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doi:10.2204/iodp.proc.314315316.102.2009

Data presentation and interpretation

Regional perspective

We used the regional 2-D seismic reflection lines to identify key tectonic features of the Nankai accretionary prism in the Kumano transect. We then extrapolated these features laterally using a high-resolution multibeam bathymetric data set (Fig. F4A) to produce a preliminary regional tectonic interpretation (Fig. F4B). We note that all interpretations should be considered preliminary because additional interpretation work is ongoing.

A composite seismic line extracted from the 3-D volume that crosses over (or close to) Stage 1 drill sites (location shown in Figs. F2 and F4B) illustrates the main regional morphotectonic zones and seismic features of the drilling transect (Fig. F6). The reflection from the top of ocean crust is imaged from the Nankai Trough in the southeast landward to a depth of ~11 km under the center of Kumano Basin. As mentioned above, potential velocity errors in the deep part of the section preclude accurate interpretation of oceanic crustal structure. Several basement highs, however, are apparent that we believe to be real, including the ridge above the interpreted landward-dipping thrust fault within the oceanic crust landward of the trench axis (Tsuji et al., in press). Above the irregular oceanic crust reflection is a seismically transparent unit that we interpret to be hemipelagic sediment of the Shikoku Basin facies (Moore et al., 2001) that is subducting with the oceanic crust. This unit can be traced landward at least 45 km under the Kumano Basin with little change in thickness (Fig. F6).

In the trench zone, younger trench deposits overlie the oceanic crust and Shikoku Basin facies (Fig. F6). As is typical of other parts of the Nankai Trough, there is a well-developed protothrust zone (PTZ), but the PTZ is overlain by a slice of trench strata previously accreted into the prism and emplaced over the trench strata by an out-of-sequence thrust (OOST). The frontal thrust zone is highly complex with a very steep slope (~10°) at the seaward edge of the prism. Landward of the trench is the imbricate thrust zone (ITZ), a series of thrust packages made up of a thick highly reflective sequence that is probably accreted trench sediment. The primary thrusts that extend through the accretionary wedge and sole into the basal décollement are highly irregular with 1.5–2.5 km downdip spacing and typical along-strike extents of ~1–3 km. These faults are recognizable by the displacement of the deformed stratigraphic horizons that comprise the accretionary wedge. The décollement is a strong continuous positive polarity reflection through most of the frontal region. A potential inactive décollement occurs ~500 m above the active décollement in a zone of low-velocity material that may be underplated hemipelagic material (Park et al., in press).

The ITZ is overlain by slope sediments that are increasingly deformed downward. The slope sediment cover generally increases in thickness from southeast to northwest (Fig. F6). Recent tectonic activity is reflected in numerous surficial landslide scars and by two apparent strike-slip faults that offset young ridges in the ITZ (Fig. F4B).

Beneath the trench upper slope and Kumano Basin, a regional OOST or splay fault system, first recognized by Park et al. (2002a), cuts across the older part of the accretionary prism in the megasplay fault zone. The megasplay fault can be traced from deep below the accretionary wedge at ~10 km depth, where it lies ~1 km above the top of the subducting crust. It cuts discontinuously up through the accretionary wedge along several branches all the way to the frontal accretionary thrust region (Moore et al., 2007). Deep segments of the splay fault are continuous along strike across the entire 12 km survey width, with shallower segments continuing typically 3–5 km along strike. Many segments along the splay fault system have reversed polarity reflections relative to the seafloor and high amplitudes relative to adjacent stratigraphic horizons. The fault segments are potentially active fluid conduits fed by deep sources, possibly including the thick underthrust sediment package (Bangs et al., in press).

The Kumano forearc basin is bounded on the southeast by a topographic valley. Beneath the valley is a complex fault zone that may have a combination of normal and strike-slip faults (Kumano Basin edge fault zone; Martin et al., 2007, 2008).

