STRATIGRAPHIC CONSTRAINTS ON THE STYLE AND MAGNITUDE OF EXTENSION IN THE SOUTHERN BLACK MOUNTAINS, DEATH VALLEY, CALIFORNIA - 10.5 MA TO PRESENT

Two types of models for extension in the southern Black Mountains (SBM) have been proposed. The first is the "rolling-hinge model", where extension of the crust is localized along a single detachment fault. The second is the "distributed-faulting model," where extension of the crust is distributed among several generations of subparallel normal faults. In the "rolling-hinge model," the metamorphic basement and Miocene intrusive igneous rocks in the SBM would be in the footwall of a single detachment fault, the Amargosa Fault, and extension within the footwall would be relatively minor. In the "distributed-faulting model," the metamorphic and igneous rocks in the SBM would be cut by several generations of normal or oblique-slip faults, and extension within the SBM would be large. Stratigraphic analysis and mapping of 10.5-4 Ma sedimentary rocks in the SBM region allows for the testing of these 2 types of models. Analysis of these rocks (exposed in a 65-km-long belt from southern Death Valley to the Kingston Range) suggests that, from 10.5 to 7.8 Ma, a <20-km-wide basin opened between the Kingston Range and Panamint Mountain blocks on a west-dipping normal-fault system. These basin strata unconformably overlie highly attenuated Proterozoic rocks within the Virgin Spring phase of the Amargosa Chaos, thus suggesting that much of the faulting in the Virgin Spring phase predated 10.5 Ma. In the SBM, the 10.5-7.8 Ma strata are overlain unconformably by an unroofing sequence that records the 7.8 Ma disruption of this older sedimentary basin and the 7.8-4 Ma tectonic denudation of the SBM. Mapping and cross-section analysis indicates that the post-7.8 Ma denudation of the SBM occurred not on the Amargosa Fault, but rather along the likely westward extension of the Sheephead Fault (SF) and a younger generation of domino faults. Mapping south of Rhodes Hill indicates that this extension of the SF cuts the Amargosa Fault and the 9-11 Ma Smith Mountain granite, and places the 7.8-4 Ma unroofing sequence in fault contact with basement (with left-lateral drag). West of Jubilee Pass, the SF places both the 10.5-7.8 and 7.8-4 Ma basin strata in fault contact with basement, with top to the northwest slip indicators. These observations suggest that the "distributed-faulting model" is the more-correct model for post-7.8 Ma extension within the SBM.