Structural and petrologic analysis of the North Fork terrane, central Klamath Mountains, California

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Content STRUCTURAL AND PETROLOGIC ANALYSIS OF THE NORTH FORK TERRANE CENTRAL KLAMATH MOUNTAINS, CALIFORNIA by Clifford Joseph Ando A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Geology) June, 1979 UMI Number: DP28549 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI DP28549 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 UNIVERSITY OF SOUTHERN CALIFORNIA T H E G R A D U A TE S C H O O L U N IV E R S IT Y PARK LO S A N G E LE S . C A L I F O R N IA 9 0 0 0 7 This dissertation, written hy ........ CL IFFORD _ .JOSEPH _ AND 0........... under the direction of hX§.... Dissertation Com mittee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillment of requirements of the degree of D O C T O R O F P H I L O S O P H Y vPh..b. Gr sU. e A 55 3 . f k O T (Qr ' -k O C- Dean DISSERTATION COMMITTEE Chairman ACKNOWLEDGEMENTS X wish to thank the members of my dissertation committee, Drs. G. A. Davis, J. L. Anderson, R. H. Osborne and D. A. Dows for their instruction and support. Special thanks must go to Greg Davis, whose guidance through the years has contributed immeasurably to the com pletion of this work. I express my gratitude also to my mother and Cris for their patience and to Tracy and Rosalie Groesbeck of Trinity Center, Cali fornia for their hospitality and dear friendship. My love and gratitude go to Katharine George of Forks of Salmon, California, whose friendship has enriched my life. Support for this dissertation was kindly provided in the form of grants from the Society of Sigma Xi and the Penrose Fund of the Geological Society of America and an internship with the California Division of Mines and Geology. CONTENTS Page ACKNOWLEDGEMENTS .............................................. ii LIST OF FIGURES................................. vi LIST OF PLATES ..... .................................... viii ABSTRACT....................................................... xi INTRODUCTION .... 1 Purpose of Study ........................................ 1 Previous Investigations ................... . .......... 5 Geography................................... 9 Accessibility ......................... ....... 9 Topography...................................... 9 Methods of Investigation ................................ 12 DESCRIPTION OF ROCK U N I T S ...................................... 13 Explanation of the Structural Sequence ................. 13 Gabbro .................................... ....... 16 General Statement .................................... 16 Lithologic Description ................................ 16 A g e ..................................................... 25 Diabasic (Microgabbroic) Rocks ........ . 25 General Statement ........................... ..... 25 Lithologic Description ................................ 28 A g e ..................................................... 33 iii Page Lower Volcanic Unit........................................ 36 General Statement....................... 36 Lithologic Description ................................ 36 A g e ..................................................... 46 Ultramafic Rocks ........................................ 53 General Statement .................................... 53 Lithologic Description ................................ 54 Age and Emplacement Mechanism..........................67 Upper Volcanic Unit........................................ 68 General Statement .................................... 68 Lithologic Description ................................ 69 A g e ..................................................... 91 Chert and Argillite........................................ 92 General Statement .................................... 92 Lithologic Description.............. 93 A g e ..................................................... 97 Carbonate Rocks .......................................... 103 General Statement .................................... 103 Lithologic Description ................................ 104 A g e .................................................... 110 Late Igneous Intrusions .................................. 113 General Statement................... 113 Lithologic Description ................................ 114 A g e .................................................... 115 iv Page PETROLOGY OF SELECTED IGNEOUS R O C K S ....................... ,.117 Approach to Study ........................... ...... 117 Analytical Methodology .......... , ........ ..... 118 Chemistry of Samples ........ ..... 119 Introduction ......... , 119 Chemical Distinction of Units and Comparison to Ophiolites ............... 125 Petrogenesis................ . , 148 STRUCTURAL ELEMENTS . . . 152 Macroscopic Structures ................. 152 Regional Folding .......... ............. 152 Faults Parallel to Regional Structural Trends .... 155 Faults Transverse to Regional Structural Trends , . , 157 Mesoscopic Structures .................. 158 Minor Folds ................. ............ 158 Foliations ................... ............ 159 Lineations ................. ............. 165 Microscopic Structures ..................... ...... 165 THE STRUCTURAL SEQUENCE AS A COGENETIC SUITE . . . 167 EVOLUTION OF THE NORTH FORK TERRANE IN THE LIGHT OF REGIONAL TECTONICS , 174 Speculations on the North Fork Terrane as an Accretionary Assemblage ............... ......... 174 Geologic History and Paleotectonic Significance of the North Fork Terrane ...... 176 REFERENCES................................. 190 v LIST OF FIGURES Page Figure 1. Map of the Klamath Mountains Province, California and Oregon, showing major geologic subdivisions ................................ 2 Figure 2. Location map showing major counties, towns, roads, and drainages in the vicinity of the study a r e a ................................................ 6 Figure 3. Generalized composite structural sequence of the North Fork terrane..............................14 Figure 4. Plot of Ti02 vs. MgO for upper volcanics, lower volcanics, diabase and gabbro ................. 126 Figure 5. Plot of FeO* vs. MgO for upper volcanics, lower volcanics, diabase and gabbro ................ 128 Figure 6. Plot of Sr vs. Ti02 for upper volcanics, lower volcanics, diabase and gabbro ................ 130 Figure 7. Total alkali vs. Si02 plot for upper volcanics, lower volcanics and diabase...........................132 Figure 8. AI2O3 vs. normative plagioclase plot of Irvine and Baragar (1971) for lower volcanics and diabase................................................135 Figure 9. FeO* vs. FeO*/MgO plot for lower volcanics, diabase and gabbro................................... 137 Figure 10. Si02 vs. FeO*/MgO plot for lower volcanics, diabase and gabbro................................... 139 Figure 11. Ti02 vs. FeO*/MgO plot for lower volcanics, diabase and gabbro................................... 141 Figure 12. AFM plot for upper volcanics, lower volcanics, diabase and gabbro................................... 144 Figure 13. Ne-01-Di-Hy-Q plot for upper volcanics, lower volcanics and diabase ................................ 146 vi Page Figure 14. Contoured equal area projection of poles to measured bedding attitudes in chert and limestone.................................... 160 Figure 15. Contoured equal area projection of poles to measured slip fibre foliation in serpentinite (A) and foliation in chert and argillite ( B ) ........................ 162 Figure 16. Regional sketch map of the California Klamaths showing major lithologic assemblages, thrust faults, klippe and fensters ............ 178 Figure 17. Schematic cross section of the central Klamath Mountains showing major structural units, thrust faults and the North Fork antiform...........................................180 vii LIST OF PLATES Page Plate 1. Geologic map of a part of the North Fork terrane, Siskiyou and Trinity Counties, central Klamath Mountains, California .......... Pocket Plate 2. Geologic cross sections of the North Fork terrane .......................................... Pocket Plate 3. Photograph of steep central Klamath topography........................................ 10 Plate 4. Photograph of resistant outcrops of gabbro . . . 17 Plate 5. Photograph of cumulate layering in gabbro .... 20 Plate 6. Photomicrograph of gabbro.......................... 22 Plate 7. Photograph of exposure of ridge-forming diabase............................................. 26 Plate 8. Photograph of typical outcrop of massive diabase............................................ 29 Plate 9. Photomicrograph of coarse diabase .................. 31 Plate 10. Photomicrograph of highly altered fine grained diabase .................................... 34 Plate 11. Photograph of Potato Mountain on the Salmon- Trinity divide .................................... 37 Plate 12. Photograph of pillowed basalt of the lower volcanic unit at Pony Buttes..................... 40 Plate 13. Photograph of interpillow red chert in the lower volcanic unit................................ 42 Plate 14. Photograph of interpillow grey carbonate in the lower volcanic unit............................ 44 Plate 15. Photomicrograph of coarse grained basalt of the lower volcanic unit............................ 47 Pag 49 51 55 57 60 63 65 71 74 76 79 81 84 86 89 94 Photomicrograph of fine grained basalt of the lower volcanic unit ..................... Photomicrograph of highly altered basalt of the lower volcanic unit ..................... Photograph of slip fibre serpentinite near the Salmon-Trinity divide ..................... Photograph of lenticular foliation in serpentinite .................................... Photomicrograph of completely serpentinized harzburgite .................................... Photograph of partially serpentinized harzburgite in upper Plummer Creek ......................... Photomicrograph of partially serpentinized harzburgite .................................... Photograph of pillowed basalt of the upper volcanic unit .................................. Close-up photograph of basalt pillow at Black Bear Creek ................................ Photomicrograph of non-porphyritic basalt of the upper volcanic unit ..................... Photomicrograph of plagioclase-phyric basalt of the upper volcanic unit ..................... Photograph of aphyric, amygdaloidal basalt of the upper volcanic unit ..................... Photograph of volcaniclastic fraction of the upper volcanic unit ....................... Photomicrograph of fine grained crystal-rich tuffaceous sediment from the upper volcanic unit............................................ Photomicrograph of fine grained tuffaceous sediment from the upper volcanic unit ........ Photograph of bedded radiolarian chert ........ Page Plate 32. Photomicrograph of recrystallized grey chert . . . 98 Plate 33. Photomicrograph of chert breccia ................. 100 Plate 34. Photomicrograph of limestone showing coated grains with concentric structure ................. 105 Plate 35. Photomicrographs of a limestone block from basalt...............................................108 Plate 36. Photomicrographs of texture of limestone clasts from chert-argillite breccia ..................... Ill x ABSTRACT The North Fork terrane at the latitude of the central Klamath Mountains is an internally disrupted and folded ophiolitic sequence, The sequence of rocks is structural and consists from lower to higher levels of: gabbro, diabase, a lower unit of mafic volcanic rocks, chert, an upper unit of mafic volcanic and volcaniclastic rocks, and chert plus chert-argillite and carbonate rocks. Serpentinized ultra- mafic rocks basically separate gabbro and diabase from higher units but locally separate other units. The structural sequence has been folded into a large west-verging antiform, here named the North Fork antiform; its core is occupied by gabbro. Whole rock chemical analysis and field relationships of gabbro, diabase, lower volcanics and upper volcanics suggest that these units are, respectively, the upper (homogeneous) part of an ophiolitic plu- tonic sequence, the fine-grained fractionated derivative of the parent melt for the gabbro, mafic oceanic crust, and alkalic seamount material. A cogenetic relationship is favored for rocks comprising the structural sequence, the sequence originating as a coherent frag ment of oceanic crust atop which a seamount was constructed and over lying oceanic sediments deposited. Rocks of the North Fork ophiolitic sequence are poorly dated and uncertainties remain as to their primary ages. The sequence is prob ably Late Paleozoic (?) or Triassic (?) in age. Disruption and folding of the North Fork assemblage took place prior to Middle to Late Jurassic time. These earlier structures, which most likely developed during accretion of the assemblage at a Late Triassic to Middle Jurassic convergent plate boundary, are truncated by high level, west-verging intraplate thrust faults of Middle to Late Juras sic age. The North Fork ophiolitic sequence contains no arc volcanic rocks nor do the immediately overlying strata contain arc-derived sedi mentary or pyroclastic material. The sequence, which appears to have formed in an oceanic environment removed from the influence of arc volcanism, was probably accreted to the continent prior to the develop ment of a Middle to Late Jurassic arc complex. At this time North Fork rocks likely occupied an intra-arc position when telescoping of the broad volcanic archipelago along intraplate thrusts produced the imbricate, thrust-bounded geometry for the major lithic assemblages of the central Klamaths. Apparently, no exposed subduction zones are present in the central Klamath Mountains. INTRODUCTION Purpose of Study The area selected for study is in Siskiyou and Trinity Counties, California, in the central part of the Klamath Mountains, a large north-south-trending geomorphic and geologic province in northern California and southwestern Oregon. In large aspect, the rock units of the Klamath Mountains are part of a long belt of Paleozoic and Mesozoic "eugeosynclinal" rocks in the western part of the North American Cordillera and are thought to be geologically coextensive with the northern Sierra Nevada of California. Irwin (1960) originally subdivided the Klamath province into four major belts or subprovinces called, from west to east in ascending structural sequence, the western Jurassic belt, the west ern Paleozoic and Triassic belt, the central metamorphic belt and the eastern Klamath belt (see Fig. 1). Boundaries between the subpro vinces are now known to be major eastward-dipping thrust faults. The study area is within the North Fork terrane, one of three terranes proposed by Irwin (1972) to comprise the southern part of the western Paleozoic and Triassic subprovince. Irwin interprets the North Fork to be the easternmost and structurally highest of the three thrust fault-bounded terranes, the other two terranes being, in descending structural order, Hayfork and Rattlesnake Creek (Fig. 1). 1 Figure 1. Map of the Klamath Mountains province, California and Oregon, showing the major geologic subdivisions of Irwin (1960) and the original western Paleozoic and Triassic belt terrane designations of Irwin (1972). 2 124° 123° ORE. 43° Port v Orford Ore. Cal’ifT 42° Yreka — area of this study 41° « W PJ CQ oA Eureka w 40° 60 0 km RC Rattlesnake Creek terrane Hf Hayfork terrane NF North Fork terrane terranes of the subdivided part of the western Paleozoic and Triassic belt of Irwin (1960) 3 The area chosen for geologic mapping is in the northern part of the western Paleozoic and Triassic subprovince (WPTS), a sector which was not subdivided by Irwin into the aforementioned terranes (Fig. 1). Rock types of the North Fork terrane in the type locality along the canyon of the Trinity River include: gabbro, diabasic rocks, mafic volcanic rocks, ultramafic rocks, chert, argillite, minor carbo nate rocks and local sandstone and pebble conglomerate. This assem blage was interpreted by Irwin (1972) to be a dismembered ophiolite suite. The major goals of this study were to map a largely unstudied northward extension of the North Fork terrane, determine whether or not the assemblage of rocks represented a disrupted ophiolitic se quence and to see how the structural and petrologic data generated from the study might affect interpretations of the evolution of the central Klamath Mountains as a geologic and tectonic province. Recent regional geologic syntheses (Hamilton, 1969; Burchfiel and Davis, 1972; 1975) address the structural history of Klamath rock units in the light of modern plate tectonics theory. A widely accepted view is that the Klamaths are an accreted terrane, the em placement of which was a consequence of convergent plate interactions during Paleozoic and Mesozoic time. Because the Klamath Mountains contain belts of Paleozoic and Mesozoic rocks whose relationships to one another are relatively unobscured by late Mesozoic plutonic intrusions, they have yielded data which impose important constraints on evolving models for the tectonic history of the western U. S. 4 Cordillera. Furthermore, the generally well preserved relationships lend themselves to detailed studies of the structural response of oceanic rocks to convergent plate tectonics. Previous Investigations The present study is concerned with rock units found in the fifteen minute Cecilville and Salmon Mountain Quadrangles (Fig. 2), north of the type North Fork terrane locality of Irwin (1972). These units were not formally included in the type description but are spatially continuous with and lithologically similar to the original units. The assemblage of rocks is structurally overlain to the east by central Metamorphic subprovince rocks along an eastward-dipping thrust fault. To the west the assemblage is separated from a terrane of mostly chert and argillite by an eastward-dipping zone of faulted and sheared serpentinized ultramafic rocks. At the time of Irwin’s (1972) formal subdivision of the south ern part of the western Paleozoic and Triassic subprovince into North Fork, Hayfork and Rattelsnake Creek terranes, geologic mapping of some of the North Fork rock types had been accomplished. Cox (1956) mapped gabbro, diabasic rocks, serpentinite and chert in the Helena Quadrangle (see Fig. 2), the type locality for Irwin's North Fork terrane. Cox (1967) later elaborated on the structure of the rocks and the nature of their juxtaposition below amphibolite facies rocks of the central metamorphic subprovince (Salmon Hornblende Schist) along his East Fork fault. 5 Figure 2. Location map showing major counties, towns, roads and drainages in the vicinity of the study area. Shown also are names and boundaries of 15 minute topographic quad rangles in and near the study area. 123° FKS. OF SALMON QUAD. S A W Y E R S BAR QUAD. \ f/' Callahan.