More than 2 km of sediment is imaged in the Kumano forearc basin. The seaward portion of the basin section is progressively tilted, likely because of repeated motion on the megasplay fault (Park et al., 2002a), and is cut by landward-dipping normal faults (Gulick et al., 2008). The region below the base of the Kumano Basin sediments is generally reflection free and is probably older accreted material.

Detailed views around NanTroSEIZE drill sites

Toe of accretionary prism: IODP Sites C0006 and C0007

Sites C0006 and C0007 are located on the first ridge landward of the frontal thrust (Fig. F7). Seismic Inline 2434 shows the setting of these sites (Figs. F8, F9).

The sedimentary section in the trench region is ~2.4 km thick (Fig. F8) and is typical of the Nankai Trough, with Shikoku Basin hemipelagics and turbidites (~1.4 km) overlying oceanic crust and subsequently overlain by a trench sediment wedge (~1 km). The trench wedge has numerous nested trench channels, and this complex of channels extends under the toe of the accretionary prism.

A well-developed PTZ affects the upper Shikoku Basin and lower trench wedge sections (Figs. F8, F9). The steeply landward-dipping thrusts have small offsets and extend from approximately the base of the trench channel complex to a strong reflection at ~6 km depth near the base of the interpreted trench wedge. We note that the landward-dipping reflections in the Shikoku Basin sequences are likely migration noise due to very low fold in this region.

The frontal thrust is a shallow-dipping (~7°–8°) detachment that extends landward ~6 km and displaces trench and upper Shikoku Basin facies over the 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.

The detailed bathymetry of the trench axis and lowermost accretionary prism (Fig. F7) shows that Sites C0006 and C0007 are on the northeast flank of a large landslide scar. Debris from this landslide form a thick, irregular deposit at the base of the scar and have caused the trench axial channel to be deflected at least 10 km seaward from its usual position at the base of the landward slope.

The region around Sites C0006 and C0007 is dominated by steep landward-dipping subsidiary thrust faults that steepen with depth. Most offsets are less than a few tens of meters, except the fault penetrated at ~4200 m at Site C0006 (Fig. F9). Northwest of Site C0006 a series of backthrusts cuts the section. Further landward of these backthrusts two additional landward-dipping subsidiary thrusts cut the section, and both sole into the main frontal thrust.

A small slope basin formed landward of Site C0006 from debris shed off the steep landward slope. Sediments in the wedge-shaped basin onlap the underlying accreted trench strata (Fig. F9).

The thrust block landward of the slope basin has been translated seaward at least 1.25 km over the basin sediments. Also, the thrust beneath this block has cut off the top of an underlying anticline (Fig. F8, Cross-line 3750), further indicating that the thrust is younger than the underlying thrust and slope sediments, making it an OOST. The surface slope of the overriding thrust block (Cross-line 3650–4000) is relatively steep (~10° seaward slope) compared to the average surface slope of the region just landward (~3°). The overriding block is cut by at least two subsidiary thrusts that splay off the main thrust. The block exhibits very low seismic amplitudes with little continuity and is distinct from the footwall block. Low seismic amplitudes often indicate distinctive lithologic units, small-scale deformation, or steep dips, as discussed further below.

Seaward edge of splay fault system: IODP Sites C0001, C0003, C0004, and C0008

These sites are all on the seaward slope of the accretionary prism, just southeast of a major ridge that forms part of the seaward boundary of Kumano Basin (Fig. F10). Two seismic inlines (Figs. F11, F12) show the setting of these sites at the seaward edge of the megasplay system. A series of thrusts within the accretionary prism offset bands of high-amplitude laterally continuous reflections interpreted as accreted trench and Shikoku Basin strata (Park et al., 2002a; Kington et al., 2008; Streiff et al., 2008). The strata are folded into hanging wall anticlines over the thrusts (Moore et al., 2007). The seismic cross-lines (Fig. F13) show that the thrusts are not perpendicular to the strike of the margin but dip at an oblique angle, thus causing oblique ramps to form. The megasplay cuts the top of these oblique ramps, providing compelling evidence for the out-of-sequence nature of the megasplay (Moore et al., 2007).

