Experimental faulting of serpentinite during dehydration: Implications for earthquakes, seismic low-velocity zones, and anomalous hypocenter distributions in subduction zones

Dehydration embrittlement of serpentine was investigated by employing triaxial deformation experiments at high pressure and temperature (P = 1-6 GPa; T = 550-820degreesC). A modified Griggs apparatus was used up to 3.4 GPa and a Walker-type multi-anvil apparatus was used at 3.5-6 GPa). The investigated specimen is a serpentinized peridotite from Val Malenco, Italy. Dehydration of the sample under differential stress resulted in faults associated with ultrafine-grained solid reaction products, formed as byproducts of antigorite dehydration. This phenomenon was observed under all conditions tested (1-6 GPa, 630-820degreesC), independent of pressure, even though the sign of total volume change (AV) of the dehydration reaction changes from positive to negative at 2.2 GPa. This observation confirms that dehydration embrittlement is a viable mechanism for triggering earthquakes independent of depth, so long as there is a hydrous mineral breaking down under differential stress. Aligned Mode-I cracks and fluid inclusion trails are common in relict olivines in the deformed serpentinite. We suggest that some of the puzzling observations, including a seismic low-velocity zone (LVZ) at the top of subducting slabs, can be attributable to aligned fluid-filled Mode-I cracks and enhanced defect mobility in olivine in the presence of water. The anomalous hypocenter distributions of earthquakes can be attributed to the degree of dehydration of hydrous minerals in combination with a certain level of stress. In addition, low-seismicity regions in subduction zones may be explained by superplastic flow under low stress along the ultrafine-grained solid reaction products that are produced during dehydration reactions.