^fr^c'' / X % ^ \ Sawyers x B a r ^ 4/ * *' "pXl Forks of Salmon h// i C ecilvilie S A L M O N M T N . QUAD C E C I L V IL L E QUAD R O N S ID E M T N . Q U A D . H E L E N A QUAD, Trinity Lake s / / French Gulch ? weavervi ^Hayfork 15 J km 7 Several studies done primarily in the Cecilville and Salmon Moun tain Quadrangles to the north of the Helena Quadrangle serve as important precursors to the present work. Davis and Lipman (1962) and Davis (1968) correlated rocks of the Stuart Fork Formation with the eastern part of the western Paleozoic and Triassic subprovince. The Stuart Fork Formation was interpreted to be the transposed and metamorphosed equivalent of greenstone and chert in Irwin’s North Fork terrane where they occur beneath the thrust contact with the overlying central metamorphic subprovince. Trexler (1968) and Davis (1968) mapped gabbro, diabasic rocks, mafic volcanic rocks, ultramafic rocks, chert and limestone in the western part of the Cecilville Quadrangle that are the northern ex tension of the eastern part of Irwin’s (1972) North Fork terrane. Hotz and others (1972) and Cox and Pratt (1973) show reconnais sance maps depicting North Fork equivalent rocks (gabbro, serpenti nite, metabasalt) in the vicinity of the Salmon-Trinity divide. They also show chert and chert-argillite west of the serpentinite zone that marks the western boundary of the North Fork terrane. The chert and chert-argillite are the northward extension of Irwin’s (1972) Hayfork terrane. The mapping for the present study in part overlaps areas mapped in reconnaissance by Hotz and others (1972) and Cox and Pratt (1973) and areas mapped in somewhat greater detail by Trexler (1968) and Davis (1968). Geography Accessibility The area of this study is a region of generally poor accessibi lity in the south-central part of the Klamath Mountains. The main road into the area is the Siskiyou County road from Callahan to Forks of Salmon (Fig. 2). Branching both north and south from this road are numerous dirt logging roads which allow fair access into the northern part of the area. The southern part of the map area is within the Salmon-Trinity Alps Wilderness Area and the only access is by poorly maintained hiking trails. Quickest access to this part of the area is from the east by trail along the Salmon-Trinity divide; one can drive from the settlement of Cecilville to Five Dollar Camp west of Cecil Point Look out and reach the trail along the divide by means of a short hike. Topography Topography in the study area is characteristic of most of the central Klamaths; it is young and generally steep with relief in many stream canyons reaching 900 to 1200m (PI. 3). The Salmon-Trinity divide trends northwest-southeast across the central part of the area and is a major drainage divide. The highest parts of the area are along this divide (Dees Peak at 2105m = 6907 ft. being the highest). The lowest elevation in the study area, about 457m (1500 ft.) is in the northwestern part along the South Fork of the Salmon River. 9 Plate 3. Photograph of steep central Klamath topography, showing best outcrops along ridge crests; snow covered Trinity Alps in background. 10 11 Major ridges and spurs trend north and south off the Salmon- Trinity divide. In many cases they are flat-topped and of similar elevation suggesting they are remnants of an old erosion surface. Streams flowing north from the Salmon-Trinity divide empty into the South Fork of the Salmon River which flows northwestward, joins the North Fork of the Salmon River at Forks of Salmon and eventually con verges with the Klamath River at Somes Bar. Streams draining southward from the divide empty into the Trinity River which flows to the west and eventually joins the Klamath River. Methods of Investigation The North Fork terrane was studied using a combination of de tailed geologic mapping, structural analysis, petrography and whole rock chemical analysis. Mapping of most of the area was carried out at a scale of 1:21,120. The extreme northern part of the area was mapped in reconnaissance at a scale of 1:62,500. All major rock units were studied petrographically. Select samples of mafic volcanic rocks, diabase and gabbro were analyzed for major elements plus select trace elements. The whole rock analyses have provided classificatory information and data by which mapped rock units can be chemically discriminated. Some of these data have been used to infer the origin of certain of the rock units. 12 DESCRIPTION OF ROCK UNITS Explanation of the Structural Sequence The assemblage of rocks studied is not an intact stratigraphic sequence. Original stratal continuity has been disrupted and the rock units tectonically shuffled; indeed many of the contacts between units are observable to be surfaces of dislocation. The sequence has been folded, with gabbroic rocks occupying the core of a large west- verging antiform here named the North Fork antiform Csee geologic map, PI. 1). At the latitude of the study area, the antiform plunges north ward and contacts between rock units in both the upright and over turned limbs generally dip steeply eastward. Although the rock units have been tectonically disrupted, a dis- cernable structural sequence does persist throughout most of the area. The structural succession (see Fig. 3 and explanation. PI. 1) from lower to higher levels is roughly as follows: gabbro (gb), diabasic rocks (db), a lower basaltic volcanic unit (V^), chert (S^), an upper volcanic unit of basalt and mafic volcaniclastic rocks (V2)» chert and argillite (S-^) and carbonate rocks (Us) . Partially to completely serpentinized ultramafic rocks (NFum, um) separate core gabbro and diabase from higher units in most places but locally occupy a variable structural position. The structural sequence within the mapped area comprises an aggregate thickness of about 4500m and will be described 13 Figure 3 Generalized composite structural sequence of the North Fork terrane. aggregate thickness 4-5 km 7/11 //// // // //////,, / / / m m ///" ' NFum i/1 1 'I mV i i 1 1 n //V// V| V n \ n \ 1 1 n \ A ,db A 0 / '/M 8 1 D f\ 7 1 1 fault- fault- fault bedded grey chert, argillite, slaty argillite; locally red and tuffaceous near base massive limestone; locally stratified with basalt cobble conglomerate near base s_____________________________ massive to pillowed vesicular basalt; locally contains volcaniclastic material bedded chert and argillite foliated serpentinized harzburgite; locally contains early tectonite fabric massive to pillowed nonvesicular basalt; contains interpillow chert and limestone; fault slices of serpentinized ultramafics along contact with diabase altered massive diabasic rocks; contains veinlets and stringers of plagioclase and quartz, plagiogranite near contact with gabbro medium to coarse grained homogeneous gabbro; locally contains cumulate layering 15 beginning with the lowest observed unit. Gabbro General Statement Gabbro (gb) is the lowest observed structural unit in the area. Its area of outcrop is narrow and elongate, occupying the central part of the antiformal core. This same unit was described by Trexler (1968) as his Cody Creek pluton in the northwestern Cecilville Quadrangle. He speculated that the outcrop length of the gabbro could be as much as 64km (40 miles) if his unit matched similar gabbros mapped by Cox (1956) in the Helena Quadrangle to the south. It now appears on the basis of mapping by Davis (1968) in the southwestern part of the Cecil ville Quadrangle and the present author in the southern part of the Salmon Mountain Quadrangle, that the units of Trexler (1968) and Cox (1956) are in fact the same. Lithologic Description Best exposures of the gabbro are found in the vicinity of hill 4902 in the north-central part of the area and on either side of the Salmon-Trinity divide between Potato Mountain and Mary Blaine Mountain. In hand specimen the gabbro is essentially a plagioclase-clinopyroxene rock with a coarse xenomorphic granular texture. Outcrops are resis tant (see PI. 4), vary from dark green to light colored depending upon the amount of plagioclase relative to pyroxene. On weathered surfaces the plagioclase has a whitish chalky appearance and the pyroxene is 16 Plate 4. Photograph of resistant outcrops of gabbro on the Salmon- Trinity divide. 17 18 light green and satiny. Most exposures of the gabbro are massive and compositionally homo geneous. However, many outcrops exhibit an approximately planar disruption fabric that is not of magmatic origin. Macroscopically this structure appears as domains of intense disruption alternating with or grading into domains of little disruption. In the thin section the disrupted part has the appearance of a fluxion foliation with grain size of the minerals reduced by comminution. In a very few localities the gabbro is observed to contain pri mary igneous layering (PI. 5), possibly of cumulate origin. The layering is expressed by an alternation of plagioclase-rich and pyroxene-rich bands and a general orientation of the long dimensions of the pyroxene crystals parallel to the banding. Ambiguous grading in the mineral bands is present at one locality and does suggest a stratigraphic top direction but this feature was not observed with enough frequency to significantly aid mapping. The pyroxene orienta tion was noted in several other localities where obvious mineralogic banding was absent, however. A typical thin section of the gabbro (PI. 6), shows the rock to be composed of about equal amounts of subhedral to anhedral plagio- clase and subhedral clinopyroxene with accessory magnetite, or in some samples, ilmenite. Grain size range of most of the plagioclase is 0.5 to 2.0mm with some grains as large as 3 to 4mm. The clinopyroxene averages 1.0 to 2.0mm. Textural evidence suggests plagioclase crystal lized first, followed by clinopyroxene and the opaque oxide. 19 Plate 5. Photograph of cumulate layering in gabbro, showing clinopyroxene-rich (dark) and plagioclase-rich (light) zones. Scale is 15,2cm long. 20 Fa* 21 Plate 6. Photomicrograph of gabbro showing typical xenomorphic granular texture; PL = plagioclase, CP = clinopyroxene. Field of view is 1.65 x 1.10mm; crossed polarizers. 22 23 The plagioclase is weakly to strongly zoned and is in general badly altered, having suffered moderate to complete saussuritization. The saussuritization is expressed by patches of fine grained epidote and albite partially or completely replacing plagioclase grains. Some samples of the gabbro show the plagioclase to be sericitized, that is, replaced by a fine grained x^hite mica, probably paragonite. Extinction angles of plagioclase were measured on a flat stage and in many cases fall into the oligoclase-adesine range but others indicate the plagioclase to be of labradoritic composition. The lower extinction angles observed are probably due to partial albitization of the original plagioclase. The clinopyroxene in the gabbro is an augite 2VZ= 50 to 60 degrees) as determined by optical methods. Larger cyrstals are un twinned and have unaltered cores with rims of magmatic hornblende formed at the expense of the pyroxene. Smaller crystals are replaced by either magmatic hornblende (with a pyroxene crystal habit) and/or fibrous uralite. In many thin sections the uralitization has been more or less complete, converting the pyroxene to a fine grained ag gregate of acicular amphibole, probably tremolite-actinolite. Rocks structurally and/or stratigraphically below the gabbro have not been observed within the map area. It is tempting to speculate, because the gabbro unit does contain some cumulate layering, that olivine-bearing gabbros might be present at unexposed lower levels. Such rocks have not been observed, however, and throughout the map area the mineralogic composition of the gabbro remains monotonously 24 consistent. Age The age of the gabbroic rocks is not known. Both Cox (1956) and Trexler (1968) believed the gabbro to be intrusive into, and hence younger than, surrounding diabasic rocks, also of unknown age. The age relationships will be discussed further in the description of the diabasic rocks. Diabasic (Microgabbroic) Rocks General Statement Diabasic or microgabbroic rocks (db) are very closely associated spatially with, and crop out more or less continuously both east and west of the gabbro on the flanks of the core of the antiform. The best continuous exposures underlie the high-standing north-south trending ridge that separates Methodist Creek and West Fork Plummer Creek drainages near the Salmon-Trinity divide (see PI. 7). The diabase unit of this study correlates in part with the west ern volcanic unit of Trexler (1968) and with diabases mapped by Cox (1956) as part of his mafic complex of Limestone Ridge. Cox reported pillow structures in his diabase unit and in his discussion described the unit as having both hypabyssal and volcanic characteristics. In mapping for this study it was found that volcanic and hypabyssal units could be distinguished in the field and so were mapped separately. 25 Plate 7. Photograph of exposure of ridge-forming diabase north of the Salmon-Trinity divide. View is to the south; West Fork Plummer Creek is at the lower left. 26 27 Lithologic Description Outcrops of the diabasic rocks are resistant, dark green to black and generally massive. Some compositional and grain size variation exists but no obvious dike or sill structures were observed in areas of excellent exposure. In fact, this general lack of internal struc ture proves to be characteristic of the diabase; primary or secondary foliations were not observed except near some contacts where a fine phyllonitic-type layering parallel to the contact was seen, this un doubtedly being the result of tectonic juxtaposition of the rock units. In hand specimen the diabase is medium to fine grained, exhi biting lath-shaped plagioclase and interstitial hornblende and/or clinpyroxene producing a subophitic texture. Most exposures are crosscut by veinlets and pockets of quartz or plagioclase plus quartz (PI. 8). Mineralogically the diabase appears to be similar to the gabbro. The contact between the diabase gabbro is observable along the South Fork of the Salmon River between Forks of Salmon and Cecilville, just upstream from the confluence of Smith Creek and the South Fork. Here the contact is an irregular zone of decrease in grain size from gabbro to diabase over 3 to 4m. The concentration of plagioclase- quartz veinlets and pockets appears to be greater in the diabase near this contact zone. A typical thin section (PI. 9) displays the subophitic texture despite the fact that most samples are altered. The 0.5 to 1.0mm lath-shaped plagioclase crystallized first. It is moderately to 28 Plate 8. Photograph of typical outcrop of massive diabase showing cross-cutting quartz-plagioclase veinlets. 29 30 Plate 9. Photomicrograph of coarse diabase showing subophitic texture; PL = plagioclase, CP = clinopyroxene. Field of view is 1.65 x 1.10mm; crossed polarizers. 31 32 completely saussuritized and constitutes 40 to 50 percent of the section. Extinction angles (flat stage) were measured on the freshest samples and indicate compositions around An^Q (sodic andesine). It is likely, however, that the An content of the original plagioclase was higher and that the lower values obtained are a result of the alter- action. The 0.2 to 0.4mm clinopyroxene is an augite (2VZ= 50 to 60 degrees); it fills the interstices between the plagioclase and accounts for 25 to 30 percent of the section. In most samples either magmatic hornblende has partially to fully replaced the clinopyroxene, or the pyroxene has been altered to a mass of fibrous uralite amphibole. Some of the hornblende has been partially replaced by chlorite. Some samples are finer grained (plagioclase 0.2 to 0.3mm; clino pyroxene 0.1 to 0.2mm) and have an intergranular texture with clino pyroxene, iron oxides, chlorite and epidote (?) filling interstices between lath-shaped plagioclase. Accessory magnetite and apatite are present in all samples and several sections are cut by veinlets of quartz or quartz plus fine grained epidote (PI. 10). Age The age of the diabase is unknown. Trexler (1968) considered it (his western volcanic unit) to be older than the gabbro by virtue of its being intruded by the gabbro. Cox (1956) formulated the same interpretation but elaborated by saying that the gabbro, diabase and associated hornblende gabbros of his mafic complex of Limestone Ridge represented a case of magmatic differentiation. The writer agrees in 33 Plate 10. Photomicrograph of highly altered fine grained diabase showing quartz-bearing veinlet transecting the slide; Q = quartz. Field of view is 1.65 x 1.10mm; crossed polarizers. 34 35 part with this latter interpretation but maintains that the igneous ages of the diabase and gabbro cannot be widely separated because of the gradational contact between them. The two units are probably co genet ic if not comagmatic. Lower Volcanic Unit General Statement The lower volcanic unit (V occurs within the map area as two large tectonic slices. Both slices partially underlie the Salmon- Trinity divide and form resistant, high-standing areas or peaks (PI. 11). Potato Mountain in the western part of the area forms the bulk of one of the slices and the Stud and Pony Buttes areas in the southern part of the area form the other. As was mentioned previously, these volcanic rocks were mapped separately from the diabasic rocks with which they are closely assoc iated spatially. The lower volcanic unit at Stud and Pony Buttes would correlate with part of the "western volcanic" unit shown in the western part of the map by Davis (1968). Also, by inference these volcanic rocks would probably correlate with the pillowed volcanic rocks described by Cox (1956) as part of his diabase unit farther to the south. Lithologic Description Very good exposures of the lower volcanics are found both on and around Potato Mountain and Pony Buttes where the outcrops have a 36 Plate 11. Photograph of Potato Mountain on the Salmon-Trinity divide showing resistant outcrop of lower volcanic unit; exposure is a fault-bounded slice. View is to the north west. 37 38 knobby appearance and weather reddish or red-brown. Internal disrup tion is readily apparent and is expressed as a phacoidal to semi- planar foliation. This foliation is best expressed near tectonic con tacts where anastomosing surfaces of slip are observed to surround zones that are more or less texturally coherent. Pillow structures are well preserved at several localities, espe cially on Pony Buttes (PI. 12) and on the south flank of Potato Mountain. Most of the pillows range in size from about 0.3m to 1.0m; they are dense and have a dark, very fine grained rind. At the Pony Buttes locality, pillows about 0.1 to 0.2m in size are interlayered with the larger pillows. No vesiculation or amygdaloidal texture was observed in any of the pillows from the lower volcanics. The absence of vesticulation in pillows is thought to indicate eruption of the basalts in deep water (Jones, 1969; Moore, 1975), probably greater than 700m (Moore and Schilling, 1973). The lower volcanic unit contains interpillow sedimentary material at several localities. Both chert (PI. 13) and carbonate (PI. 14) were observed. The chert is either dark grey or brick red and the carbonate is medium grey. Thin sections of sampled interpillow sedi ments reveal that recrystallization has in general destroyed most of the primary texture. Hand specimens of the lower volcanics are dense, light to medium green, very fine grained and sometimes cut by fine veinlets of quartz. Quite often the mineralogy is not observable even with a hand lens but 39 Plate 12. Photograph of pillowed basalt of the lower volcanic unit at Pony Buttes. Pillows are non-vesicular. 