Pressure-Induced Change in Mechanical Anisotropy Within the Foliation Plane of Antigorite as an Indicator of the Brittle‒Ductile Transition

Abstract

The response of antigorite deformation to preexisting fabrics in subduction zones is unclear. We deform antigorite schist with foliation and lineation (X is parallel to the lineation, Y is in the foliation and perpendicular to the lineation, and Z is normal to the foliation) at different angles to the maximum principal stress (σ₁) via a Paterson gas-medium apparatus under a confining pressure of 200 MPa, a temperature of 600°C, a strain rate of ∼10⁻⁵ s⁻¹, and a preheating time of 6 hr. The mechanical results indicate that σ(Z) ≫ σ(X) ≈σ(Y), indicating nearly transverse isotropy. Observations of the recovered samples revealed that less ductile and more brittle deformation occurred simultaneously, indicative of a brittle-to-semibrittle regime. Microcracks formed parallel to the foliation likely increase anisotropy. The slightly greater strength, more kinks and/or occurrence of slow stick-slip along the Y direction suggest that the a-axis is in a hard-friction direction, supporting the results of an atomic force microscopy (AFM) study by Campione and Capitani (2013, https://www.doi.org/10.1038/ngeo1905) on single-crystal antigorite under ambient conditions. In contrast, our previous deformation at 1.3 GPa shows that the b-axis is in a hard-slip direction, consistent with the results from transmission electron microscopy (TEM) observations by Amiguet et al. (2014, https://www.doi.org/10.1002/2013jb010791) on antigorite deformed under 1 and 4 GPa. This comparison suggests that mechanical anisotropy within the foliation changes with pressure, likely reconciling the opposite results obtained by the two studies and indicating brittle‒ductile transitions near the mantle wedge corner where deep slow earthquakes related to antigorite may have occurred.