Permian and Triassic rocks of the Mojave Desert and their implications for timing and mechanisms of continental truncation

Abstract

Upper Paleozoic and lower Mesozoic rocks in the Mojave Desert and adjacent areas record the change from a passive continental margin to a subduction margin in the southwestern United States. This change was apparently accompanied by truncation and reorientation of the continental margin in the Mojave Desert and western Basin and Range Province. From Late Proterozoic to Mississippian time the western margin of North America had a northeast‐southwest trend through Nevada and eastern California. Truncation of the margin began in Pennsylvanian time when a northwest trending, left‐lateral strike‐slip fault zone formed across the Antler orogenic belt, the Cordilleran miogeocline, and the more cratonal character Mojave Desert. Rocks of the Antler belt were apparently displaced southward from east central California to their present position in the northwestern Mojave Desert. Sedimentary rocks deposited during this interval record an active continental borderland, characterized by alternately subsided and uplifted marine basins. This transcurrent faulting continued to Late Permian time. Magmatic activity and deformation started in the Mojave Desert in Late Permian time and affected both miogeoclinal‐cratonal and displaced Antler rocks. Magmatic activity ceased and marine sedimentation resumed in Early Triassic time. Lower Triassic rocks constitute an overlap sequence on both deformed and undeformed miogeoclinal‐cratonal rocks and possibly on displaced Antler rocks. Facies trends in these rocks are northwest‐southeast, indicating the new margin trend. Truncation and reorientation of the margin were completed therefore by Early Triassic time, or possibly as early as Late Permian time. Upper Permian magmatism and deformation in the Mojave Desert are older than, and distinct from, the early Triassic Sonoma Orogeny of Nevada. Lower Triassic rocks in the Mojave Desert are post tectonic with respect to deformation in that region, whereas correlative Lower Triassic rocks in Nevada are pretectonic to syntectonic with respect to the Sonoma Orogeny. This difference reflects the variation in deformation and tectonic setting along a newly developed Cordilleran margin. The southern part of the Cordillera was apparently a subduction margin as early as Late Permian or Middle Triassic time, while the central (Nevada segment) was the site of collision tectonics.