Stress states in subduction zones: Extrapolation of flow laws and piezometric relations of quartz to high-P/low-T metamorphic conditions

Abstract:

Strength profiles across the continental lithosphere have been extensively discussed based on laboratory rock-mechanics data but little is known about the stress states in subduction zones, which are characterized by the presence of aqueous fluids and siliceous sedimentary rocks. Here we focus on the deep parts of the interplate megathrusts that consist of high-P/low-T metamorphic rocks. For example, the depth range from 45 to 60 km of the interplate megathrust in NE Japan, which caused the 2011 M9.0 Tohoku-oki earthquake, corresponds to the blueschist facies zone with temperatures ranging from 300 to 400 C (Shimizu, 2014). To evaluate the stress states during interseismic periods, we used the dislocation creep flow law of wet quartz proposed by Luan and Paterson (1992). We applied the flow law without making a water fugacity correction, because the strength of quartz in high-P/low-T conditions would be governed by concentrations of water-related species within the crystals rather than water fugacity. Thus the present model gives conservative estimates. The result of calculation indicates the shear strength over 200 MPa at the depth of 45 km and 50–100 MPa at 60 km depth.

Recrystallized grain size in quartz rocks has been considered as an indicator of paleo-stress but physical basis of the piezometric relations is still in debate. In the theoretical models of grain size piezometers, formation of new grains and grain growth associated with strain-energy or surface-energy driven grain-boundary migration (ρGBM or γGBM) have been considered as elementary processes of dynamic recrystallization (DRX). Recent dynamic equilibrium models predict negative temperature dependence of the mean grain size (e.g., Shimizu, 2008); however, Platt and Behr (2011) pointed out that the processes of ρGBM and γGBM are locally antagonistic and questioned the model assumptions. Here we evaluate relative importance of surface energy in DRX of quartz and proposed a new model that includes possible effects of surface energy drag. The revised piezometric relation is used for paleo-stress estimates in the Sanbagawa metamorphic belt, Japan.