The geology and structural significance of the Arch Mountain area, northern Black Mountains, Mohave County, Arizona

Edward Eschner, University of Nevada, Las Vegas


The Arch mountain detachment fault represents the brittlely deformed, upper crustal part of a regional detachment fault system that has a breakaway zone adjacent to the Grand Wash Cliffs. I interpret the fault and lower plate mylonites at Saddle Island to be the plastically deformed, deep level part of the regional fault system that became exposed by progressive structural denudation, isostatic uplift and erosion. I suggest the following: (1) regional uplift and arch formation south of Lake Mead during the late-Cretaceous to early Tertiary; (2) early to mid Tertiary detachment faulting, development of the Las Vegas Valley shear zone and westward transport of the Paleozoic and Mesozoic rocks represented at Petroglyph Wash, Boulder Canyon, the Frenchman Mountain and Sheep Mountain structural blocks and at the Spring Mountains; (3) continued detachment faulting and westward transport of the River Mountains volcanics, and the development of the Lake Mead fault system with related high-angle normal faults. (Abstract shortened with permission of author.).