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doi:10.2204/iodp.proc.314315316.220.2012

Geological settings

During Expedition 315, the upper accretionary prism was cored at two sites: IODP Site C0001 at the seaward edge of the Kumano Basin uplift (outer arc high) and IODP Site C0002 at the southern margin of the basin. Holocene to late Pliocene silty clay to clayey silt is unconformably underlain by the late Miocene upper accretionary prism composed mainly of mudstone and bounded by a thick sand layer at 207 m core depth below seafloor (CSF) at Site C0001 (Ashi et al., 2008; Kinoshita et al., 2008). An unconformity between the forearc basin and accretionary prism is identified by the lithology change from basal Pliocene mudstone above to late Miocene interbeds of mudstone and sandstone below at 922 m CSF at Site C0002 (Ashi et al., 2008). During Expedition 316, the seaward edge of the Kumano Basin uplift where the megasplay fault is branching from the plate interface between the subducting Philippine Sea plate and overlying Eurasian plate was cored at IODP Site C0004. A 6 cm thick microbreccia, indicating concentrated shear within the megasplay fault zone, was found in the middle Pliocene hemipelagic section at 291 m CSF. IODP Site C0008 is located ~1 km seaward of Site C0004, providing a reference site for the sediments underthrusting beneath the megasplay fault (Kimura et al., 2008).

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