40 41 Plate 13. Photograph of interpillow red chert in the lower volcanic unit. Scale is 15.2cm long. 42 43 Plate 14. Photograph of interpillow grey carbonate in the lower volcanic unit. Scale is 15.2cm long. 44 - ~ --- - --- -4 J in some samples small laths of plagioclase can be seen. Thin sections of reveal that the primary mineralogy is par tially preserved. Coarser grained samples (PI. 15) contain up to about 15 percent clinopyroxene (sub-calcic augite?) phenocrysts ranging in size from 0.5 to 1.5mm; some are twinned. The groundmass consists of saussuritized subhedral to euhedral zoned plagioclase, granular clinopyroxene, magnetite and secondary chlorite and quartz. Finer grained samples from pillow localities (PI. 16) contain a very small (less than 1) percentage of clinopyroxene and/or plagioclase pheno crysts ranging in size from 0.2 to 0.4mm. Clinopyroxene appears to have crystallized before plagioclase. The groundmass is either a felted mass of altered plagioclase microlites with subsidiary granular clinopyroxene and iron oxide or a very fine grained mixture of plagio clase, clinopyroxene, iron oxide and chlorite that is difficult to identify. Most samples contain fine veinlets of quartz plus chlorite or quartz plus calcite (PI. 17) and some samples contain small, ir regularly-shaped vugs filled with calcite or chlorite and epidote(?). Age The age of the lower volcanic unit is at present not known. Interpillow sedimentary material collected from within the study area has yielded no datable faunas. Recrystallized remains of radiolaria have been found in the red chert but the primary morphology of these has been destroyed. A recent discovery from the southern part of the North Fork terrane may bear on the question of the age of the lower volcanics, however. Irwin and others (1977) and Irwin (1977) report 46 Plate 15. Photomicrograph of coarse grained basalt of the lower volcanic unit showing large clinopyroxene phenocrysts; CP = clinopyroxene, PL = plagioclase: dark material is mostly chlorite and magnetite. Field of view is 1.65 x 1.10mm; crossed polarizers. 47 48 Plate 16. Photomicrograph of fine grained basalt of the lower vol canic unit from a pillow locality showing wispy plagio clase microlites (light colored, lath-shaped grains); dark material is fine grained chlorite and magnetite; CP = granular clinopyroxene, CC - calcite amygdule. Field of view is 1.65 x 1.10mm; crossed polarizers. 49 50 Plate 17. Photomicrograph of highly altered basalt of the lower volcanic unit showing irregular, discontinuous veinlets of quartz plus calcite transecting the section (light colored); dark material is mostly fine grained magnetite and chlorite; CP = granular clinopyroxene. Field of view is 1.65 x 1.10mm; crossed polarizers. 51 52 that red cherts associated with mafic rocks of the North Fork ophio- lite are probably late Triassic in age based on the presence of the radiolarian Eptingium as determined by E. A. Pessagno, Jr. It is pos sible that the lower volcanic unit (V^) correlates with some of the ophiolite-related volcanic rocks from the southern part of the North Fork terrane. Ultramafic Rocks General Statement Serpentinized ultramafic rocks (NFum, urn) comprise a major portion of the North Fork terrane. Within the study area they are present mostly in two more or less continuous, north-south-trending belts. One occupies most of the prominent Plummer Creek drainage in the east ern part of the area and the other occupies most of the Methodist Creek drainage in the western part of the area. In addition, small slices of serpentinite occur interleaved with other rock units and satellitic to the main ultramafic bodies. Both zones of ultramafic rocks cross the Salmon-Trinity divide and extend southward into the Helena Quadrangle. The eastern zone is continuous with serpentinites interpreted by Cox (1956) as intruding his North Fork fault. The western zone is continuous with serpenti nites occupying his Twin Sisters fault. Subsequent use of North Fork terminology was made by Trexler (1968) and Davis (1968) in the Cecil- ville Quadrangle. However, because the author interprets the ultra mafic zones to a folded, once continuous sheet, he prefers nonapplication of separate terms for Twin Sisters and North Fork zones. Instead, these will be referred to as western and eastern outcrop zones of the North Fork ultramafics (NFum), respectively. Lithologic Description Major outcrops of serpentinized ultramafic rocks usually underlie areas of subdued topography and relatively sparse vegetation, a rela tionship that is common in the Klamath Mountains. Soil cover, where present, is generally red-brown and poorly developed. Outcrop expres sion of the serpentinites varies according to degree of serpentiniza- tion and state of disruption. Near contacts, which are usually sharp and show evidence of movement, the serpentinites possess a prominent slip-fibre foliation parallel to the contacts (PI. 18). This slip- fibre-type serpentinite is generally a mottled green and yellow color and is pervasively slickensided. Such rocks are 100 percent serpen tinized and all textural features of the ultramafic protolith have been destroyed. Small slices of serpentinite interleaved with other rock units are almost exclusively of slip-fibre type. Farther from contacts the outcrops are more resistant and have a rounded blocky appearance. Weathering imparts to these outcrops a red-brown or greyish-brown color. Disruption is less pervasive than near contacts and is expressed either as a lenticular, non-penetrative type foliation (PI. 19) or as relatively undisrupted blocks surrounded by slip-fibre serpentinite. Hand specimens are dark green to black on a fresh surface and despite the almost complete serpentinization of the protolith, some hint of the primary mineralogy is left in the form of 54 Plate 18. Photograph of slip fibre serpentinite near the Salmon- Trinity divide. Outcrops typically lack vegetation; large resistant block is a knocker of the lower volcanic unit. 55 56 Plate 19. Photograph of lenticular foliation in serpentinite showing elongate, blocky appearance. Scale is 15.2cm long. 57 58 bastite pseudomorphs. The bastites range in size from several mm to 2 or 3cm, have a surface sheen representing cleavage faces of the original orthopyroxene and often show evidence of kinking. Examination of thin sections from both outcrop zones of the North Fork serpentinites reveals that the ultramafic protolith for both units was probably harzburgite. Although most samples are 95 to 100 percent serpentinized, well developed mesh texture and relict orthopyroxene can be seen in unsheared samples. A typical thin section (PI. 20) contains 15 to 20 percent bas tite pseudomorphs averaging 3 to 5mm, 80 to 85 percent mesh textured serpentine and accessory magnetite and red-brown spinel. Veinlets defining the mesh texture contain the magnetite, which is probably secondary, and some cross-fibre chrysotile. The red-brown spinel forms cores rimmed by an opaque spinel (probably magnetite) and is very likely a relict primary mineral. Of the two zones of ultramafic rocks, the western zone is more disrupted. In the Methodist Creek drainage the western zone is one of complex lithologic intermixing of serpentinized peridotite, chert, volcanic rocks and carbonate rocks. Some of the smaller mafic blocks have been rodingitized and their original textures are almost unrecog nizable. In contrast, the eastern zone of serpentinites appears to be less internally disrupted, reaching a maximum outcrop width of over 1.5km in Plummer Creek drainage. In the upper part of Plummer Creek drainage, there is a fabric in the ultramafic rocks which appears to predate the slip fibre 59 Plate 20. Photomicrograph of completely serpentinized harzburgite showing mesh textured serpentine (S) and bastite pseudo- morph (B); bastite shows relict cleavage of the original orthopyroxene. Field of view is 1.65 x 1.10mm; crossed polarizers. 60 61 foliation. It is defined by a pyroxene foliation which strikes at a high angle to the pervasive slip fibre foliation (PI. 21). Also seen in some outcrops is a lineation defined by trains of chrome spinel in the plane of the foliation. No compositional layering or cumulate textures were observed in the ultramafics. It is significant that samples from these outcrops clearly reveal the mineralogic composition of the ultramafic protolith for the ser- pentinites, confirming conclusions reached by examining completely serpentinized rocks. Thin sections KM-125 (PI. 22) and KM-158 are 80 to 85 percent serpentinized and contain relict magnesian olivine and enstatite. Mesh textured serpentine and remnant olivine grains make up most of the section. The olivine exhibits undulatory extinct ion and appears to have been an anhedral aggregate of more or less equant interlocking grains averaging 1 to 1.5mm in diameter. The 2 to 4 mm orthopyroxene is partially serpentinized, contains fine ex solution lamellae of clinopyroxene and is rimmed by talc. The exso lution lamellae parallel (100) and the lamellae from grain to grain appear to be aligned in about the same direction suggesting that the (100) of the orthopyroxene is systematically oriented. The pyroxene grains are bent or kinked about axes oriented approximately perpen dicular to the long dimensions of the exsolution lamellae in thin section. The sections also contain subsidiary red-brown Cr-spinel rimmed and transected by magnetite and veinlets containing secondary magnetite. Olivine at one time made up the bulk of the samples and orthopyroxene accounts for about 15 to 20 percent, making the rock 62 Plate 21. Photograph of partially serpentinized harzburgite in upper Plummer Creek showing remnant orthopyroxene weather ing in relief; outcrop contains early tectonic fabric defined by pyroxene orientation. Scale is 15.2cm long. 63 64 Plate 22. Photomicrograph of partially serpentinized harzburgite (thin section KM-125) showing remnant orthopyroxene (OP), olivine (OL) and chrome spinel (SP) ; section is 70-80% serpentinized. Field of view is 1.65 x 1.10mm; plane light. 65 66 a harzburgite. The modal composition of these ultramafic rocks (dominant olivine, subsidiary orthopyroxene, accessory Cr-spinel) is similar to composi tions reported for Alpine-type harzburgites; examples include Burro Mountain (Loney and others, 1971), Troodos (Moores and Vine, 1971) and Vourinos (Moores, 1969). The harzburgites do not lend themselves to petrofabric study due to the high degree of serpentinization. Based on the modal composi tion, the lack of obvious cumulate textures or compositional banding, and on the presence of the preferred mineral orientations, the harz burgites might reasonably be interpreted as Alpine-type. Similar features have been described by Moores (1969, 1973), Loney and others (1971), Ave Lallement and Carter (1970) and Medaris (1972) for other ultramafic rocks considered to have been syntectonically recrystal lized in the solid state. If fresh enough samples could be found, a study of the olivine microfabric in the harzburgites would confirm or refute this interpretation. Age and Emplacement Mechanism Cox (1956), Trexler (1968) and Davis (1968) all considered the North Fork serpentinites to have been intruded along the North Fork fault zone. The mechanism of intrusion was not clearly defined al though Davis at that time preferred a crystalline mush-type of injection. Cox interpreted the serpentinites to postdate the gabbro and diabase in the Helena Quadrangle and to have been emplaced concommitantly with development of his North Fork Fault. 67 Although it is agreed that the age of emplacement of the ser- pentinite postdates the ages of the rocks in contact with it, it cannot be assumed that the ultramafic protolith for the serpentinites also postdates the enclosing rocks. The age of the harzburgite is entirely open to question. The absence of pronounced thermal effects along contacts of the serpentinites militates against high temperature intrusion and the presence of rodingitized country rocks or zones of reaction attributed to low temperature (Coleman, 1967) argues in favor of cold emplace ment of the North Fork ultramafics. The writer favors the interpreta tion that the serpentinites are discrete fault (possibly thrust) bounded slices that have been modified by later faulting during folding of the structural sequence. Two interpretations which differ rather subtly have been discussed. In the first, serpentinites from an unknown source are thought to intrude a developing fault zone. In the second, serpentinized harzburgites of probable Alpine-type are jux taposed prior to folding by thrust (?) faulting against rocks with which they may have been closely related. That is, contrary to the ideas of Cox (1956) Trexler (1968) and Davis (1968) the writer believes the serpentinized ultramafics are genetically related to the gabbro, diabase and volcanic rocks now in fault contact with them. Upper Volcanic Unit General Statement Rocks belonging to the upper volcanic unit are the same as those 68 assigned to the "western metavolcanic" unit of Trexler (1968) and Davis (1968) in the western Cecilville Quadrangle. In the Cecilville Quadrangle the upper volcanics comprise a lenticular unit reaching about 1.5km in maximum outcrop width which pinches out to the south and extends an unknown distance north of the study area. The unit in part depositionally overlies bedded cherts of the sedimentary unit ("western chert" unit of Trexler, 1968 and Davis, 1968) and in part tectonically overlies the North Fork serpentinites. The upper vol canics are in turn unconformably overlain by limestone and chert belonging to S^. In western part of the area, rocks of the upper volcanics plus associated limestones and chert are found as slices within western outcrops of the North Fork serpentinites. Recalling the observation that the western zone of ultramafics is more disrupted than the east ern zone, it is apparent that the upper volcanic unit and its assoc iated sedimentary rocks east of the eastern zone of ultramafics are in an intact stratigraphic relationship whereas this same section is only partly preserved within and near the western zone. Nevertheless the upper volcanic unit is distinctive and is recognizable even in tectonic slices where primary textures are preserved. Lithologic Description In the field two distinct sub-units of the upper volcanics are discernable: 1) a massive to pillowed basalt fraction and 2) a mafic volcaniclastic fraction. Contacts or contact zones between the fractions were generally not mapped but observed localities of 69 pillowed or volcaniclastic fractions are noted on the geologic map (PI. 1). The best and most easily accessible exposures of the upper vol canics are along the South Fork of the Salmon River at Black Bear Creek and between Plummer Creek and Limestone Bluffs. Both pillowed and volcaniclastic rock types are observable at these localities. Good exposures of the upper volcanics are also found on the Salmon- Trinity divide east of Election Camp and along the divide trail below hills 5374 and 6037 northwest of Potato Mountain. The massive to pillowed basalt fraction appears to constitute the bulk of the upper volcanic unit. Outcrops are dark green to black and are generally less disrupted than outcrops of the lower volcanics (V^). Hand specimens vary from dense to scoriaceous; some are fresh and some (especially those from within the western zone of North Fork ultramafics) are highly weathered. Macroscopically, the mineralogy is generally not detectable except for plagioclase laths visible in some hand samples. Pillow basalts were reported by Trexler (1968) as constituting a part of the upper volcanics (his "western metavolcanic" unit). The localities reported were: 1) in French Creek near the Salmon River 2) along the South Fork of the Salmon River opposite Plummer Creek 3) along the South Fork of the Salmon River at Black Bear Creek and 4) on the Salmon-Trinity divide east of Election Camp. The best pillowed basalts observed by the author in this part of the map area are at the Black Bear Creek locality (PI. 23) where pillows averaging 70 Plate 23. Photograph of pillowed basalt of the upper volcanic unit at the confluence of Black Bear Creek and the South Fork of the Salmon River. Tops and bottoms of some pillows are clearly evident. 71 72 0.5 to 0.6m across are well exposed in roadcuts. Tops are generally upright and most dip gently eastward. The basalt is dense and fine grained and, in striking contrast to the pillows in the lower volcanic unit, contains abundant vesicles and calcite amygdules near the rims of the pillows (PI. 24). The vesicles are from one to several mm in diameter and are arranged in concentric zones near the pillow margins. The features resemble the concentrations of subspherical to radially elongate besides described by Moore and Schilling (1973) in Reykjanes Ridge basalts erupted in water depths less than about 700m. Pillows were also observed in the upper volcanic unit on the Salmon-Trinity divide and along the divide trail in the vicinity of hill 5374 north west of Potato Mountain. Here the tops of the pillows are upright and horizontal or face westward. Eastward facing pillows were observed in a tectonic slice of the upper volcanics within the western zone of the North Fork serpentinites immediately west of Potato Mountain. Microscopic textures of the basalt vary from sample to sample. However, three basic textural types seem to characterize the upper basalts. The first type (PI. 25) is nonporphyritic and contains about 50 percent 0.4 to 0.5mm subhedral to anhedral, zoned, poorly twinned plagioclase, 25 percent 0.1 to 0.4mm elongate, titaniferous (?) clino- pyroxene and 15 percent opaque oxide, probably magnetite or ilmenite. Textural relationships suggest that clinopyroxene crystallized before plagioclase. The plagioclase is slightly saussuritized and less than 1 percent of the clinopyroxene is altered to amphibole. The rock has an intersertal texture with secondary chlorite and zeolitic minerals 73 Plate 24. Close-up photograph of basalt pillow at Black Bear Creek showing concentric rings of vesicles and calcite amygdules near the pillow margins. Knife is about 9cm long. 74 75 Plate 25. Photomicrograph of non-porphyritic basalt of the upper volcanic unit showing elongate clinopyroxene (CP), sub- hedral plagioclase (PL) and magnetite (MT). Field of view is 1.65 x 1.10mm; crossed polarizers. 