MIOCENE CLOCKWISE ROTATIONS AND LEFT-LATERAL FAULTING RELATED TO THE LAS VEGAS VALLEY SHEAR ZONE, FRENCHMAN MOUNTAIN BLOCK, LAKE MEAD DOMAIN, NEVADA

The Frenchman Mountain block lies in the western Lake Mead domain in southern Nevada, immediately east of Las Vegas. Most interpretations suggest that the block was translated 60 – 80 km to its current location during Miocene normal and strike-slip faulting. Previous studies have modeled how and when the Frenchman Mountain block translated to the west from near or on the Gold Butte block, with most workers agreeing that east tilting occurred during the later stages of translation during deposition of the upper Horse Spring Formation and red sandstone. This study focused on the lower Horse Spring Formation near the Dry Wash and Boulevard faults in the eastern Frenchman Mountain block. To better understand the stratigraphy and deformation, a 1:5000 geologic map was created, data from primary and secondary structures were collected, and three tuffs were dated using Ar geochronology (Rainbow Gardens tuff at 18.41 Ma; Thumb Member tuffs at 15.30 and 14.53 Ma). Four megabreccia deposits in the study area were likely derived from Gold Butte and deposited at about 15.5 to 14.6 Ma when the Frenchman Mountain block was adjacent to, or on top of, the Gold Butte block; these ancient rock avalanches occurred during the peak of detachment faulting on the South Virgin – White Hills detachment fault. The Dry Wash and Boulevard faults are oblique, left-lateral, strike-slip faults with 1.7 and 3.1 km offset. Map and facies relationships show that the Thumb Member deposits were faulted during deposition at ~15.6 to 14.5 Ma, with increased fault activity after 14.5 Ma and before 8 Ma. Structural analysis of the eastern Frenchman Mountain block results in a new model of progressive clockwise rotation and normal and left-lateral faulting during translation of the block along the eastern Las Vegas Valley shear zone, the primary right-lateral fault of the western Lake Mead domain. The model is primarily based on clockwise block rotation of domains in a bookshelf manner with concurrent left lateral faulting south of the Boulevard fault, while the Sunrise structural block to the north was faulted and rotated clockwise in a broken-block fashion. This rotational model predicts that the Frenchman Mountain block began at ~16 Ma as a north-northeast elongate block that was reduced in length from about 20 to 14 km during translation and rotations from 15.5 to 8 Ma.