INFLUENCE OF TERTIARY STRUCTURES ON QUATERNARY FAULTS, LAS VEGAS BASIN, NEVADA

The Las Vegas basin is a N-S elongated extensional basin with significant differences in the E, W, and N structural boundaries. The study examines how Tertiary structures influence the Quaternary (Q) faults that bound the basin and the impact that relationship may have on seismic hazards.

The E side of the basin is formed by the ~N-S striking, W-dipping Frenchman Mountain fault (FMF). This side of the basin has short steep alluvial fans suggesting that the FMF accommodates major offset and traps sediments sourced from the E side of the basin. Kinematic indicators show that the FMF is a down-to-the-west normal fault. The FMF is cut by an ~ENE striking fault between Frenchman Mountain and the River Mountains. The apparent left-lateral offset of Miocene units across this fault is greater than that of Q units. Thus, this strike-slip fault appears to have accommodated slip in both the Miocene and Q.

The W side of the basin has long, gently sloping fans that are cut by at least 3 faults of significant Q offset: the Eglington, Decatur, and Cashman / Whitney Mesa. Offset along these faults is less than that of the FMF. These faults dip E and strike ~ N-S except near their northern termini. From S to N, these faults bend from a ~N to a NE strike. The present NE traces of these faults end near a NNW-striking fault which influences the basin geometry, based on gravity data (Langenheim et al., 2001, Tectonics). This relationship implies that the faults propagated to the N and curved to the NE as they approached an older NNW-striking fault. Thus, the older, perhaps Miocene, NNW-striking fault altered the local stress field such that the propagating Q faults curved toward it.

The WNW-striking Las Vegas Valley shear zone (LVVSZ) defines the N side of the basin. Locally, fault scarps exist along the LVVSZ, but the majority of the offset appears to be pre-Q based on differences in apparent offset between Q and older units. Thus, the LVVSZ appears to provide a pre-Q structural control on the northern end of the basin.

In summary, the Las Vegas basin is an asymmetric graben with the largest offset along a Q fault on the E side. The northern terminus of the basin is defined by the Tertiary or older LVVSZ. A pre-Q NNW-striking fault within the basin localizes the termini of W-dipping faults in the W part of the basin, and thus, limits the surface rupture lengths and potential earthquake magnitudes along the faults in the W part of the basin.