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The central Basin and Range of the southwestern United States is known for large-magnitude Cenozoic extension and a unique combination of normal and major strike-slip faults. The Lake Mead region constitutes the eastern portion of this domain and has been the site of numerous mapping and detailed structural studies, which have led to several models explaining the complex faulting and folding of the region, as well as the tectonic drivers of this deformation. The syntectonic basin fill of the Oligocene-Miocene Horse Spring Formation records a considerable portion of this deformation. A more detailed understanding of the Horse Spring Formation is important to determining the deformation history of the area and to constraining regional tectonic reconstructions. In this study, we present results of detailed mapping and stratigraphic analyses of the Lower Horse Spring Formation in the Longwell Ridges area, Nevada. Detailed measured sections combined with 1:5,000 scale mapping allow us to recognize and document lithofacies and their detailed architecture within the Lower Horse Spring Formation and highlight the extreme lateral and vertical facies changes within this portion of the formation. New 40Ar/39Ar ages and volcanic ash geochemical data support these analyses. These data record deposition within a range of environments, including alluvial-fan, lacustrine, and fluvial settings. Deposition occurred within an asymmetric basin with a main bounding fault lying east of the modern Overton Arm of Lake Mead. Activity on this fault began around 17 Ma and increased significantly at ca. 15.5 Ma.

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