Stratigraphy and paleogeographic significance of metamorphic rocks in the Shadow Mountains, western Mojave Desert, California

摘要

Stratigraphic correlations presented here for the ductilely deformed and metamorphosed rocks exposed in the Shadow Mountains indicate that they formed on the North American continental margin and are not exotic or significantly displaced from their site of origin. These strata represent a depositional history that spans Late Proterozoic and Paleozoic passive margin development, late Paleozoic transitional passive to active plate margin tectonics, and late Paleozoic–early Mesozoic establishment of a convergent margin along the western edge of the North American craton. The stratigraphic sequence in the Shadow Mountains is represented by a basal siliceous and calcareous section that is correlated with upper Proterozoic and Cambrian miogeoclinal strata. These strata are overlain unconformably(?) by a calcareous sequence correlated with rocks of Pennsylvanian and Permian age of borderland affinity. The uppermost sequence comprises hornfels and calcareous rocks that rest unconformably across older strata and are correlated with Permian and Triassic rocks that likely record uplift and erosion of a magmatic arc. These stratigraphic correlations have several important paleogeographic and tectonic implications for southwestern North America. First, passive margin, Late Proterozoic and early Paleozoic miogeoclinal facies extend as far west in the Mojave Desert as the Shadow Mountains. Second, the presence of Pennsylvanian and Permian borderland basin sediments suggests that the area was affected by late Paleozoic tectonics, probably associated with the transition from a passive to an active plate-margin setting. Third, inferred Late Permian–Triassic rocks record the onset of convergent-margin tectonism and magmatism in this region.