INITIATION AND EARLY EVOLUTION OF A FOREARC BASIN IN SW JAPAN BASED ON 3D SEISMIC DATA ANALYSIS

An outstanding question is whether sedimentation or tectonism is the main control on forearc basin evolution. Mapping of the main sedimentary packages within the lower Kumano forearc basin in high-resolution 3D seismic (with Integrated Ocean Drilling Program (IODP) well data) allows an evaluation of this question.

The Kumano basin overlies the inner Nankai accretionary prism, where the Philippine Sea Plate subducts beneath Japan. This area is the focus of the IODP NanTroSEIZE drilling program, and is known for its prominent “megasplay” fault that bounds the basin seaward. The Kumano basin is thus used as a testing ground for forearc basin development. The mapping results show that the lowermost sediments of the basin (LB1) were deposited in a thrust-bound slope basin environment between ~3.8 Ma and ~2.06 Ma. The slope basin sediments are unconformably covered by a relatively thin and sheet-like interval that could correspond to slope cover and has been interpreted as mass transport deposits (LB2). Its top has an age of ~1.3 Ma, partly coeval with deposition of the early upper Kumano basin sedimentary sequences (UB1). UB1 sequences appear to track the known slip history along the megasplay. Landward, a third sedimentary package within the lower basin (LB3) postdates LB2 and correlates with an interval of high wood content; this section may have a different sediment source than the upper Kumano basin sequences. LB3 onlaps against an anticline previously identified as a buried thrust, which appears to have reactivated at ~1 Ma, after movement along the megasplay ceased. This basin shows that forearc basins can initiate as slope basins in an outer-wedge environment and undergo provenance changes and inner wedge deformation before the more systematic progradation of the basin section fill. The lower Kumano sections correlate spatially and temporally with regionally recognized tectonic events (megasplay nucleation, buried thrust reactivation) rather than with documented changes in sedimentation rates into the basin, suggesting tectonics created the accommodation space and led to the initiation of the forearc basin. Despite potential controls on the wedge mechanics via sediment supply variations, tectonics appeared to continue driving key stratigraphic developments within the basin through the middle Pleistocene.