Gouge-zone or solid-rock: An experimental view on fault frictional behavior

Abstract:

Natural faults always include zone(s) of breccia, gouge or cataclasite that localize the slip. In contrast, many rock mechanics experiments are conducted on experimental faults made of rough, solid blocks without fault-rock zones. We experimentally compare the frictional strength of solid experimental faults with fault-zones made of granular material in high-velocity/long-distance runs.

The frictional evolution of solid and granular dolomite fault was tested in a rotary apparatus at slip velocity up to 1 m/s and normal stress up to 7 MPa. The granular samples were composed of the 125-250 microns fraction of the crushed dolomite. They were sheared in a confined, rotary cell with continuous monitoring of CO₂ and H₂O and mechanical data. The tests showed that the granular samples required longer slip-distances and higher velocities to evolve to a frictional strength similar to the solid samples. Yet, both sample types display similar evolution trends, including slip-weakening at velocities above ~0.05 m/s, and drastic velocity-weakening as slip velocity approached 1 m/s. At velocity above 0.3 m/s, a shining principal-slip-zone developed spontaneously with identical microstructure in both solid and granular sample: thickness < 1 micron and sintered, 20-40 nm nano-grains. This development was associated with intense emission of CO₂ (Fig. 1). In a similar testing approach and conditions, we tested the friction evolution of granular granite from the San-Andreas fault-zone at Tejon-Pass, CA. These samples remained strong, μ =0.8-0.9, at velocities up to 0.8 m/s and slip–distances up to 3 m, in contrast to known frictional evolution of solid faults made of granite and tonalite. We envision that at the present slip-velocity/normal-stress, the dolomite samples reached a stage of thermally activated phase-transition and associated weakening, whereas the granitic samples were below such transition.

Fig. 1. Evolotion of slip-velocity, friction, temperature and CO₂ emission in run 3165 with granular Kasota dolomite. Normal stress is 3.3 MPa. The time delay of CO₂ data reflect gas flow from shear chamber to gas analyzer.