PATH-DEPENDENT STRENGTH AND DEFORMATION BEHAVIOR OF SHALLOW SUBDUCTION FAULT ROCK (Invited Presentation)

The mechanical properties of subduction zone fault rocks at shallow depths are proposed to affect different megathrust slip behaviors, including tsunamigenic earthquakes and shallow slow slip. However, the in-situ rheology of basalt-derived fault rocks as well as their path-dependent behavior during subduction are not well understood. We used deformation experiments to quantify (1) the rheology of metabasaltic gouge and chlorite under shallow megathrust conditions and (2) how different stress paths linked to elevated fluid pressures affect different slip modes. We conducted triaxial shear experiments on chlorite-rich gouge and exhumed cataclasite from the Rodeo Cove thrust zone in California. Samples were sheared at 100 ^oC, 130 or 160 MPa confining pressure, pore pressures from 10 to 120 MPa, slip rates from 0.001 to 10 µm/s, and to 7.5 mm displacement. For tests at high pore pressure, we also varied consolidation state to simulate maintenance of excess pore pressures at the shallow end of the megathrust (normally consolidated) and the release of dehydrated fluids further downdip (overconsolidated). The rheology of chlorite is frictional throughout these conditions and shifts from rate-weakening behavior at slow slip velocities to rate-strengthening at faster rates when normally consolidated. The cataclasite, which is composed of other clays besides chlorite, is significantly weaker and rate-strengthens across the tested slip velocities. In addition, gouge that is overconsolidated exhibits rate-strengthening behavior, whereas samples that remain normally consolidated under high pore pressure exhibit unstable slip behavior. This indicates that fluid overpressures resulting from dehydration reactions will tend to promote more stable slip compared to disequilibrium compaction. We interpret deformation mechanisms causing these changes in behavior and suggest that pore pressure history plays a key role in affecting slip modes along the shallow megathrust.