Deformation, Structure, and Hydrogeology in the Nankai Accretionary Prism, Japan: Initial Results of ODP Leg 196 Logging While Drilling and Installation of Long-term Hydrogeological Observatories

Leg 196 was the second of a two-leg program of coring, logging, and installing advanced CORK (ACORK) long-term hydrogeological observatories in the Nankai Trough, the type example of a convergent margin accreting a thick section of clastic sediments. The two-leg program was built on results from Leg 131 and was designed to define the interrelationship of deformation, structure, and hydrogeology in the Nankai accretionary prism. Leg 196 focused on logging while drilling (LWD) and installation of ACORKs at two sites near the toe of the Nankai prism: Site 808, cored during Leg 131 at the deformation front, and Site 1173, cored during Leg 190 as a reference site 12 km seaward. At Hole 1173B we collected LWD data to basement at 737 meters below seafloor (mbsf). Here the LWD data verify a subtle porosity increase with depth from 122 to 340 mbsf, followed downhole by a sharp decrease in porosity and return to a normal consolidation trend. The sharp decrease in porosity correlated with the diagenetic transition from cristobalite to quartz and is marked by a strong seismic reflector that is reproduced well by a synthetic seismogram based on the LWD data. Resistivity-at-the bit (RAB) images of the borehole show no evidence of a propagating protodecollement but, rather, reveal a basinal state of stress dominated by steeply dipping fractures and normal faults of variable strike. In Hole 808I we acquired LWD data to just below the decollement (1035 mbsf); poor drilling conditions precluded further penetration. Borehole Resitivity At Bit (RAB) images provide unparalleled structural and stratigraphic detail across the frontal thrust and decollement that indicate northwest-southeast shortening consistent with the seismic reflection data. RAB images also document borehole breakouts that show a northwest-southeast oriented maximum principal in situ stress direction, nearly parallel to the maximum principal stress direction inferred from microfaults in cores and from the plate convergence direction. Resistivity curves suggest that the frontal thrust zone has compacted, presumably due to faulting. In contrast, the resistivity data suggest that the decollement zone is dilated. These resistivity anomalies in the frontal thrust and decollement zones cannot be explained by variations in pore water composition and need to be verified by the density and porosity logs, after careful correction for borehole washouts. In Hole 1173B, a four-packer, five-screen ACORK installation was successfully emplaced. It was configured for monitoring the hydrogeological state and processes in basement and the stratigraphic projection of the decollement in the Lower Shikoku Basin formation. The ACORK in Hole 808I was configured with two packers and six screens and was intended to penetrate just to the décollement, with an emphasis on determining the hydrogeological state and processes at the frontal thrust, a fractured zone 160 m below the frontal thrust, and the decollement. Owing to extreme deterioration of drilling conditions and underreamer failure, actual penetration concluded ~36 m short of the target depth.