U-Th and (U-Th)/He zircon geochronology redefines the timing of volcanic activity in the Salton Trough (Southern California, USA), the subaerial extension of the incipiently oceanic Gulf of California. U-series disequilibrium corrected (U-Th)/He zircon analyses for a granophyre ejecta clast from the Red Island rhyolite dome indicate an eruption age of 2480 ± 470 a (calendric dates between 0 and 940 Before Common Era, BCE; error at 95% confidence). This eruption age is supported by U-Th zircon crystallization ages for two obsidian-bearing lavas: Red Island (the host for the granophyre) and Obsidian Butte, a prehistoric quarry for obsidian that is widely distributed in southern California and northern Mexico archaeological sites. Lavas and granophyre display overlapping zircon crystallization age distributions that support field and compositional evidence that they are cogenetic and contemporaneous. The (U-Th)/He eruption age is younger and significantly more precise than previous ages for these volcanoes, and is the first indication that the eruption of obsidian flows coincided with human presence in the region. A late prehistoric eruption age agrees with the absence of the Obsidian Butte lithic source among early prehistoric cultural artifacts, previously attributed to submergence of the quarry location during hypothesized persistent flooding by ancient Lake Cahuilla.

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