NEW LINKAGE OF THE ARROW CANYON RANGE AND WILDCAT WASH FAULTS, SOUTHERN NEVADA: IMPLICATIONS FOR SEISMIC HAZARDS

Two active north-striking, west-dipping normal faults, the Arrow Canyon Range fault (ACRF) and Wildcat Wash fault (WWF), pose a seismic hazard to nearby communities in southern Nevada. They lie just 50 - 65 km northeast of Las Vegas, Nevada. Previous interpretations of the ACRF and WWF suggest that (1) the fault tips underlap, (2) no strain transfer occurs between the faults and (3) they are late Quaternary in age. However, little detailed mapping or paleoseismic studies previously have been completed.

We tested the hypothesis that the faults have late Quaternary surface ruptures and the fault tips underlap with no strain transfer. New data include detailed field mapping, topographic profiling and logging of natural dissections and a borrow pit across the fault. The map data, profiles, and logs document fault scarps that cut Quaternary alluvial fans and terrace deposits. The oldest alluvial fan surfaces are offset as much as 30 m. Younger fan surfaces and terraces are offset up to 3 m. However, total offset is much greater because locally the fault places Quaternary alluvial fans against Ordovician age bedrock. One wall of a borrow pit that is 3.1 m deep shows two strands of the WWF and two colluvial wedges. One natural dissection across the WWF with a depth of 2.1 m exposes four colluvial wedges indicating at least that many surface rupturing earthquakes.

New map data show that the ACRF and WWF have overlapping fault tips with immature strain transfer between them. The fault tips overlap by at least 0.8 km and are spaced 3.2 km apart across strike. The area between the overlapping fault tips contains a reverse fault with an attitude of N63°E, 69° S and a broad, gentle fold with an attitude of 0.2°, N70°E. The presence of these structures suggests the initiation of a relay ramp and the beginning of strain transfer between the faults. Consequently, the faults appear to be in an early stage of linkage that has profound implications for seismic hazards to communities in southern Nevada. Linkage increases the total fault length, and thus, the potential rupture length. Therefore, the largest possible earthquake magnitude that may occur along the combined ACRF and WWF is greater than on either individual fault. The magnitude of the combined surface rupture lengths is estimated at M 6.8 using the Wells and Coppersmith fault regression.