vernon, R. M, and Tobisch, O. T.,1989, A review of criteria for the identification of magmalic and tectonic foliations in granitoids: Journal of Structural Geology, v. 11, p. 349-363. Pigage, L. C., 1975, Metamorphism of the Settler Schist, southwest of Yale British Columbia: Canadian Journal of Earth Sciences, v. 13, p. 405-421. Pitcher, W. S., and Berger, A. R., 1972, the geology of Donegal: a study of granite emplacement and unroofing. Wiley, New York, 261-296, 435 p. Pitcher, W. S., 1979, The nature and ascent and emplacement of granitic magmas: Journal of the Geological Society of London v. 136, p. 627-662. Plummer, C. C., 1980, Dynamothermal contact metamorphism superposed on regional metamorphism of the pelitic 1 48 rocks of ihe Chiwaukum Mountains area: Washington Cascades: Geological Society of America Bulletin, Part II, v. 91, p. 1627 1668. Plummer, C. C\, Geology of the crystalline rocks Chiwaukum Mountains and vicinity, Washington Cascades, University of Washington Ph. D. thesis, 204 p. Pollard, D. D., and Aydin, A., 1988, Progress in understanding jointing over the past century: Geological Society of America Bulletin,, v. 100. p. 1181-1204. Pongsapich, W, 1974, Geology of the eastern part of the Mount Stuart Batholith, central Cascades, Washington: unpublished Ph. D. thesis, Univ. of Washington. Pratt, R. M., 1958, Geology of the Mount Stuart area: Ph. D. thesis. University of Washington, 229 p. Ramsay, J. G., holding and fracturing of rocks., 1967, McGraw Mill, New York, Ramsay, J. G., and Huber, M. I.. 1987, Techniques of Modern Structural Geology; v. I. Strain Analysis, Harcourt Brace Johvanovich, London. Ramsay, J. G., and Huber, M. I., 1987, Techniques of Modern Structural Geology; v. 2, holds and hractures, Harcourt Brace Johvanovich, London. 149 Ridley, J., 1986, Parallel stretching lineations and fold axes oblique to a shear displacement direction- a model and observations. Journal of Structural Geology, v. 8, p. p. 647-653. Rubin, C. M., Saleeby, J.B., Cowan, E i. S., Brandon, M. T., and McGroder, M. F., 1990, Regionally extensive mid- Cretaceous west-vergent thrust system in the northwestern Cordillera: Implications for continent margin tectonism: Geology, v. 18, p. 276-280. Saleeby, J.B., 1983, Accretionary Tectonics of the North American Cordillera: Annual Review of the Earth and Planetary Sciences. Sanderson, D.J., and Marchini, W. R. D., 1984, Transpression, Journal of Structural Geology, v. 6, p. 449-458. Shackleton, R.M., and Reis, A.C., 1984. The relation between regionally consistent stretching lineations and plate motions. Journal of Structural Geology, v. 6, p. 111-117. Shaw, H. R., The fracture mechanism of magma transport from the mantle to the surface, in Physics of Magmatic Processes: Hargraves, R. B., Priceton Univ. Press, Princeton N. J., p. 201-264. 1 5 0 Shimamoto, T., and Ikeda, Y., 1976, A simple algebraic method for strain estimation from deformed ellipsoidal objects. 1, basic theory: Tectonophysics v. 36, p. 315-337. Southwick, D. L., 1974, Geology of the alpine-type ultramafic complex near Mount Stuart. Washington: Geological Society of America Bulletin, v. 85, p. 391-402. Speer, J. A., 1982, Metamorphism of the pelitic rocks of the Snyder Group in the contact aureole of the Kiglapait layered intrusion, Labador: Effects of buffering partial pressures of water: Canadian Journal, of Earth Sci, v. 19, p. 1888-1909. Tabor, R. W., Frizzel, V. A., Vance, J. A., and Nassler, C. W., 1984, Ages and stratigraphy of the lower-middle Tertiary sedimentary and volcanic rocks of the central Cascades Washington:Application to the tectonic history of the straight creek fault: Geological Society of America Bulletin, v. 95, p. 26-44. Tabor R. W., Zartman, R. E., and Frizzel, V. A., 1987a, Possible tectonostratigraphic terranes in the North Cascades crystalline core, Washington, in Schuster,J. E., ed., Selected papers on the