Inhibition of subduction thrust earthquakes by low-temperature plastic flow in serpentine

Serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults; however, recent frictional experiments have shown a marked increase in its shear strength with increasing depth, implying its deformation via unstable sliding at a plate interface in subduction zones. Here we report the results of a series of simple-shear experiments designed to determine the dominant deformation style of low-temperature serpentine species under P-T conditions that correspond to the mantle wedge corner in cool subduction zones (ca. P=1GPa and T=200°C). We found that with increasing shear strain, the dominant flow mechanism of the serpentinite evolves from (1) inhomogeneous semi-brittle flow by strain localization into sample-scale shear bands to (2) homogeneous ductile flow by intracrystalline deformation within individual serpentine grains, with the development of a planar shape fabric and a strong crystallographic-preferred orientation. TEM observations revealed that grain size reduction in lizardite crystals occurs due to (001) interlayer glide, represented by the occurrence of tiny sheet-like sectors oriented parallel to (001)_liz lattice fringes, which evolved from large, elongated sectors with long axes oriented normal to (001)_liz. Given that a low-viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains, the above results indicate that the presence of low-temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes, as stress is preferentially accommodated by plastic flow. This hypothesis explains the occurrence of an anomalous non-seismic region (devoid of large thrust earthquakes) in locally hydrated forearc mantle within northeast Japan.