76 77 being interstitial. The second type (PI. 26) is a plagioclase-phyric basalt con taining 25 to 30 percent 1.0 to 3.5iran subhedral phenocrysts. The tex ture is intergranular, the groundmass composed of clinopyroxene, an opaque oxide, plagioclase, secondary chlorite plus zeolitic (?) mater ial and secondary hematite. The plagioclase phenocrysts were first to crystallize and were followed by roughly concurrent crystallization of the groundmass plagioclase and clinopyroxene. Some of the opaque minerals (ilmenite) are large and skeletal and the rock is slightly vesicular, some of the vesicles being filled with chlorite. The third textural type (PI. 27) was taken from pillow localities and is an almost aphyric amygdaloidal basalt containing about 50 per cent 0.1 to 0.4mm microlitic plagioclase, 10 to 15 percent opaques, 25 percent mixed secondary chlorite, opaques and zeolitic (?) mater ial and 10 to 15 percent irregular to spherical calcite amygdules. The chlorite and zeolites appear to replace the primary mafic minerals and are interstitial to the felted, lath-shaped plagioclase. Some of the opaques are large and skeletal, perhaps suggesting they were at one time skeletal mafic minerals. Some samples appear to contain altered granular clinopyroxene and some contain chlorite filled vugs in addition to the calcite amygdules. The mafic volcaniclastic fraction of the upper volcanic unit con sists of basalt breccias, lapilli tuffs and crystal rich to lithic volcaniclastic sedimentary rocks. It is not known whether these rocks represent a continuous horizon within the upper volcanic unit or even 78 Plate 26. Photomicrograph of plagioclase-phyric basalt of the upper volcanic unit showing plagioclase phenocrysts (PL); groundmass is lath-shaped plagioclase (light colored) plus chlorite, magnetite and hematite (dark material). Field of view is 1.65 x 1.10mm; crossed polarizers. 79 80 Plate 27. Photomicrograph of aphyric, amygdaloidal basalt of the upper volcanic unit showing plagioclase microlites (PL) and calcite amygdule (CC); dark material is mostly mag netite, hematite and chlorite. Field of view is 0.60 x 0.40mm; crossed polarizers. 81 - - -------- 82 whether they occupy a consistent stratigraphic position. Volcani- clastic sediments were observed below the pillow basalts at the Black Bear Creek locality along the South Fork of the Salmon River, however; this is consistent with Trexler’s (1968) observation that the tuffa- ceous part of his "western metavolcanic" unit occupied the lower part of the section. Outcrops (PI. 28) of the tuffaceous rocks vary from light to medium green to purplish-brown in color and have a distinctive clastic appearance. Clasts are angular to subangular and vary in size from less than 1mm to over 4 or 5cm. The finer grained rocks can properly be called crystal-rich tuffaceous sediments and the coarser grained rocks lithic lapilli tuffs and volcanic breccias. Bedding or layering is present in the finer grained rocks; indi vidual beds measure about 1cm to several cm in thickness. Bedding was only observed in a few localities, notably at Black Bear Creek on the South Fork of the Salmon River and along the divide trail northwest of Potato Mountain. Bedding was not observed in the coarser grained volcaniclastic rocks. Microscopic study of the finer grained volcaniclastic confirms that they are crystal-rich or fine grained lithic tuffs. Typical samples (PI. 29) contain 30 to 50 percent subangular to angular lithic fragments or crystal fragments ranging in size from less than 1mm to about 2cm. Invariably the lithic clasts are basaltic volcanic rocks having textures very similar to the basalts elsewhere in the upper volcanic unit (i.e.aphyric, plagioclase-phyric microporphyritic, 83 Plate 28. Photograph of volcaniclastic fraction of the upper volcanic unit showing angular clasts. Knife is about 9cm long. 84 85 Plate 29. Photomicrograph of fine grained crystal-rich tuffaceous sediment from the upper volcanic unit; PL = plagioclase, MV = altered mafic volcanic lithic fragments, CHL = chlor- itic matrix. Field of view is 1.65 x 1.10mm; plane light. 86 87 vesicular to amygdaloidal). Crystal fragments are mostly plagioclase but some samples contain considerable clinopyroxene fragments as well. The matrix for the clasts consists of mostly chlorite with varying amounts of microcrystalline quartz, iron oxide, granular epidote (?) and calcite. Much of the material is very fine grained and may be altered fine ash. An interesting rock collected from the upper volcanic unit below hill 5374 northwest of Potato Mountain is worth mentioning because of its environmental significance. It is a laminated volcaniclastic rock with 1 to 2cm thick layers. Some of the layers are plagioclase crystal-rich tuffaceous sediments similar to those just described. Other layers, however, contain a mixture of about 10 percent plagio clase crystal fragments 0.1mm or less in size and 10 to 15 percent recrystallized radiolaria (?) and spines (?) 0.1mm in diameter and several times this in length (see PI. 30). These fragments are in a matrix which appears to be silt or clay and contains some cryptocrys talline material. The spines are composed of microcrystalline quartz and are concentrated with the plagioclase in layers. It seems thus that the volcaniclastic rocks of the upper vol canic unit are not all of pyroclastic origin. The texture and com position of the rock just described suggests it may be partly of pyro clastic origin and partly of epiclastic origin producing a tuffaceous mudstone. Trexler (1968) interpreted his "western metavolcanic" unit as a complex eruptive sequence deposited as islands or in a subaqueous 88 Plate 30. Photomicrograph of fine grained tuffaceous sediment from the upper volcanic unit showing spines (?) and radio laria (?) composed of microcrystalline quartz; section cut perpendicular to bedding. Field of view is 1.65 x 1.10mm; plane light. 89 L _______ ------- 90 environment. G. A. Davis (personal communication) suggests the unit might be a seamount. These interpretations seem entirely reasonable; in addition, the writer subscribes to the belief that the upper vol canic unit in part contains the interface between a purely volcanic environment and a dominantly sedimentary environment. Further aspects of this will be described in the section on the carbonate rocks. Age The age of the upper volcanic unit is not known. It has not been dated directly and no fossil-bearing interpillow sedimentary material has been found. Carbonate rocks with preserved original texture are found both depositionally overlying the upper volcanics on the east ern limb of the North Fork antiform and intercalated with volcani- clastic sediments of V2 on the western limb. As yet, no datable fossils have been recovered from these carbonates. 91 Chert and Argillite General Statement Occupying a position high in the structural succession is a se quence of chert, argillite and slaty argillite (S^)* This unit corre lates with the "western chert" unit of Trexler (1968) and Davis (1968) and is continuous with metachert and slate reported by Cox (1956, 1967) east of his North Fork fault zone and west of his Twin Sisters zone in the Helena Quadrangle. Sedimentary rocks of S-^ extend both east and west of the map area. In an eastward (and structurally upward) direction, rocks designated S-^ are overlain by a discontinuous (?) zone of argillite and isolated blocks of glaucophane-lawsonite blueschists first recognized by Davis (1968). To the west, the S^ sedimentary unit is terminated against an inferred boundary in part occupied by glaucophane schist (G. A. Davis and P. H. Cashman, personal communication). Due to the fact that some chert and argillite occur as tectonic slices in serpentinite or as isolated bodies in apparent depositional contact with volcanic rocks, no attempt was made to establish a rigor ous stratigraphic column for S]_. Correlation of isolated chert bodies with more continuous expanses of chert and chert-argillite was generally impossible because of the monotonously similar appearance of chert and chert-argillite outcrops wherever they were observed. Instead, all chert and argillite structurally below the zone of glauco phane schists was mapped as S-^. 92 Lithologic Description Bedded chert plus argillaceous chert and slaty argillite com prises the bulk of the Sj sedimentary unit. In the eastern part of the area, bedded chert predominates. Most of these cherts are strati- graphically higher than the upper volcanic unit (V2) but some chert lies below the upper volcanics in an apparent intact stratigraphic position. In addition, there occur fault slices of bedded chert either within the North Fork serpentinites or along the contact of the ser- pentinites with the upper volcanics. Individual chert beds average 3 to 8cm in thickness and are sepa rated in many outcrops by thin partings of argillaceous material (PI. 31). Pinch and swell structure is common as is discontinuous bedding. Some exposures contain subisoclinal to isoclinal folds averaging about 0.3 to 1.0m limb to limb. Hinges of most folds trend generally north or south and plunge at low angles. These will be dis cussed more fully in the section on minor folds. Most of the cherts are grey and occur in outcrops traceable for a maximum of a few tens of meters. Two occurrences of red chert were noted within the study area, however. The first is as part of a se quence of bedded chert and tuffaceous chert stratigraphically above limestones (Ls) in the northeastern part of the area. Outcrops were observed along the road from Cecilville to Forks of Salmon at Limestone Bluffs. Red chert also occurs near the mouth of Matthews Creek, about 5km north of the Limestone Bluffs locality (Irwin and others, 1978). 93 Plate 31. Photograph of bedded radiolarian chert showing typical thin bedded character with argillaceous partings between chert beds; outcrop contains open folds. 94 95 The second occurrence of red chert is as interpillow sedimentary material in basalts of the lower volcanic unit (V^). These cherts are structurally below the North Fork serpentinites, however, and are not included in the S-^ sedimentary unit. In the western part of the map area, the S-^ sedimentary unit appears to contain much more argillite relative to eastern exposures and is more disrupted. Slices and fragments of S-^ bedded chert and argillaceous chert occur in the western outcrop zone of the North Fork serpentinites and along the contact between the lower volcanics (V^) and the diabase (db). West of the western zone of ultramafics, bedded chert in is relatively rare. The well bedded chert within and around the serpentinites gives way westward to slaty argillite and siliceous argillite containing an admixture of different lithologic types. Specifically, fragments of pillow basalt, altered mafic vol canic rocks, bedded chert, chert breccia and gabbro are present in the argillite along Hotelling Ridge at the extreme western edge of the map area. A penetrative phacoidal-type foliation is present in the argi llite creating a distinct north-south structural grain. The terrane of admixed argillite, chert and volcanic rocks ex tends westward well past Forks of Salmon where pronounced lithologic intermixing and penetrative deformation appears to be a characteristic style for the unit (Cashman, 1974). The present state of the chert- argillite units west of the map area is thought to be a result of both primary sedimentary mixing of rock types and superimposed penetrative deformation (P. H. Cashman, 1977, personal communication). 96 Thin sections of S-^ bedded chert show that both the red and grey varieties are composed of microcrystalline to cryptocrystalline quartz (PI. 32). Veinlets of slightly coarser grained quartz cut the sections and small (0.15 to 0.2mm) circular zones of microcrystalline quartz are abundant in some samples. The zones (circular in thin section but probably representing cross sections of subspherical or cylindrical objects) may represent recrystallized radiolaria. Sections more rich in argillite contain a dark, muddy matrix some of which has been recrystallized to very fine grained white mica. Some of these argillite-rich rocks are actually breccias. A sample collected along a logging road just southeast of hill 3681 is a matrix supported breccia containing about 30 percent recrystallized chert clasts and 15 percent plagioclase-phyric microporphyritic basalt clasts averaging about 1cm in diameter (PI. 33). The clasts are angular and are imbedded in a dark matrix. An anastomosing foliation similar to that which is present in larger outcrops of the argillite cuts the section and appears to be flattened about the chert and basalt clasts. Age Age relationships in have been determined mostly by studies of fossil assemblages. Rocks of the Hayfork and North Fork terranes of Irwin (1972) were at one time thought to be of late Paleozoic (Late Pennsylvanian to Early Permian) age based on fusilinids and forami- nifers found in pods and lenses of limestone occurring in both terranes (Irwin, 1972; Coleman and Irwin, 1974). Such limestone bodies are 97 Plate 32. Photomicrograph of recrystallized grey chert showing radiolaria (?) which are now microcrystalline quartz (R) and a fine grained quartz veinlet transecting the section. Field of view is 1.65 x 1.10mm; crossed polarizers. 98 99 Plate 33. Photomicrograph of chert breccia showing angular clasts of recrystallized chert (CH) and mafic volcanic rock (MV); matrix is argillite-rich. Field of view is 1.65 x 1.10mm; crossed polarizers. 100 101 generally not traceable along strike for more than several hundred meters and most appear isolated in a matrix of tuffaceous chert and argillite. Recent work by Irwin and others (1977) indicates that earlier conclusions regarding the ages of rock units in the Hayfork and North Fork terranes were probably in error. Radiolarian faunas extracted from siliceous tuff and chert from the southern part of the North Fork terrane have been dated as Early to Middle Jurassic (probably post- Pliensbachian) in age. Furthermore, it has become more reasonable to interpret the limestone pods and lenses of late Paleozoic age as exotic blocks derived from an older terrane (Irwin, 1977). This interpreta tion is consistent with the conclusions reached by Irwin and others (1977) and Irwin (1977) that ophiolite-related volcanic rocks in the southern North Fork terrane are probably Late Triassic in age based on radiolaria found in red chert associated with those volcanics. Additional age control on cherts of the sedimentary unit is provided by radiolarians and conodonts collected from the undivided northern part of the western Paleozoic and Triassic subprovince near the mouth of Matthews Creek, north of the Browns Meadow fault (see PI. 1). Irwin and others (1978) report a Middle Permian age for these faunas. These are apparently the only known cherts of Paleozoic age in the western Paleozoic and Triassic subprovince. 102 Carbonate Rocks General Statement Carbonate rocks (Ls) crop out discontinuously within the map area (PI. 1). The largest continuous exposure was described by Davis (1968) and Trexler (1968) in the western Cecilville Quadrangle as their west ern limestone unit. In the Cecilville Quadrangle the carbonate unit occupies a position low in the sedimentary section; along the north ward extension of Limestone Ridge, it is deposited unconformably on basalts of the upper volcanic unit (V£). A basal conglomerate of rounded basalt pebbles and cobbles and a zone of basalt inclusions in the lower part of the carbonate are visible along the Salmon-Trinity divide trail east of High Prairie. Bedded cherts of S-^ deposition- ally overlie the carbonate unit which pinches out to the south at the Salmon-Trinity divide and to the north just north of the South Fork of the Salmon River. At both of these localities, cherts of S-^ di rectly overlie basalts of V2* Other exposures of carbonate occur north of the confluence of Methodist Creek and the South Fork of the Salmon River in the northern part of the map area and in the vicinity of hill 5374 northwest of Potato Mountain in the western part of the area. At these localities the carbonate are intercalated with basalt and volcaniclastic sedi ments of the upper volcanic unit (V£). 103 Lithologic Description The carbonates are limestones and are characteristically medium grey and massive but locally are dark grey and well stratified; stra tification varies from 0.3 to 0.6m beds to centimeter scale lamina tions with thin argillaceous partings. The units appear to be mostly undisrupted but in places contain minor folds whose hinges trend generally north. Where clearly exposed the folds are overturned west ward . Texturally the limestones from different exposures are very similar. Many samples are too recrystallized to classify accurately but some samples possess enough primary textural attributes to allow an attempt at classification. Using the classification of Folk (1962b) the samples can be placed in the sparse biomicrite, packed biomicrite or poorly washed biosparite categories; they contain a range of from 15 to 70 percent allochemical material, 15 to 20 percent micritic matrix, 10 to 40 percent sparry calcite and 15 to 30 percent microspar. Many samples whose textural features are not destroyed by recrys tallization appear to be matrix supported. Framework grain types are varied but by far dominating the others and often constituting as much as 60 percent of the sample are 0.3 to 1.0mm subround to elliptical (in cross section) coated grains (ooids) with well developed concen tric structure (PI. 34). Most are intact but in a few samples some of the ooids are broken. Nuclei for the grains are mostly sparry cal cite and pelmatazoan fragments. 104 Plate 34. Photomicrograph of limestone showing coated grains with concentric structure; rock is matrix supported; matrix for framework grains is microspar. Field of view is 1.65 x 1.10mm; plane light. 105 106 Other identifiable allochems include pelmatazoan fragments (mostly columnals) and large (0.3 to 1.0mm) benthic foraminifera (?) The remaining framework grains are mafic volcanic clasts in less than 1 percent to about 5 percent abundance. These volcanic clasts are tex- turally very similar to the basalts from V£. The sparry calcite in the samples occurs as large patches, nuclei for coated grains and as veinlets cross-cutting the thin sections. It is believed that a large fraction of the sparry calcite is of secon dary origin. The microspar in all of the samples appears dirty and is interstitial to the framework grains. It is interpreted that much if not all of this microspar was at one time micritic matrix and has suf fered recrystallization. Two minor occurrences of limestone were discovered within the study area. The first is as a small lens in basaltic volcanic rock northwest of Potato Mountain in the western part of the area. Here the limestone is a sparse biomicrite containing about 45 percent allo chems, 20 to 25 percent cloudy, patchy microspar and 10 percent sparry calcite as veinlets. The rock appears to have been matrix supported, the framework grains being large (2.5 to 6.0mm) pelmatazoan columnals and an assortment of fragments of brachiopods, gastropods, bryozoans and small foraminifera (PI. 35). The second additional limestone occurrence is as pebble to cobble sized clasts in chert-argillite breccia. Samples were collected from near hill 3681 in the western part of the area where the chert-argil lite breccia described in the preceding section crops out. Samples 107 Plate 35. Photomicrographs of a limestone from basalt showing variety of allochems: (A) bryozoan fragment, (B) pel matazoan columnal, (C) recrystallized brachiopod fragment, (D) small foram. Field of view for A-D is 1.50 x 1.0mm; all are in plane light. 