Abstract
Basin and Range extension in the Lake Mead region, Nevada, is unique in that strike-slip faulting exerts major control on extensional processes. The south Virgin Mountains (SVM) in eastern Lake Mead are cut by multiple NE-striking faults that form the Lake Mead left-lateral fault system (LMFS). Here, details of early basin formation and the onset of left-normal oblique faulting of the LMFS are well preserved because they are not overprinted by younger deformation as is the case to the west. In the 1980s, Bob Bohannon defined the Oligo-Miocene Horse Spring Formation for the Lake Mead region, and in the 1990s Beard focused within the SVM on the interaction between lower basin strata and onset of faulting. Starting in the 2000s, an NSF-funded university research group, led by the highly enthusiastic team of Paul Umhoefer, Lisa Lamb, Tom Hickson, Bill McIntosh, Nelia Dunbar, along with Sue Beard (USGS), began a new effort in the Lake Mead area. Within the SVM, Lamb et al. (2018) elevated the Rainbow Gardens Member to formation status, and MS students of Umhoefer's (Matt Schmidt, 2014; Lane Tucker, 2017; Daniel Conrad, 2018) as well as undergraduate students led by Lamb and Hickson, focused on detailed stratigraphic and structural studies of Oligo-Miocene geology. These NSF-funded studies are being used to update USGS 100k and 24k geologic maps from the 1980s to the 2000s. In particular, the eastern termination and stepover structures of the individual LMFS strands are key structural features of the SVM. The interaction of sedimentation and faulting at the NE end of the Lime Ridge fault (LRF) strand was mapped originally by Beard and improved upon by Schmidt, using higher precision Ar-dating and detailed facies mapping. Schmidt determined that extension initiated nearly equal 17-16.3 Ma along east-side down normal faults within a stepover zone between the LRF and the Piedmont fault, a LMFS strand to the NNE. As displacement progressed, deformation shifted to left normal oblique faulting along the LRF nearly equal 15.4 Ma. By nearly equal 14 Ma, most deformation had shifted westward and the SVM were uplifted, such that basins were inverted and cannibalized into younger basins to both the east and west (Beard, 1996). In honor of Paul Umhoefer, who encouraged us for years to do this, we present the melding of mapping and ideas along the eastern LMFS as they evolved through the decades.