Stratigraphy and Structure of the Mendenhall Gneiss, South-Central San Gabriel Mountains, California By Jeffrey T. DeLand Undergraduate Senior Thesis presented to the faculty of the Geological Sciences Deparetment, California State Polytechnic University, Pomona, California May 22, 2003 i Abstract STRATIGRAPHY AND STRUCTURE OF THE MENDENHALL GNEISS By Jeffrey T. DeLand Department of Geological Sciences California State Polytechnic University, Pomona An undergraduate thesis presented on the Stratigraphy and Structure of the Mendenhall Gneiss including structural and lithologic data colleted in remote regions of the Angeles National Forest, southern California. Geologic field mapping of basement rocks in the south central San Gabnel Mountains has yielded data enabling correlation of the largest continuous section of metamorphic rock with displaced rock bodies located on opposing sides of strike-slip fault zones. Palinspastic reconstructions provide insights that can be used for correlation with mountain ranges such as the Chocolate and Orocopia Mountains located more than 200 km south, and on the north side of the San Andreas Fault. New data supports tighter constraints on piercing points along the San Gabriel Fault with 218 ± 3 Ma Triassic Mt. Lowe intrusive rocks in contact with 1700 Ma Paleoproterozoic Mendenhall Gneiss. Large and small scale structural features provide information which can be used to aid in reconstruction of late Cenozoic brittle deformation events, as well as help in the determination of which land mass (Antarctica, Australia, Siberia or China) rifted from the western Laurentian margin during the 750 Ma break up of Rodinia. ii TABLE OF CONTENTS Abstract……………………………………………………………………………………..ii List of Figures………………………………………………………………………………iv Introduction…………………………………………………………………………………1 Previous Studies……………………………………………………………………………5 Rock Units and Field Relations Paleoproterozoic Fine Grained Banded Gneiss………………………………………9 Paleoproterozoic Granite Augen Gneiss .....................................................................15 (Mz?) or (pC?) Foliated Diorite or Hornblende Diorite..............................................17 Triassic Plutonic and Intrusive Rocks.........................................................................19 Cretaceous Plutonic and Intrusive Rocks....................................................................22 Structural Analysis Plot Sets 1-8…………………………………………………………………...26-32 Methods…………………………………………………………………………...24 Foliation…………………………………………………………………………..33 Lineation……………………………………………………………………….....34 Faults……………………………………………………………………………...35 Regional Implications / Interpretations / Significance……………………………...…….36 Conclusions……………………………………………………………………………….47 Future Studies……………………………………………………………………………..49 Acknowledgments………………………………………………………………………...50 Bibliography…………………………………………………………………………........51 Plates Plate 1 - Geologic Map of the Study Area………………………………………..52 Plate 2 – Cross Section A-A’…………………………………………………..…53 Plate 3 – Cross Section B-B’……………………………………………………..54 Plate 4 – Cross Section C-C’……………………………………………………..55 Plate 5 – Cross Section D-D’……………………………………………………..56 Appendices Structural Data…………………………………………………………………57-58 iii LIST OF FIGURES 1. Geologic Map of the San Gabriel Mountains…………………………………………….3 2. Location Map of the Eastern Transverse Ranges showing Study Area .............................3 3. Simplified Geologic Map of Southern California showing Study Area.............................4 4. Metamorphic Facies Diagram showing Type Metamorphism............................................6 5. Index Map of Central San Gabriel Mountains showing Study Area..................................7 6. Photograph of Mendenhall Gneiss showing texture...........................................................8 7. Photograph of Mendenhall Gneiss showing foliation and fault........................................10 8. Photograph of Mendenhall Gneiss showing folding.........................................................11 9. Photograph of Mendenhall Gneiss showing folding.........................................................12 10. Geologic Map showing localities of Mendenhall Gneiss………………………………13 11. Photograph of granite augen gneiss showing foliation…………………………………16 12. Photograph of gneissic diorite showing cross cutting relationships ...............................17 13. Photograph of gneissic diorite xenolilth in Cretaceous Granite.......................................l8 14. Photograph of Mt. Lowe intrusive rocks with diorite dike……………………………..19 15. Photograph of gneissic diorite in Mt. Lowe…………………………………………….20 16. Distribution of Dating Localities for Triassic Mt. Lowe.................................................20 17. Concordia Diagram showing