EVIDENCE FROM INDIAN WELLS VALLEY AND COSO RANGE FOR MIOCENE INITIATION OF THE EASTERN ESCARPMENT OF THE SOUTHERN SIERRA NEVADA, CALIFORNIA

The eastern front of the Sierra Nevada Range forms the western margin of the Basin and Range province. Timing and mechanisms of uplift of the Sierra Nevada has been studied by many workers using a variety of approaches. We present evidence that the eastern margin of the Sierra Nevada in its southern reaches from the Garlock fault northward to Owens Lake formed as a normal fault active in late Miocene time. We base this interpretation on subsurface data gathered in the Indian Wells Valley and study of surface outcrops in the western Coso Range.

Data from the Indian Wells Valley come from seismic reflection lines, deep and shallow drill holes, and potential field studies. Seismic reflection lines across the Indian Wells Valley show a low-angle normal fault zone dipping eastward from the Sierran front. Seismic facies calibrated to deep drill holes show westward coarsening deposits that are up to 1.5 km thick. Ar-Ar dating of material recovered from drill holes reveals that most of these rocks were deposited between 7 and 4 Ma. Gravity data show that these rocks thicken significantly just north of the reflection line. Outcrop study was done on the Mio-Pliocene Coso Formation in the northwestern Coso Range. These rocks pinch out eastward onto the Coso Range, but thicken significantly to the west. Facies studies show these nonmarine rocks to have been deposited in a fluvio-lacustrine setting receiving significant volcanic input. Ar-Ar and (U-Th)/He ages bracket deposition to between 6 and 4 Ma, although some slightly older deposits are know from areas to the north. Structural studies show some syn-sedimentary normal faults cutting these deposits.

Depositional characteristics of the Coso Formation are consistent with subsurface studies: these data show that the fault (imaged as a low-angle normal fault in seismic data) became active as the eastern margin of the southern Sierra Nevada starting in late Miocene time. This fault defining the eastern margin of the Sierra Nevada controlled basin development and facies characteristics. How development of this escarpment is related to overall surface uplift of the Sierra is uncertain, but must reflect creation of significant elevation in the region.