DEVELOPMENT OF LAS VEGAS BASIN, NEVADA WITH IMPLICATIONS FOR SEISMIC HAZARDS

The purpose of this study is to document and analyze the relationship between faulting and sedimentation in the development on an extensional basin and the implications of that development for seismic hazards. We focus on Las Vegas basin, Nevada because it is both a typical example of a basin formed by normal and strike-slip faults, and the ~2 million people in the valley would be significantly impacted by an earthquake in the valley or nearby. We used ~1500 well logs, geophysical measurements and surface data from air photos, maps and the field to document the 3D architecture of the basin-fill sediments and the geometries of Miocene and active faults.

The basin-fill sediments are cut by the W-dipping Frenchman Mountain fault (FMF) on the E and the E-dipping Las Vegas fault system (LVFS) in the central part of the basin. The fault attitudes and geometries in our 3D model are based on surface exposures, changes in well log units and offsets in bedrock determined from gravity data (Langenheim et al., 2001). Data from 25 well logs allow location of the unconformity between (Pliocene -) Quaternary (Q) and Miocene sediments. The unconformity was positioned above rock types unique to the Miocene section including volcanic units and thick gypsum layers.

Our 3D geometric model shows that the basin is a composite basin with distinct Miocene and Q histories. The deepest part of the basin (~4.5 km) is located in the NE part of the basin and most of the basin-fill is Miocene or older in age which suggests that the location of the depocenter and thickest Miocene stratigraphy was controlled by the FMF, which had slip in both Miocene and Q time. The steep northern boundary of the basin fill is controlled by the Las Vegas Valley shear zone which had ~ 4 km of down-to-the-SW normal slip to accommodate the fill thickness, although regional data show ~50 km of right-lateral strike slip in the Miocene. The thickest part of the Q sediments is located in the central part of the basin suggesting that the LVFS became active after the FMF and caused the main depocenter to move W. This relationship implies that the LVFS is more active than the FMF. Although the FMF is usually modeled as the intrabasin fault that poses the greatest threat to Las Vegas, our new data suggest that the LVFS may be more active, and thus, may pose the greater threat.