Tectonic Processes in Upper Crustal Normal Fault Systems

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9628262 Wernicke The South Virgin Mountains, Nevada, occupy the easternmost portion of the highly extended Lake Mead region. We request funds for completion of detailed geologic mapping and geochronologic studies in the South Virgin Mountains Nevada. In terms of exposure, stratigraphic resolution, topographic relief and a regional datum for reconstruction, the South Virgin Mountains are among the best examples of upper crustal imbricate normal fault structure in the world. Mapping to date by the PI and graduate student Robert Brady suggests that domino-style, "rolling-hinge," and rollover kinematics are all probably active within the system, but may be operative at different times and places across strike of the rift. Hence the area offers the potential to test the applicability of these models, and possibly determine how structural position within a series of tilted blocks influences their development. In addition to geologic mapping, we propose to complete Ar/Ar and U-Th/He dating of selected samples to establish absolute ages of basin formation and unroofing of the blocks, respectively. In the case of the latter method, which has a uniquely low closure temperature of about 70oC in the mineral apatite, this project will be the first application of He dating to the problem of extensional tectonics.