LEG 131
Nankai Trough

The Nankai Trough is a topographic manifestation of the subduction between the Shikoku Basin, part of the northwest-moving Philippine Sea Plate, and the Southwest Japan Arc. To the east, the trough converges with a major arc-arc collision boundary between Honshu Island and the Izu- Bonin Arc. The backarc basin formed behind the Izu-Bonin Arc by east-west directed spreading, accompanied by a late phase northeast-southwest spreading episode in the late Oligocene to middle Miocene. The fossil spreading axis lies in the center of the basin and has been subducted at the middle of the trough. During Leg 131, one site (Site 808) was drilled to study the interplay of the various parameters that influence structural evolution of accretionary prisms with special emphasis on hydrogeology.

The Shikoku Basin basaltic basement in the Nankai region is middle Miocene in age (~ 15.6 Ma) and is overlain by middle Miocene to Pleistocene hemipelagic mudstones. The onset of major turbidite deposition occurred after 0.5 Ma. The first major thrust fault occurs within an overturned fold at 365 mbsf and has a vertical stratigraphic throw of 145 m. The decollement zone is approximately 20 m thick and occurs at 960 mbsf within a lithologically homogeneous hemipelagic sequence. The style of sediment deformation changes with time and/or position within the accretionary prism; the complete interval between the thrust and the decollement shows pervasively deformed sediments with abundant minor faulting at depth, whereas complex shear bands predominate near the fault thrust. There is evidence that the shear bands predate faulting.

Porosity increases downward across the thrust and density correspondingly decreases, indicating that sediments have not re-equilibrated to the loading caused by the thrust displacement, and confirming a young age. Immediately below the decollement, porosity again increases downward and bulk density correspondingly decreases, indicating higher pore pressures in this zone. The composition of interstitial pore water changes with lithology and at the two major faults; a large smooth decrease in chlorinity toward the decollement can be interpreted as evidence of fossil fluid flow or the result of an in-situ reaction. There is no evidence of active localized fluid flow, in contrast with results obtained from the Barbados accretionary prism; fluid expulsion in the Nankai prism may be primarily from diffusive flow. In-situ temperature measurements give a calculated heat flow consistent with conductive cooling models of the Philippine Sea lithosphere. However, hydrocarbon-gas distributions suggest higher maturity levels than those expected from the present geothermal gradient; the thermal gradient may have been higher in the past.