Asymmetric diagenetic changes in a half-graben basin: the Kanmon Group (Lower Cretaceous), SW Japan

The Kanmon Group (Lower Cretaceous) is a nonmarine sedimentary succession in the Inner Zone of south-west Japan. It was deposited in a basin having a half-graben structure with major depocenters on the west side of the bounding fault. The degree of some diagenetic changes of the Kanmon Group sandstones is largely dependent on the depositional position in an asymmetric basin, i.e., the distance from the basin-bounding fault. Illite crystallinity values and compaction of Kanmon Group sediments in the vicinity of the boundary fault are much lower (more thermally altered) and more severe, respectively, than those away from it. In addition, calcite phases near the fault are more diverse and complex. These results indicate that the boundary fault system strongly influenced diagenetic changes in the Kanmon Basin. The basin-bounding fault is considered to have opened episodically by tensional stress caused by oblique subduction of the proto-Pacific plates (Izanagi and Kula plates) throughout basin evolution, and it was used as a pathway for diagenetic fluids, causing more extensive and advanced mineralogical alterations.