Measure and Date of Movement on Las Vegas Valley Shear Zone, Clark County, Nevada

Abstract

Large-scale offsetting, by fracturing and bending of major geologic units, indicates clearly that a fault zone with large right-lateral displacement extends northwest across a pattern of ranges bordering Las Vegas Valley. Evidence indicating the date and measure of the movement is supplied by an unusual sedimentary unit within the Frenchman Mountain block east of Las Vegas. The Thumb Formation, dated isotopically as being Miocene in age (±17 m.y.), contains large landslide masses of brecciated metamorphic and granitic rock. These masses moved southward from an area in which only a thick section of sedimentary strata is now widely exposed. The granite is recognized as part of the distinctive Gold Butte Granite, a conspicuous unit in the South Virgin Mountains near the Nevada-Arizona border. The only present exposures of that unit near the projected location of the shear-zone axis are more than 64 km east-southeast from the landslide masses within the Thumb section. The thick Muddy Creek Formation, the basal part of which is dated as being early Pliocene, was laid down after movement on the shear zone ended.