Deformation of Antigorite and Its Geological Implications

Abstract

Abstract The deformation model of antigorite and its relationship to intermediate‐depth earthquakes are still controversial. Here, we conducted deformation experiments on foliated iron‐poor antigorite at confining pressures of 1,100–1,500 MPa, temperatures of 400–600°C, and axial displacement rates of 3.75 × 10 −6 –7.24 × 10 −4 mm/s using a solid‐medium Griggs‐type apparatus. As the temperature and/or pressure increases, the stress exponent decreases, and all samples show stable fault slip. Microstructural observations show ductile behavior indicated by kinking and bending of the originally straight foliation, and brittle characteristics, such as Riedel shear, comminution, and microcracking. Therefore, antigorite deformation is controlled by a semibrittle regime. Antigorite decomposed to forsterite and enstatite in samples sheared at temperatures ≥550°C but not in samples triaxially compressed at 600°C. This observation, combined with phase boundary information, implies that the deformation geometry (e.g., triaxial compression or shear) may change the phase stability of antigorite. Following dehydration, samples still show stable fault slip, indicating that dehydration does not directly trigger earthquakes. In cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces seismic activity by producing water and talc (or a talc‐like phase) in the process of shear friction because such phases within the fault zone may reduce the rate dependence, thereby inducing unstable slip. The comparison of the semibrittle strengths of antigorite and dunite and the weak strain rate dependence of stress suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge.