Sediment Deformation and Hydrogeology at the Nankai Accretionary Prism: Synthesis of ODP Leg 131 Shipboard Results

Abstract

The Nankai accretionary prism is arguably the best-known clastic prism in the world. The main objective of ODP Leg 131 was to provide data on the deformational processes and associated hydrogeology of the prism. Such information can be incorporated into the existing geophysical and geological data set to characterize the evolution of sediments in this tectonic setting. The objective was approached by drilling Site 808, at which the frontal thrust and décollement of the prism toe were penetrated. Drilling also succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, and with good core recovery. Both the frontal thrust and the décollement were clearly recognizable in the cored material. Bedding dips, faults, and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to northwest-southeast shortening. Below the décollement no significant deformation features were observed. Physical properties data show major discontinuities at the décollement, notably an increase in porosity below the décollement. This might indicate excess pore pressure in the subducted section. An increase in porosity below the frontal thrust is less marked and might reflect only the youthfulness of this feature. Attempts to make downhole measurements were severely hampered by unstable hole conditions, but useful constraints have been placed on the thermal regime, some calibration of laboratory physical properties to in-situ conditions has been provided, and in-situ stress and pore pressure were measured in the uppermost sediments. Evidence of channelized fluid flow is inconclusive because no vein structures were observed in the cores, no unequivocal geochemical anomalies indicative of flow were found, and thermal measurements are not significantly different from those predicted by a purely conductive heat-flow model. We conclude that dewatering probably occurs mainly through diffuse flow at this site.