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We present field and microstructural data from an exhumed subduction complex in the Ryukyu arc, Japan, where epidote-blueschist, Triassic, Tomuru metamorphic rocks with block-in-matrix structure crop out. With the aim to constrain epidote-blueschist rheology, we investigate fabric development and infer deformation mechanisms of blocks and matrix through microstructural analyses on the main fabric-forming minerals: glaucophane, epidote and albite. The blocks have a poorly developed, discontinuous foliation. In contrast, the matrix has a well-developed, continuous foliation. Glaucophane is the principal foliation-forming phase, arranged in interconnected layers that surround epidote and albite, which suggests that glaucophane controls epidote-blueschist rheology. Deformation mechanisms inferred from petrographic and electron backscatter diffraction (EBSD) analysis are similar in blocks and matrix, with glaucophane deformed by diffusion creep accompanied by reaction-driven creep, albite by diffusion creep, and epidote by brittle fracture and incipient dislocation creep, based on degrees of internal misorientations, lattice preferred orientation (LPO) development and grain size. During deformation, shear stress must have been relatively low, which is implied by the predominance of diffusion and reaction creep as the dominant deformation mechanisms, and by the small viscosity contrast between blocks and matrix, reflected by their similarity in dominant deformation mechanism. • Epidote-blueschist rheology is guided by diffusion creep of glaucophane. • Microstructures imply low shear stress at deformation on the deep plate interface. • Diffusion creep is unlikely to accommodate stress transients on the plate interface. [ABSTRACT FROM AUTHOR]

The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE–SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c -axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40–7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110–130 MPa at ∼300 °C. The e -twin morphology, twinning ratio, and distribution of the glide direction on the e- twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ 1 and σ 3 axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40–80 MPa at 150–200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70–50 Ma. [ABSTRACT FROM AUTHOR]

A shear zone developed in the Teshima granite in the Ryoke metamorphic belt of Japan contains isolated K-feldspar inclusions showing various shapes within deformed host quartz grains. We present a shape-change model for isolated K-feldspar inclusions, which is a mathematical equation that considers stress, inclusion size, temperature, and the duration of deformation. To calculate the shape-change pattern (i.e., the aspect ratio and size of inclusions) with respect to deformation duration using the model, we used values for deformation temperature (~500–600 °C) and paleo-stress (90 ± 10 MPa) estimated from deformed quartz microstructures within the shear zone. Shape-change patterns were obtained by calculating the model at the estimated range of temperatures with an interval of 25 °C and at the estimated paleo-stress. Shape-change patterns were subsequently corrected by compensating for the effect of post-deformation annealing during exhumation of the shear zone. Deformation duration was estimated by identifying the shape-change pattern calculated by the model that most closely corresponded to the observed shape-change pattern for the shear zone and considering the cooling rate of the Ryoke belt. The resultant deformation duration of the shear zone is estimated as 16,500–8400 yr under conditions of T = 550 °C and σ = 90 ± 10 MPa. Shear strain in the shear zone is calculated as 50–260 by multiplying the shear strain rate by the deformation duration. • Mathematical model explaining shape changes of isolated K-feldspar inclusion. • The model applied to a shear zone developed in the Teshima granite. • Comparison of shape change patterns between the model and the natural data. • Deformation conditions of the shear zone estimated from the comparison. [ABSTRACT FROM AUTHOR]

Abstract: In order to assess the influence of remote mean stress correlated with depth of burial on the principal mode of failure at fault tips during fault slip in a lithologically homogeneous, fractured rock mass, the growth mechanisms of strike-slip faults have been studied at outcrop-scale in the siliceous mudstones of northern Hokkaido, Japan. We take a multifaceted approach combining i) geological characterization of fractures by fracture mapping in outcrop and fracture logging of boreholes (drilling depth: ≤1020m), ii) rock mechanical characterization by laboratory tests on core samples, and iii) theoretical analyses using the Griffith–Coulomb criterion. These suggested that the principal mode of failure in the mudstones is dependent, not only on rock strength, but also on remote mean stresses. During and/or after uplift and erosion the faults grew mainly by linking with adjacent faults via numerous splay cracks, formed by tensile failure above roughly 400m depth. In contrast, below this depth, the faults grew predominantly by shear failure. Such growth mechanisms are consistent with the fact that hydraulic tests performed in boreholes show that highly permeable sections (hydraulic transmissivity: >10−5 m2/s) are restricted to depths of less than 400m. [ABSTRACT FROM AUTHOR]