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 (σ n ) of 100 to 400 MPa, pore fluid pressures (P f ) of 50 to 220 MPa, and at temperatures (T) of 22 to 600°C, using stepped displacement rates (v) from 0.3 to 100 μm/s. The gouges are characterized by a coefficient of friction (μ) of 0.2-0.3 at T ≤ 400°C and 0.3-0.4 at 500-600°C, while (a-b) values showed positive values for nearly all conditions tested, except at 300°C. Microstructures of gouges sheared at T ≤ 300°C show evidence for widespread comminution, compared with a lower porosity at 600°C. Experiments using a stack of single crystal sheets showed μ ≤ 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.

We report a new type of ultramafic pseudotachylyte that forms a fault- and injection-vein network hosted in the mantle-derived Balmuccia peridotite (Italy). In the fault vein the pseudotachylyte is now deformed and recrystallized into a spinel-lherzolite facies ultramylonite, made of a fine (< 2 mu m) aggregate of olivine, orthopyroxene, clinopyroxene, and spinel, with small amounts of amphibole and dolomite. Electron backscattered diffraction study of the ultramylonite shows a clear crystallographic preferred orientation (CPO) of olivine. The fault vein pseudotachylyte overprints a spinel-lherzolite facies amphibole-bearing mylonite, indicating that shear localization accompanying chemical reaction had taken place in the peridotite before seismic slip produced frictional melting. The occurrence of amphibole in the host mylonite and that of dolomite as well as amphibole in the matt-ices of ultramylonite and pseudotachylyte may indicate that fluid was present and had evolved in its composition from H2O-rich to CO2-rich during ductile deformation with metamorphic reactions, which may account for the observed rheological transition from ductile to brittle behavior. The spinel-lherzolite facies assemblage in mylonites, P-T estimations from pyroxene geothermometry and carbonate reactions, and the type of olivine CPO in deformed pseudotachylyte indicate that both the preseismic and the postseismic ductile deformations occurred at similar to 800 degrees C and 0.7-1.1 GPa

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