POSSIBLE EVIDENCE FOR PALEOSEISMICITY ON THE WHIPPLE DETACHMENT, CALIFORNIA

Low-angle normal faults (LANFs) accommodated significant amount of Neogene extension in the southwest U.S. (e.g., Axen, 2004; Lister & Davis, 1989). How LANFs slip is a long-standing paradox in rock mechanics because their low angle means they are unfavorably oriented in an Andersonian stress field. While current literature and a lack of observational evidence suggest that LANFs should not slip seismically (Axen 2020) (Collettini 2011), large (Mw > 7) earthquakes have been documented along the Mai’iu fault in Papua New Guinea (Biemiller et al., 2020), which suggests it may be necessary to incorporate LANFs in earthquake hazard assessments.

Here we investigate the Whipple Mountain detachment, CA, to test for evidence of past LANF seismic slip on this fault. The Whipple detachment fault accommodated ~50km of top-to-the-NE slip and is postulated to have initiated at the ductile-brittle transition zone (Axen 2020). The brittle detachment contains a cm- to m-thick fault core consisting of ultracataclasites, gouge, and pseudotachylyte. These fault rocks exhibit consistent crosscutting relations and structural positions with respect to the upper plate. We establish a fault rock stratigraphy, attempt to correlate fault rock layers between exposures, and use microstructural observations to test for a melt origin for the some of the fine-grained layers. One gouge layer immediately beneath the upper plate contact is present in multiple exposures separated by several kilometers. Injections of the gouge from this layer cut up into the hangingwall, indicating mobilization of the gouge during faulting. These results can provide the basis to test whether the Whipple LANF slipped in moderate to large earthquakes.