PLIOCENE - QUATERNARY TOPOGRAPHIC TRANSIENTS IN THE SOUTHEASTERN SAN JOAQUIN VALLEY, CALIFORNIA, LINKED TO MANTLE LITHOSPHERE REMOVAL

Thermo-mechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada, California, predict complex regional patterns of subsidence and uplift through both time and space. Detrital apatite (U-Th)/He thermochronometry, together with other paleothermometry estimates, is used to evaluate these models and to investigate such topographic transients. We present results from sediments sampled on the Kern Arch, a crescent-shaped uplift located in the southeastern San Joaquin Basin, immediately adjacent to delaminating mantle lithosphere at depth. Apatite (U-Th)/He ages from Oligo-Miocene sandstones collected in Kern Arch well cores indicate post-depositional heating to temperatures beyond those corresponding with their present burial depths. When integrated with available geologic and stratigraphic constraints, temperature – time modeling of thermochronometry data suggests partial He loss from apatites at temperatures of 70° – 90°C, followed by exhumation to present burial temperatures of 35° – 60°C since ca. 6 Ma. Thermal histories derived from modeling of apatite (U-Th)/He data agree well with independent results from vitrinite reflectance analysis and chlorite thermometry, which both indicate recent burial and heating of sediments to minimum temperatures of 70°C. Our results imply 1.0 – 1.6 km of rapid (~ 0.4 mm/yr) burial and subsequent exhumation of southeastern San Joaquin sediments in latest Miocene - Quaternary time. Stratigraphic and geomorphic relations further constrain the principal burial episode to post – 2.5 Ma and exhumation to post – 1 Ma. Our results are consistent with estimates of surface subsidence and uplift from Sierran delamination models, which predict a minimum of 0.7 km of subsidence in regions presently associated with mantle lithosphere at depth, and a minimum of 0.8 km of rock uplift in regions where delamination has recently occurred. We attribute the marked pulse of tectonic subsidence in the San Joaquin Basin to viscous coupling between the lower crust and a downwelling mass in the delaminating slab. The ensuing episode of denudation is interpreted to result from the northwestward peeling back of the slab and the associated replacement of dense lithosphere with buoyant asthenosphere.