STRATIGRAPHY AND PALEOGEOGRAPHIC SIGNIFICANCE OF METAMORPHIC ROCKS INTHE SHADOW MOUNTAINS, WESTERN MOJAVE DESERT, CALIFORNIA

Stratigraphic correlations presented here for the ductilely deformed a nd metamorphosed rocks exposed in the Shadow Mountains indicate that t hey formed on the North American continental margin and are not exotic or significantly displaced from their site of origin. These strata re present a depositional history that spans Late Proterozoic and Paleozo ic passive margin development, late Paleozoic transitional passive to active plate margin tectonics, and late Paleozoic-early Mesozoic estab lishment of a convergent margin along the western edge of the North Am erican craton. The stratigraphic sequence in the Shadow Mountains is r epresented by a basal siliceous and calcareous section that is correla ted with upper Proterozoic and Cambrain miogeoclinal strata. These str ata are overlain unconformably(?) by a calcareous sequence correlated with rocks of Pennsylvanian and Permian age of borderland affinity. Th e 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 ar c. These stratigraphic correlations have several important paleogeogra phic and tectonic implications for southwestern North America. First, passive margin, Late Proterozoic and early Paleozoic miogeoclinal faci es extend as far west in the Mojave Desert as the Shadow Mountains. Se cond, the presence of Pennsylvanian and Permian borderland basin sedim ents suggests that the area was affected by late Paleozic tectonics, p robably associated with the transition from a passive to an active pla te-margin setting. Third, inferred Late Permian-Triassic rocks record the onset of convergent-margin tectonism and magmatism in this region.