THIN SHARP SHEAR SURFACE OF THE SHALLOW PORTION OF OUT-OF-SEQUENCE THRUST

Out-of-sequence thrust (OST) plays a great role in thickening of an accretionary prism, formation of a forearc basin and tsunami generation. The purpose of this study is to investigate the lithology, texture and evolution of OST in order to know the role in the seismo-genesis. I had mainly a feild observation in the subduction zone, Kure merange unit of the cretaceous Shimanto accretionary complex, southwest Japan, and revealed the thermal structure based on the vitrinite reflectance, microstructure analyses.

More than ten small echelon OSTs was found by detaild geologic and thermal structural analysis. Because the maximum temperature of 150± 30° C was estimated at the hanging wall of this faults based on the vitrinite reflectance, shallow portion of the OST would have been exposed in this outcrop. The echelon fault system has large total offset of thermal difference of 100° C in temperature, and they cut previous geologic structure. Although small thermal difference was found in some thrusts, any specific thrust to make large offset was not developed within the faults system. Generally thickness of the fault zone is within 50 cm and the fault rocks consists of cataclasite, fault gouge, fault brecca and pseudotachylyte. Quartz, calcite and zeolite veins are well developed within the brecciated sandstone blocks and along the fault surface. These fault rocks are characterized of deformation with pressure solution, foliated clasts and some millimeter of cataclasite cut the clasts directry. Shear surface of the most of the fault are localized and fractured repeatedly.

This fault system would happen rapid slip with formation of pseudotachylyte repeatedly to make large total offset of the thermal structure. Although Fault surface heals untilit it can be lapid slip, weaker than the host rocks.