NEW GEOLOGIC MAPPING IN THE MONTGOMERY MOUNTAINS, SOUTHERN NEVADA

New geologic mapping in the Montgomery Mountains northwest of Pahrump, Nevada, refines our understanding of the Mesozoic and Cenozoic geologic history of the region. The Six Mile fault, previously interpreted as a Mesozoic thrust, is actually a Tertiary low-angle detachment fault. Across Amargosa Flat, this fault links with the Point of Rocks detachment system at the north end of the Spring Mountains. Two smaller scale low-angle normal faults that may be part of this regional detachment system have been identified in the upper and lower plates of the Six Mile fault and are here named the Ash Meadows and Johnnie detachments, respectively. New age-dating and detailed mapping of Tertiary supradetachment basin-fill deposits indicate these detachments formed before 13.0 Ma. The Ash Meadows anticline, previously described as Mesozoic, is reclassified as Tertiary because it deforms the Ash Meadows detachment fault and is spatially associated with the Tertiary State Line fault. We divide the previously described Mesozoic Montgomery thrust fault into two separate thrust faults that are separated by the younger Six Mile detachment, making correlation questionable. New mapping between Stewart Valley and Ash Meadows defines several previously unidentified strands of the State Line fault zone. Studies along these fault strands indicate at least 15 km of right-lateral offset and movement ranging from at least 13.0 Ma to Holocene. New mapping of Quaternary stratigraphy and structure has been added throughout the area.