PERMO-TRIASSIC PALEOGEOGRAPHY AND TECTONICS OF THE SOUTHWESTERN UNITED STATES by JAMES DOUGLAS IALKER B.S. Massachusetts Institute of Technology (1980) M.S. Massachusetts Institute of Technology (1981) Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in Partial Fulfillment of the Requirements of the Degree of DOCTOR OF PHILOSOPHY IN GEOLOGY at the MASSACHUSETTS INSTITUTE OF TECHOLOGY June 1985 @ Massachusetts Institute of Technology 1985 Signature of Author: ADepart-ft of Earth,"' Atmospheric, and Planetary Sciences June 25, 1985 Certified by: B.C.L Burchfi~.Y Thesis Supervisor Accepted by: ------------r VT.R. Madden Departmental Graduate Advisor OCT 10 '85 2 PERMO-TRIASSIC PALEOGEOGRAPHY AND TECTONICS OF THE SOUTHWESTERN UNITED STATES by JAMES DOUGLAS WALKER Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on June 25, 1985 in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Geology ABSTRACT Upper Permian and Lower Triassic rocks in the Mojave Desert and adjacent areas record the change from a passive margin to a subduction margin in the southwestern United States. This change was preceded by truncation of the continental margin and reorientation of the margin and paleogeographic trends in the Mojave Desert and western Basin and Range Province; areas to the east apparently were unaffected. The western continental margin of North America trended northeast-southwest through Nevada and California from late Precambrian to Devonian time. During the Antler Orogeny in latest Devonian and earliest Mississippian time, eugeoclinal rocks were thrust onto the continental margin as a structurally complex package called the Roberts Mountain Allochthon. The position and nature of the continental margin changed during this event, but their overall trends were unaffected. The Roberts Mountain Allochthon became part of the continental margin of North America. Processes leading to the truncation of the continental margin began in Pennsylvanian time when a strike-slip fault zone formed across the Antler belt and the miogeocline. A continental borderland developed in east-central California during this event. Rocks of the Roberts Mountain Allochthon and underlying slope-facies strata were displaced southward during the strike-slip faulting to their present position in the northwestern Mojave Desert; they are presently exposed in the El Paso Mountains, Pilot Knob Valley, and Lane Mountain area. In Late Permian time a magmatic arc developed in the Mojave Desert on the displaced Roberts Mountain Allochthon rocks and on cratonal-miogeoclinal rocks in the Victorville area and San Bernardino Mountains (?). Magmatism was accompanied by deformation and metamorphism. The name Sidewinder event is proposed for this arc activity and deformation. The presence of magmatic rocks, deformation, and metamorphism in the western Mojave Desert may tie the displaced rocks into their present position by Late Permian time. Marine sedimentation resumed in the Mojave Desert during a lull in arc magmatism in Early Triassic time. Deposition of Lower Triassic rocks in the western Mojave Desert took place in shallow-marine basins surrounding mountains and hills. In the eastern Mojave Desert sedimentation was in shallow-marine basins only locally flanked by hills; relief is recorded by clasts derived from Precambrian crystalline rocks in Lower Triassic conglomerates in the Soda Mountains and Clark Mountains. In the Mojave Desert, lithofacies belts trend northwest-southeast while to the east of these areas lithofacies follow Paleozoic northeast-southwest trends. The Lower Triassic rocks constitute an overlap sequence for the Sidewinder event and for juxtaposition of rocks of the Roberts Mountain Allochthon with rocks of the craton-miogeocline in the western Mojave Desert. The name Sidewinder overlap sequence is proposed for the Lower Triassic rocks. The presence of Lower Triassic rocks in the western Mojave Desert that are correlative with rocks to the east precludes large post-Early Triassic movements of the western Mojave Desert relative to the North American craton. Arc magmatism resumed in Middle Triassic time. Volcanic and plutonic rocks of this age are present in the Victorville area, Soda Mountains, Devils Playground, Inyo Mountains, Argus Range, Saddelbag Lake area, Yerrington, and probably southeastern California and southern Arizona. Upper Permian rocks related to the Sidewinder event in the western Mojave Desert record the first occurrance of Phanerozoic arc magmatism in the southwestern part of the United States. The Sidewinder event is older than the Sonoma Orogeny of Nevada. Lower Triassic rocks in the Mojave form an overlap sequence for the Sidewinder event; correlative rocks in Nevada are pretectonic to posttectonic to the Sonoma Orogeny. Thesis Supervisor: Dr. B.C. Burchfiel Title: Schlumberger Professor of Geology TABLE OF CONTENTS Abstract .............................................................. 