Unstable fault slip induced by lawsonite dehydration in blueschist: Implication for the seismicity in the subducting oceanic crusts

Abstract:

Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the subducting mantle In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Moho. These observations have stimulated interest in potential relationships between blueschist-facies metamorphism and seismicity, particularly through the dehydration reactions involving lawsonite. The rheology of these high-pressure and low-temperature metamorphic minerals is largely unknown.

We conducted experiments on lawsonite accompanied by monitoring of acoustic emission (AE) in a Griggs-type deformation apparatus. Deformation was started at the confining pressure of 1.0 GPa, the temperature of 300 ˚C, and constant displacement rates of 0.16 to 0.016 μm/s, that correspond to equivalent strain rates (ε) of 9 × 10^–5 to 9 × 10^–6 1/s. In these experiments, temperature was increased at the temperature ramp rate of 0.5 to 0.05˚C/s above the thermal stability of lawsonite (600˚C) while the sample was deforming to test whether the dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (i.e., stick-slip) occurred during dehydration reactions in the lawsonite gouge layer, and AE signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R₁ and B shear), and the fault surface shows mirror-like slickensides. The unloading slope (i.e., rate of stress drop as a function of slip) during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate. A thermal-mechanical scaling factor in the experiments covers the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers to induce seismicity in cold subduction zones.