Frictional Properties of Simulated Chlorite Gouge at Hydrothermal Conditions: Implications for Subduction Megathrusts

Chlorite is abundant at hypocentral depths in subduction zones and is likely to play a key role in controlling megathrust slow slip and catastrophic rupture. However, no data exist on the frictional properties of chlorite(-rich) fault rocks under the hydrothermal conditions relevant for the subduction seismogenic zone. We report results from experiments conducted under such conditions, using chlorite powders prepared from single crystal clinochlore (Mg-chlorite), as well as limited experiments using a stack of single crystal sheets. Shear experiments were carried out at effective normal stresses (sigma(n)) of 100 to 400 MPa, pore fluid pressures (P-f) of 50 to 220 MPa, and at temperatures (T) of 22 to 600 degrees C, using stepped displacement rates (v) from 0.3 to 100 mu m/s. The gouges are characterized by a coefficient of friction (mu) of 0.2-0.3 at T <= 400 degrees C and 0.3-0.4 at 500-600 degrees C, while (a-b) values showed positive values for nearly all conditions tested, except at 300 degrees C. Microstructures of gouges sheared at T <= 300 degrees C show evidence for widespread comminution, compared with a lower porosity at 600 degrees C. Experiments using a stack of single crystal sheets showed mu <= 0.008 at low displacements (<3 mm) followed by hardening, while microstructures are suggestive of slip along (001), folding and tear of cleavage planes, and gouge production. Our results have important implications for the mechanisms controlling megathrust fault slip under greenschist facies conditions in a subduction zone and shed new light on the strain accommodation mechanisms within sheared gouges versus single crystals composed of phyllosilicates.