MIO-PLIOCENE STRATIGRAPHIC AND STRUCTURAL EVIDENCE REFUTES SIMPLE ROLLING-HINGE MODEL FOR POST-7.8 MA EXTENSION IN THE SOUTHERN BLACK MOUNTAINS AND INDICATES LARGE-SCALE EXTENSION DISTRIBUTED AMONG MANY FAULTS

Stratigraphic analysis and mapping in the footsteps of Lauren Wright and Bennie Troxel allows for the rigorous testing of the two types of models for extension in the southern Black Mountains (SBM) that have been proposed. The first of these models 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 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. Analysis of 10.5-4 Ma sedimentary rocks in the SBM 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 in the Virgin Spring phase of the Amargosa Chaos, thus suggesting that much of the Virgin Spring phase faulting 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, cross-section analysis, and fission-track dating indicate that the post-7.8 Ma denudation of the SBM occurred not on the Amargosa Fault, but rather along a younger generation of domino faults and the likely westward extension of the Sheephead Fault (SF). 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. These observations suggest that a "distributed-faulting model" similar to that proposed by Wright and Troxel (1973) is the more-correct model for post-7.8 Ma extension within the SBM.