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The Structural and Metamorphic History of the Oakhurst Roof Pendant, Mariposa and Madera Counties, California

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The Structural and Metamorphic History of the Oakhurst Roof Pendant, Mariposa and Madera Counties, California UNLV Theses, Dissertations, Professional Papers, and Capstones 12-1972 The structural and metamorphic history of the Oakhurst roof pendant, Mariposa and Madera counties, California Lee R. Russell Texas Tech University Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Geology Commons, and the Tectonics and Structure Commons Repository Citation Russell, Lee R., "The structural and metamorphic history of the Oakhurst roof pendant, Mariposa and Madera counties, California" (1972). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1443. http://dx.doi.org/10.34917/3432680 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. This Thesis has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. THE STRUCTURAL AND METAMORPHIC HISTORY OF THE OAKHURST ROOF PENDANT, MARIPOSA AND MADERA COUNTIES, CALIFORNIA by LEE R. RUSSELL, B.A. A THESIS IN GEOLOOY Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of i. the Requ~rements for .the Degree of MASTER OF SCIENCE Director December, 1972 .. ,.,... --- ----------·~ AC: £tJ5' 13 !97Z (\)o. 217 eop, Z ACKNOWLEOOMENTS I would like to express my most sincere gratitude to Dr. S, E. Cebull for his priceless help in directing this thesis. The many hours of discussion with him on interpre­ tations provided me both inspiration and motivation. His scholarly and instructive criticism of the manuscript was invaluable. It has been a pleasure to work with him. In addition, thanks go to G, Mark Gower for drafting assistance and to Jerry B. Oldham for his photographic help. 1i CONTENTS ACKNOWLEDGMENTS ii LIST OF ILLUSTRATIONS v ABSTRACT vii I. INTRODUCTION l Purpose and Scope of Investigation l Location and Geography l Techniques of Study 3 Geologic Setting 4 Geology of the Western Sierra Nevada Metamorphic Belt 5 Stratigraphy 5 Synkinematic Regional Metamorphism 9 Structural Geology 10 Synopsis of Paleozoic and Mesozoic Geologic . ' History 13 Some Important Previous Work of Special Relevance to the Study of the Oakhurst Roof Pendant 15 II. DESCRIPTION OF MESOSCOPIC STRUCTURAL AND LITHOLOGIC RELATIONSHIPS IN THE OAKHURST ROOF PENDANT 19 Lithology 19 Structure 24 III. DESCRIPTION OF MICROSCOPIC MINERALOGICAL AND TEXTURAL RELATIONSHIPS 30 Eastern Unit 30 iii .• ";J)__, Central Unit--Hornblende Schist 33 Western Unit 39 "Other Rocks" 43 IV. INTERPRETATION OF MESOSCOPIC AND MICROSCOPIC FABRICS 48 Mesoscopic Fabric 48 Microscopic Fabric 54 Relationship Between the Mesoscopic and Micro- scopic Fabrics and Chronologies 60 V, CONCLUSIONS, POSSIBLE REGIONAL SIGNIFICANCE, AND SUGGESTIONS FOR FURTHER S7'UDY 64 Conclusi.ons 64 Possible Regional Significance 66 Suggestions for Further Study 67 LIST OF REFERENCES 69 iv • LIST OF ILLUSTRATIONS Figures 1. General geologic index map of the central Sierra Nevada region 2 2. Generalized stratigraphic scheme for the rocks of the western metamorphic belt 6 3, Generalized cross-section of the western meta- morphic belt along the Merced River 7 4. Photograph of four' hand specimens that illustrate the transposition of early layered structures 20 5. Contoured Schmidt net plots of foliations from the three units of the Oakhurst roof pendant 25 6, Schr.ddt net plots of lineation data 27 7. Schmidt net plots of minor fold axes and axial planes in rocks of the Oakhurst roof pendant 29 8, Index map of the geology immediately surrounding the Oakhurst roof pendant, showing locations of important radiogenic dates 28 9, Photomicrograph illustrating static recrystalliza- tion in the rocks of the eastern unit 32 10. Photomicrograph of a shear fold in a quartz-rich layer from an eastern unit sample 32 11. Photomicrograph of a hornblende porphyroblast with inclusions of diopside and hornblende 35 12. Photomicrograph of a hornblende porphyroblast surrounded by an ~-plane 35 13. Photomicrograph of two hornblende porphyroblasts showing fragments cataclastically sheared or pulled from the porphyroblasts 36 14. Photomicrograph of diopside poikiloblastically growing over quartz 38 15. Photomicrograph of a large hornblende porphyroblast that has largely altered 'to biotite 38 v ~.•4i5lJA.t i I 22 I WIIR_t iS nan. w M &I UJ£! @¢ JA • 16. Photomicrograph of an actinolite porphyroblast displaying inclusions of diopside and actinolite 42 17, Photomicrograph of statically altered ultramafic rock, possibly originally serpentinite 46 18. Generalized cross-section, drawn along line A-A' on the isogonal contour map of foliation dips (Plate 2), illustrating the pronounced foliation fan and the interpreted antiform or nappe-like structure 51 19. Schmidt net plot depicting the relationship of the synoptic foliation plane of the hornblende schist unit to the contoured maximum of horn- blende lineations 53 20. Alternative interpretive metamorphic histories for M 56 1 21, Interpretive metamorphic history for M2 59 22. Synoptic diagram showing an interpretive meta­ morphic history for the Oakhurst roof pendant and the relationship between the mesoscopic and microscopic fabrics '63 Plates 1. Geologic map of the Oakhurst roof pendant, back Mariposa a.nd Madera Counties, California pocket 2. Isogonal contour map of the central and back southern sections of the study area pocket vi 4 ~ ..1Q4,-.