DID CLIMATE PLAY A CAUSAL OR COINCIDENT ROLE IN THE ORIGIN AND DISPLACEMENT HISTORY OF THE EGLINGTON FAULT, LAS VEGAS, NEVADA?

Abstract

The Eglington fault in Las Vegas, Nevada is a normal fault with displacement propagating to the surface as a broad warp that is mostly covered by urbanization. It is part of the intra-basinal Las Vegas Valley fault system, a series of Quaternary faults that lack consensus as to their origin, displacement history, and earthquake recurrence. The fault is preserved within Tule Springs Fossil Beds National Monument, where it displaces groundwater discharge deposits of the middle-late Pleistocene Las Vegas Formation (LVF) by up to 4.2 m. Given that LVF deposits record dramatic hydrologic changes that occurred in response to abrupt climate oscillations during the late Quaternary, this study dually interprets the timing of displacement on the fault and identifies climatic events that are temporally coincident with that event. Specifically, the fault warps bed D2of the LVF, a unit that represents widespread marshes that filled the valley between 31.68 and 27.58 ka. These marshes desiccated in response to abrupt warming during Dansgaard-Oeschger (DO) events 4 and 3, leading to formation of a pervasive, carbonate cap by ~27 ka. Warping of bed D2occurred after the cap hardened, and likely after the final collapse of the full-glacial marshes coincident with DO-2 at ~23.5 ka, but before 13.37 ka as evidenced by the presence of spring deposits of bed E1dthat are inset into the incised topography of bed D2within the warp zone. Together, these features and their corresponding ages provide robust constraints on the timing of movement along the fault, allow us to calculate displacement rates of 0.3-0.4 mm/yr for the latest Pleistocene, and demonstrate that the Eglington fault has not been active during the Holocene. Dramatic lowering of groundwater levels at the end of the last glacial period may have triggered the fault displacement through unloading of vertical stress. The temporal synchroneity of groundwater lowering and warping suggests that, rather than a solely seismic origin, there may be a case for pursuing the hypothesis of climatically modulated tectonics in the Las Vegas Valley. The new observations and age constraints presented here should be considered in future updates of the USGS national seismic hazards model and whether the Eglington fault is considered a seismic source for the Las Vegas Valley.