108 109 were also collected from near the limestone locality in the northern part of the area, the description of which appears earlier in this section. The breccias containing the limestone clasts are grain supported with argillite and carbonate matrix. Additional clast types in the breccias are mafic volcanic lithic fragments and chert fragments. The breccias are interstratified with chert-argillite and in one locality, silty carbonate rocks. The limestone clasts themselves are mostly grain supported packed biomicrites containing mostly 0.2 to 1.0mm coated grains (ooids) plus minor percentages of pelmatazoan fragments and mafic volcanic lithic clasts as framework grains and cloudy microspar (interpreted to have been micritic matrix) interstitial to the framework grains (PI. 36). These clasts are very similar texturally to the in-place limestone des cribed previously with one major difference. Many of the coated grains, unlike the grains in the other limestones, contain recrystallized chert and mafic volcanic clasts as nuclei. Age As was discussed in the previous section, fusilinid-bearing car bonate blocks occurring in argillite matrix of the North Fork terrane are regarded by Irwin (1977) as exotic blocks of late Paleozoic age. Good summaries of locations of limestone bodies and genera of fossil forms can be found in Irwin (1972) and Irwin and Galanis (1976). The Early to Middle Jurassic ages obtained by Irwin and others (1977) for North Fork chert and siliceous tuff suggest that the bulk of the Plate 36. Photomicrographs of texture of limestone clasts from chert-argillite breccia: (A) coated grains in a matrix of microspar, (B) coated grains with both chert (CH) and mafic volcanic (MV) nuclei, (C) coated grain with a mafic volcanic nucleus. Field of view for A is 1.65 x 1.10mm; B is 0.50 x 0.40mm; C is 0.50 x 0.30mm; crossed polarizers. Ill 112 sedimentary section within the terrane is of Mesozoic rather than Pale ozoic age. If these age relationships are correct, it is probable that some of the carbonate rocks in the present study area are not of late Pale ozoic age. Specifically, the large limestone body at Limestone Bluffs rests depositionally on basalt of the upper (V2) volcanic unit and carbonate rocks are intercalated with basalt and volcaniclastic sedi ments of V2 in the northwestern and extreme northern parts of the map area. By inference these volcanic rocks are of early Mesozoic age based on the discoveries discussed earlier. As yet no datable faunas have been found in these limestones. A point of interest that should be noted is that crinoids of questionable Triassic age were reported from the Limestone Bluffs area by Seyfert (1965); to date, the writer has been unable to confirm this. Late Igneous Intrusions General Statement Several igneous intrusions (gr) which postdate rocks of the North Fork terrane are present at the latitude of the study area. The intru sions take the form of either small stocks or dikes that appear to be related to the stocks. The stocks are resistant, form highstanding topographic areas or peaks and occupy areas of 2.5 square km or less. Three such stocks were mapped. The first underlies hill 3681 in the western part of the area and the third the high-standing topography east of Election Camp in the southeastern part of the area. 113 Dike rocks cutting across foliations, bedding or contacts in older rock units can be seen where there is good exposure within the map area. No attempt was made to systematically map these dikes, but where observed they are generally subvertical with a northeast-southwest trend. They appear to be related to the igneous stocks because they are similar in mineralogic composition to them and occur in swarms or in greater concentration near them. Lithologic Description The stocks were not studied in detail although their most ob vious characteristics were noted in the field. Each pluton contains a variety of rock types ranging from pyroxene diorite to quartz and biotite-bearing hornblende diorite and exhibits some textural varia tion. Some primary igneous layering and ill-defined primary folia tion are present. By far the most common mineral assemblage visible in a random hand specimen is hornblende-plagioclase with accessory opaques and sphene. The texture varies from xenomorphic to hypautomorphic granu lar, the latter type being more common and exhibiting elongate to prismatic hornblende and tabular plagioclase. The plagioclase has a chalky appearance due to extensive alteration. Thin sections of these rocks typically contain 5 to 30 percent 2.0mm relict clinopryoxene (augite), 15 to 45 percent 1.0 to 3.0mm hornblende, 40 to 50 percent 1.0 to 2.0mm plagioclase (andesine or sodic labradorite) and 0.1 to 0.2mm accessory opaques (probably magnetite), sphene, apatite and epidote. Plagioclase, clinopyroxene, 114 apatite and the opaques appear to have crystallized first, followed by hornblende and sphene. Much of the hornblende is a magmatic reaction amphibole replacing the clinopyroxene but some with acicular to elong ate prismatic habit has probably crystallized directly from the melt. Some samples exhibit a poikilitic texture with hornblende enclosing grains of plagioclase and clinopyroxene. A few samples contain hornblende and plagioclase plus 3 to 5 per cent quartz and about 1 percent biotite; accessories are apatite, sphene and an opaque. Crystallization sequence is the same as pre viously noted, with quartz crystallizing last in the interstitial spaces. In all samples collected, the plagioclase is severely saus- suritized and the mafic minerals are partially altered to chlorite. The dike rocks are finer grained than the stocks and generally exhibit porphyritic texture. Phenocrysts are elongate prismatic to acicular 0.5 to 1.5mm hornblende or sometimes 1.0 to 1.5mm clinopy roxene and the groundmass is tabular to anhedral plagioclase with in terstitial hornblende, minor clinopyroxene and opaques. Collected samples of the dike rocks are pervasively altered. The mafic minerals show evidence of alteration to chlorite and the plagioclase is severely saussuritized or sericitized. Age None of the stocks or dike rocks within the study area have been dated. However, these rocks can probably be assigned to the Middle or Late Jurassic because they are very similar in composition and occurrence to dated plutons of this age outside the study area. 115 The plutons within the study appear to lie within the Ironside Mountain plutonic belt of Hotz (1971) yielding ages of 165 to 167 m.y. but they are very near the western boundary of his Trinity Mountains plutonic belt which yields ages from 127 to 140 m.y. In addition, the undated plutons of the study area are in close proximity to 152 to 157 m.y. plutons in the Forks of Salmon area. 116 PETROLOGY OF SELECTED IGNEOUS ROCKS Approach to Study Chemical data bearing on the classification of rocks and the de rivation of petrogenetic information are abundant in the literature and can often complement or constrain genetic interpretations based on field and petrographic data. With specific reference to areas of petrology which might bear on this study, igneous rocks (especially basalts) from modern oceanic environments are being characterized by their chemical properties (Hart and others, 19 72; Jakes and White, 1972; Bonatti and others, 1971; Hawkins, 1974; Schilling, 1975). Also, on land ophiolites or ophiolitic sequences, presumably representing ancient fragments of oceanic crust and upper mantle, are being chemi cally as well as physically compared with modern oceanic crustal sequences (Bailey and others, 1970; Coleman, 1971; Moores and Vine, 1971; Bailey and Blake, 1974; Moores and Jackson, 1974; Miyashiro, 1975; Kay and Senechal, 1976). Sone authors (Pearce and Cann, 1971; 1973; Pearce, 1975; Floyd and Winchester, 1975; Winchester and Floyd, 1976; Smewing and others, 1975) are of the opinion that rock types from modern petro-tectonic settings (e.g. island arc, mid-ocean ridge etc.) have a chemical "fingerprint” peculiar to them which is sometimes recognizable in ancient rock suites. 117 Because the North Fork terrane and its lateral equivalents con tain significant proportions of mafic volcanic and plutonic rocks (Cox, 1956; Trexler, 1968; Davis, 1968; Irwin, 1972) and because a cogenetic relationship among them was implied by Irwin (1972) when he suggested these rocks were part of a disrupted ophiolite suite, the diabases, gabbro and mafic volcanic rocks become likely candidates for additional study using modern analytical methodology and interpretive techniques. Additionally, the writer knows of no published analyses of mafic igneous rocks from the North Fork terrane or its equivalents. This provides further incentive for studying this assemblage of rocks in more detail when one considers the current high levels of interest in Klamath Mountains geology. Therefore a pilot study consisting of whole rock major and select trace element analyses of a few samples from the North Fork terrane was carried out to determine whether or not the assemblage of mafic igneous rocks was ophiolite-related and whether the volcanic rocks mapped in the field had followed more than one petrogenetic path. Analytical Methodology Rock samples from the gabbro (gb), diabase (db), lower volcanics (V-^) and upper volcanics (V£) were analyzed for eight major (Si, Al, Fe, Mg, Ti, Ca, Na, K) and two trace (Sr, Rb) elements using atomic absorption spectrophotometry. Rb was determined by flame emission due to improved sensitivity and detection limits required for analysis of the low Rb rocks thought to occupy the terrane. Samples were 118 prepared using the HF-H3BO3 acid disolution technique outlined by Ber- nas (1968), Langmyhr and Paus (1968) and Anderson (1977, personal communication). This method involves direct dissolution of rock powder with aqua regia and hydroflouric acid in a teflon-lined stain less steel bomb. Stabilization of silica was accomplished by complex- ing the hydroflouric acid with boric acid. Two subsequent dilutions (1:8000 and 1:40,000) of the initial (1:400) solution containing the samples were made allowing all the elements listed above to be deter mined from one initial solution. Working standards were made from stock solutions and provided with a matrix compatible with sample solutions in terms of fluoroboric acid complex and ionization suppressant content. Multiple standards for each element were made in order to bracket samples as closely as pos sible. Standards and blanks were run repeatedly throughout a given analysis; frequency of repetition was dependent upon instrument stabi lity and the inherent nature of the element being determined (Si and A1 usually have a noisy output). USGS standard W-l was run as an internal standard using data of Flanagan (1973). Instrumentation was a Perkin-Elmer model 370 double beam spectro photometer with digital readout and an adjustable integrating mode of 1, 3 or 10 seconds. Samples were run in the 3 second mode. Chemistry of Samples Introduction Major and trace element data for the analyzed samples appear in 119 Table 1 and normative compositions are listed in Table 2. Norms were calculated using a minimum Fe20 3/Fe2C>3 + FeO ratio based on alkali con tent (J. L. Anderson, 1977, personal communication). SiC>2 in the samples ranges from 49.7(7) to 54.9(5) percent and there is a bimodal grouping of the samples very close to these two extreme values. MgO ranges from 3.11 (an aphyric, plagioclase-rich basalt) to 11.90 percent (a clinopyroxene-rich gabbro). Ti02 ranges from 0.22 to 4.12 percent, AI2O3 ranges from 14.95 to 16.50 percent, K^O ranges from 0.04 to 4.66 percent and total alkalis (Na20 + K2O) ranges from 1.11 to 8.37 per cent . Looking at the data for basalt and diabase (Table 1) one sees that there exist two subgroupings which correspond very well with units mapped in the field (see PI. 1 for sample localities). The first sub group contains the diabases (samples KM-152 and KM-169) the lower (V-^) volcanics (samples KM-160 and KM-166). These samples contain greater than 50 percent Si02 and low Ti02 (less than 2.0%). The second sub group contains the upper (V2) volcanics (samples KM-163a, KM-170 and KM-173) with less than 50 percent Si02 and high Ti02 (greater than 3.0%). Total iron (as FeO) is slightly higher in the second subgroup as is (Na20 + K^O). It is realized, however, because the basalts and dibases are slightly to moderately altered and/or have been subjected to low grade metamorphism, that Na20 and K^O may have been mobilized (Hart, 1970; Matthews, 1971; Pearce, 1975; Scott and Hajash, 1976). The second subgroup also appears to be distinguishable on the basis of Sr content, which is high (greater than 300ppm) compared to the first subgroup. 120 Table 1 Whole rock major and trace element analysis for gabbro, diabase, lower volcanics and upper volcanics; major elements in wt. %, trace elements in ppm. 121 SAMPLE gab (gb) diabase (db) lower volcanics (V,) upper volcanics O ',) USGS-W1 KM-72 KM-152 KM-169 KM-160 KM-166 KM-163a KM-170 £ - ■ KM-173 Si°2 50.2(0) 54.9(5) 54.6(2) 54.4(4) 53.6(2) 50.8(4) 49.9(3) 49.7(7) 52.6(4) Ti°2 0.22 1.28 1.24 1.38 0.98 3.16 3.01 4.12 1.24 A12°3 15.59 16.50 15.18 15.58 14.95 15.72 15.04 15.66 14.93 FeO* 4.84 8.81 8.58 8.99 8.28 10.15 10.25 11.90 9.88 MgO 11.89 6.31 6.22 6.18 7.30 3.11 7.90 4.79 6.27 CaO 15.31 8.09 7.92 7.67 10.40 6.06 8.42 6.72 10.29 Na20 1.07 3.35 3.56 4.50 3.38 3.71 3.30 4.94 1.96 K2° 0.04 0.93 0.18 0.38 0.13 4.66 0.61 0.19 0.60 Sum 99.16 100.22 97.50 99.12 99.04 97.41 98.46 98.09 97.81 Rb 1 22 2 4 4 153 13 8 23 Sr 129 190 109 114 104 383 558 312 138 Rb/Sr 0.01 0.12 0.02 0.04 0.04 0.40 0.02 0.03 K/Rb 400 423 900 950 325 305 469 238 FeO*/MgO 0.41 1.40 1.38 1.45 1.13 3.26 1.30 2.48 K20+Na20 1.11 4.28 3.74 4.88 3.51 8.37 3.91 5.13 * total iron as FeO Table 2. Normative mineralogy for gabbro, diabase, lower volcanics and upper volcanics. 123 KM-72 KM-152 KM-169 11 0.30 1.78 1.76 Or 0.20 5.45 1.10 Ab 9.55 30.00 32.75 An 37.28 27.18 25.55 Mt 0.90 2.48 2.30 Di 30.40 10.28 11.80 Hy 17.92 18.72 18.62 01 3.45 Q 4.11 6.12 Ne An/Ab+An 0.80 0.48 0.44 KM-160 KM-166 KM-163a KM-170 KM-17: 1.92 1.38 4.56 4.26 5.90 2.25 0.80 28.50 3.70 1.15 40.45 30.50 28.10 30.10 45.55 21.28 25.33 12.95 24.83 20.55 2.73 2.18 4.19 2.82 3.71 13.48 21.16 14.60 14.12 10.92 16.52 16.50 17.98 9.34 3.26 2.19 2.88 1.37 2.15 3.84 0.34 0.46 0.32 0.45 0.31 Chemical Distinction of Units and Comparison to Ophiolites Figures 4 and 5 show the variation of MgO with TiC^ and FeO*, res pectively. Separation of the subgroups previously noted is good; Figure 4 reflects the high MgO and low Ti0 2 contents of the gabbro and the high Ti02 of the upper volcanics, while Figure 5 depicts the low FeO* of the gabbro and the high FeO* of the upper volcanics relative to the lower volcanics and diabase. Sr versus Ti0 2 is plotted in Figure 6. Here one clearly sees that Sr and Ti0 2 in the upper volca nics are high relative to the lower volcanics and diabase and that the data are distributed into distinct subgroups. It thus appears that the North Fork terrane at the latitude of the study area contains at least two volcanic units, separable on the basis of both field and chemical criteria, and a diabase unit which is similar in chemical com position to the lower volcanic unit. On a total alkali (Na20 + K^O) versus Si02 diagram (Fig. 7), the chemical distinction between the upper volcanics and the lower vol canics plus diabase is represented in another way. All analyses of the lower volcanics and diabase are subalkalic, plotting below the alkaline basalt-subalkaline or tholeiite of Irvine and Baragar (1970) and the alkaline basalt-tholeiite line of Macdonald and Katsura (1964). In contrast, the upper volcanics are alkalic with two of the analyses plotting above the line of Irvine and Baragar (1971) and all three analyses plotting above the Macdonald and Katsura (1964) line. It is consistent that the alkalic upper volcanic rocks are also high in Ti02 and FeO* compared to the lower volcanics and the diabases. Similar 125 Figure 4. Plot of Ti02 vs. MgO for upper volcanics, lower volcanics, diabase and gabbro, showing separation of upper volcanics from lower volcanics plus diabase. Note the high Ti02 of the upper volcanics relative to the lower volcanics and diabase. 126 4 6 2 8 10 12 M gO ® upper volcanic unit (V^) • lower volcanic unit (V-j) A diabase (db) ■ gabbro (gb) Figure 5. Plot of FeO* vs. MgO for upper volcanics, lower volcanics, diabase and gabbro, showing separation of upper volcanics from lower volcanics plus diabase. Note the high FeO* of the upper volcanics relative to the lower volcanics and diabase. 128 10 8 FeO* 6 4 2 2 6 8 10 4 12 M gO <j upper volcanic unit (V2 ) • lower volcanic unit (V^) ▲ diabase (db) ■ gabbro (gb) 129 Figure 6 Plot of Sr vs. TiO^ for upper volcanics, lower volcanics diabase and gabbro, showing separation of upper volcanic from lower volcanics plus diabase. 1000 (ppm) 100 0.1 1.0 HO2 (wt. %) ® upper volcanic unit (V£) # lower volcanic unit (V^) ▲ diabase (db) ■ gabbro (gb) 131 Figure 7. Total alkali vs. SiC^ plot for upper volcanics, lower vol canics and diabase, Curves are the alkaline basalt- subalkaline or tholeiite line of Irvine and Baragar (.1971) and the alkaline basalt-tholeiite line of Macdonald and Katsura (1964). Also plotted (open circles) are Deccan basalts of Vallance (1974) and Carlsberg Ridge basalts of Cann (1969). 132 9.0 8.0 7.0 Irvine and Baragar (1971) 6.0 Vallance (1974) Macdonald and Katsura (1964) 5.0 Cann (1969) 4.0 3.0 3 upper volcanic unit (V2 ) 0 lower volcanic unit (V^) ▲ diabase (db) 2.0 .0 45 65 40 50 55 60 Si 0^ 133 relationships were noted by Bailey and Blake (1974) for basalts and diabases from the Coast Range ophiolite in California. Irvine and Baragar (1971) separate calc-alkalic and tholeitic rocks on an AI2O3 versus normative plagioclase (An/An + Ab + 5/3 Ne) diagram. When plotted on this diagram (Fig. 8), the subalkaline ana lyses (lower volcanics and diabase) straddle the boundary between the calc-alkalic and tholeiitic fields. When FeO* versus FeO*/MgO is plotted for the subalkaline analyses (Fig. 9), the data straddle Miya- shiro's (1974) boundary between the calc-alkalic and tholeiitic fields. On an Si02 versus FeO*/MgO plot (Fig. 10), the analyses cluster in the calc-alkalic field close to the boundary with the tholeiitic field. Ti02 versus Fe0*/Mg0 is shown on Figure 11. Here the analyses closely parallel the trend of abyssal tholeiitic basalts. Shown for compari son are the Skaergaard trend (strong Fe enrichment) and the trend of Asama volcano (Miyashiro, 1975), a distinctly calc-alkalic trend. The diagrams (except for Si02 vs. Fe0*/Mg0) do not clearly define whether the lower volcanics and diabses are distincly of tholeiitic or calc-alkalic affinities. This is not surprising when one considers that Si02, Na20, K^O and FeO* can be mobilized during hydrothermal al teration and/or low grade metamorphism (Bischoff and Dickson, 1975; Pearce, 1975; Scott and Hajash, 1976). It is significant, however, that when Ti02 (which is thought to be relatively immobile during al teration and low grade metamorphism) is plotted against Fe0*/Mg0, the data parallel the trend for abyssal tholeiite. The question of whether one is dealing with true calc-alkalic basalts or basalts that have undergone significant alteration becomes 134 Figure 8. AI2O3 vs. normative plagioclase plot of Irvine and Bara gar (1971) for lower volcanics and diabase. The data lie astride the boundary between calc-alkaline and tholeiite fields, 135 136 22.0 20.0 calc-alkaline 18.0 6.0 14.0 th o le iitic 2.0 10.0 90 80 70 60 50 40 30 20 10 0 Normative Plagioclase (An/An+Ab+5/3 Ne) • lower volcanic unit (V-j) ▲ diabase (db) Figure 9. FeO* vs. FeO*/MgO plot for lower volcanics, diabase and gabbro. Note that the data for lower volcanics and dia base plot on the boundary between calc-alkaline and thol- eiite fields of Miyashiro (1974). Shown for reference (open circles) are ophiolitic pillow basalts from Coleman (1977). 