@h%; t1ZP!X1.lt&IW.¥4A$.k .• .U4¢MUAXOX Ll __ .• ~PLt4MJIIMW ''*-~ ..• I 4-¥[email protected]•W.;;c:;;:;ill£LbQ.J t£4¥$4U JS;)i# ... ABSTRACT The Oakhurst roof pendant, near Oakhurst, California, consists of three pre-Cretaceous clastic metasedimentary rock units, surrounded and intruded by rocks of the Sierra Nevada batholith. T-..ro zones of probable "sheared granitic" and "ultramafic" rocks trend northwesterly and are approxi- mately aligned with the Foothills fault system farther north. The dominant mesoscopic feature of the pendant is a northwesterly.striking foliation which forms a downward converging fan. Earlier-formed hornblende lineations have been transposed into this foliation plane, as shown by Schmidt-net projections. Rocks comprising the three units show three textural stages: (1) Early amph~~oles and diopside overgrown by later static amphibole pophyroblasts, Hornblende lineations in one of these units show a great-circle Schmidt-net distri- but ion. (2) A superimposed, well-developed, cataclastic ~-plane (the mesoscopic foliation). (3) Late, static growth of poikiloblastic micas and diopside, straight well-crystallized micas, near polygonal quartz and feldspar, epidote and biotite after amphibole vii and diopside, and chlorite after biotite. The "sheared granitic" and "ultramafic" rock bodies display the same prominant foliation and static and retro­ grade metamorphic textures as do rocks of the three major units, These data are interpreted as reflecting three domi­ nant post-sedimentary structural (D) and thermal (M) events: (1) M /D consists of episodes of synkinematic 1 1 metamorphism,·which resulted in an early foliation, , 11 and of static metamorphism, M . M is of the epidote­ 1 1 amphibolite facies. (2) n resulted in the cataclastic development of the 2 prominent fanning foliation, ; the fan is indicative of s2 an .antiformal structure. This antiform and foliation developed as a consequence of subvertical movement, as indi- cated by the geometry of transposition of hornblende linea- tions into .§. The "sheared granitic" and "ultramafic" 2 • rock bodies were emplaced along two shear zones during n2 These zones could be the roots of thrust sheets and possibly an extension of the Foothills fault system, which has been mapped as terminating 20 miles to the northwest, (3) M was a static metamorphic event associated with 2 batholithic intrusion. Locally, this event was characterized viii ..,J;'_:\'15Jift!J.41!!1i-----&"'1--&!llii ___ , !!1!2.1!'11 --l!IIJiii!!A~~-Ntr!!l.--!1. II!!L!l'P -~ ··· · a tQ__ 4! 42.4MJ¢A 5 SSQU, .. t.44Aii¢$Lk t .Q • initially by high temperatures, but retrograde conditions generally prevailed, M2 was dominantly of the albite­ epidote-hornfels facies, but locally of the hornblende­ hornfels facies. Radiogenic dates of batholithic rocks near the pendant date M2 as Late Jurassic-Middle Cretaceous. Therefore, M ;n and n are pre-Late Jurassic. 1 1 2 M /D may be a southerly expression of .the Late 1 1 Permian-Early Triassic Sonoma Orogeny. D apparently 2 reflects the deformation associated with the classical Nevada.n Orogeny, and M2 corresponds to late or post­ orogenic granitic intrusion of the Sierra Nevada batholith. ix i!ZS!Jb&M l&&UJ\Qf&i& MJCGLUJJ(.J. J1 r. - .. fil'RQ,tt;;;t¥4!4XWU!£&$_ .#41 C. &,W41tW 2£bJI%a%114i_.ZiC CHAPTER I INTRODUCTION Purpose and Scope of Investigation The purpose of this investigation was to study the structural and metamorphic history of a roof pendant in the Sierra Nevada batholith. Hereafter this pendant will be referred to as the Oakhurst roof pendant. It is located '20 miles southeast of the mapped termination of the Melones fault zone, a major component of the Foothills fault system (Clark, 1960; Fig. 1), and is made up of undifferentiated pre-Cretaceous·metasedimentary rocks (Strand, 1967) that are surrounded and partly intruded by granitic rocks of the batholithic complex. Because of the location of the pendant, the possibility that the fault zone may extend south an unknown distance (Cloos, 1932) was pursued as a secondary objective of the thesis. Location and Geography The area of study is in east-central California, between the towns of Oakhurst, Coarsegold, and Ahwahnee, 40 miles northeast of Fresno and 20 miles southeast of Mariposa (Fig. 1), California Routes 41 and 49 are the major paved highways into the area.
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