Discussion—Origin and Evolution of the Osaka Basin

Abstract

On the basis of the seismic interpretation presented in the previous chapter, the subsurface architecture of Osaka Bay in southwest Japan is described in detail in this chapter. Although this Plio-Pleistocene basin has a two-layered simple system of Cretaceous granitic basement and fluvio-tidal/deltaic sediments as a first approximation, discrepancies between the gravitational and seismic analyses imply the existence of concealed pre-Neogene basins within the northern part of the acoustic basement. The main findings regarding the three-dimensional characteristics of significant structural trends are summarized as follows. (1) A number of active faults fringing the northwestern margin of Osaka Bay have been classified into the Awaji–Rokko Fault System to the west and the Osaka Bay Fault to the east accompanied by dextral and reverse displacements, respectively. However, the present research shows that both lateral and vertical slips occur on each of the continuous fault strands, which are collectively regarded as a buffer zone of prevailing transpressive tectonic stress. (2) Among sporadic basement highs acting as domain boundaries of the late Pleistocene basin, a N–S warping zone on the southern coast extends into the bay area and has the most important paleoenvironmental influence, as exemplified by the formation of a freshwater lake ca. 500–400 ka. (3) A seismic section passing through the Kitan Strait shows upward-splaying faults. Together with the specific separation senses of the faulted blocks, they are interpreted as constituting a large transcurrent fault. Its connectivity with onshore active fault traces indicates that the observed rupture is a part of the Median Tectonic Line (MTL) with dominant right-lateral motions through the late Quaternary. In an open sea half-graben, the northern depocenter of which is cut by the MTL, there is a conspicuous sedimentary interface, on which a truncation pattern of reflection terminations was clearly identified. On the basis of the neotectonic history of southwest Japan, this unconformable boundary is related to intermittent N–S contractional events dating to the Pliocene. (4) Gentle horsts and grabens off the southeastern coast of Awaji Island are aligned in a N–S trend and regarded as a wide shortening zone under E–W compression. Although a remarkable negative gravity anomaly around this structure suggests longstanding cumulative deformation, the development of Holocene depocenters on the gravity minimum points to ongoing structural buildup. A chronological table of the Plio-Pleistocene noted tectonic events around the Kinki Triangle suggests occasional changes of regional tectonic regimes, which have occurred at shorter intervals since ca. 1 Ma. Finally, the authors present a structural model of the present Osaka sedimentary basin as a fault-surrounded active tectonic region.