2 List of Figures ....................................... 7 Acknowledgements ..................................................... 9 Chapter 1: Outline of Problem Introduction ........................................ 12 Paleozoic Continental Configuration ............................. 15 Initial 87Sr/ 86Sr Ratio ................................... 16 Facies Distributions ...................................... 19 Tectonic Setting ..................... ...................... 21 Margin Truncation ....................................... 23 Northwestern Mojave Rocks and Antler Belt ....................... 23 Early Arc Activity ............................................. 26 Outer Truncation ................................................ 27 Geologic Time Scale ............................................ 28 Summary ........................................ 31 Chapter 2: Regional Setting Introduction ........................................ 32 Colorado Plateau Structural Block .............................. 32 Mojave Desert Structural Block ................................. 35 Basin and Range Province ....................................... 36 Chapter 3: Southern Nevada and Mojave Desert Introduction .................................................... Colorado Plateau ............ ............................. ······· Spring Mountains ............ ............................. ······· Clark Mountain .............. ............................. ······· Providence Mountains ........ ............................. ······· New York Mountains .......... ............................. ······· Devils Playground ............................. ······· Soda Mountains .............. ............................. ······· Cave Mountain ............................. ······· Victorville ................. ............................. 1 Shadow Mountains .... ....... ···· ···· ······ ····· ···· Northwestern Mojave Rocks .. ···· ···· ······ ····· ···· El Paso Mountains ... ···· ···· ······ ····· ···· Lane Mountain ...... ... ··········· Summary ............ ... ··········· Chapter 4: Related Areas Introduction ................ ··············9 ···· ······ .... ··· 86 Southeastern California ..... ·············· ···· ······ .... ··· 86 Death Valley Area ........... ·············· ···· ······ .... ··· 90 Argus Range and Darwin Hills ·············· ···· ······ .... ··· 95 Inyo Mountains .............. ··············| ···· ······ .... ·· 101 Mina-Western Nevada ......... ·············· ···· ······ .... ·· 103 Southeastern Sierran Pendants ············· ···· ······ .... ·· 108 Saddlebag Lake Pendant ·············· ···· ······ .... ·· 109 White Mountains ............. ·· 110 Eastern Nevada, Utah, and Arizona .............................. 111 Somona Orogeny ....................................... 119 Chapter 5: Permian and Triassic Paleogeography and Tectonic Settings Introduction ........................................ 124 Late Permian Paleogeography ................................... 124 Late Permian Tectonic Setting ................................. 132 Early Triassic Paleogeography ................................. 134 Early Triassic Tectonic Setting ................................ 142 Miogeocline in Mexico ......................................... 146 Limits of Terrane Accretion ................................. 147 Middle Triassic Arc ........................................... 147 Suggestions for Nomenclature .................................. 148 Chapter 6: Margin Modification .................................... 151 Chapter 7: Structural Problems General Structural Problems .................................... 168 Deformation of the Mojave Desert ............................... 175 Chapter 8: Conclusions ............................................. 187 References ....................................... 189 Appendix ........................................ 204 LIST OF FIGURES AND PLATES Figure 1: Late Precambrian to Mississippian paleogeographic trends in the southwestern United States ................. Figure 2: Names used for Permian and Lower Triassic stages in this study. ............. o............. e... ... ...... 30 Figure 3: Structural blocks in the western United States ............ 34 Figure 4: Location map for areas mentioned in text. .. 00 ...... .. 00 39 Figure Key to patterns on stratigraphic columns. 41 Figure Grand Canyon section ...................... 43 Figure Spring Mountain section . .................. 47 Figure Section in the northern Providence Mountains. 51 Figure New