We determine the features and distribution of fault rocks along the Median Tectonic Line (MTL), SW Japan, to establish the 3D architecture of the fault zone across the brittle–plastic transition. Cataclasites exposed close to the lithological boundary fault (the MTL) can be divided into those formed by sinistral faulting at temperatures of ∼300 °C and those formed by dextral faulting at ∼250 °C. Mylonites distributed to the north of the cataclasites were formed by sinistral faulting and can be divided into lower-temperature mylonite (L-T mylonite) close to the MTL and higher-temperature mylonite (H-T mylonite) distant from the MTL, where deformation temperatures were lower and higher than 400 °C, respectively. Structures formed by sinistral faulting are oblique to those formed by dextral faulting, indicating that the former structures are older than the latter. Structures formed by sinistral faulting underwent deformation around the brittle–plastic transition. Thus, the MTL fault zone records deformation through a crustal section. Microstructural observations suggest that the differential stress just below the brittle–plastic transition (L-T mylonite) was ∼200 MPa and that this value may not change substantially in the deep crust (H-T mylonite).

我们确定了沿日本西南部中线构造线 (MTL) 的断层岩的特征和分布，以建立跨越脆性-塑性过渡的断层带的 3D 结构。暴露于岩性边界断层（MTL）附近的碎裂岩可分为由左旋断层在~300°C 温度下形成的碎屑岩和由右旋断层在~250°C 温度下形成的碎屑岩。分布在碎裂岩北部的糜棱岩是由左旋断层作用形成的，可分为靠近MTL的低温糜棱岩（LT糜棱岩）和远离MTL的高温糜棱岩（HT糜棱岩），变形温度较低且分别高于 400°C。左旋断层形成的构造倾斜于右旋断层形成的构造，表明前者的结构比后者更古老。由左旋断层形成的结构在脆塑性转变周围发生变形。因此，MTL 断层带通过地壳剖面记录变形。微观结构观察表明，刚好在脆性-塑性转变（LT 糜棱岩）下方的微分应力约为 200 MPa，并且该值在深地壳（HT 糜棱岩）中可能不会发生实质性变化。