Paper No. 6-4
Presentation Time: 8:30 AM-6:00 PM
CLIMATICALLY DRIVEN DISPLACEMENT ON THE EGLINGTON FAULT, LAS VEGAS, NV
The Eglington fault is one of several intrabasinal faults in the Las Vegas Valley fault system and is the only one recognized as a source for significant earthquakes in the USGS national seismic hazards model. Its broad warp displaces late Pleistocene paleo-spring deposits of the Las Vegas Formation, which record hydrologic fluctuations that occurred in response to millennial and submillennial-scale climate oscillations throughout the late Quaternary. The sediments allow us to constrain the timing of displacement on the Eglington fault and identify hydrologic changes that are temporally coincident with that event. The fault deforms deposits that represent widespread marshes that filled the valley between ~31.7 and 27.6 ka. These marshes desiccated abruptly in response to warming and groundwater lowering during Dansgaard-Oeschger (D-O) events 4 and 3, resulting in the formation of a pervasive, hard carbonate cap by 27.0 ka. Vertical offset by as much as 4.2 m occurred after the cap hardened, and most likely after younger marshes desiccated irreversibly due to a sudden depression of the water table during D-O 2, beginning at 23.3 ka. Displacement is further constrained to before 19.5 ka as evidenced by undeformed spring deposits that are inset into the incised topography of the warp. Coulomb stress calculations over a wide range of dip angles (40-70°), friction values (0.1-0.7), and water table drop estimates (10-33 m) show that the sudden vertical load change caused by groundwater withdrawal promotes failure on the Eglington fault for all dip angles when coefficient of friction values exceed 0.55. When coefficient of friction values are between 0.45 and 0.55, the results are variable and are dependent on the dip angle. Displacement is inhibited for all dip angles when coefficient of friction values are less than 0.45. These results validate the hypothesis that the substantial groundwater decline during D-O 2 unclamped the Eglington fault through unloading of vertical stress of the water column, causing it to slip and deform the deposits of the Las Vegas Formation. The synchroneity of events suggests that climatically modulated tectonics operated in the Las Vegas Valley during the late Quaternary and the future seismic potential of the Eglington fault should be evaluated in light of these data.