SECONDARY FAULTS AND SLIP SENSE ALONG THE FRENCHMAN MOUNTAIN FAULT, SOUTHERN NEVADA

The Frenchman Mountain Fault (FMF) is a steeply west-dipping fault that bounds the eastern side of Las Vegas Valley and lies in an area of urban expansion. The fault has been inferred to produce earthquakes possibly as large as M7.0. Given the potential hazard to Las Vegas and the limited amount of previous study, it is critical to gather data from the FMF prior to the destruction of exposures by urbanization. The purpose of this study is to quantify the sense of slip along the FMF and assess secondary footwall faults.

In some exposures, the FMF places Quaternary alluvial fan deposits in the hangingwall against Precambrian metamorphic basement rocks. The footwall basement rocks are tremendously faulted and sheared. The measured strikes of >100 minor footwall faults mostly range from N40^oW to N40^oE. They are primarily dip-slip faults, but some have minor strike-slip components. These faults are typically non-planar; many have ramp-flat geometries. Most fault sections dip between 30^oW and vertical, but a few dip as gently as 5^oW. The geometries appear to be controlled by competency contrasts between schist and gneiss within the basement. Also, the range of fault orientations might be a result of changes in geometry of the FMF throughout its development. Analysis of the footwall faults shows that (1) many are similar in orientation to the FMF and (2) others are Riedel shears, including R, R' and P shears.

In addition, 31 kinematic indicators were measured. Most are slickenlines and slickenfibers. The majority trend between N60^oW and S50^oW.

The Riedel shears, along with the other kinematic indicators, constrain the overall sense of slip along the FMF. The slip is generally normal (down-on-the-west), essentially dip slip, and the direction ranges from WNW to WSW.