Downloadable ContentDownload PDF
Pliocene-Quaternary subsidence and exhumation of the southeastern San Joaquin Basin, California, in response to mantle lithosphere removal
Thermomechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada region, California (USA), predict a complex spatiotemporal 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 Tertiary strata from the Kern arch, a crescent-shaped active uplift located in the southeastern San Joaquin Basin, along the western flank of the southern Sierra Nevada. 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 Oligocene-Miocene sandstones collected in Kern arch well cores indicate postdepositional heating to temperatures beyond those corresponding with their present burial depths. When integrated with available geologic and stratigraphic constraints, temperature-time modeling of thermochronometric 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. By constraining the late Cenozoic geothermal gradient to ?25 °C/km, our results imply 1.0-1.6 km of rapid (?0.4 mm/yr) subsidence and sedimentation, and then subsequent uplift and exhumation of southeastern San Joaquin Basin strata in latest Miocene-Quaternary time. Stratigraphic and geomorphic relations further constrain the principal burial episode to ca. 2.5 Ma or later, and exhumation to ca. 1 Ma or later. Subtle differences in the maximum temperatures achieved in various wells may reflect differing degrees of tectonic subsidence and sedimentation as a function of growth faulting and 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 ?0.7 km of tectonic subsidence in regions retaining mantle lithosphere adjacent to the area of delamination, and a minimum of ?0.8 km of rock uplift in regions where delamination occurred recently. 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 exhumation is interpreted to result from the northwestward peeling back of the slab and the associated replacement of dense lithosphere with buoyant asthenosphere that drove rapid rock and surface uplift.