GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 25-10
Presentation Time: 9:00 AM-5:30 PM


SPRINGER, Kathleen B. and PIGATI, Jeffrey S., U.S. Geological Survey, Denver Federal Center, Box 25046, MS 980, Denver, CO 80225

The Eglington fault is one of several intrabasinal faults in the Las Vegas Valley, Nevada and is the only fault recognized as a seismic source for significant earthquakes in the 2014 USGS National Seismic Hazards Model. Its broad warp displaces middle Pleistocene to early Holocene 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 also allow us to constrain the timing of displacement on the Eglington fault and to identify hydrologic changes that are temporally coincident with that event. The fault warps 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 a pervasive, hard carbonate cap by ~27 ka. Fault displacement by as much as 4.2 meters occurred after the cap hardened, and likely after the valley-wide marshes collapsed irreversibly due to sudden depression of groundwater coincident with DO-2 at ~23.5 ka. The timing of displacement is further constrained to before 19.4 ka as evidenced by the presence of younger, undeformed spring deposits inset into the incised topography within the warp zone and likely before 23.0 ka based on the presence of nearby spring discharge. There is no evidence of Holocene fault displacement. We hypothesize that significant and permanent lowering of groundwater levels during the ~500 year-long megadrought that took place during DO-2 triggered the displacement on the pre-existing Eglington fault through unloading of vertical stress of the water column. The synchroneity of the abrupt hydrologic change and warping on the Eglington fault suggests that climatically modulated tectonics operated in the Las Vegas Valley during the late Quaternary.