2-PHASE EVOLUTION OF THE SHADOW VALLEY BASIN, SOUTH-EASTERN CALIFORNIA - A POSSIBLE RECORD OF FOOTWALL UPLIFT DURING EXTENSIONAL DETACHMENTFAULTING

Miocene strata of the Shadow Valley Basin rest unconformably on the up per plate of the Kingston Range - Halloran Hills detachment fault syst em in the eastern Mojave desert, California. Basin development occurre d in two broad phases that we interpret as a response to changes in fo otwall geometry. In southern portions of the basin, south of the Kings ton Range, phase one began with near synchronous initiation of detachm ent faulting, volcanism and basin sedimentation shortly after 13.4 Ma. Between c. 13.4 and c. 10 Ma, concordantly bedded phase one strata we re deposited onto the subsiding hangingwall of the detachment fault as it was translated 5-9 km south-westward with only limited internal de formation. Phase two (c. 10 to 8-5 Ma) is marked by extensional dismem berment of the detachment fault's upper plate along predominantly west -dipping normal faults. Phase two sediments were deposited synchronous ly with upper-plate normal faulting and unconformably overlie phase on e deposits, displaying progressive shallowing in dip and intraformatio nal onlap. Northern portions of the basin, in the Kingston Range, expe rienced a similar two-phase development compressed into a shorter inte rval 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 100 00 0 years immediately prior to c. 12.4 Ma. Differences in the duration o f basin development in and south of the Kingston Range apparently rela te to position with respect to the detachment fault's breakaway; north ern basin exposures overlie the upper plate adjacent to the breakaway (0-15 km) whereas southern basin exposures occur far from the breakawa y (20-40 km). We interpret the phase one to phase two transition as re cording breakup of the detachment fault's hangingwall during footwall uplift. We propose a model for supradetachment basin evolution in whic h early, concordantly bedded basin strata are deposited on the hanging wall as it translates intact above a weakly deforming footwall. With c ontinuing extension, tectonic denudation along the detachment fault le ads 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, mechanical ly unfavourable for simple upper-plate translation. Additionally, we a rgue that continuing hangingwall thinning during phase two places geom etrical 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 thermochronolog ical and geobarometric methods.