ANALYSIS OF BRECCIA LANDSLIDES WITHIN THE LOWER HORSE SPRING FORMATION, FRENCHMAN MOUNTAIN, LAKE MEAD, NEVADA

The Frenchman Mountain block lies in the western Lake Mead domain in Nevada, immediately east of Las Vegas. Past interpretations suggest that the block was translated 60 – 80 km west to its current location during Miocene extension from near or on the Gold Butte block. Our recent studies focused on the lower Horse Spring Formation in the Frenchman Mountain block and give much more detailed stratigraphic and chronologic context to the key megabreccias used for correlation to the Proterozoic rocks of Gold Butte. Five megabreccias in the study area were likely derived from Gold Butte and deposited at about 15.5 to 14.6 Ma based on a sedimentation rate of 450 m/my between two dated tuffs using Ar geochronology: 15.30 Ma tuff 4 m above breccia 2B and 14.53 Ma tuff ~30 m above breccia 4. Based on past thermochronology and our new work, these rock avalanches were derived from the footwall block of Gold Butte during the peak of detachment faulting on the South Virgin – White Hills detachment fault. Thickness of some of the breccia beds decreases from south to north and initial analysis of soft-sediment folds related to the landslides both suggest northward motion of the landslides off of Gold Butte block. The calculated sedimentation rate based on tuffs implies that a southeast-derived Paleozoic-clast conglomerate is 15.6 Ma, which is interpreted to be the beginning of increased detachment faulting and a progradation of the Paleozoic strata formerly capping the Gold Butte Proterozoic rocks just before breccia deposition. Detailed facies analysis reveals that the Rainbow Gardens Member limestones and various Thumb Member facies change to the north indicating that the study area was in the basin center to northward margin of the basin ~18 to 15.3 Ma. Local syn-deposition growth faulting shows local faulting in the basin center at the time of conglomerate deposition. Using Fryxell’s (1992) mapping of Gold Butte we can detect likely source areas for each breccia that change from east to west across Gold Butte; this is compatible with landslides that were shed off the top of Gold Butte systematically over 1 million years as the footwall of the detachment was exhumed. All our data and previous analysis of the breccias suggest strongly that the Frenchman Mountain block was located within a few kilometers to the north of the Gold Butte block at 15.5 – 14.6 Ma.