137 15 10 5 1 2 3 4 5 6 7 Fe0*/Mg0 # lower volcanic unit (V-j) ▲ diabase (db) ■ gabbro (gb) 138 Figure 10. Si02 vs. Fe0*/M 0 plot for lower volcanics, diabase and gabbro. Note that the data plot in the calc-alkaline field near Miyashiro's (1974) boundary between calc- alkaline and tholeiite fields. Open circles are ophio- litic pillow basalts from Coleman (1977). 139 80 70 60 50 1 2 3 4 5 6 7 Fe0*/Mg0 • lower volcanic unit (V^) ▲ diabase (db) ■ gabbro (gb) 140 Figure 11. TiC^ vs. FeO*/MgO plot for lower volcanics, diabase and gabbro. Note that the data closely parallel the abyssal tholeiite trend. The Skaergaard trend and the trend of the calc-alkaline Asama Volcano (Miyashiro, 1975) are shown for comparison. Open circles are ophiolitic pillow basalts. 141 O 2 3 4 FeO*/MgO # lower volcanic unit (V-j) A diabase (db) ■ gabbro (gb) apparent when one plots ophiolitic pillow basalts on some of these same diagrams. Coleman (1977) has done this for SiC^ versus FeO*/MgO, FeO* versus FeO*/MgO and SiO^ versus (Na^O + K^O). The data plot well into the tholeiitic and calc-alkalic fields and, in the case of SiC^ versus (Na^O + K^O), into the alkalic basalt field. This problem will be discussed further in the section on petrogenesis. Figure 12 is an AFM plot for all analyses. The lower volcanics and diabases cluster well within the composite field representing ba salt and diabase analyses from the Coast Range ophiolite (Bailey and Blake, 1974), Vourinos (Moores, 1969), Troodos (Moores and Vine, 1971; Gass and Masson-Smith, 1963) and the Newfoundland ophiolites (Irvine and Findlay, 1972; Smitheringale, 1972; Williams and Malpas, 1972). Individual samples from the upper volcanics either do not cluster with the lower volcanics and diabases or plot outside the composite field. The single gabbro analysis plots near the M apex within the composite field for analyzed ophiolitic gabbros; this position reflects the rela tively high MgO content, probably due to the abundance of modal clino- pyroxene in the sample. Basalts and diabases are plotted in the system Ne-01-Di-Hy-Q in Figure 13. Here again one clearly sees the separation of the sub groupings. Samples KM-160 and KM-166 are lower volcanics and KM-152 and KM-169 are diabase; these are all quartz normative. Samples KM-163a, KM-170 and KM-173 belong to the upper volcanics and are oli vine normative. In addition, sample KM-163a is nepheline normative. 143 Figure 12. AFM plot for upper volcanics, lower volcanics, diabase and gabbro, showing the ophiolitic affinities of the lower volcanics, diabase and gabbro. 144 145 fie ld of ophiolitic basalts & ® upper volcanic unit (V2 ) lower volcanic unit (V-j) diabase (db) gabbro (gb) field of ophiolitic gabbros M Figure 13. Ne-01-Di-Hy-Q plot for upper volcanics, lower volcanics and diabase, showing contrast in normative compositions between upper volcanics and lower volcanics plus diabase. 146 147 O upper volcanic unit (V9) Ne -*■ • lower volcanic unit (V,) ▲ diabase (db) 01 Hy Petrogenesis As was previously noted, sample populations are very small and the chemical data generated are limited, which severly constrains what can be said about magma generation and the source of melts. Petro- graphic observations reveal that the samples (particularly the lower volcanics and diabase) are altered. In thin section, veinlets of quartz and in some samples, epidote or epidote plus quartz, are visible. Secondary minerals (altered plagioclase, chlorite, actinoli- tic amphibole) present in the lower volcanics and diabase are the same as those reported in spilitized basalts from India (Vallance, 1974) and the Carlsberg Ridge (Cann, 1969). Chemically compared with oceanic basalts (Kay and others, 1970; Hawkins, 1976) the lower volcanics and diabase are enriched in Si02 and Na20 and slightly depleted in CaO. Enrichment in Na20 and deple tion in CaO has been noted in both ancient (Vallance, 1974) and modern (Cann, 1969) spilitized basalts. It is clear from work such as this that changes in chemistry of a rock that has undergone spilitiza- tion can have profund effects on its classification. For example, data from Vallance (1974) and Cann (1969) plot in the alkalic basalt field on the (Na20 = ^0) versus Si02 diagram (Fig. 7). These basalts were probably tholeiitic before alteration as Vallance (1974) appa rently can trace the chemical degradation from tholeiite to spilite in a suite of Indian Deccan pillow basalts. It is suggested that the enrichment in Na20 and Si02 and the depletion in CaO in the lower volcanics and diabase from the North 148 Fork terrane might be ascribed to alteration and/or metamorphism assoc iated with spilitization. Analyses of these units resemble analyses of spilites from the California Coast Ranges (Bailey and Blake, 1974). These are plotted for comparison on the FeO* versus FeO*/MgO and SiC>2 versus FeO*/MgO diagrams (Figs. 9 and 10, respectively). The Coast Range ophiolite spilites occupy both the tholeiitic and alkalic fields on both plots. An alternative explanation for the Si02 enrichment is that it represents a higher degree of silica saturation in the rocks due to fractionation. Although most oceanic or abyssal tholeiites are olivine normative (Engel and others, 1963; Kay and others, 1970; Hawkins, 1976), some samples are quartz normative and have been interpreted as fractionated products of magmas which were originally olivine norma- tive(Hawkins, 1976). It is possible that the lower volcanics and diabase from the North Fork terrane are fractionated products of a more silica undersaturated basalt magma. The lower volcanic unit contains phenocrysts of clinopyroxene and the diabase contains subophitic intergrowths of clinopyroxene and plagioclase. Shido and others (1971) report that some analyzed abyssal tholeiitic basalts have clinopyroxene phenocrysts. Furthermore, the presence of clinopyroxene as phenocrysts may be due to fractionation of early crystallized olivine and plagioclase. The early crystalli zation of these minerals in a basalt magma drives the composition of the residual liquid to the cotectic between the crystallization fields of olivine and plagioclase (Green and Ringwood, 1967). Further 149 simultaneous crystallization of olivine and plagioclase moves the liquid composition away from the plagioclase and olivine apices to wards the clinopyroxene apex resulting in the crystallization of clino pyroxene . Major element chemistry of the lower volcanics and diabase does not unequivocally document an ocean floor origin. Yet when one also considers the geologic evidence (non-vesiculated pillows, interpillow radiolarian chert and lack of associated volcanogenic sedimentary material), the units are probably best interpreted as altered or spilitized abyssal basalts and associated hypabyssal diabasic rocks comprising layer two of oceanic crust. The single gabbro sample is distinctly high in CaO and MgO and low in Ti02, FeO*, Na20 and K2O relative to the lower volcanics and diabase. On the FeO* versus FeO*/MgO (Fig. 9), Si02 versus FeO*/MgO (Fig. 10) and Ti02 versus Fe0*/Mg0 (Fig. 11) diagrams, the sample plots near the origin. It is tentatively interpreted that the gabbro sample represents a cumulate (clinopyroxene as the cumulate phase) of the lower volcanic and diabase units. The upper volcanics are classifiable as alkalic basalts according to the criteria (high total alkalis vs Si02) of Macdonald and Katsura (1964) and Irvine and Baragar (1971). The analyses (especially the high Ti02 and Sr contents) fall into the range reported by Miyashiro (1975) for ocean island tholeiitic series basalts and very closely resemble those reported from the California Coast Ranges by Bailey and Blake (1974) as being oceanic island-type basalts. They also resemble 150 an average for oceanic island basalts and analyses of alkalic basalts from the Mamonia Complex of the Troodos Massif, Cyprus, thought by Pearce (1975) to represent within-plate basalts. The compositional variability in the upper volcanics and indicated by the scatter of data points on the various diagrams (AFM, total alkali vs. SiC^, Ne-01-Di-Hy-Q) possibly means that some of the upper basalts are transitional between subalkaline and alkaline types. Nevertheless, the above evidence coupled with the observation that the uppoer volcanic unit contains vesicle-rich pillow basalts plus crys tal-rich and lithic tuffaceous deposits, and is overlain deposition- ally by shallow water carbonates, suggests very strongly that it represents oceanic island or seamount material within the North Fork terrane. 151 STRUCTURAL ELEMENTS Macroscopic Structures Regional Folding As was noted earlier a definable structural sequence of, from lower to higher levels, gabbro and diabase (gb and db), mafic volcanic rocks (V-|), ultramafic rocks (NFum and um) , mafic volcanic and volcani- clastic rocks (V2) , chert and argillite (S-^) and limestone (Ls) is recognizable within the map area. Despite the disruption and lahk of lateral continuity in some units, a repetition of the structural se quence can be seen along an approximately east-west traverse of the map area. The structurally higher rock units are more or less symmetrically disposed about the gabbro which comprises a continuous north-south- trending unit in the central part of the map area. Most of the con tacts between units are subvertical or dip steeply eastward. The symmetry and orientation of units defines an antiform, cored by gabbro, steeply overturned westward and which, at this latitude, plunges gently northward. The symmetry and orientation of units can best be seen in the field on a traverse along the Salmon-Trinity divide trail between Hotelling Ridge and Five Dollar Camp. The overturned western limb of the antiform is more highly dis rupted than the eastern limb. As was alluded to in the descriptions 152 of the various rock units, the western part of the map area (which is underlain by the overturned limb of the antiform) contains tectonic slices and fragments of the units comprising the structural sequence. All major units are found on either side of the core gabbro and they occur in essentially the same structural position on both limbs of the fold. It would therefore appear that folding of regional magnitude is present in the Western Paleozoic and Triassic subprovince in the area of the south-central Klamath Mountains. It is significant, however, that this discovery is not without precedent. Diabasic rocks which are continuous with those in the study area were mapped by Cox (1956) and were thought originally by him to occupy the core of a large anti cline. He believed the core of an anticline in the Chanchellula Formation of Hinds (1932) (S-^ unit of this study) to be intruded by diabase; the core was thought to be flanked on the west and east by later, high angle reverse faults (Twin Sisters and North Faults, res pectively) . Cox (1956) assumed the strips of metasedimentary rocks on either side of the diabase to be of about the same age and postu lated the folding and later faulting as a tentative explanation, this being permitted by existing evidence for correlation of the two strips of metasediments and intrusive origin of the diabase. The evidence was not abundant and Cox apparently abandoned the idea of the anti cline because it is not described in his subsequent writings nor does it appear on published maps of his area (Cox, 1967; Cox and Pratt, (1973). 153 An interesting interpretation was presented by Griscom (1972, p. B46) who obtained computer simulations of linear magnetic anomalies associated with the two north-south-trending zones of serpentinite (NFum) at the approximate latitude of the study area. His results sug gested that the two zones converge at depth, forming a synformal struc ture. The present author entertains this as a viable interpretation but suggests that the present geometry of the eastern limb (an east facing, eastward dipping sedimentary sequence lying structurally above the eastward-dipping zone of serpentinite) would be difficult to obtain by simple synformal folding. It is believed by the writer and his co-workers (Ando and others, 1976; 1977) that the antiform which appears well documented in the Cecilville and Salmon Mountain Quadrangles is continuous into the Helena Quadrangle where units mapped by Cox (1956) are involved in antiformal folding of essentially the same geometry as that recognized farther to the north. Further, it may be deduced from the maps of Cox (1956, 1967) that the antiform at the latitude of his area plunges to the south, the overall geometry defining a doubly-plunging structure. Differences in interpretations made by Cox (1956) and the writer and his co-workers arise mainly in the areas of timing of events and gene tic relationships of rock units to one another. The recognition of a regional fold cored by rocks belonging to the northward extension of Irwin's (1972) North Fork terrane has important implications concerning the basis for Klamath geologic subdivisions. Recalling that the North Fork terrane at the latitude of the type 154 locality along the Trinity River is the easternmost and structurally highest of Irwin's (1972) western Paleozoic and Triassic subprovince terrane designations, one sees that sedimentary rocks of S west of the western zone of North Fork serpentinites (NFum) must belong to a northward extension of the Hayfork terrane of Irwin (1972). Recalling also that metamorphosed and transposed greenstone and chert of the Stuart Fork Formation (Davis and Lipman, 1962) have been correlated with the North Fork terrane (Davis, 1968) the conclusion can be reached that at the latitude of the present study area, the Hayfork, North Fork and Stuart Fork terranes are structurally equivalent (Ando and others, 1976). Rocks that correlate with the North Fork terrane and Stuart Fork Formation occupy the core and eastern limb, respectively, and rocks which would correlate with the Hayfork terrane occupy the western limb of the antiform. On this basis it is preferred (Ando and others, 1977) that Irwin's (1972) terrane designations for the western Paleo zoic and Triassic subprovince not be applied at the latitude of the study area. Furthermore, contrary to the interpretations of Irwin, the fault contact between the Hayfork and North Fork terranes appears not to be a fundamental structure in terms of subdividing the west ern Paleozoic and Triassic subprovince into terranes. Faults Parallel to Regional Structural Trends Small scale regional reconnaissance or compilation maps of the Klamath Mountains (Irwin, 1966; Hotz, 1971) clearly show that the dominant structural trends in the western Paleozoic and Triassic 155 subprovince are north-south. Major bounding faults of the Klamath sub provinces are north-south trending. Within the study area, faults which separate mapped units tend to reflect this north-south struc tural "grain", Some of the mapped units are in effect slices bounded above and below by faults. Most notable of these faults are those which bound the folded North Fork ultramafic sheet. The writer prefers not to treat the ultramafic rocks as occupying "fault zones". Rather, it is thought more reasonable that both western and eastern outcrops of the North Fork serpentinites be regarded as discrete structural units. Further more, these units are interpreted to be the remnants of a once con tinuous, fault possibly thrust bounded sheet that has been anti- formally folded. As mentioned earlier, the writer suggests non application of the original Twin Sisters and North Fork terminology, Continued usage of separate names for the western and eastern zones of ultramafic rocks could perpetuate the interpretation that these are separate units. The faults that bound the ultramafic bodies, as well as faults that juxtapose other major rock units within the study area, dip steeply eastward at about 50 to 60 degrees. It is also apparent that some, if not all of these faults have been folded and may have at one time been low angle. To date, few kinematic data that might bear on sense of fault movement have been found. However, one set of struc tures with possible kinematic significance has been mapped just east of Potato Mountain on the Salmon-Trinity divide where, on a north- south trending, eastward-dipping fault surface between rocks of the 156 lower volcanic unit (V^) and serpentinized ultramafic rocks, slick- enside striae are observed to plunge approximately down the dip. At this time it seems permissible if not viable to suggest that rock units in tectonic contact with one another within the study area were juxtaposed by faulting of reverse geometry, possibly thrusting. This suggestion is consistent with the thrust-dominated style of faulting recognized by Davis (1968) for the south-central Klamaths. Minimum displacements on these faults are difficult to judge, but if, as will be suggested in a later section, the assemblage of rocks present are genetically related to one another, displacements are probably not large. Futhermore, although at least some of these faults predate the antiformal folding, it is believed that they de veloped during a protracted deformational episode that produced the folding and that the tectonic slices have been shuffled but are not extremely far-traveled relative to one another since initial detach ment . Faults Transverse to Regional Structural Trends Faults which are oriented transverse to the dominant north-south structural grain are very rare within the map area. Those that do occur are exclusively high angle normal faults and clearly postdate the north-south trending structure. Several normal faults of small displacement were recognized but deemed not significant at the map scale of about 1:21,100. By far the most prominent cross fault present in the map area is the Browns Meadow fault first recognized by Davis (1961). It was later traced 157 westward by Davis (1968) and Trexler (1968) into the Cecilville Quad rangle where displacements are dominantly dip-slip with the northern side down (see PI. 1). In the Cecilville Quadrangle, the Browns Meadow fault clearly postdates the antiformal folding and emplacement of the ultramafics. Rocks of the upper volcanic unit (V^), the diabase (db) and sedimentary rocks of S-^ as well as the ultramafics are offset by the fault, exposing higher structural levels on the northern (hanging) wall. The writer has been unsuccessful in tracing the Browns Meadow fault northwesteard into the Salmon Mountain Quadrangle. If, as seems apparent, displacements decrease northwestward along its trace, the fault may die out by the time it reaches the Salmon Moun tain Quadrangle. Mesoscopic Structures Minor Folds Where good exposures exist, it can be seen that minor folds are common in some of the stratified sedimentary rocks of the study area. These folds are almost exclusively found in bedded chert and limestone sequences and appear to be of dominantly flexural type. The style of the bedded cherts is manifested by open to subisoclinal folds approaching chevron geometry, often with a weakly developed axial planar spaced cleavage. In limestone the fold closures are more rounded and the folds display more elements of a passive (i.e. mani fested by more similar geometry) deformational mechanism than do the cherts. 