Mélange and its seismogenic roof décollement: A plate boundary fault rock in the subduction zone—An example from the Shimanto Belt, Japan

Abstract

The Mugi Mélange located in western Shikoku of the Shimanto Belt shows systematic Y‐P deformation fabrics formed by microshear and pressure solution that penetrate throughout the mélange pile. Magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility (AMS) are highly oblate. Maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the mélange and the mean pole of P surfaces, respectively. This agreement suggests that the Mugi Mélange was formed as a result of underthrusting of trench filling sediment. Vitrinite reflectance ranges from 2.52% to 3.08%, which corresponds to a maximum paleotemperature of ∼180–200°C. Pseudotachylyte, evidence of a seismogenic slip, was found in the upper boundary roof fault of the Mugi Mélange. However, there is not a thermal gap between the mélange and the overlying coherent piles, and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the mélange beyond the boundary fault, which suggests that paleoisotherms parallel the boundary fault orientation. The isotherms in the seismogenic zone are estimated as subparallel to the plate boundary décollement. Therefore the setting of the cataclastic boundary fault, which includes pseudotachylyte, appears to be a major plate boundary thrust or a subhorizontal splay fault. A probable geologic setting that accounts for the Mugi Mélange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time: interseismic slip in the mélange and seismic slip along the roof fault.