NEXT WEEK!!!!! CREST speaker - Dr. Robinson Cecil (CSU Northridge) Robinsons talk will be in ITV-C (Library basement) at 1pm on Monday, March 10th. Robinson will be talking about the tectonic significance of the Kern River Formation here in Bakersfield, and how its burial and exhumation history can be used to constrain crust-mantle processes beneath our feet. The Kern River Fm is the host of the Kern River oilfield, so this is petroleum geology, tectonics, and petrology rolled into one talk. Robinson is an assistant professor at CSU Northridge having completed her research at University of Arizona and CalTech - she has excellent insights into research opportunities in these schools. ABSTRACT: Thermomechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada region, 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 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 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 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 in latest Miocene - Quaternary time. 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 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 exhumation is interpreted to result from the northwestward peeling back of the slab and the associated replacement of dense lithosphere with buoyant asthenosphere, which drove rapid rock and surface uplift.
Posted on: Wed, 05 Mar 2014 23:32:40 +0000