EVIDENCE FOR EARTHQUAKES ALONG THE LAS VEGAS VALLEY FAULT SYSTEM, SOUTHERN NEVADA

A 10-m high exposure of the upper part of a large, compound fault scarp in Las Vegas Valley reveals evidence for paleoearthquakes. This is important because there has been a long-standing debate as to the origin and seismogenic potential of faults within Las Vegas Valley. The exposure is along the northern end of the Valley View fault, where several secondary faults were exposed. In the north face is a fault with offsets of several meters, and at least two other smaller faults, all showing apparent normal separations. The largest fault strikes N45-55E, and dips to the southeast 73 degrees near the top of the fault, curving downward to a lower dip of about 40 degrees at the bottom of the exposure. The rake of slickensides is S28E, indicating the fault has principally normal displacement, with a smaller left-lateral component. The ratio of normal slip to left-lateral slip is ~5:1. Three faults were exposed in the southwest corner of the excavation that are the approximate continuation of faults exposed in the north face. These faults have differential offsets indicating at least two episodes of faulting. The most recent event offsets greenish-brown silts containing a black mat that was dated at 14,690 ± 570 ybp, and had apparent offsets of about 1 m. The penultimate event has about twice the amount of offset as the most recent event, created a small depression along one fault that was filled with clayey silts, and occurred in latest Pleistocene. Minor ductile deformation observed in the excavations indicates these sediments would have deformed plastically if the loading of the fault was slow, as by compaction. The dominance of brittle offsets, however, indicates the displacement was rapid, and we interpret these secondary faults to have moved abruptly during earthquakes in the latest Pleistocene. This supports a tectonic component to this fault. Because the main fault trace was not exposed, offsets and the number of events observed are minimums for the fault zone.