Fracture-induced pore fluid pressure weakening and dehydration of serpentinite

Abstract

We investigate the strength, deformation processes, and pore fluid weakening during localized shear of antigorite serpentine. Recent work has shown that some phyllosilicates, including antigorite, undergo a reverse transition from ductile to localized deformation at the pressure-temperature conditions of deep slow slip and tremor in subduction zones. Here, we investigate the processes that lead to and occur during localized deformation. Because high pore fluid pressure is hypothesized to control the location and style of fault slip at these conditions, we investigate the role of pore fluids on these deformation processes. We present the results of undrained general shear experiments on antigorite-rich serpentinite deformed to varying strains at 500°C, 1 GPa pressure, and with 0 to 2 wt.% added pore water. At all fluid conditions, the serpentinite exhibits strain hardening during distributed deformation and subsequent strain weakening associated with the formation of a prominent shear fracture zone. The magnitude of strain weakening correlates with increasing pore water content. We evaluate two end-member scenarios for how the effective stress influences strength during localized deformation and find that either an increase in fluid pressure or increase in the parameter α in the effective stress law can explain the weakening. At all fluid conditions, we also find evidence for localized dehydration of antigorite within the fracture zones, at pressures and temperatures where antigorite is considered stable. Although the extent of the reaction did not measurably affect fault strength in our experiments, at the time scales of in-situ deformation in the Earth, reaction weakening and associated pore fluid pressurization may occur.