THE LAS VEGAS VALLEY FAULT SYSTEM: A RESULT OF DIFFERENTIAL COMPACTION OR TECTONIC RUPTURE?

The Las Vegas Valley fault system (LVVFS) cuts through the Las Vegas basin, Nevada, and has been interpreted to be formed by either differential compaction or tectonism. These models result in vastly different seismic hazards. Understanding the seismic potential of the LVVFS is critical to seismic safety, building costs and understanding regional tectonics. Subsidence in Las Vegas Valley fluctuated between 6 cm and few mm per year since 1935, the first monitoring efforts. The lack of correlation studies between subsidence rates and fracturing poses a question of whether this subsidence amount is big enough to form the LVVFS scarps. If the amount of differential subsidence across the faults is too small to create the scarps, an alternative, a tectonic origin of the LVVFS, needs to be considered. This study attempted to evaluate the differential compaction of sediments in Las Vegas Basin and the origin of the LVVFS.

Differential compaction faults form along lithologic boundaries that separate coarse sediments from finer ones or sediments of different thicknesses, and causes reduction in porosity. Well data show that two of the LVVFS faults, Decatur and Eglington, have nearly similar deposits on both sides, suggesting that differential compaction is unlikely to form the fault scarps. To further assess the compaction model, the porosity reduction in the basin was evaluated using neutron and resistivity well logs and thin-sections. Well logs showed unusually low porosity in all sediment types, which might deceptively indicate a long history of compaction. Thin-sections from the Valley View fault zone, on the other hand, showed no single sign of compaction. They rather showed a significant amount of early cemented silt, fine sand, and clay. Additionally, sediments in outcrops of the Las Vegas Formation, the uppermost basin-fill unit, are cemented, mostly with calcite. Accordingly, the basin fill sediments are not compacted enough to produce such the 12- 40 ft. scarps of the LVVFS and cementation is the main cause of the low porosity showed by well logs. In which case, the LVVFS must have formed by tectonic processes before the onset of groundwater pumping induced subsidence.