Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of southwest Japan

Abstract

The inner belt of Southwest Japan is located in the region north of the Median Tectonic Line (MTL), which is the largest strike-slip fault in the Japanese Islands belonging to an island-arc system. The inner belt is divided into ten or more small blocks bounded by active faults and geological tectonic lines. The blocks are 20 to 80 km in width and approximately 200 km in length. The width and the length have remained nearly constant during the long history since the Late Cretaceous. Significant igneous activity has occurred along boundaries of the blocks from the Late Cretaceous to the present. With the use of a geometrical strike-slip and block rotation model, the small blocks were rotated in such a way that they are to be rearranged in parallel with each other at the time of the block formation. The inner belt of Southwest Japan was then placed north of its present location to the position occupied in mid-Cretaceous time. With this configuration the inner belt is found to be situated between two major strike-slip faults; the MTL and the Sikhote-Alin fault. Faulting and igneous activity that occurred from the Late Cretaceous to the present can be geometrically interpreted by the strike-slip and block rotation model which induces left-lateral slip movement of the two major bounding faults. Main plutonic activity occurs at triangle gaps generated by the block rotation. Other igneous activity occurred along block terminations block boundary faults and Riedel shear fractures inside the block. Bending of Southwest Japan occurred along three block boundaries: the Itoigawa-Shizuoka Tectonic Line, the Tsurugawan-Isewan Tectonic Line and the Hanaore-Kongo fault line, during the opening of the Sea of Japan Blocks raised by underthrusting during the rotation of the inner belt due to the opening of the Sea of Japan at about 15 Ma have continued to tilt, even at present. Most earthquake faults were generated along block boundary faults, while a few earthquake faults also appearing along Riedel shear fractures that originated from the block boundary faults.