Abstract
Extensional tectonics in the Lake Mead region is characterized by a complex history of detachment (low-angle), normal (high-angle), and strike-slip faulting. The strata of the late Oligocene to Miocene age Horse Spring Formation--comprised by, from base to top, the Rainbow Gardens, Thumb, Bitter Ridge Limestone, and Lovell Wash Members--record this history in considerable detail. Clast composition of Thumb Member megabreccias in the Frenchman Mountain block constrain its starting position near Gold Butte, in the South Virgin Mountains, at 15.5 to 14.5 Ma. A number of models have been proposed to explain the tectonic history of the region that rely strongly on this initial position of the Frenchman Mountain block and inferred kinematics of subsequent faulting. Another test of these models is to retrodeform the sedimentary basin(s?) that formed prior to deposition of Thumb Member megabreccias according to the inferred deformation path of the Frenchman Mountain block, thereby creating a paleogeographic map of the initial, Rainbow Gardens age, basin. The facies architecture of this retrodeformed basin should be geologically reasonable and consistent with reasonably well-constrained, paleogeographic interpretations of the underlying strata and structure, as well as subcrop distributions. Basin analysis of the Rainbow Gardens Member, when applied to the retrodeformed basin, show stratal thinning to the north and northwest, consistent with paleocurrent indicators that suggest a basin margin to the northwest and a depocenter located in the southeast. This interpretation is also supported by stable isotopic analyses of lacustrine carbonates of the upper Rainbow Gardens Member. The retrodeformed basin was a N-S elongate basin that lay between the front of the Sevier thrust belt to the west and NE, the Kingman Arch to the south, incipient cliffs of the Colorado Plateau to the east, and possibly the Caliente volcanic field to the north. Collectively, our new lithostratigraphic and sedimentological analyses shed considerable light on the nature of the first Oligocene to early Miocene sedimentary basin(s?) of the Lake Mead region and have strong implications for the tectonic evolution of the area.