a b s t r a c tSedimentary deposits in the distal Kumano forearc basin of the Nankai accretionary margin off Kii Peninsula, Japan, have been imaged using three-dimensional (3D) seismic data. The seismic data, along with logging and core data from the Integrated Ocean Drilling Program (IODP) show that the unconformity between the accretionary prism and overlying forearc sediments is time-transgressive. The unconformity at Site C0002 separates 5 Ma prism rocks from 3.65 Ma basin deposits; at Site C0009 it separates 5.6 Ma prism from 3.8 Ma basin sediments. Acoustic reflections in the basal deposits are subparallel to the underlying accretionary prism; the acoustic facies varies in thickness from 50 to 750 m. The mudstone deposits and laterally equivalent turbidites are interpreted as lower trench-slope deposits. The condensed slope sediment (SS) section decreases in age from 3.5 to 1.5 Ma at Site C0002 to 1.5 e0.9 Ma at C0009.Acoustic sequences within the lower forearc basin (LFB) contain higher proportions of silt and sand turbidites and progressively onlap the SS unit along a low-angle discontinuity (KL) in a landward direction. Because of the landward onlap of the LFB unit, the oldest LFB strata at C0002 are older than 1.67 Ma, whereas those at C0009 are younger than~0.9 Ma. Thus, the amount of time missing or characterized by condensed sedimentation across the KL unconformity decreases in duration in the landward direction. The landward-onlapping sequences tilt progressively landward in response to regional uplift along an out-of-sequence thrust (OOST; mega-splay) fault. Regional tilting shifted the basin's depocenter progressively landward, expanding that part of the basin from~10 km in width to >30 km. The onset of sand-silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at~1.9 Ma. Conversely, turbidites of the Upper Forearc Basin (UFB) progressively onlap LFB in a seaward direction. Furthermore, the respective thicknesses of the LFB and UFB units switch from the seaward side of the basin (C0002) farther landward (C0009): the LFB unit is > 800 m thick in the seaward region, whereas it is only 200e300 m thick in the landward region; the UFB unit is < 50 m thick in the seaward region, and up to 600 m thick in the landward region. Thus, Kumano Basin responded in both space and time to a complex interplay between tectonics and sedimentation. The stratigraphy records a balance between the effects of prism uplift along the basin's distal edge with the rerouting of channels and canyons along the basin's proximal edge.

[1] Analyses of normal faults in the Kumano forearc basin of the Nankai Trough reveal multiple normal fault populations in a region generally thought to be under compression. Most faults have offsets of less than 20 m and dips of [60][61][62][63][64][65][66][67][68][69][70] and show no growth structures, indicating that the faults were active for short periods of time. The oldest generation of faults is older than~0.9 Ma and strikes~50-60. The next oldest faults strike~160-170 , are older than 0.44 Ma, and are related to local uplift along the western edge of the region. The youngest faults cut the seafloor; shallow faults near the SE margin of the basin curve from 100 in the middle of the survey area to~145 at the SE corner of the area. The pattern of the two youngest fault populations is consistent with the regional stress pattern (maximum horizontal stress subparallel to the trench). Orientations of older fault populations are caused by uplift of the underlying accretionary prism, implying that the forearc basin region is not as stable as previously thought. Reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2%, concentrated near the seaward edge of the basin. The active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge.

Accretionary prisms commonly grow seaward, with the strata of the inner prism consisting of older, previously accreted outer prism rocks overlain by thick fore-arc basin strata. We focus on the Nankai Trough inner accretionary prism using three-dimensional (3-D) seismic data and logging data from the Integrated Ocean Drilling Program (IODP). We update the 3-D seismic volume using well velocity data to better constrain deeper horizons. Interpretation of these horizons reveals multiple folds with axial surfaces that strike near parallel to modern outer prism thrust faults, and we interpret that these folds formed as a result of thrust faulting. Reactivation of one inner prism thrust fault continued until at least 0.44 Ma, after the modern fore-arc basin formed, indicating that the inner prism had continued deformation until that time. Structural restorations of these folded seismic horizons demonstrate that 580 m of slip occurred on this steeply dipping reactivated thrust after fore-arc basin formation. Structural interpretation and analysis of logging-while-drilling data, including borehole images, in the deep inner prism revealed intense deformation of a generally homogenous lithology characterized by bedding that dips steeply (608-908), intersected by faults and fractures that have a range of dips and densities. Our study of the deep Kumano Basin provides new insights into the structure of the inner prism and reveals that although the inner prism has partially preserved inherited outer prism structures, these older folds and faults are steeply rotated and cut by multiple fracture populations during subsequent deformation.

The Emperor seamounts form part of the hotspot generated Hawaiian-Emperor seamount chain in the Pacific Ocean. The fixed hotspot hypothesis (Morgan, 1971) suggests the chain formed at a deep mantle hotspot presently located off the southeast flank of Hawai'i, in the vicinity of Loih'i seamount. Differences in orientation between the Hawaiian Ridge and the Emperor seamounts have been attributed to changes in the direction of absolute motion of the Pacific plate from more northerly during 50-83 Ma to more westerly during 0-50 Ma (Morgan, 1971). Paleomagnetic data from Deep Sea Drilling Project (DSDP) drill and sample sites, however, suggest that while the Hawaiian Ridge, which includes the Hawaiian Islands, formed close to the present day latitude of Loih'i, paleolatitudes progressively increase from ∼2° at Koko, through ∼8° at Suiko, to ∼19° at Detroit indicating that the Hawaiian hotspot may have migrated south during emplacement of the Emperor seamounts rather than stayed fixed (Tarduno et al., 2003). Subsequent studies have suggested that the Hawaiian-Emperor seamount chain formed by some combination of changed Pacific plate motions and hotspot wander (