age of Josephine Mountain rocks.....................................23 18. Stereographic Plot of Mendenhall Gneiss.......................................................................33 19. Stereographic Plot showing metamorphic lineation........................................................34 20. Middle Miocene palinspastic reconstruction of San Gabriel Mtns.................................39 22. Paleoproterozoic reconstruction of Laurentia (Siberia Connection) ..............................45 23. Neoproterozoic reconstruction of Laurentia (Cathaysia)................................................46 24. Diagram showing possible connection of China and Laurentia (Cathysia) ...................47 iv I. Introduction The Mendenhall Gneiss was first studied while early California geologists began mapping an adjacent anorthosite - gabbro - syenite intrusive sequence during early exploration of the western San Gabriel Mountains (Miller, 1934, 1946), but was not studied in any detail until Oakeshott (1958) described the type locality around Mendenhall Peak (Barth, et al., 1995). This type locality has been the focus of many studies, (Barth, et al., 2001, see also figure 1) while other localities of the Mendenhall Gneiss remain largely unexplored. The purpose of this project is to map the stratigraphy and structure of one of these locations on the north side of the San Gabriel Fault. Using brunton compasses and topographic basemaps, data was taken on rock foliations and lineations, fault orientations along with slip direction lineation when available, as well as axial planar surfaces to construct a basement rock geologic map. The research area includes the largest unfaulted, continuous section of Paleoproterozoic metamorphic rock in the San Gabriel Mountains that can be compared and correlated with other areas. Late Cenozoic strike slip faulting along the San Andreas, San Jacinto and San Gabriel Faults have given way to the increasing study of structural relationships for the purpose of correlation. Various lithologic units may offer piercing points along certain faults which when palinspastic reconstructions are made show distinct patterns that can be useful in determination of the amount of movement on lateral faults as well as formulating the tectonic and/or depositional history of a region (Ehlig, 1981; Nourse, 2002, see figure 2). Detailed mapping of the San Gabriel Mountains basement terrain may also offer evidence as to which present-day continent existed on the western continental margin of Laurentia prior to Neoproterozoic rifting of the supercontinent 1 Rodinia. The San Gabriel Mountains offer a distinct challenge to the geologist in that multiple rock units of varying ages may be associated along multiple fault systems, and record many separate tectonic events. The present-day San Gabriel Mountains are believed to have been formed by Late Cenozoic thrust faulting along the south boundaries, for example the Sierra Madre and Cucamonga fault systems. These reverse faulting events may have been caused by the left lateral transform movement that offset the San Gabriel fault in the “Big Bend” region (Ehlig, 1981). This type of scenario combines transform movement with compression of the adjacent tectonic plates to form a rotational structural environment known as transpression and transrotation. Subsequent erosion and brittle deformation of the basement terrain, along with 160 – 240 km of movement laterally by the San Andreas Fault (Matti and Morton, 1993; Dillon and Ehlig, 1993; Powell, 1993), have shaped the crystalline rocks into what we see today. Figure 1 shows a location map including the major lithologic units and faults in the study area. The study area is marked in a red and black rectangle, and it can be seen on the Chileo Flats USGS 7.5 minute topographic quadrangle. It is located along Angeles Crest Highway (Highway 2) in the Angeles National Forest, just north of the San Gabriel Fault in the central San Gabriel Mountains. Figure 2 shows the location of the San Gabriel Mountains relative to the San Andreas Fault and San Bernardino Mountains; also possible correlative units are located in mountain ranges throughout Southern California (see figure 3). 2 Figure 1 Geologic Map of the San Gabriel Mountains modified from Ehlig (1981) showing approximate location of study area. Inset Map shows terranes of the San Gabriel Mountains; abbreviations are SGT = San Gabriel terrane; CT = Cucamonga terrane; SAT San Antonio terrane; PS = Pelona Schist. Abbreviations for Cretaceous Plutons in San Gabriel terrane are JMI = Josephine Mountain intrusion; VMS = Vetter Mountain stock; WMB = Waterman Mountain/Mt. Wilson batholith. Figure 2 Location map of the central and eastern Transverse Ranges of southern California,