Despite the deformation within the accretionary prism, porosity at Site 1178 decreases gradually from ~63%-70% at the seafloor to 26%-35% at 673 mbsf, a typical burial-compaction trend across slope and accreted sediments (Moore, Taira, Klaus, et al., 2001). At the toe of the Nankai accretionary prism (Site 808) (Fig. F1), however, the offset along the active frontal thrust has disturbed the preexisting porosity profile (Shipboard Scientific Party, 1991). A disturbed preexisting porosity profile is not present at Site 1178. This requires that stress has been applied long enough for fluid to seep away and that recently active faulting did not occur at Site 1178. In contrast to the large thrust-slice zone (LTSZ), where active out-of-sequence thrusts develop, no faults appear to crosscut slope sediments imaged in a seismic profile across the LDRZ (Fig. F1B). This suggests that faulting at Site 1178 occurred before deposition of the slope sediments prior to 5.54 Ma. One of the main results of drilling during ODP Leg 190 is the recognition of the rapid seaward growth of the Nankai accretionary prism; the outer 40 km of the prism (from the LTSZ to the prism toe) accreted within the past 2 m.y. (Moore, Taira, Klaus, et al., 2001). Therefore, faulting at Site 1178 is considered to have occurred when Site 1178 was located at the frontal part of the accretionary prism. The repetitions in biostratigraphy suggest that the faults at Site 1178 formed in association with the development of in-sequence thrusts at the frontal part of the prism. At Sites 1174 and 808 (Fig. F1), deformation bands appear to reflect pervasive strain related to faulting at the toe of the Nankai accretionary prism (Taira et al., 1992; Moore, Taira, Klaus, et al., 2001; Ujiie et al., in press). Although the deformation style is different, the bedding-oblique foliation in the deformed interval (400-506 mbsf) may record pervasive strain associated with faulting at the frontal part of the prism.
The timing of faulting and magnetic fabric development at Site 1178 implies that tectonic fabrics formed at the frontal part of the Nankai accretionary prism have been preserved ~65 km landward of the deformation front. Studies of ancient accretionary complexes have shown that offscraped sequences buried more than a few kilometers document the progressive development of tectonic fabrics during frontal accretion and the subsequent penetrative deformations associated with intra-prism compression (e.g., in the Tertiary Shimanto accretionary complex) (DiTullio and Byrne, 1990; Ujiie, 1997; Lewis and Byrne, 2001). By contrast, the accreted sediments at Site 1178 do not record any modification of preexisting tectonic fabrics and/or overprinting of deformation during subsequent rapid seaward growth of the Nankai accretionary prism. This suggests that penetrative deformation associated with seaward growth of the prism was not significant at shallow burial depths in the prism.