GEOLOGY OF THE DEATH VALLEY JUNCTION 30’ X 60’ QUADRANGLE — THE BASIS FOR INTERPRETING THE VOLCANO-TECTONIC EVOLUTION OF THE CENTRAL DEATH VALLEY AREA

The Death Valley Junction 30’ x 60’ quadrangle encompasses stratigraphic and structural geologic features recording the Cenozoic volcano-tectonic evolution of central Death Valley. Most notably, the central Death Valley rhombochasm is a 65x80-km rhombic pull-apart basin occupying the releasing step-over between the northern Death Valley—Furnace Creek and southern Death Valley faults. Stewart (1983) documented this feature by palinspastically restoring offset thrust fault segments and isopachs, thereby closing the rhombochasm. The central Death Valley volcanic field records the contemporaneous magmatism that occurred during extension and strike-slip strain that formed the rhombochasm. In the evolution of this tectonic-magmatic feature, changes in volcanic and structural styles, rates, and loci were synchronized spatially and temporally. The volcanic field covers an area of 3600 km², and consists of >700 km³ of lava flows, domes, and pyroclastic deposits of four major eruptive stages: Stage 1 (11-9 Ma: rhyolite and andesite), Stage 2 (9-7.5 Ma: dacite>basalt>andesite), Stage 3 (7-5 Ma: dacite>basalt), and Stage 4 (4.5-0.7 Ma: basalt). The loci of eruptive centers migrated northwestward synchronously with northwestward migration of tectonically controlled, adjacent depocenters. Stage 1 and 2 volcanism is broadly correlative to the supradetachment stage of rhombochasm development. Related instrusions include exposed upper-plate hypabyssal and lower-plate plutonic bodies. Stage 3 and 4 volcanism occurred during two tectonic stages in which higher-angle faults cut across the detachment fault, forming basins that are nested within the original detachment-floored area of the rhombochasm. Time-transgressive changes from dominantly silicic and intermediate magmas in Stages 1 and 2 to dominantly mafic and lesser intermediate magmas in Stages 3 and 4 coincided with decreases in eruptive volumes and rates, suggesting decreasing recharge to the magma system through time. Because of these changing volcanic rates, early syn-detachment basin fill in the rhombochasm is dominantly volcanic, where post-detachment basin-fill is mainly sedimentary. Only the westernmost part of the rhombochasm is still tectonically active, reflected in Quaternary faulting of basin fill in central Death Valley.