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Group B, Poster #082, Earthquake Geology

What can clay minerals tell us about fault mechanics and deformation during the earthquake cycle? Insights from the Hidden Spring fault

Lydia R. Bailey, Alexis K. Ault, Sidney Hemming, Alexandra A. DiMonte, & Greg Hirth
Poster Image: 

Poster Presentation

2023 SCEC Annual Meeting, Poster #082, SCEC Contribution #12818 VIEW PDF
Clay minerals are common constituents of fault damage and govern the frictional strength, slip stability, permeability network of fault zones, and commonly exhibit velocity strengthening behavior. K-Ar geochronology of illite from fault gouge is commonly used to determine the ‘last major episode of fault slip’ along ancient, exhumed fault zones. This tool has the potential to extend paleo-seismic records beyond radiocarbon and luminescence dating, and can provide insights into how fault zones evolve through time. However, we currently do not understand when clays form during the earthquake cycle, and how continued deformation and fluids alter existing clay mineralogy, textures, and fault str...ength. This impacts our ability to reliably interpret K-Ar dates of clay from fault gouge.

To address this challenge, we investigate the active Hidden Spring fault (HSF) in the southern San Andreas fault (SSAF) system in California, which cuts through both Late Cretaceous Orocopia Schist and Pleistocene Upper Palm Springs Formation. The HSF has experienced triggered aseismic slip in response to slip along the SSAF. We combine detailed field observations with field-emission secondary electron microscopy imaging (FE-SEM); powder X-ray diffraction; and K-Ar geochronology on clay-rich fault gouge. Prior thermochronology and geologic relationships suggest present-day exposures have resided in the upper ~1 km in the last 1 Myr, providing a framework for investigating clay formation and deformation in the shallowest portion of fault zones.

The HSF in the basement and sedimentary rocks is characterized by a branching heterogeneous gouge zone with sub-vertical foliation and local strike- and dip-slip lineations; isoclinal folding and crenulation development at cm- and mm-scales in the Orocopia Schist; and multiple, anastomosing discrete clay- and salt-coated slip surfaces. FE-SEM reveals these surfaces comprise um to sub-um-diameter locally comminuted detrital micas, authigenic illite and/or non-comminuted, aligned, and kinked nm-scale palygorskite fibres. Palygorskite post-dates authigenic illite and is spatially associated with salt precipitation. Observations to date collectively support multiple slip events and fluid-rock interactions captured within mm spatial scales. The complex fault architecture we observe at all scales motivates high spatial resolution K-Ar analysis presently underway to place clay formation and deformation in the context of earthquake cycles.