Based on our seismic reflection data, we subdivide the accretionary prism along the Muroto Transect into several tectonic/structural domains (Fig. F5, F6): Shikoku Basin, Nankai Trough Trench axial zone, protothrust zone (PTZ), imbricate thrust zone (ITZ), out-of-sequence thrust (OOST) zone, large thrust-slice (LTS) zone, and landward-dipping reflector (LDR) zone.
The Shikoku Basin region seaward of the Nankai Trough is characterized by several distinct seismic stratigraphic units. The uppermost unit is <100 ms thick at the edge of the Nankai Trough and thins seaward. This unit is the distal facies of the Nankai Trough axial deposits described below. Underlying this thin unit are two seismic sequences that were drilled at Site 1173. The upper sequence has high-amplitude, laterally discontinuous internal reflections. The lower sequence has very low amplitude reflections that are parallel to the underlying oceanic crust reflection. Both of these sequences are hemipelagic strata deposited far seaward of the Nankai Trough axis. Both sequences continue westward where they are buried by trench turbidites.
Seaward of the high basement topography of the Kinan Seamounts is a unit stratigraphically below the Shikoku Basin hemipelagic units described above. It is characterized by a well-stratified sequence ~0.7 s thick (labeled "Shikoku Basin turbidites" on Fig. F6). This section may correlate with the Pliocene-Miocene turbidite unit identified along the Ashizuri Transect (Fig. F1) and recovered at Sites 297 and 1177 within the lower Shikoku Basin facies (hereafter called the Pliocene-Miocene turbidite unit). Seaward of the trench axis, buried basement topography of the Kinan Seamount chain is imaged as a broad ridge overlain by 0.7 s (750 m) of sediment. Seaward of the ridge, the sediment thickness increases to ~1.75 s (2 km); the deepest ~1.0 s of sediment onlaps the ridge, indicating that the ridge was formed before sedimentation began.
The trench axis of the Nankai Trough is characterized by a thick, seaward-thinning wedge of trench turbidites that overlie the Shikoku Basin sequence described above. The wedge is 550 ms (~500 m) thick at the western boundary of the trench and thins to ~110 ms (~100 m) at its seaward edge. The turbidite unit was supplied mostly through an axial transport system from a source region to the northeast in the Izu collision zone, as indicated by a mixture of volcanic, sedimentary, and metamorphic provenance (Taira and Niitsuma, 1986; Underwood et al., 1993). This unit diachronously onlaps the Shikoku Basin sequence (Bray and Karig, 1985).
The trench strata and the upper hemipelagic deposits are stripped off the subducting plate and accreted. The accretionary complex is divided into several segments based on structural styles. Between the deformation front and the frontal thrust is a zone of diffuse structural thickening, the PTZ. This area represents a zone of incipient deformation and initial development of the décollement within the massive hemipelagic unit. Above the décollement, the sediment thickness increases landward, probably as a result of tectonic deformation with the development of small faults and ductile strain, as documented by Morgan and Karig (1995a, 1995b). Trench strata are tilted seaward at the seaward edge of the PTZ because of this thickening.
Landward of the PTZ, a zone of well-developed seaward-vergent imbricate thrust packages can be recognized. The thrust packages are sigmoidal in cross section and are 700-800 m thick with a mean angle of ~30°. The thrusts are associated with hanging-wall anticlines that form linear ridges that are laterally continuous for 15-20 km (Figs. F4, F5, F6). There are twelve major thrust packets, with a thrust spacing of ~1.5-2.0 km. The thrusts sole into a well-defined décollement ~0.25-0.30 s above the oceanic crust. The frontal thrust forms the seaward edge of the ITZ. Site 808 (Leg 131) cored the frontal part of the ITZ. A bottom-simulating reflector (BSR) is developed just landward of the frontal thrust and appears discontinuously throughout the ITZ.
From 20 to 30 km landward of the frontal thrust, the imbricate thrust packages are overthrust by a younger generation fault system. Because this fault system cuts the preexisting sequence of imbricate thrusts, it is called an OOST (Park et al., 2000). The prism thickens from 1.6 to 1.9 s (~1.9-2.3 km). A 6.25-km-wide slope basin with ~0.6 s of sediment is ponded behind this OOST. Strata within the basin are tilted landward, with dips that increase with depth, indicating continued relative uplift along the OOST during sedimentation. Significant deformation also appears within the underthrust Shikoku Basin hemipelagite. The hemipelagic unit seems to be tectonically thickened, probably as a result of duplexing.
Approximately 30-35 km landward of the frontal thrust, at least four distinctive OOSTs separate tectonic slices of either previously imbricated packages or relatively coherent sedimentary sequences. The thrust packets abruptly increase to ~0.7-1 s in thickness, with the thrust spacing increasing to 2-3 km. These coherent thrust slices are formed of strata that closely resemble the deeper section seaward of the Kinan Seamounts. Underneath these thrust slices, there are packages of strong reflectors that may be composed of thickly underplated Shikoku Basin hemipelagic units, and the décollement cuts down closer to the oceanic crust. Slope sediment in this zone shows landward tilting, suggesting recent active uplift. BSRs are patchy and weakly developed in this zone.
This LDR zone is characterized by landward-dipping, semicontinuous strong reflectors. The zone appears to be divided into several discrete packages by thrust faults overlain by a thick sequence of slope sediments. Because the uppermost slope sediment layer is relatively undeformed, some of these faults may not have been active for some time. This zone might be composed of sediments that are more consolidated and rigid than those in zones closer to the trench. A BSR is well developed throughout this zone and diminishes abruptly at the boundary between this zone and the LTS zone.