STRUCTURAL AND BASIN EVOLUTION OF THE UPPER HORSE SPRING FORMATION ALONG THE EASTERN TERMINUS OF THE LAS VEGAS VALLEY SHEAR ZONE, LAKE MEAD DOMAIN, NEVADA

The NW-striking, dextral Las Vegas Valley shear zone (LVVSZ) is a regional strike-slip fault that accommodated westward extension during the Miocene. The LVVSZ terminates to the east in the Gale Hills of the Lake Mead area, near a strand of the NE-striking, sinistral Lake Mead fault system. In this area, Miocene sedimentary rocks of the upper members of the Horse Spring Formation (ca. 16.5 – 12 Ma) record complex syn- and post-depositional strain as well as at least two basin partitioning events. Basin and structural evolution of the eastern Gale Hills developed in 5 main events based on local structural and stratigraphic relationships determined from detailed mapping (1:5,000) and stratigraphic analyses. 1) From ca. 16.5 – 15.0 Ma, NNE-striking, normal faulting accompanied dominantly clastic deposition of the Thumb Member. 2) From ca. 15.0 – 14.5 Ma eastward propagation of the LVVSZ into the eastern Gale Hills caused higher subsidence rates north of the LVVSZ and initiated oblique-slip on local NNE-striking normal faults. Deposition during this time was dominated by fine-grained clastics. 3) From 14.5 – 13.7 Ma, north-down motion along the eastern terminus of the LVVSZ created a fault-bounded southern margin of the Bitter Ridge Limestone Member lake. This relationship is documented by a SE-coarsening facies change in the upper third of the Bitter Ridge Limestone Member in the SE Gale Hills. 4) From 13.7 – 12.0 Ma, increased strain along the eastern terminus of the LVVSZ caused partitioning of the Bitter Ridge Limestone lake and a switch from carbonate deposition to mixed clastic and carbonate deposition of the Lovell Wash Member. Subsidence continued north of the LVVSZ while uplifts of pre-Tertiary rocks to the south of the shear zone formed the source of boulder conglomerates, which interfinger laterally with algal limestones. Increased strain likely caused local uplifts and further basin partitioning along the footwalls of NNE-striking oblique-slip faults in the eastern Gale Hills. 5) After ca. 12.0 Ma, increased strain along the LVVSZ and interaction with the Lake Mead fault system caused multiple orientations of folds in the eastern Gale Hills. This strain caused uplift and basin inversion north of the shear zone and subsidence and basin formation south of the shear zone as the two fault systems formed a tectonic wedge.