SPACE-TIME PATTERNS AND GEODYNAMIC DRIVERS OF MIOCENE TO PLIOCENE STRAIN IN DEATH VALLEY, CA FROM LOW TEMPERATURE THERMOCHRONOLOGY

We use apatite and zircon (U-Th)/He thermochronology to evaluate temporal and spatial patterns and geodynamic drivers of Miocene to Pliocene strain within the Death Valley area of eastern California. We analyzed 48 samples from the footwalls of the Amargosa-Black Mountains and Panamint-Emigrant detachment faults in the Black and Panamint mountains, respectively. Samples were collected from a range of lithologic units, including Proterozoic gneiss and metasedimentary rocks, Cretaceous gneiss and leucogranite, and Tertiary diorite and granite. Zircon He ages from the Black Mountains record uninterrupted footwall cooling and exhumation from 9 to 3 Ma. Numerical modeling of these data indicates that this cooling occurred during two phases. From 10-6 Ma, cooling was rapid, with rates ranging from ~5-10 mm/yr. This period is followed by slower exhumation (<5 mm/yr) since 6 Ma. The cumulative exhumation is estimated to be 10-16 km, similar to geobarometric estimates for the Willow Springs pluton (Holm et al., 1992). Samples from the central Panamint Range also show two periods of cooling. Zircon He ages record late Miocene cooling and an exhumed He partial retention zone, whereas apatite He ages are invariant with paleodepth and document rapid cooling at ca. 4 Ma. The results suggest that the Panamint Range has experience 180+˚C of cooling or a minimum of 7.2 km of exhumation since ~12 Ma. When examined in the context of published fault timing data, the new data indicate that the transition from Basin and Range extension to dextral transtension associated with the eastern California shear zone occurs as a westward migrating wave. The transition begins at ~8 Ma, in ranges to the east and north of the Black Mountains, coincident with a major change in the kinematics of the plate boundary. Dextral transtension then migrates westward into eastern Death Valley at 6 Ma. Data from Panamint Range and several ranges to the west of Death Valley indicate that transtension initiated across a broad region simultaneously at ca. 3-4 Ma. These Pliocene ages match the timing of lithospheric delamination in the central and southern Sierra, suggesting that by ~4 Ma internal or intraplate factors, rather than external controls like plate boundary kinematics, may be have become the dominant geodynamic driver.