LIMITS ON THE DISPLACEMENT OF THE MORMON PEAK DETACHMENT FROM DETAILED GEOLOGIC MAPPING OF SEVIER-AGE CONTRACTILE STRUCTURES, MEADOW VALLEY MOUNTAINS, NEVADA

The Mormon Peak detachment, a large-displacement Miocene normal fault in the Basin and Range province of southern Nevada, formed nearly parallel to, but not precisely along, the Sevier-age (Cretaceous) Mormon thrust fault. To properly characterize the formation of this complex fault system, the structures resulting from each episode of deformation must be temporally distinguished. The Meadow Valley Mountains, to the west of the Mormon Mountains, lie in the hanging wall of the Mormon Peak detachment. Due to the extrusion of numerous volcanic units just prior to and during extensional deformation, the area provides an opportunity to distinguish the timing of key structures formed during the two phases of faulting. Here I present a detailed geologic map of the area in the Meadow Valley Mountains closest to the detachment that exhibits evidence of deformation both the Cretaceous contractile and Miocene extensional events. Examples of pre-Tertiary deformation include a NW-striking fold within Paleozoic units overlain by flat-lying Tertiary units, as well as a decrease in angle between Paleozoic bedding and Tertiary bedding from west to east across the range, interpreted as a fault-bend at the top of the thrust ramp. Tertiary deformation is recorded by numerous high-angle normal faults striking NNW to NNE in the easternmost part of the range, generally with west-side-down displacement, as well as significant tilting (35-50° towards the NE) of eastern exposures of volcanic units. Analysis of stereograms of Tertiary and pre-Tertiary units indicate that at the time of extrusion of the Tertiary units, the pre-Tertiary units formed a NE-trending monoclinal flexure, with gentle NNW dips along the eastern flank of the range, steepening to moderate NW dips in the central part of the range. These relations, given the well-constrained position of the top of the thrust exposed in the detachment’s footwall, suggest that displacement on the Mormon Peak detachment is less than c. 24 km.