geology of Washington: Washington Division of Mines and Geology Bull, v. 77, p. 107-127. Tabor, R. W., Frezzel, V. A., Whetten J. T, Waitt, R. B., Swanson, D. A., Byerly G. R., Booth D. B., Hetherington M. J., and Zartman, R. E., 1987b, Geologic map of the Chelan 30- 1 5 1 minute by 60-minute quadrangle, Washington: U. S. Geological Survey Map, I-1661. Taylor, N. W., Paterson S. R., and Miller, R. B., 1991. Structural analysis of the Chiwaukum Schist, Mount Stuart region. Central Cascades, Washington: Geological Society of Ameria., Abstracts with Programs, v. 23 no. 2. Taylor. N. W.. Paterson S. K.. and Miller, R. B„ 1992, Transposition cycles in the Chiwaukum Schist, Mount Stuart region. Central Cascades. Washington: Geology Society of America Bulletin, Abstracts with Programs, v. 24, no. S. Tobisch, O. T. and Paterson, S. R., 19X8, Analysis an d interpretation of composite foliations in areas of progressive deformation: Journal of Structural Geology, p. 7 4 3-734. Tullis, J., and Yund. R.A., 1987, Transition from cataclastic flow to dislocation creep of feldspar: Mechanisms and microstructures: Geology v. 13, p. 606-609. Umhofer, P.J., 1987, Northward translation of "Baja British Columbia" along the L.ate Cretaceous to Paleocene Margin of Western North America: Tectonics, v. 6. p. 377-394. Vance, J. A.. Dungan, M. A., Blanchard D. P.and Rhodes J. M., 1980, 'Tectonic setting and trace element geochemistry of I 5 2 Mesozoic ophiolitic rocks in western Washington: American Journal of Science, v. 280-A, p. 359-388. Vernon, R.H., 1988a. Evidence of syndeformational contact metamorphism from porphyroblast-matrix microstructural relationships: Tectonophysics v. 5, 158, p. 113 - 126. Vernon, R. H., 1988b, Microstructural evidence of rotation and non-rotation of mica porphyroblasts: Journal of Metamorphic Geology, v. 6, 595-601. Vernon, R. H., 1978, Porphyroblst-matrix microstructural relationships in deformed metamorphic rocks: Geol. Rundschau, v. 67, p. 288-303. Vernon, R. H., 1988c, Sequential growth of andalusite and cordierite porphyroblasts, Cooma Complex, Australia: Journal of Metamorphic Geology, v. 5, p. 255-269. Weber, K., 1981, Kinematic and metamorphic aspects of cleavage formation in very-low grade metamorphic slates: Tectonophysics, v. 78, p. 291-306. Whetten, J. T., Zartman, R. E., Blakey, R. J., Jones, D. L., 1980, Allochthonous Jurassic ophiolite in northwestern Washington: Geological Society of America. Bull., v. 1, p. 35 9-368. 1 5 3 Whitney, D. L., McGroder, M. F., 1989, Cretaceous crustal section through proposed Insular-Interm ontane suture, North Cascades Washington: Geology, v. 17, p. 555-558. Williams P. F. and Campagnoni, R., 1983, Deformation in the Bard areaif the Sesia Lanzo Zone, Western Alps, during subduction abnd uplift: J. Metamorphic Geology, v. 1, p. 117-140. Williams, P. F., 1985, Multiply deformed terranps- problems of correlation: Journal of Structural Geology: v. 7, 269-280. Winkler, H. G., 1979, Petrogenesis of Metamorphic Rocks: Springer-Verlag, New York, 348 p. Yardley, B. W. D.,1989, An Introduction to Metamorphic Petrology; Longman Earth Sci. Series: New York, John Wiley and Sons, 248 p. 1 5 4 INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. 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Creator Taylor, Nicholas William (author)

Core Title Structural geology of the Chiwaukum schist, Mount Stuart region, central Cascades, Washington

School Graduate School

Degree Master of Science

Degree Program Geology

Degree Conferral Date 1994-12

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag geology,OAI-PMH Harvest

Format masters theses (aat)

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Paterson, Scott R. (committee chair), Anderson, Joseph Lawford (committee member), Davis, Gregory A. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-5729

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