Japan Marine Science and Technology Center, Japan

Ocean Drilling Program Legs 190 and 196 were a two-part program to study deformation and fluid flow in the central Nankai Trough off Shikoku Island. During Leg 190 two reference sites were drilled outboard of the trench (Sites 1173 and 1178), one site into the protothrust zone (Site 1174), two sites into a trench slope basin above a major out-of-sequence thrust (Sites 1175, 1176), and one site into an older portion of the accretionary prism. During Leg 196, Sites 1173 and 808 (drilled through the frontal thrust during Leg 131) were revisited, employing logging while drilling and installing two Advanced Circulation Obviation Retrofit Kits (ACORKs). Our reference sites defined the stratigraphic framework and physical properties baseline of the accreting/subducting Shikoku Basin sedimentary section. The proto-thrust and frontal thrust sites documented the dewatering and deformation processes at the toe of the accretionary prism. Porosity comparisons between Sites 1173 and 808 suggest that elevated fluid pressures occur beneath the decollement at Site 808. Initial measurements from the ACORK at Site 808 indicate a pressure pulse apparently from the decollement. Negative chloride anomalies at Sites 1174 and 808 could be due to fluid flow from deeper in the prism, but active smectite dehydration could also be responsible for the anomalies. Resistivity imaging of the frontal thrust shows borehole breakouts with principal stress orientations consistent with core-scale structures and plate convergence directions. Combined with a three-dimensional seismic reflection survey, our drilling data suggest that the decollement developed because of overpressuring beneath a diagenetic phase transition. Seismic data further suggest that the velocity-density inversion at the decollement at Sites 1174 and 808 is obliterated after underthrusting to seismogenic depths. The slope basin sites provide key evidence that the large out-of-sequence thrust was very recent and that the outer 40 km of the accretionary prism was built within the past 1 m.y.