The overthrusting block, penetrated at Sites C0001, C0003, and C0004, is characterized by low seismic amplitudes and the lack of laterally continuous seismic reflections similar to the overthrust block in the toe region described above. Several low-amplitude landward-dipping reflections are interpreted as thrusts within this block that is believed to be older accreted material brought up from depth along the megasplay fault (Moore et al., 2007). Two higher amplitude zones, also interpreted as thrusts, were penetrated at Site C0003. At Site C0001, low-amplitude seaward-dipping reflections dominate (Figs. F14, F15). We interpret these reflections as backthrusts because of their dips (nearly perpendicular to the thrusts) and because a package of prominent landward-dipping reflections is cut off by one of these reflections (Fig. F14, Cross-line 5540–5560).

The megasplay surface is not a simple planar fault but is a complex series of anastomosing segments (Figs. F16, F17). Both amplitudes and geometries of the fault segments change rapidly along strike and downdip. At Site C0004, it is a zone ~100 m thick with reflections at the top and base (Fig. F17).

The top of the megasplay block is highly irregular in three dimensions. We believe that most of this roughness is due to slumping of material from the top of the block, a process that continues today on thrust anticlines on the lower portion of the prism slope.

The megasplay block has overridden younger slope sediments (Fig. F16) that extend at least 1250 m landward under the block. These sediments were penetrated at Sites C0004 and C0008. The block is also overlain by the youngest slope sediments, which were sampled at Sites C0001 and C0004.

The slope sediment cover is cut by numerous small-offset normal faults (Fig. F18). Although the faults are imaged in the 3-D inlines (e.g., Fig. F16, Cross-line ~5190–5200, 3.0 km), they dip approximately parallel to the inlines, so they are best displayed in the cross-lines (Fig. F17, F18). In addition, two northwest-striking strike-slip faults offset surface ridges in the ITZ (Fig. F4) and truncate reflection horizons within the 3-D data volume.

The slope sediments are also affected by slumping. Many slump scars are visible in the regional bathymetry (Fig. F4A) and on the 3-D seismic lines (e.g., Figs. F16, F17), and regional onlap surfaces within the slope sediments occur in several places (Figs. F14, F15, F16, F19).

Kumano forearc basin: IODP Site C0002

Site C0002 is located near the seaward (southeast) edge of the Kumano forearc basin (Fig. F20). A seismic inline imaging the drill site (Fig. F21) shows ~900 m of landward (northwest)-tilted, high-amplitude, laterally continuous reflections that we interpret as a turbidite sequence (Gulick et al., 2008). The turbidites lap onto an older sequence ~50–100 m thick that is relatively transparent with lateral thickness variations. We interpret this sequence to be hemipelagic sediments deposited on the lower trench slope prior to formation of the forearc basin.

Underlying the sedimentary sequence is a strongly deformed zone that has few laterally continuous seismic reflections. Landward of Site C0002 (Cross-line ~6230–6330) is an anticline that we interpret to be the top of a hanging wall structure similar to those imaged in the accretionary prism to the southeast. These strata may have been deposited in a slope basin environment and then uplifted to form the base of the forearc basin (Gulick et al., 2008).

The forearc basin sequence is cut by a series of landward (northwest)-dipping normal faults (Gulick et al., 2008). We cannot identify any depth variations in offsets along these faults that would indicate that they are growth structures. Many of the faults extend to the surface and some cut the entire basin sequence into the underlying prism. Several normal faults are also imaged in the cross-line direction (Fig. F22), indicating that their strikes are oblique to the trend of the margin (see the “Expedition 314 Site C0002” chapter).

The seismic cross-line through Site C0002 shows that surface sediments in the basin are affected by slumping with large slump scars evident northeast and southwest of Site C0002. These slump scars are also evident in the regional bathymetry (Figs. F4A, F20).

A strong bottom-simulating reflector (BSR) occurs throughout Kumano Basin (G. Gaillot et al., unpubl. data). This high-amplitude negative polarity BSR is strongest to the southeast of Site C0002 and has minimal amplitude at the site.

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