158 Large numbers of bedding attitudes of fold hinges could not be measured in the field because of poor exposures in parts of the study area. Most measured fold hinges trend at small angles to either north or south and plunge at small angles. Implication for a wavelength of folding larger than that of the small scale folds (which average less than lm limb to limb) is found in variable bedding attitudes measured in chert. Poles to measured bedding attitudes define a very wide east- west trending girdle which is roughly consistent with most measured fold hinges (Fig. 14). Consistent direction of overturning of minor folds was not observed. Foliations Descriptions of the style of primary and secondary surfaces pre sent in the various rock units appear in the individual lithologic descriptions. Most foliations are of secondary origin and appear to have a consistent orientation within the different rock units. As with sedimentary bedding in chert, large numbers of attitudes could not be measured because of poor exposures in many parts of the area. Where well expressed however, foliations strike either north-south or at small angles to north and generally dip steeply eastward; some dip steeply westward or are vertical (see Fig. 15). Some primary folia tion is present in late intrusive igneous rocks and is defined by orientation of elongate mafic minerals. The expression or style of foliation changes from unit to unit, this change probably being attributable to differences in response of the various rock types to the deforming stresses. It is not known 159 Figure 14. Contoured equal area projection of poles to measured bedding attitudes in chert and limestone; wide east-west girdle consistent with north and south trending fold axes. Contours are 1.5, 3, 4, 5.5 and 7% of 1%. 160 161 Figure 15. Contoured equal area projection of poles to measured slip fibre foliation in serpentinite (A) and foliation in chert and argillite (B). Plots reflect the dominant steep eastward dip of foliation. Contours are (A) 1.5, 3.5, 5.0, 7.0, 10.5% and (B) 1.5, 3.5, 7.0, 8.5, 10.5%. 162 .163 whether foliations of similar orientation in different rock units are genetically related. It can be argued that some of them are not. For instance, some cleavage in bedded chert is axial planar to minor folds which, in the opinion of the author may be parasitic to the larger antiformal structure. However, slip fibre foliation in serpentinite which probably developed during its thrust emplacement must predate the antiformal folding. To summarize, secondary foliations are manifested in the fol lowing ways: 1) in the gabbro, discontinuous zones of intense dis ruption resembling fluxion foliation with mineral grains highly com minuted grade into zones of little or no disruption; 2) obvious foliations were not observed in the diabase unit except near tecto nic contacts where a fine phyllonitic layering is developed parallel to the contacts; 3) the lower volcanic unit (V^) contains anasto mosing zones of minor slip surfaces creating a phacoidal-type folia tion; 4) serpentinized ultramafic rocks contain either penetrative slip-fibre foliation or a non-penetrative blocky to lenticular dis ruption depending generally upon proximity to tectonic contacts. A foliation which is probably the result of recrystallization during an earlier metamorphic event is found in the cores of some large ultra- mafic bodies; it is defined by pyroxene orientations and strikes at a high angle to the dominant north-south slip fibre foliation; 5) no manifestation of foliation was observed in outcrops of the upper vol- canics (V£); 6) chert-argillite of contains a penetrative anastomosing to phacoidal foliation, whereas well bedded cherts with 164 thin argillaceous partings where folded sometimes contain an axial planar, spaced (fracture) cleavage. Lineations Well defined, pervasive lineations were not observed in any of the rock units in the study area. At a few localities, however, chert rod lineations paralleling minor fold hinges in bedded chert were measured and at one locality a boudin lineation in chert-argillite was measured. Pinch and swell structure is common in bedded cherts and probably represents incipient boudinage. A lineation defined by chromian spinel trains in the central parts of the ultramafic bodies was observed. This lineation repre sents part of the earlier metamorphic fabric described in the section on ultramafic rocks. Microscopic Structures In general, microscopic structures useful in unravelling the sys tematic deformation in the study area were not observed. Foliations that appear penetrative in hand specimens are expressed in much the same style in thin section; this is particularly true of the highly disrupted parts of the chert-argillite sequences in S^. As was alluded to in the section on ultramafic rocks, petrofabric study of the early metamorphic fabric in the cores of the bodies would be most useful in working out some of the systematic deformation in these units. However, because this fabric probably predates the 165 emplacement of the ultramafic bodies, petrofabric study would contri bute only to an understanding of their early (pre-emplacement) meLamorphic history. 166 THE STRUCTURAL SEQUENCE AS A COGENETIC SUITE As was briefly stated in the introduction, the close spacial association of ultramafic rocks, gabbro, diabase, mafic volcanic rocks, chert plus chert-argillite and limestone in the North Fork terrane prompted Irwin (1972) to interpret the assemblage as a dismembered ophilite suite. This was reasonable in view of the knowledge that other assemblages interpreted to be ophiolites had been observed to consist of very similar rock types in stratigraphic vontinuity (Rein hardt, 1969; Davies, 1971; Moores and Vine, 1971). The writer favors the implication made by Irwin (1972) that the assemblage of rocks comprising the North Fork terrane are genetically related. However, because the rock units have been tectonically mixed, unequivocal documentation that they are cogenetic may not be possible. It is considered most reasonable that they be treated as a once stra- tigraphically intact "package" rather than to treat them as initially separated, fare-traveled tectonic units. Data generated as part of this study allow reasonable conjectures regarding the origin of the North Fork rock suite to be made. It was suggested earlier that the North Fork harzburgites are probably Alpine-type ultramafics. Although the harzburgites were not analyzed chemically, it is also suggested that these rocks may represent frag ments of depleted upper mantle material that once formed the base of 167 the North Fork ophilitic sequence. Analyzed Alpine-type ultramafic units from the California Coast Ranges (Bailey and Blake, 1974) as well as the Klamath Mountains (Snoke and others, 1977) and other parts of the western Cordillera of the United States (Himmelberg and Loney, 1973) have been interpreted as chemically depleted fragments of ocean ic upper mantle material (Coleman, 1971). Because rocks structurally and/or stratigraphically below the gabbro have not been observed within the map area, it is difficult to address the evolution of the gabbro or its relationship to other rock units with much authority. The writer believes that the gabbro, which is similar chemically to clinopyroxene-bearing gabbros of oceanic affinities from other California ophiolites (Bailey and Blake, 1974), may be the upper part of an ophiolitic plutonic sequence. By way of analogy, gabbros in intact stratigraphic positions are found near the top of differentiated plutonic sequences in ophiolites from the Calif ornia Coast Ranges (Hopson and others, 1975) and the Sierra Nevada foothills (Saleeby, 1974). The absence of pervasive cumulate layering in the gabbros sug gests that they do not represent the lower level of the plutonic se quence where one would expect to find cumulate gabbro, olivine- plagioclase clinopyroxenite and cumulate ultramafic rocks. Rather, the gabbro seems to resemble homogeneous gabbro of the type described by Dewey and Kidd (1977) from the western Newfoundland ophiolites. They believe these gabbros to have been generated in the extreme upper part of a magma chamber in the axial region of an accreting plate 168 boundary (spreading center) and to be plated onto the base of the over- lying sheeted complex during the accretion process. The contact between gabbro and diabase in the North Fork terrane appears to be gradational over a short distance. The single analyzed gabbro sample is possibly a clinopyroxene-rich cumulate of the diabase. A more detailed chemical study is needed to determine whether the gab bro and diabase are tied genetically but an interpretation consistent with available data is that they were derived from the same parent melt, the diabase being a product of fractionation and chilling of this melt. In terms of a model, the melt, after undergoing fractionation would have been intruded and chilled against a lid or capping of rocks at higher levels (presumably pillowed basalt). Thus one would find chilled diabase above gabbro with a gradational contact between the two. Unfortunately the diabase and upper volcanics are invariably in fault contact within the map area and it cannot be proven that the dia base was chilled against these pillowed basalts. Furthermore, outcrop thicknesses of the steeply dipping diabase exceed two km in the study area and it is considered unlikely that a margin of this thickness could develop by simple chilling of the gabbro against a lid of pillow basalts. Alternatively, if the gabbro is regarded as being similar to homo geneous gabbro described by Dewey and Kidd (1977), and an analogy is made with their accretion model, the contact between diabase and gab bro may be one of underplating or accretion of the gabbro onto the roof of a magma chamber. The diabase, in the context of this model, 169 would have been fed from the magma chamber, injected as a sheeted com plex and underplated by homogeneous gabbro. A puzzling aspect of the diabase-gabbro transition in the North Fork terrane is the absence of significant multiple injection features in the diabase and the lack of diabase dikes cutting the gabbro. These relationships contrast markedly with the commonly reported presence of these features in the upper parts of most other ophiolites and could argue against the underplating model. However, it is the writer’s opinion that an injection type emplacement mechanism (rather than a fractionation and chilling model) cannot be discounted at this time. The absence of dikes of diabase in the gabbro might be construed as evidence that the diabase was injected in a sheet-like fashion bet ween a (presumed) lid of pillowed basalt and solidified homogeneous gabbro. A possible analog would be the sheeted sill of low angle dike complex in the Point Sal ophiolite (Hopson and others, 1975). A mechanism of intrusion similar to the one developed by Walker (1975) for shallowly inclined intrusive sheets on Iceland can be en visioned. On Iceland, rising batches of magma were apparently divert ed laterally along surfaces of equal density beneath a capping of pre-existing lavas rather than being extruded to the surface along vertical fractures. If an intrusion mechanism such as this was responsible for em placement of the North Fork diabases, the absence of evidence for multiple intrusion and the lack of dikes in the gabbro still remains puzzling. An untested explanation for the lack of chill contacts 170 is that successive batches of magma may have arrived so rapidly that sufficient cooling of the host rock to produce chill margins did not occur. Rather, a thickened sheet of diabase that more or less homo genized texturally as it cooled, may have developed. Another relationship that suggests the diabases may have intruded as a large sill-like feature is the presence northwest of Limestone Bluffs of outcrops of gabbro both above and below the diabase. Strati- graphic relationships between the two gabbro bodies are uncertain as the higher one is truncated by the Browns Meadow fault and the inter vening diabase is apparently lacking in internal structure. Also, intrusive features along contacts between gabbro and diabase that might suggest relative age have not been found. However, the outcrop pat tern of the units remains as mild evidence for the "sill" origin of the diabase. Field data collected to date appear insufficient to prove that either of the models discussed is correct. Both are considered possi bilities but some form of sill-like injection of the diabase and underplating by the gabbro are preferred by the writer. Chemical data for the lower (V-^) and upper (V2) volcanic units support the idea that they are, respectively, fragments of tholeiitic oceanic crust and alkalic island or seamount material. V-^, in all likelihood, represents the upper part of the volcanic section of the North Fork ophiolitic sequence upon which chert and chert-argillite were subsequently deposited. V2 is probably a remnant of an ancient seamount built in part on pelagic sedimentary rocks (chert) belonging 171 to the Ssedimentary unit. Davis (1968) has suggested that V^ (his western metavolcanic unit) was once exposed above wave base, citing as evidence the presence of conglomerate containing basalt pebbles and cobbles atop the basalt, and overlying limestone contain ing shallow water faunas. The writer concurs with this interpreta tion. Trexler (1968) interpreted the limestone unit to be a tectonic lens beneath the cherts. The writer considers this highly unlikely in view of the unconformable relationships found along the lower con tact of the carbonate unit by Davis. A more reasonable interpretation is that the limestone is a local deposit on basaltic volcanic rocks of the seamount. Further study of the limestones, particularly those from the west ern limb of the North Fork antiform, reveals that they at least in part comprise shoaling deposits marginal to the seamount. This is supported by the presence of ooid-bearing limestones containing nuclei of fragments of shallow water faunas and in some cases, nuclei which are fragments of mafic volcanic rock. Many samples do not contain large amounts of clean sparry calcite cement but do contain abundant dirty-appearing microspar which probably originated as micritic matrix. On this basis it seems reasonable to suggest that these samples were once parts of shoal deposits and have been transported to somewhat deeper water on the flanks of the seamount. Further evidence for resedimentation comes from the occurrence of the ooid-bearing limestone clasts in argillite matrix described under S-^. Active erosion and sedimentation on the flanks of the seamount is 172 represented by the occurrence of the clasts of aphyric basalt in chert-argillite breccia described under and the interlaminated crys tal-rich tuffs and mudstones noted under V£. The resedimented "sub- units" appear to occupy a position higher in the stratigraphic sequence than limestone bodies present on the western limb of the antiform and may partly represent the encorachment of the deeper water chert-argi llite depositional environment upon the seamount and its associated shallow water carbonates exposed farther to the east. That this en croachment did in fact take place is evidenced by the presence of chert and chert-argillite strata depositionally above the main limestone and the upper volcanics. 