PROVENANCE OF MIOCENE STRATA OF THE EL PASO BASIN, SOUTHERN CALIFORNIA, AND CONSTRAINTS ON THE UPLIFT HISTORY OF THE SOUTHERN SIERRA NEVADA

Detrital zircon U-Pb and Hf analyses of Miocene strata of the El Paso Basin provide new insight into the paleogeographic and paleotopographic history of the southern Sierra Nevada and northern Mojave regions. The El Paso Basin is uniquely situated north of the Garlock Fault and east of the southern Sierra Nevada (SSN); Miocene basin strata can therefore record important changes in the Late Cenozoic tectonic history of adjacent basement blocks. Miocene strata consist of the volcaniclastic Cudahy Camp Formation and the overlying fluvio-lacustrine Dove Spring Formation. An ~18 Ma sandstone collected from the base of the Cudahy Camp Fm yielded a unimodal detrital zircon (DZ) U-Pb peak age of ca. 250 Ma. This age indicates local sourcing from the Permo-Triassic El Paso Mountains to the south. Volcaniclastic units in the upper Cudahy Camp Fm and the lower Dove Spring Fm contain north-directed paleocurrents and are dominated by zircon grains with Miocene ages (17 – 13 Ma), which are interpreted to derive from the Eagle Crags volcanic center south of the Garlock Fault. Higher in the Dove Spring section, there is an abrupt change in provenance marked by the influx of zircon grains with ages ranging from ca. 110 – 78 Ma and DZ peaks at ca. 88 Ma. Plutons with ages of 110 – 80 Ma are characteristic of the voluminous Late Cretaceous flare-up in the Sierra Nevada batholith, but are also abundant throughout much of the Mojave block, making provenance ambiguous. Hf isotope analysis of dated detrital zircons was performed in order to help distinguish between source areas with overlapping U-Pb ages. We identify two groups in our Cretaceous DZ dataset: group 1 consists of zircons with U-Pb ages ranging from 115 – 94 Ma and initial εHf values of -7 to +2; group 2 consists of zircons with U-Pb ages ranging from 94 - 74 Ma and initial εHf values of -10 to -3. We interpret group 1, the older and less evolved (higher εHf) zircons as being sourced from plutons in the Sierra Nevada. We interpret group 2, the younger and more evolved (lower εHf) zircons, as being derived from plutons in the Mojave Block. The onset of zircons from group 1 at ca. 10 Ma suggests an uplift and erosion event in the SSN at that time, which is 3-7 m.y. earlier than some sedimentary, geomorphic, and thermochronologic estimates but consistent with thermochronology from the Tehachapi range west of the study area.