TECTONIC REORGANIZATION IN THE DEATH VALLEY AREA AT 4 MA

The Death Valley region has undergone several phases of tectonic reorganization over the last 16 Ma with the penultimate reorganization occurring ~4.5-3 Ma. Quantitatively improving the timing and extent of uplifting regions and the companion basin deposition elucidates tectonic patterns and has implications for species distribution. In this study, we present updated ⁴⁰Ar/³⁹Ar dating and geochemical data that better constrains the tectonics and paleodrainage in the Death Valley region. Earlier K/Ar work dated basalt flows as young as 4.2 Ma in the Nova Formation in the Panamint Range. These basalts erupted from the Darwin Plateau in the Argus Range, flowed east toward Death Valley, and established that Panamint Valley formed after 4.2 Ma. We dated basalt flows in the Nova Basin between 5.40 ± 0.01 Ma to 4.06 ± 0.01 Ma (whole-rock ⁴⁰Ar/³⁹Ar). One 4.73 ± 0.04 Ma basalt flow, with trace element composition consistent with Darwin Plateau basalts, overlies an alluvial fan conglomerate with Hunter Mountain batholith boulders in central Death Valley. These data define two coalescing basins with differing headwaters that persisted until at least 4 Ma. To the northwest, we obtained a 3.92 ± 0.05 Ma date on a basalt that erupted in the Saline Range and flowed east into the Last Chance Range. This demonstrates that 4 Ma the Saline Range was a highland and the Last Chance Range a topographic low. In the northern Last Chance Range, tephrochronology and clast provenance, indicate rapid range uplift initiated about 3.5 Ma. In Deep Springs Valley, a conglomerate with an intercalated tephra bed indicate that the ridge separating Deep Springs Valley and Eureka Valley was a topographic low 3.1 Ma and a fluvial system extended from the White Mountains southeast into Eureka Valley. Tectonically, these data show that the penultimate tectonic reorganization began after 4 Ma adjacent Death Valley and after 3 Ma to the northwest.