Testing the Role of Plasticity on the Frictional Strength of Calcite Gouge with Increasing Normal Stress
Sophia WrightSubmitted September 7, 2025, SCEC Contribution #14372, 2025 SCEC Annual Meeting Poster #TBD
Understanding the microphysics of fault friction has implications for scaling laboratory measurements of frictional strength and stability to spatial and temporal scales relevant to tectonic movement. Fault gouge can deform by fracturing, granular processes, and plastic deformation. Twinning in calcite provides a useful microstructural record of plastic deformation in crustal rocks and can serve as a proxy for stress during crystal-plastic deformation. We performed friction experiments on calcite gouge and pre-stressed calcite gouge using an L-block direct-shear configuration with a displacement rate of 0.1-3.0 µm/s in a triaxial deformation apparatus at room temperature and normal stresses ranging from 10 to 150 MPa. Additionally, we conducted a hydrostatically pressurized experiment at 70 MPa in order to understand the influence of the initial compaction stage on twin density and orientation. The pre-stressed calcite gouge tests whether previous plastic deformation in the form of twinning alters the friction coefficient, allowing for a quantitative understanding of plastic deformation in gouge deformation. Post-experiment thin sections were imaged using optical microscopy, allowing for the quantification of twin density, grain size, and twin orientation from photomicrographs. Our results demonstrate that twin density increases with normal stress, with the sample sheared under the lowest normal stress (10MPa) having an average twin density of 0.096 μm-1, while the highest normal stress sample (150 MPa) has an average twin density of 0.335 μm-1. Twin orientations suggest that twinning accommodates shear deformation, with sheared samples showing a strong alignment of twin orientations, while non-sheared samples display a homogenous distribution. These results suggest that plastic deformation plays an increasing role in accommodating strain at higher normal stresses, where twinning becomes more active. We predict that the effect of pre-stressing the gouge to a higher twin density will influence frictional strength and stability at high normal stress, but will have a minimal effect at lower normal stress due to the dominance of granular deformation processes.
Citation
Wright, S. (2025, 09). Testing the Role of Plasticity on the Frictional Strength of Calcite Gouge with Increasing Normal Stress. Poster Presentation at 2025 SCEC Annual Meeting.
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