DETRITAL ZIRCON ANALYSIS OF MIOCENE STRATA OF THE EL PASO BASIN, SOUTHERN CALIFORNIA, AND NEW CONSTRAINTS ON UPLIFT OF THE SOUTHERN SIERRA NEVADA

Stratigraphic and detrital zircon analyses of Miocene strata of the El Paso Basin provide new constraints on the tectonic history of the northern Mojave and southeastern Sierra Nevada. The uplift history of the southern Sierra is debated; thermochronologic studies argue for uplift in early to middle Cenozoic time, whereas geomorphic and sedimentologic evidence suggests post ~7 Ma uplift. Strata of the El Paso Basin, which is uniquely situated north of the El Paso Mountains (EPM) and east of the southern Sierra Nevada, record changing tectonic conditions in the surrounding basement blocks. Miocene strata consist of the volcaniclastic Cudahy Camp Formation and the overlying fluvial lacustrine Dove Spring Formation. We measured and described 824 m from the base of the Cudahy Camp Formation into the Dove Spring Formation, measured 44 paleocurrent directions at 3 sites, and collected samples for detrital zircon (DZ) analysis to determine: 1) the provenance of sandstones, 2) the paleogeography of the El Paso Basin, and 3) the tectonic activity of adjacent basement blocks.

The two lowest DZ samples, which have estimated depositional ages of 17 Ma and 12.5 Ma, yielded unimodal U-Pb peak ages of 17.3 Ma and 15.1 Ma, respectively. These ages, together with north-directed paleocurrents, indicate sourcing from local volcanic centers in the northern Mojave block. Higher in the Dove Spring Formation, a sandstone with an estimated depositional age of 12 Ma yielded a DZ spectrum with peak ages at 340-310 Ma, 250 Ma, and 13.8 Ma. The Carboniferous and Permo-Triassic peaks are interpreted to derive from the EPM basement and the Miocene grains from volcanic centers to the south, also consistent with N-NW directed paleocurrent data. A sandstone, collected at 170 m above the previous sample, with an inferred depositional age of ~11.6 Ma yielded DZ peak ages at 88 Ma and 14 Ma. This last sample is interpreted to signal influx from a southeastern Sierra Nevada source, thereby marking an abrupt shift in basin provenance. These data provide evidence for uplift of the southern Sierra Nevada at ca. 12-11.6 Ma, consistent with the onset of rotation of the basin and uplift in the Tehachapi Mountains, but 4-5 million years earlier than previous estimates.