Structural Framework of the ODP Leg 131 Area, Nankai Trough

Abstract

A reprocessed multichannel seismic reflection line across the ODP Leg 131 area allows well-controlled structural interpretations of the accretionary prism toe. A 2.5-km-wide protothrust zone is characterized by thickening and seaward tilting of the trench wedge strata, associated with a significant amount of layer-parallel shortening and fluid expulsion. The frontal thrust is imaged as a fault-plane reflection and ramps upward from a décollement within the Shikoku Basin hemipelagic section. Over this ramp, the overthrusting sediments form a fault-bend fold on which ODP Site 808 is located. The spacing of the first three thrusts at the toe of the prism is approximately 1.5-1.7 km. Thrust spacing decreases arcward to less than 1 km as horizontal shortening progresses. The polarity of the reflection marking the basal décollement is reversed under the trench and accretionary prism. This polarity reversal is generated by a zone of low velocity beneath the décollement, indicating that the subducting basin hemipelagic section has not dewatered to the extent that the section above has, and therefore may be overpressured.