University of Tokyo, Japan

During Leg 190, accreted sediments at an evolved portion of the Nankai accretionary prism were penetrated. Deformation structures and anisotropy of magnetic susceptibility (AMS) data from Site 1178 indicate that structural and magnetic fabrics formed at the frontal part of the prism have been preserved, even approximately 65 km landward of the deformation front. As the sediments approached the deformation front, the bedding/fissility-parallel magnetic fabric took on a northeast-southwest-trending subhorizontal maximum AMS axis because of northwest-directed bedding-parallel compression. Subsequent faulting, represented by brecciated zones, took place at the frontal part of the accretionary prism, which is presumably equivalent to the imbricate thrusts in the prism. During this process, a steeply dipping bedding-oblique foliation developed in a fault zone, >100 m in thickness. The foliation was formed as a flattening plane associated with shearing deformation and alternates with brecciated zones, giving a geometric relationship similar to S-C or P-Y fabric. The magnetic fabric oriented parallel to the bedding-oblique foliation records pervasive strain related to faulting. Bedding and bedding-parallel magnetic fabric together with fissility were rotated about a horizontal axis during frontal accretion, resulting in a seaward-verging magnetic fabric with northeast-southwest-trending subhorizontal maximum AMS axes. Overprinting of the deformation and/or modification of preexisting fabrics during subsequent rapid seaward growth of the Nankai accretionary prism has not been recorded in the accreted sediments at Site 1178, suggesting that later penetrative deformation was not significant at shallow burial depths in the prism.