131 Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems

This session explores key structural processes that govern the temporal, mechanical, and rheological evolution of crustal/lithospheric extensional and strike-slip systems with a focus on novel field observations, microstructural and analytical methodologies, and numerical modeling.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 217D

Fault Fabric, Friction, and Flux: How Fault Rheology Shapes Shallow Slip Behavior in Strike-slip Fault Systems

Paper number: 131-7
Program: Technical Sessions
Time: 3:20 PM – 3:40 PM

Authors

Abstract

Geodetic and geophysical data reveal that continental strike-slip fault systems accommodate deformation through a spectrum of behaviors, from large earthquakes to slow slip events (SSEs) and creep. What controls this variability? Fault geometry and stress state influence deformation at many scales, but fault material properties and rheology critically modulate slip behavior, particularly in the shallow (<3 km) crust. Evaluating the role of rheology requires directly linking fault rock characteristics to slip rates and styles. Shallowly exhumed fault rocks and rupture interface materials from large earthquakes provide access to natural fault zones deformed at known or inferred rates. These samples can be integrated with deformation experiments at near in situ conditions, geochemical and chronometric tools, and geophysical datasets to constrain the impact and evolution of fault physical properties.

We highlight two strike-slip fault systems where observed long-term and coseismic slip behavior is influenced by fault material properties and rheology. Along the southern San Andreas fault, a distinctive “red clay” gouge is present at the surface and depths of geodetically observed SSEs. Field observations show this gouge localizes strain at multiple scales. Friction experiments indicate it is weak when saturated, has low healing rates (<0.001/decade), exhibits velocity-dependent frictional stability, and produces laboratory SSEs with friction and stress drop values that broadly overlap geodetic data. These results suggest the gouge could nucleate shallow SSEs or facilitate rupture propagation from depth.

For comparison, the 2023 Mw 7.6 Elbistan earthquake ruptured ~150 km of the Çardak–Yeşilyurt fault system in Türkiye through diverse lithologies. Segments with localized, high-offset, and supershear rupture velocities correspond to mature fault fabrics rich in smectite. In areas with more distributed surface rupture and lower displacement, fault rocks are less evolved. Two years of CO₂ flux surveys along the rupture show elevated emissions restricted to the surface rupture that have since declined rapidly, reflecting post-seismic reductions in permeability as microcracks close. The spatial patterns of flux magnitude and subsequent loss correlates with mapped rupture characteristics, providing a novel, real-time proxy for material property evolution and fault healing. Together, these examples suggest that shallow fault rheology influences slip behavior and evolves rapidly over time, with implications for how we understand the structural evolution and seismic hazard of strike-slip fault systems.

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

doi: 10.1130/abs/2025AM-414372

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