173 EVOLUTION OF THE NORTH FORK TERRANE IN THE LIGHT OF REGIONAL TECTONICS Speculations on the North Fork Terrane as an Accretionary Assemblage Following the Permo-Triassic Sonoma orogenic event, a Mesozoic volcanic arc was developed along the continental margin of North America. This arc, which is represented in the Klamath Mountains by Eastern Klamath subprovince units, was built on older (Paleozoic) arc materials and was tied to the continent (Burchfiel and Davis, 1972; 1975). Rocks of the Central Metamorphic subprovince had previously been juxtaposed by thrusting (during the Devonian-Mississippian) be neath the Eastern Klamath arc and occupied an outboard position rela tive to it following the Sonoma event. Parental North Fork terrane rocks probably originated somewhere west of the continental margin just described, were brought eastward as part of the downgoing oceanic plate and accreted along a subduction zone presently not exposed in the Klamath Mountains. The continuity of arc volcanism before, during and after the Sonoma event argues in favor of eastward polarity sub duction for this time period (Davis and others, 1978). Ando, and others (1976) have shown that a large antiform, steeply overturned westward, is the major structure in the North Fork terrane. Although the paucity of kinematic data collected to date places severe constraints on what can confidently be said about emplacement 174 of North Fork rock units, the westward vergence of the antiform is a kinematic indicator in itself. It is assumed that this westward ver gence is not an artifact of later rotation, an assumption that is supported by other evidence for dominantly west-directed structures in the Klamaths (convex westward traces of thrust faults and rock units and sequential decrease in age of terranes from east to west). It is suggested that the North Fork terrane was accreted during eastward underthrusting of the Eastern Klamath "arc" by a more wes terly oceanic assemblage of rocks. For reasons outlined in the following section, the treatment of the Eastern Klamath volcanic arc assemblage as a discrete Mesozoic arc is much too simple. Eastern Klamath "arc" is loosely used here to facilitate discussion of the geometric relationship of the North Fork terrane to Central Meta morphic and Eastern Klamath rock assemblages. It is not known whether folding of the North Fork assemblage was coincident with its emplacement but it is entertained here as a possi bility. The westward vergence of the structure suggests it may have been emplaced in the accretionary prism between the "arc" and the trench. Recent studies (Seeley and others, 1974; Kulm and Fowler, 1974; Karig, 1974b) have shown that trenchward verging structures are common features in modern subduction environments. Further corro boration is found in the low grade of metamorphism of some North Fork rocks (Davis, 1968) and the essentially unmetamorphosed character of others (this study); the assemblage was therefore probably accreted at high structural levels in a zone of high strain but relatively low heat flow. 175 Geologic History and Paleotectonic Significance of the North Fork Terrane Currently available data allow better resolution of the timing of events affecting the North Fork terrane than was previously pos sible. However, the primary ages of rocks in the Western Paleozoic and Triassic subprovince are not well known; sedimentary strata are yielding important fossil dates but as yet there appear to no known primary igneous ages for North Fork ophiolitic rocks. As was stated earlier, it is now known that ophiolite-related radiolarian cherts in the southern part of the North Fork terrane (Hayfork Quadrangle) are probably as young as Late Triassic or Early Jurassic and that tuffaceous cherts from the same terrane are prob ably Early to Middle Jurassic (Irwin and others, 19 77; Irwin and others, 1978). By inference the ophiolite-related cherts and vol canic rocks of the North Fork terrane farther to the north (area of this study) are of probable Late Triassic to Early Jurassic age. However, the presence near the mouth of Matthews Creek (PI. 1) of Middle Permian radiolaria and conodonts (as reported by Irwin and others, 1978) in cherts structurally above the ophiolitic core of the North Fork antiform dictates that care be exercised in assuming a Triassic age for the ophiolite. For this reason, the author con siders assumptions regarding the primary age of the North Fork ophiolite to be unwarranted at this time. Rocks of the Forks of Salmon and Wooley Creek plutons intrude the western limb of the North Fork antiform northwest of the study 176 area and yield ages of 167 m.y. and 152 to 154 m.y., respectively, as reported by Hotz (1971). Thus the folding and disruption of the North Fork ophiolitic sequence and its overlying chert-volcanic- limestone-chert sequence must have taken place prior to the onset of the 152 to 167 m.y. (Middle to Early Late Jurassic) plutonic intru sion. The thrust plate of Central Metamorphic subprovince rocks (upper plate for the Siskiyou thrust zone of Davis, 1968) truncates the up right eastern limb of the steeply oriented North Fork antiform (see Figs. 16 and 17). It has been shown (Davis, 1968) that the Siskiyou thrust fault in the Cecilville Quadrangle has been folded and that chert-argillite and greenstone in the lower plate (Stuart Fork For mation) have been metamorphosed to chlorite or biotite subfacies of the greenschist facies. Isotopic ages of from 133 m.y. yo 240 m.y. (K-Ar whole rock and mineral ages) were obtained by Lanphere and others (1968) for rocks of, or probably correlative with, the Stuart Fork Formation. They believed that ages ranging from 133 m.y. to 158 m.y were most reasonable and interpreted the metamorphism of parental Stuart Fork rocks as having occurred in the Jurassic. It now appears clear that the Siskiyou thrust, in postdating the folding of the North Fork assemblage, is younger than Early to Middle Juras sic) . Because the Siskiyou upper plate is folded (Davis, 1968) and intruded by plutons ranging in age from 127 m.y. to 140 m.y. (Hotz, 1971), its emplacement is probably no younger than latest Jurassic. West of the study area, it can be shown that two other major 177 Figure 16. Regional sketch map of the California Klamaths showing major lithologic assemblages, thrust faults, klippe and fensters (modified after Irwin and others, 1977). 178 123 124 W JS W P T W JS Yreka W P T E K S Callahan W J S E K S C M S T u rn NF-Hf C M S E K S W JS Weaverville E K S Fran Redding # Ore. area of map 50 km E K S eastern Klamath subprovince Trinity ultramafic complex T u rn C M S SF central metamorphic subprovince Stuart Fork Formation NF-Hfl North Fork-Hayfork terrane Rattlesnake Creek terrane western Jurassic subprovince R C W J S Fran Franciscan assemblage 179 Figure 17. Schematic cross section of the central Klamath Mountains showing major structural units, thrust faults and the North Fork antiform. It is implied that the flat-lying major thrusts either steepen at higher levels or are modified by high angle faulting. Radiometric ages and minimum distances of travel of thrusts given where known. Diagram not to scale. 180 181 165-167 24-32 Siskiyou thrust C.R. thrust (114-120 my) 450-480 ++ + + + +++ • * - 380*12 127-140+ ophiolite 35 km Condrey Mtn. schist Franciscan (?) E K S = eastern Klamath subprovince Tum = Trinity ultramafic complex C M S = central metamorphic subprovince SF = Stuart Fork Formation NF-Hf = North Fork-Hayfork terrane R C = Rattlesnake Creek terrane W JS = western Jurassic subprovince Fran = rocks of the Franciscan assemblage western Paleozoic and Triassic subprovince of Irwin (1960) Klamath thrust faults are also of Jurassic age (Fig. 16). The Rattlesnake Creek ophiolite of Late Triassic-Jurassic age (Irwin and others, 1977; Irwin and others, 1978) appears to be depositionally overlain by turbidites, hemipelagic muds and volcaniclastic sediments (Charleton, 1978) correlative with the Hayfork-Bally metaandesite of Irwin (1977). This assemblage is in probable thrust contact with higher North Fork-Hayfork rocks of the antiform along an eastward- dipping zone (Cox, 1967; Ando and others, 1977). The thrust is likely of Late Jurassic age as it separates upper plate Hayfork (= North Fork) rocks intruded by the Forks of Salmon pluton (167 m.y., Lan- phere and others, 1968) from lower plate rocks correlative with Irwin’s (1977) Hayfork Bally metaandesite of 156 m.y. (M. Lanphere as referenced in Irwin, 1977). The thrust fault which juxtaposes the Ironside Mountain complex (165 to 167 m.y.) with underlying rocks of the western Jurassic sub province (Galice Formation of Late Oxfordian to Early Kimmeridgian age) is also of Late Jurassic age. This is substantiated by the Middle and Late Jurassic ages of the upper and lower plates, res pectively, and the fact that the fault is overlapped to the south by earliest Cretaceous (Valanginian to Hauterivian) rocks of the Great Valley sequence. Thus the Middle to Late Jurassic was a time of major crustal shortening by thrust faulting in the central Klamath Mountains. The existence of major west-directed thrust faults in the central Kla- maths has been known for some time (Davis, 1968) but the recently 182 determined fossil ages for rocks in this area have provided better control on the age of thrust faulting than was previously possible. An interesting outgrowth of the study of thrust geometries in the central Klamath is the discovery that at present the thrusts occupy high crustal levels. The major thrust plates in the Cecilville area are relatively thin more or less subhorizontal and are struc turally imbricated through a vertical crustal thickness of less than 3km (Ando and others, 1977). In the Cecilville area a stacked se quence of four thrust plates exists (Fig. 17). From top to bottom these are: 1) Eastern Klamath subprovince rocks, 2) Trinity ultra mafic sheet, 3) Central Metamorphic subprovince and, 4) rocks of the western Paleozoic and Triassic subprovince. The first two thrust plates are preserved in a klippe north of the settlement of Cecilville and the third and fourth plates comprise the upper and lower plates, respectively, of the Siskiyou thrust fault south of the Cecilville area (Davis, 1968; Ando and others, 1977). Farther north in the Condrey Mountain area, Klein (1977) has made a good case for the schists of Condrey Mountain being the meta morphosed equivalent of the Galice Formation of the western Jurassic subprovince. Here the schists occupy a window in the western Paleo zoic and Triassic subprovince and are overlain by a thrust plate of amphibolite and ultramafic rocks (Medaris, 1966; Kays, 1968; Hotz, 1971). It appears, therefore, that all four of the major Klamath subdivisions plus the Trinity ultramafic sheet are structurally imbri cated through a very small vertical thickness. 183 Considerable cumulative shortening along these faults is indi cated by the following minimum displacements: 1) Trinity ultramafics over central Metamorphic subprovince, about 32km (20 miles, Davis, 1968), 2) Siskiyou thrust 24 to 32km (15 to 20 miles, Davis, 1968), and 3) western Paleozoic and Triassic subprovince over western Juras sic subprovince, 35km (about 22 miles, Klein, 1977). The cumulative shortening by thrust faulting in the assemblage of plates discussed (about 100km) the small thickness of the stacked sequence, and the apparent subhorizontal orientation of the thrusts are highly suggestive that in the central Klamaths, the style of thrusting is manifested by thin, flat-lying sheets. At least one of these thrusts is reportedly modified by later high angle faulting (the western Paleozoic and Triassic subprovince over Galice Forma tion thrust of Klein, 1977). Careful study of the traces of other Klamath thrust faults may reveal that they are also modified by later high angle faulting. The inference is made in Figure 16 that the major thrusts either flatten at depth or are modified by high angle faults where they break the earth’s surface. The writer agrees with Davis and others (1978) that none of the thrust faults in the central Klamaths directly represents a subduc- tion zone (plate boundary). They attribute the thrusts to shortening within a Late Jurassic to Early Cretaceous magmatic arc, rather than at a plate boundary. This can probably best be shown for the Siski you thrust which postdates thrust faulting within, and antiformal folding of, the North Fork ophiolitic sequence. The possibility 184 exists that a subduction zone of Late Triassic to Early Jurassic age once separated the North Fork ophiolitic assemblage from the conti nent to the east. However, as stated in Davis and others (1978), such an inferred plated boundary is presently obscured by the Siskiyou thrust plate. Recent work by Suppe (1977) suggests that recognition of sub duction zones in the geologic record is perhaps not so straightfor ward as previously supposed. According to Suppe, the Coast Range thrust of the northern California Coast Ranges, long presumed to rep resent an exposed ancient subduction zone, is actually a complex series of cross-cutting thrusts that modify the decollement separat ing Great Valley and Franciscan rocks. These later thrusts suc cessively step down from higher to lower structural levels, none of them representing the subduction zone itself. Relationships such as those described by Suppe (1977) may be present in the Klamaths; what were once regarded as plate boundaries may in fact be intraplate structures developed in the leading edge of the upper plate. Similar relationships appear to be present in the central Klamaths where deformation in ophiolite assemblages accreted at a convergent plate boundary is postdated by the major thrust faults. The view of the Klamath province as a broad volcanic archipelago during the Jurassic Period (Irwin and others, 1977) is considered reasonable in light of the comments made by Davis and others (1978). They (Davis and others) maintain that although the Klamath region 185 was one of tectonic complexity during the Jurassic Period, it is diffi cult to envision all of the various thrust fault-bounded assemblages as exotic compoennts. Yet at the same time, a coherent plate tecton ics setting for all the diverse elements of the Klamath province cannot be prescribed. In turn, relationships in the North Fork terrane place certain constraints on an evolving paleogeographic model for the central Kla maths for Late Triassic and Jurassic time. The North Fork terrane at the latitude of the study area appears to represent oceanic crust and associated sediments on which a seamount was constructed. The sea mount was subsequently exposed to erosion above wave base as an oceanic island and collected shallow water carbonates on its summit and flanks. A period of subsidence then ensued, the seamount basalts and carbonates being overlapped by the Late Triassic cherts of Irwin and others (1977). In the opinion of the writer, the North Fork terrane in the area of study contains no volcanic rocks of island arc affinities, nor does it contain clastic or volcaniclastic contributions from an arc source. Other studies ophiolites of the Klamath region, in contrast, either appear to be basement for arc complexes or are postulated to have formed in a behind the arc position. Examples of the first occurrence are the Trinity ophiolite of Ordovician age (Hopson and Mattinson, 1973), the Preston Peak ophiolite of Triassic (?) - Jurassic (?) age (Snoke, 1977) and the Rattlesnake Creek ophiolite of Triassic age (Charleton, 1978). An example of the second occurrence is the 186 Josephine ophiolite of Jurassic age, thought by Vail and Dasch (1977) to have formed behind an arc represented by the Rogue and Galice For mations . The North Fork ophiolite of Triassic (?) age appears to fit neither of the these occurrences. Thus its presence within the Middle Jurassic arc complex of Davis and others (1978) as a fragment of oceanic crust with no obvious arc affinities poses some fundamental questions. First, owing to uncertainties regarding its age, is it a piece of older (Late Paleozoic? Triassic?) oceanic crust trapped with in a Middle Jurassic arc complex, perhaps as a result of outstepping of subduction? Was the North Fork ophiolite generated in an intra arc position? If so, why is there no contribution of arc-derived volcani- clastic sediments in the overlying strata? Within the context of the model presented by Davis and others (1978) the North Fork ophiolite must have occupied an intra-arc position during the Middle Jurassic. If, as the data suggest, its accretion took place prior to the intrusion of Middle Jurassic plutons, it must have been positioned between the Eastern Klamath "arc" (active until at least Bajocian time) and the Triassic (?) to Middle Jurassic "arc" built on the Rattlesnake Creek ophiolite. Perhaps it was far enough away from these active parts of the arc complex not to receive debris from either source. However, the presence (as olisto- liths?) of Early Triassic (?) blueschist blocks, Late Paleozoic fusilinid-bearing carbonate blocks and Late Paleozoic cherts within higher strata of the North Fork terrane are highly suggestive of a nearby older source terrane that could shed debris into these strata. 187 Also of significance is the occurrence, west of the western limb of the North Fork antiform, of higher chert and argillite strata of the Hayfork (= North Fork) terrane in apparent fault contact with a western sequence of sandstone and siltstone interpreted to have been deposited near an intermediate volcanic source (Cashman, 1978). Some of these rocks are probable equivalents of those reported by Charleton (1978) farther south and are suggestive of a western (arc?) source contributing detritus to higher North Fork strata. Perhaps the Klamath region during the Middle to Late Jurassic was in fact a broad volcanic archipelago. If so, it must have been one where roughly coeval individual island chains were constructed across oceanic basement of different ages. Middle Jurassic eastern Klamath volcanic rocks lie above a Paleozoic and Triassic stratigraphic sec tion that overlies the Ordovician Trinity ophiolite. To the west, Triassic (?) to Middle Jurassic arc rocks are deposited on the Rattle snake Creek ophiolite of Triassic age and Late Jurassic arc rocks stratigraphically overlie the Josephine ophiolite of Jurassic age. At present there seems no simple explanation for the decrease in age of oceanic basement westward beneath the arc complex. However it is conceptually more satisfying to the writer that the Middle and Late Jurassic Klamath arc complex was built across previously sutured (?) oceanic crust of different ages than to postulate a multiplicity of narrow, far-travelled and separately accreted arcs. Comments to this effect have been aptly put by Davis and others (1978). Finally, the view of major thrust faults in the Klamaths as being 188 intraplate structures has interesting implications for what can be learned of the subduction process from ancient terranes. If one accepts this view, it follows that the actual subduction zone or plate boundary may never be seen in the ancient rock record. It is clear that during protracted periods of plate convergence, accreted packages of arc materials, oceanic crust and oceanic sedi ments are left at a plate edge as the locus of subduction steps out ward. Yet it is apparent from the observations of Suppe (1977) and Davis and others (1978) that later modification of the earlier record of subduction takes place. What then remains is a complex array of accreted materials cut by high level thrusts that postdate the accre tion, the actual plate boundary being totally obscured. 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W. Oregon and implications for pre-Navadan paleogeography: Geol. Soc. America Abs. with Programs, v. 9, p. 519-520. Vallance, T. G., 1974, Spilitic degradation of a tholeiitic basalt: Jour. Petrology, v. 15, p. 79-96. Walker, G. P. L., 1975, Intrusive sheet swarms and the identity of crustal Layer 3 in Iceland: Geol. Soc. London Jour., v. 131, p. 143-161. Williams, H., and Malpas, J., 1972, Sheeted dikes and brecciated dike rocks within transported igneous complexes, Bay of Islands, west ern Newfoundland: Canadian Jour. Earth Sci., v. 9, p. 1216-1229. Winchester, J. A., and Floyd, P. A., 1976, Geochemical magma type dis crimination: application to altered and metamorphosed basic igneous rocks: Earth and Planetary Sci. Letters, v. 28, p. 459-469. 197 A 2500 2000 1500 1000 500 meters above sea level ~ a:: . . .0 . . . . LL - I- z <{ ~ a:: 0 LL. B I I- a:: 0 z 2000 1500 NF um 1000 500 0 meters above sea I eve I . . . . . . . . . ~ 0::: 0 LL f- .. z <( ~ . 0::: ...... . 0 LL .. . NF um no vertical exaggeration B' 2000 1500 1000 500 0 2500 2000 1500 1000 500 Geologic Cross Sections of the North Fork Terrone ANDO, PLATE 2

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Creator Ando, Clifford Joseph (author)

Core Title Structural and petrologic analysis of the North Fork terrane, central Klamath Mountains, California

Contributor Digitized by ProQuest (provenance)

Degree Doctor of Philosophy

Degree Program Geological Sciences

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

Tag Geology,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c29-344690

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Identifier DP28549.pdf (filename),usctheses-c29-344690 (legacy record id)

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University of Southern California Dissertations and Theses