33-4 Fault Rock Record of Paleoseismicity on The Whipple Detachment, California

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

Presenting Author:

Authors:

Abstract:

Low-angle normal faults (LANFs) accommodated significant amount of Neogene extension in the southwest U.S. 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, large (Mw > 7) earthquakes have been documented along the Mai’iu fault in Papua New Guinea (Biemiller et al., 2020), which suggests these phenomena do occur. Investigating the fault rock assemblage of ancient examples of LANFs may provide pivotal insight to help resolve this paradox.

Here we investigate the Whipple Mountain detachment, CA, to test for evidence of paleo-slip on this LANF. The Whipple detachment fault accommodated ~50km of top-to-the-NE slip (Gans and Gentry 2016) and is postulated to have initiated at the ductile-brittle transition zone (Axen 2020). The brittle detachment varies in core thickness, approximately sub-meter scale up to 1 meter, consisting of ultracataclasites, gouge, and pseudotachylyte. These fault rocks exhibit consistent crosscutting relations and structural positions with respect to the upper plate that record evidence of slip history. 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 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. The presence of quenched oxides and sulfides indicative of pseudotachylyte formation across the entirety of exposures further indicates large scale slip occurred. These results can provide the basis to test whether the Whipple LANF slipped in moderate to large earthquakes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.