Diverse Slip Behaviors in Granitoid Fault Rocks Controlled by Local Heterogeneity

Daniel Ortega-Arroyo, ETH Zurich
Matej Pec, Massachusetts Institute of Technology
Hoagy O'Ghaffari, Massachusetts Institute of Technology
Sofia Cubillos, Massachusetts Institute of Technology
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Abstract

Fault zones exhibit a wide spectrum of slip behaviors, from slow creep (mm/yr) to fast, coseismic rupture (m/s). While modern geophysical observations can capture this variability, the underlying physical mechanisms, as well as the respective fingerprints of the various slip modes in the geological record—particularly those related to slow-slip— are not fully understood. Here, we aim to elucidate the processes giving rise to the different slip modes and their respective microstructures through rock deformation experiments. A total of 14 tests were conducted on granitoid gouges deformed at a constant loading rate under 500 MPa confining pressure and room temperature. Despite nominally identical boundary conditions, samples failed either through aseismic creep, slow but accelerated slip, and /or stick-slip. Detailed microstructural analyses using SEM-BSE and EDS compositional mapping revealed key distinct features and deformation microstructures associated with each different slip mode:
Stable creep with no perceptible weakening occurred in samples that locally exhibited the lowest quartz content among all our tests and where the strain was distributed across multiple ultrafine-grained principal slip zones (PSZs) scattered throughout the samples.
Slow-slip events, marked by slow but accelerated weakening (~2 µm/s) and subtle shear heating, occurred on samples that localized deformation along a narrow (2-5µm), low-porosity, nanocrystalline to partially amorphous PSZ embedded within a wider (<100 µm) high-strain region.
Stick-slips, which were characterized by rapid slip velocities (m/s), audible acoustic emissions, and sharp temperature spikes, occurred on samples that developed frictional melts. Notably, samples that failed exclusively through abrupt stick-slip behavior contained the highest quartz concentrations and the most pronounced strain localization.
Our observations suggest that subtle variations in fault zone structure, strain localization, and degree of comminution —influenced by the mineral distribution— can have a significant effect on a fault's slip behavior. These findings highlight the role of local heterogeneity in modulating fault stability and offer direct constraints on the mechanics and microstructural fingerprints of slow slip hosted within granitic fault zones.

Session

Experiments of all sorts