Structure of the Nankai Trough Accretionary Zone from multichannel seismic reflection data

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Abstract
New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12–16 km wide and 350–750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by “kink” folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthrusting sediments form fault‐bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5–2.5 km. The second thrust cuts up from an inferred décollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothrust zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal décollement. The underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the décollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thrusts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the western area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured décollement in the eastern area.