Three-Dimensional Architecture of the Median Tectonic Line in Southwest Japan Based on Detailed Reflection Seismic and Drilling Surveys

Abstract

The subsurface morphology of an arc-bisecting tectonic zone has been unraveled by means of well-organized seismic investigation with the aid of borehole stratigraphic control. The Median Tectonic Line (MTL) active fault system in southwestern Japan, one of the world's largest intraplate transcurrent faults, is driven by the recent oblique subduction of the Philippine Sea Plate. Six tied seismic profiles covering the mountainous range, the southern foothill of which is truncated by the MTL, were used to visualize the Quaternary basins on both feet of the Izumi Mountains. North-and east-trending basement deformation was confirmed on the northern and southern sides of the watershed, respectively; this deformation reflects the spatial diversity in tectonic stress. Seismic data on the southern Izumi flank revealed a low-angle fault parallel to the MTL active fault system; this fault may be interpreted as a dormant structure that developed from 6 to 2 Ma under the intermittent increases of the compressive regime. A kink zone in the upthrown block of the thrust was identified on seismic profiles and continuously traced through field geological survey. This zone confirms the prevailing contractional phase related to the transient convergence mode of the oceanic plate.