The Nankai Trough is the topographic expression of the subduction boundary between the Shikoku Basin and the Southwest Japan Arc (Fig. 1). The Shikoku Basin is part of the Philippine Sea plate, which is subducting to the northwest under Japan at a rate of 2-4 cm/yr (Karig and Angevine, 1986; Seno, 1977), approximately normal to the plate margin. Active sediment accretion is presently taking place at the Nankai Trough. The record of accretion extends landward to Shikoku Island, where older accretionary prism rocks are exposed. The Cretaceous and Tertiary Shimanto Belt is characterized by imbricated thrust slices of trench turbidites and melanges composed of ocean-floor basalts, pelagic limestone and radiolarian chert and shale, and hemipelagic shale (Taira et al., 1988). The youngest part of the Shimanto Belt is early Miocene in age. The Shimanto Belt is interpreted as a direct ancient analog of the Nankai accretionary prism.
The well-resolved seismic profiles demonstrate several characteristic structural subdivisions across the accretionary prism. Based on the multichannel seismic (MCS) Profile 141 obtained by the Ewing 9907/8 cruise, the accretionary prism can be divided into several tectonic domains from the trench landward (Fig. 3): Nankai Trough axis zone, proto-thrust zone (PTZ), imbricate thrust zone (ITZ), first out-of-sequence thrust (OOST) zone, large thrust slice zone (LTSZ), and landward dipping reflectors zone (LDRZ).
Nankai Trough Axis Zone
Legs 87 (Site 582) and 131 results indicate that the stratigraphy of the trench floor is composed of the following lithologic units in descending order: trench turbidites (Holocene-Pleistocene), turbidite-hemipelagite transition (Pleistocene), hemipelagite with tephra layers (early Pleistocene late Pliocene), massive hemipelagite (mid-Pliocene to mid-Miocene), acidic volcaniclastics (15 Ma), and pillow basalts (16 Ma).
An additional unit is recognized within the surrounding Shikoku Basin sequence that is not present in the local trench stratigraphy on MCS Profile 141. This unit is characterized by a well-stratified sequence about 0.7 s thick. DSDP Leg 31 recovered Pliocene sands from a correlative seismic unit indicating that this unit may be composed of Pliocene-Miocene turbidites (hereafter called Pliocene Miocene Turbidite Unit).
Proto-Thrust Zone (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 due to tectonic deformation with the development of small faults and ductile strain as documented by Morgan and Karig (1995a, 1995b).
Imbricate Thrust Zone (ITZ)
Landward of the PTZ, a zone of well developed seaward-vergent imbricate thrusts can be recognized. The thrusts are sigmoidal in cross section with a mean angle of about 30° and typical thrust spacing of 0.5 km. The seaward edge of the ITZ marks the deformation front. One DSDP and one ODP leg were dedicated to coring at the frontal part of the imbricated thrust zone: Site 583 of DSDP Leg 87 and Site 808 of ODP Leg 131.
Site 583 is situated on the hanging wall of the frontal thrust. Although drilling failed to penetrate the décollement zone, good quality physical properties measurements were obtained from all of the holes, providing evidence that sediments dewater under tectonic stresses as they are accreted (Bray and Karig, 1988). The pore-water concentration depth profiles from these sites are far from being detailed enough to provide insight into the nature of fluid flow at this segment of the Nankai Trough (Kastner et al., 1993). The significant geochemical findings were that organic-fueled diagenesis is intense, and that at ~600 mbsf methane concentrations and the C1/C2 ratios abruptly decrease. Interestingly, similar abrupt decreases were observed at the décollement zone at Site 808. Fluid flow from a deep-seated source could explain these observations.
Site 808 (Fig. 4), which penetrated the whole prism and reached oceanic basement at 1290 mbsf during Leg 131, was particularly successful in terms of physical properties and structural geology measurements because of relatively high recovery rates and also because the sediments yielded consistently high-quality paleomagnetic data (Taira et al., 1991; 1992). These data allowed individual core sections and, in some cases, individual structural samples to be oriented relative to the present geographic coordinates. Physical properties generally varied smoothly downhole, except for sharp discontinuities across the frontal thrust and décollement zones. Discrete structures showed distinct concentrations in the vicinity of the fault zones as well as at several horizons above the décollement zone.
Pore waters were recovered throughout the sediment section at Site 808, including the frontal thrust, décollement zone, and underthrust package. Depth profiles for chemical concentrations and isotopic ratios (particularly D, O, Sr, and He) do not support active fluid flow along the décollement, despite its distinct reverse polarity seismic reflection, nor along the frontal thrust. They do, however, support lateral fluid flow (1) below the décollement at the approximate depth of the minimum in Cl concentration (~1100 mbsf) and (2) above the décollement along a horizon marking the lithological boundary between the volcanic-rich and -poor members of the Shikoku Basin sediments (~820 mbsf). The nature of the fluid flow, whether steady state or episodic, is as yet unresolved. Cores recovered from Site 808 also revealed that fractures within the décollement zone have not been mineralized; the overpressured décollement appears to form a leaky dynamic seal preventing significant lateral or vertical fluid flow. This contrasts with the situations at Barbados and Peru where the major tectonic structures have been mineralized, implying continuous confined fluid flow.
First Out-of-Sequence Thrust (OOST) Zone
About 20 km landward from the deformation front, 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. Important and significant deformation also appears within the underthrust Shikoku Basin hemipelagite. The hemipelagic unit seems to be tectonically thickened, probably as a result of duplexing.
Large Thrust Slice Zone (LTSZ)
This zone is characterized by the development of at least four distinctive out-of-sequence thrusts that separate tectonic slices of either previously imbricated packages or relatively coherent sedimentary sequences. The coherent slices are composed of ~0.7-s-thick (maximum) stratified layers that closely resemble the Plio-Miocene Turbidite Unit recognized in depressions in the Shikoku Basin. Underneath these thrust slices, there are packages of strong reflectors that may be composed of thickly underplated Shikoku Basin hemipelagic units. Slope sediment in this zone shows landward tilting suggesting recent active uplift. Bottom-simulating reflectors (BSRs) are weakly developed in this zone and are patchy.
Landward Dipping Reflectors Zone (LDRZ)
This zone is characterized by landward dipping, semicontinuous strong reflectors. This zone seems to be divided into several discrete packages by thrust faults. 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 more rigid or consolidated sediments compared with the previous zones closer to the trench axis. A BSR is well developed throughout this zone and diminishes abruptly at the boundary between this zone and the LTSZ.
The structural domains described above show variation along the strike of the prism. Along two parallel transects, separated by about 100 km, sharp differences in prism architecture and structure are evident. The western transect, which includes Leg 87 sites (Fig. 5), displays a well developed PTZ, containing a series of subparallel dipping discontinuities of unknown origin. These features are not evident within the eastern PTZ (Leg 131 and Profile 141 region). Differences in prism taper and seismic character of the décollement along the two transects suggest that the mechanical behavior of the prism differs along strike and that this variability may result from significant differences in pore pressures and fluid-flow regimes at the two locations.
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