Basin formation at a contractional bend of a large transcurrent fault: Plio-Pleistocene subsidence of the Kobe and northern Osaka Basins, Japan

Abstract

The basin formation process at a contractional bend of a large transcurrent fault is studied here from the viewpoint of extensive subsidence history. The Kobe and northern Osaka Basins in southwest Japan are located at the easternmost left-stepping bend of the dextral Median Tectonic Line (MTL), a fault system activated throughout the Quaternary. Many subordinate faults in these basins accommodate compressive stress, whereas differential vertical motion of fault-bounded blocks results in complex sedimentation patterns within the basins. We compiled stratigraphic information of the Late Pliocene to Pleistocene Osaka Group using core samples of 19 drill holes distributed in the study area. On the basis of magnetostratigraphy, tephrochronology and paleoenvironmental analysis, 15 datum horizons of transgression, which are recognized as bases of marine clays (Ma −1 to 13), in cores are assigned to the oxygen isotope events. An interval accumulation rate between two successive horizons represents a cycle of eustatic sea level changes, and thus provides us with an indicator of tectonic subsidence rate. Subsidence curves and temporal changes in subsidence rates at the drill holes visualize differential subsidence in detail. The northern Osaka Basin is divided into western and eastern depocenters and the central N–S Uemachi Basement high, which grew rapidly during the Ma 7/8 interval (ca. 550 kyr), whereas the subsidence pattern in the Kobe Basin is controlled by constant vertical displacements on the NE–SW Wada-misaki Fault. An interbasinal tectonic episode of sharp decline in subsidence is also detected in the Ma 5/6 interval (ca. 670 kyr), coinciding with the Mantidani Unconformity in the Osaka Group.