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doi:10.2204/iodp.proc.314315316.102.2009

Discussion

Frontal thrust zone

The frontal thrust region in the Kumano transect is highly anomalous compared to the Nankai prism west of this transect (Okino and Kato, 1995; Kimura et al., 2007) where the frontal thrust marks the landward boundary of the PTZ, dips landward at ~25°–35°, and extends only 1–2 km landward (e.g., Aoki et al., 1982; Moore et al., 1990; Gulick et al., 2004). In the Kumano transect, however, the frontal thrust is a shallow-dipping (<10°) detachment that extends landward ~6 km and displaces trench and upper Shikoku Basin facies over the deeper PTZ and younger trench sediments (Figs. F8, F9). At its seaward edge, the frontal thrust is essentially parallel to bedding in both the hanging wall and footwall, denoting a classic “flat on flat” overthrust geometry, in contrast to the “ramp on ramp” geometry observed along most of the Nankai Trough.

Another anomalous feature of the Kumano transect frontal thrust zone is the development of an OOST <5 km landward of the frontal thrust. The lack of seismic continuity in the overthrust block may indicate that the block has been thrust up from deeper within the prism with seismic reflections being disrupted by microfracturing as the release of pressure allows fractures within the block to open.

We identified a large landslide just west of the 3-D box (Fig. F4B). This landslide was probably caused by the subduction of a small seamount imaged in a nearby 2-D line (ODKM03-I, Taira et al., 2005). This topography is typical of landslides caused by seamount subduction in other convergent margins (e.g., Dominguez et al., 2000). The large embayment in the seafloor just west of the 3-D box is likely the result of recent subduction of a ridge or seamount with resulting collapse as it passed under the frontal ridge (e.g., Lallemand and Le Pichon, 1987; Park et al., 1999; Dominguez et al., 2000). A likely phenomenon associated with this ridge or seamount is a significantly thicker sedimentary section at the trailing margin of the topographic feature (Ike et al., 2008a, 2008b). It is likely that the combination of ridge/​seamount subduction with thin sediments on the landward flank of the feature and thick sediments on the seaward flank has greatly altered the “normal” structural regime at the toe of the prism.

Frontal prism

The frontal prism is formed of accreted trench turbidites and underlying Shikoku Basin hemipelagic deposits. We identified lateral or oblique thrust ramps in this region, indicating that thrust motion is at an oblique angle to the strike of the thrusts. The ramp anticlines, although blanketed with slope sediments in the mid-slope region, form a series of ridges and troughs whose trend defines the strike of the oblique ramps. This topographic trend dominates the region from the frontal ridge landward to the splay fault. Both the frontal ridge and the splay fault cut off the oblique ramp trends, perhaps reflecting a recent change in the regional stress pattern.

Megasplay

Displacement along the megasplay is at least 1250 m in the horizontal direction and at least 750 m in the vertical direction (as indicated by the amount of slope sediment overridden), so the minimum displacement along the thrust is 1450 m. As opposed to the underlying accretionary prism thrusts, there is no indication of lateral slip along the megasplay, so our inlines, oriented approximately parallel to the direction of slip, should provide an accurate representation of splay fault slip.

Normal faulting seaward of megasplay thrust

Many shallow normal faults cut the slope sediments in the frontal splay fault region. These faults are best observed on the seismic cross-lines over Sites C0004 and C0008 (Figs. F17, F18). The faults generally strike ~290°–310° approximately perpendicular to the overall trend of the prism with steep dips to both the northeast and southwest. These normal faults may be due to extension as the underlying prism blocks ride up the underlying lateral ramps.

Kumano forearc basin

Above the splay fault under Kumano Basin is a unit that consists of steeply dipping discontinuous reflections that we interpret to be an older part of the accretionary prism. There are many indications that the older prism is similar to the current frontal prism, such as the structure on Figure F21 between Cross-lines 6240 and 6340, which we interpret to be the top of a hanging wall anticline. There are also several segments of landward-dipping reflections within the old prism.

Overlying the older prism is a hemipelagic unit that in some places has seaward dips and in other areas fills topographic lows in the underlying prism. We interpret this unit to be slope sediments that filled in small basins on the lower trench slope before this part of the prism was uplifted by slip along the splay fault.

The slope sediment sequence is onlapped by a thick succession of turbidite sediments that are tilted landward, presumably because of uplift at the seaward flank of the basin by slip along the megasplay (Park et al., 2002a; Gulick et al., 2008). This uplift has shifted the present locus of sedimentation landward in the basin. The basin sediments are cut by normal faults, many of which cut the surface sediment. The faults show no evidence for differential offset with depth. We thus believe that most of the faulting is very young.

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