The Nankai subduction zone off of southwest Japan forms an "end-member" sediment-dominated accretionary prism. Here, a sedimentary section ~1 km thick (Figs. F1, F2) is accreted to or underthrust beneath the margin in the style of a fold and thrust belt (Moore et al., 2001). The Philippine Sea plate underthrusts the margin at a rate of ~4 cm/yr along an azimuth of ~325° (Seno, 1977) down an interface dipping 3°–7° (Kodaira et al., 2000), causing repeated earthquakes (magnitudes >8) with an average recurrence interval of ~180 yr (Ando, 1975). Currently the margin is locked with little convergence between the Muroto Peninsula and the Philippine Sea plate (Mazzotti et al., 2000). The convergent margin of southwest Japan has a geologic record of accretion of deep sea deposits extending to at least the Cretaceous (Taira et al., 1988). However, rocks cored during Leg 190 (Fig. F2) and even those subducted to seismogenic depths entered the subduction zone no earlier than the Pliocene (Moore, Taira, Klaus, et al., 2001).
In the area of Leg 190/196 drilling or the Muroto Transect (Fig. F1), the basin to margin transition can be divided into the Shikoku Basin and overlying trench fill, the protothrust zone, the imbricate thrust zone, the frontal out-of-sequence thrust zone, the large thrust slice zone, and the landward-dipping reflector zone (Fig. F2). A condensed summary of these tectonic provinces from Moore et al. (2001) and Moore, Taira, Klaus, et al. (2001) follows.
The Philippine Sea plate entering the Nankai Trough along the Muroto Transect is near the axis of an extinct spreading center marked by the Kinan Seamounts (Okino et al., 1999). As documented at Site 1173, the 16-Ma oceanic crust of the Shikoku Basin is overlain by, successively, volcaniclastics, a middle Miocene to mid-Pliocene massive hemipelagite, an upper Pliocene to lower Pleistocene hemipelagite with tephra layers, and a Pleistocene turbidite to hemipelagite transition sequence and a Pleistocene to Holocene trench turbidite unit.
Entering the protothrust zone, the ~1-km-thick sedimentary section initially deforms above a décollement zone or detachment surface developed in the latest Miocene massive hemipelagite layer. The lower portion of this massive hemipelagite is underthrust along with underlying volcaniclastics and oceanic crust. Coring and seismic studies demonstrate that this initial deformation above the décollement zone consists of small thrust faults associated with subtle folding at seismic scales (Park et al., 2000) and development of minor faults at core scale (Moore, Taira, Klaus, et al., 2001; Morgan and Karig, 1995).
Major thrust faulting and growth of the accretionary prism initiate at the frontal thrust and continue upslope (Fig. F2). Immediately landward of the frontal thrust, the imbricate thrust zone consists of a series of well-developed seaward-vergent imbricate packets spaced several kilometers apart. The imbricate thrust zone is ~20 km wide; Site 808 penetrates its frontal thrust. Out-of-sequence thrusts overprint the imbricate thrusts, starting ~20 km landward of the deformation front. Farther landward from the deformation front, out-of-sequence thrusts cutting thick thrust packages define the large thrust slice zone. These out-of-sequence thrusts were probably initially imbricated from thick turbidite sands of the Shikoku Basin (Moore, Taira, Klaus, et al., 2001). The landward-dipping reflector zone upslope from the large thrust slice zone is less well imaged than more seaward portions of the prism. However, the landward-dipping reflectors probably represent thrust boundaries.