Shear-Enhanced Electrical Conductivity of Synthetic Quartz-Graphite Gouges: Implications for Electromagnetic Observations in Carbonaceous Shear Zones

Graphite is considered as a material that promotes fault weakening and electrical conductivity (sigma) enhancement at fault zones. We studied how shear deformation may affect the evolution of friction and electrical conductivity of synthetic quartz (Qz)-graphite (Gr) mixtures and, more importantly, whether the sigma of the mixtures present visible changes at the beginning of the simulated fault slip. Long-displacement friction experiments were performed on 1.2-2.3 mm-thick gouge specimens of varied Gr volume fraction (X-Gr = 0-100 vol.%) under identical normal stress (2 or 5 MPa), slip rate (similar to 1.0 mm/s), and N-2-flushing conditions. The experimental results suggested that the sigma of the specimens with >= 4.6 vol.% X-Gr abruptly increased under limited shear displacement. With continued shear, the steady-state electrical conductivity (sigma(ss)) increased by more than seven orders of magnitude when X-Gr > 3.4 vol.%, while the steady-state frictional coefficient remained high (0.54-0.80) except for the specimens with X-Gr > 13.6 vol.%. The post-mortem microstructures revealed that the high sigma(ss) observed in the intermediate Gr content specimens (3.4-13.6 vol.%) is associated with an ad-hoc fabric (graphite-cortex clasts) present in the principal slip zone. For high Gr content, excess Gr flakes fill the pores and help develop mechanically lubricated surfaces. We propose that low Gr content (i.e., as low as 3.4 vol.%) can cause high conductivity anomalies in natural shear zones. Overall, the findings suggest that the initiation of slips within carbonaceous shear zones can be detected by identifying unusual temporal signals using electromagnetic stations.