Deformation mechanisms and rheology of chlorite-actinolite schist and blueschist: Implications for creep behavior along the slab-mantle interface

Abstract

Chlorite-actinolite schist (CAS) and blueschist represent major lithologies along the subduction plate boundary at the slab-mantle interface, yet their deformation mechanisms and rheological properties remain poorly constrained. We investigated a subduction melange in western Kyushu, Japan, deformed along the slab-mantle interface under epidote-blueschist facies conditions at similar to 500 degrees C, comparable to slow slip source regions. Subduction-related shear in the melange is concentrated in multiple 0.01-1.5 m-thick CAS layers, which exhibit S-C fabric and either bound metasediment from metabasite or are intercalated within metasediment or metabasite. Microstructures of CAS are characterized by fine-grained actinolite showing aluminum zoning parallel to the S-surface and its truncation, solution seams, and strain shadows, all consistent with viscous shear accommodated by dissolution-precipitation creep. Glaucophane in blueschist is deformed by microboudinage, accompanied by diffusion of sodic-calcic to calcic amphiboles into boudin necks, and exhibits low internal misorientations and a weak crystallographic preferred orientation, indicating diffusion creep limited by microboudinage. Rheological analyses based on flow laws for dissolution-precipitation creep and microboudinage diffusion creep under peak metamorphic conditions indicate that CAS is mechanically weaker than blueschist and deforms at strain rates approximately two orders of magnitude higher under stress conditions inferred for modern subduction plate interfaces. The mechanical weakness of CAS is consistent with the observed concentration of viscous shear along multiple CAS layers. The higher strain rates of CAS suggest that whereas blueschist can accommodate only steady-state creep, CAS is capable of hosting accelerated creep, including slow slip.