Evolution of tectono-sedimentary systems in the Kumano Basin, Nankai Trough forearc

Abstract

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.

Data report: a brittle (normal?) shear zone from Hole C0002P: deformation structures and their relationship with calcite veins (IODP Expedition 348, Nankai Trough accretionary prism)

“…Box = region with 3-D seismic data, red = Expedition 338 sites, blue = NanTroSEIZE Stage 1 and Stage 2 sites, yellow arrows = estimated farfield vectors between Philippine Sea plate and Japan (Seno et al, 1993;Heki, 2007), stars = locations of 1944 and 1946 tsunamigenic earthquakes. B. Schematic cross-section of Nankai accretionary prism, based on Moore et al (2015). Figure F2.…”

Gas-In-Place Estimate for Potential Gas Hydrate Concentrated Zone in the Kumano Basin, Nankai Trough Forearc, Japan

“…Kumano Basin's evolutionary history is complex, but it is generally accepted that the initial accommodation space was formed when uplift along the megasplay fault created a barrier to sediment delivery to the trench at around ~1.95 Ma [26][27][28][29]. Biostratigraphic age constraints obtained at Sites C0002 and C0009 mark the basin sediments as Quaternary to modern in age [26,29].…”

“…Biostratigraphic age constraints obtained at Sites C0002 and C0009 mark the basin sediments as Quaternary to modern in age [26,29]. At present, the basin is filled with ~2 km of pelagic and hemipelagic muds intercalated with clastic turbidites and thin ash layers [26].…”

Submarine Mass Movements and their Consequences

“…Accretion of a thick section (>1 km) of terrigenous sediment has built a wide accretionary prism (Aoki et al 1982). Kumano Basin has formed since~1.95-2.0 Ma behind a ridge that is believed to have been uplifted by movement along the mega-splay fault (Gulick et al 2010;Moore et al 2015). Kumano Basin has formed since~1.95-2.0 Ma behind a ridge that is believed to have been uplifted by movement along the mega-splay fault (Gulick et al 2010;Moore et al 2015).…”

“…The evolution of Kumano Basin is documented by seismic stratigraphic analysis and ocean drilling (Gulick et al 2010;Underwood and Moore 2012;Moore et al 2015). The Quaternary section is divided into 12 landward-dipping seismic stratigraphic units (K1-K12 after Gulick et al 2010).…”