Two-phase evolution of the Shadow Valley Basin, south-eastern California: a possible record of footwall uplift during extensional detachment faulting

Miocene strata of the Shadow Valley Basin rest unconformably on the upper plate of the Kingston Range - Halloran Hills detachment fault system in the eastern Mojave desert, California. Basin development occurred in two broad phases that we interpret as a response to changes in footwall geometry. In southern portions of the basin, south of the Kingston Range, phase one began with near synchronous initiation of detachment faulting, volcanism and basin sedimentation shortly after 13.4 Ma. Between c. 13.4 and c. 10 Ma, concordantly bedded phase one strata were deposited onto the subsiding hangingwall of the detachment fault as it was translated 5–9 km south-westward with only limited internal deformation. Phase two (c. 10 to 8–5 Ma) is marked by extensional dismemberment of the detachment fault's upper plate along predominantly west-dipping normal faults. Phase two sediments were deposited synchronously with upper-plate normal faulting and unconformably overlie phase one deposits, displaying progressive shallowing in dip and intraformational onlap. Northern portions of the basin, in the Kingston Range, experienced a similar two-phase development compressed into a shorter interval of time. Here, phase one occurred between c. 13.4 and 12.8–12.5 (?) Ma, whereas phase two probably lasted for no more than a few 100000 years immediately prior to c. 12.4 Ma. Differences in the duration of basin development in and south of the Kingston Range apparently relate to position with respect to the detachment fault's breakaway; northern basin exposures overlie the upper plate adjacent to the breakaway (0–15 km) whereas southern basin exposures occur far from the breakaway (20–40 km). We interpret the phase one to phase two transition as recording breakup of the detachment fault's hangingwall during footwall uplift. We propose a model for supradetachment basin evolution in which early, concordantly bedded basin strata are deposited on the hangingwall as it translates intact above a weakly deforming footwall. With continuing extension, tectonic denudation along the detachment fault leads to an increasing flexural isostatic footwall response. We suggest that isostatic footwall uplift may drive internal breakup of the upper plate as the detachment fault is rotated to a shallow dip, mechanically unfavourable for simple upper-plate translation. Additionally, we argue that continuing hangingwall thinning during phase two places geometrical constraints on the timing, amount and, thus, rate of footwall uplift. Kinematically determined footwall uplift rates (0.5–4.5 mm/yr) are comparable with rates determined independently by thermochronological and geobarometric methods.