GEOLOGIC EVOLUTION AND REGIONAL IMPLICATIONS OF SOUTHERN OLD DAD MOUNTAIN, MOJAVE DESERT, CALIFORNIA

We present new geologic mapping and related studies for Old Dad Mountain in the eastern Mojave Desert, California. The area contains poorly understood Mesozoic and Cenozoic structures and rocks. A typical Proterozoic to early Mesozoic succession is exposed (basement rocks to Triassic Moenkopi Formation). The section is overlain by undated volcanic rocks (now metamorphosed) almost certainly of Mesozoic age. In addition, the volcanics are intruded in their southern exposures. There are three major structures observed at Old Dad: 1) the late(?) Mesozoic Powerline Canyon shear zone that juxtaposes Precambrian basement rocks on the east against to Mesozoic metavolcanic and plutonic rocks on the west; 2) a probable Sevier structure, the Playground thrust, that transports Paleozoic carbonate rocks to the southeast atop the Powerline Canyon shear zone and related rocks; and 3) the late Cenozoic Old Dad normal fault that is most likely associated with Miocene extension. Mapping was mostly consistent with the geology described by Dunne (1977). In addition, we collected samples of the Sands Granite and two metavolcanic units for U-Pb geochronology (results are pending).

The mapping and new ages will place the age of structures and rocks in much better context with the regional tectonics. The Sands Granite is intruded into the metavolcanic units and both are deformed in the Powerline Canyon shear zone and truncated by the Playground thrust. Due to these relationships, the Powerline Canyon shear zone is probably Cretaceous or older in age, but regional correlations are uncertain. The Powerline Canyon shear zone is also similar to the active, dextral Soda-Avawatz fault zone in that it juxtaposes Mesozoic metavolcanic rocks on the west again Proterozoic rocks on the east. It is clear that the Powerline Canyon structure is not active, but may record older motions possible on the Soda-Avawatz fault, or may be similar to faults in the Clark and New York Mountains.