Thesis

Provenance of Miocene Strata of the El Paso Basin, Southern California: Constraints on the Late Cenozoic Paleogeography of the Southern Sierra Nevada Region

Detrital zircon U-Pb and Hf analyses of Miocene strata in the El Paso Basin provide new constraints on the paleogeography of the northern Mojave and southern Sierra Nevada region. The El Paso Basin is located between the fault-bounded southeastern flank of the Sierra Nevada and the northern margin of the Mojave Desert, which is bounded by the Garlock fault and the El Paso Mountains immediately north of it. Due to its unique location, El Paso Basin strata can record important indicators of tectonic activity in surrounding basement blocks and geologic structures. Basin strata consist of the Paleogene Goler Formation and overlying Miocene Ricardo Group, made up of the volcaniclastic Cudahy Camp Formation and fluvio-lacustrine Dove Spring Formation. A sandstone near the base of the Cudahy Camp Fm yielded a unimodal detrital zircon U-Pb peak age of ca. 250 Ma, indicating local sourcing from the Permian-Triassic El Paso Mountains. Higher in the section, a sandstone from the upper Cudahy Camp Fm yielded a unimodal DZ peak age of ca. 18 Ma. That age supports previous correlation of the Cudahy Camp Fm with the Eagle Crags Volcanic Field, which is south of the Garlock fault and now offset ~64 km to the northeast. The lower Dove Spring Formation is dominated by locally derived boulder and pebble conglomerate with 14 Ma DZ age peaks. The sediment texture, DZ age population, and north-directed paleocurrents indicate local sourcing from the northern Mojave, likely the Lava Mountains area. Higher in the Dove Spring Formation, there is an abrupt change in provenance at ca. 12 Ma marked by the influx of Cretaceous zircon grains with ages ranging from ca. 124-74 Ma and dominant age peaks at ca. 88 Ma. Plutons with ages of 110-85 are characteristic of the voluminous Late Cretaceous Sierra Nevada batholith, but are also abundant throughout much of the Mojave block, making provenance ambiguous based on DZ age alone. Hf isotope analyses of dated detrital zircon grains were performed in order to help distinguish between source areas with overlapping U-Pb ages. Initial Hf values in Late Cretaceous detrital zircons range from -10.7 to +7.1, generally becoming more negative through time. Based on available basement geochronology and isotope data, most of these grains are interpreted to derive from the southeastern Sierra Nevada, though derivation of some of these grains from sources in the Mojave Block cannot be precluded. Analysis of detrital zircons in the El Paso Basin leads to the following interpretations: 1) basin development rejuvenated in the Early Miocene marked by deposition of the Cudahy Camp Formation in response to uplift of the El Paso Mountains and possible normal movement on a nascent Garlock fault; 2) the provenance of sediment was north-directed through the middle Miocene from volcanic centers south of the Garlock fault; 3) left-lateral slip on the Garlock fault activated by 12.5 Ma, providing the Lava Mountains a pathway to the El Paso Basin; and 4) the arrival of Late Cretaceous zircons between ca. 12-10 Ma suggests uplift of the southern Sierra Nevada, perhaps via activation of normal movement on the Sierra Nevada Frontal fault system; consistent with Late Cenozoic uplift models, but 2-7 m.y. earlier than previous estimates.

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