Paper No. 3
Presentation Time: 2:15 PM
POST – 6 MA SUBSIDENCE AND EXHUMATION OF THE SOUTHEASTERN SAN JOAQUIN BASIN, CALIFORNIA, IN RESPONSE TO MANTLE LITHOSPHERE REMOVAL
New thermo-mechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada, California, predict a complex spatio – temporal pattern of vertical surface displacements. We evaluate these models by using (U-Th)/He thermochronometry, together with other paleothermometry estimates, to investigate such topographic transients. We target sediments from the Kern Arch, a fan-shaped uplift located in the southeastern San Joaquin Basin, along the western flank of the southern Sierra. Kern Arch stratigraphy provides a unique record of subsidence and exhumation in a sensitive region immediately adjacent to the delaminating mantle lithosphere at depth. Detrital 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. Assuming a regional late Cenozoic geothermal gradient of 25°C/km, 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. Subtle differences in the maximum temperatures achieved in various wells may reflect differing degrees of tectonic subsidence and sedimentation as a function of distance from the range front. Our results are consistent with estimates of surface subsidence and uplift from Sierran delamination models, which predict a minimum of 0.8 km of subsidence in regions presently associated with mantle lithosphere at depth, and a minimum of 0.6 km of surface 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.