Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out‐of‐sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well‐drained, sand‐rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated décollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

PandSwave velocity measurements of water-rich sediments from the Nankai Trough, Japan

“…Eastwood and Castagna , ]. In the geoacoustic results of our study, overconsolidation due to material removal from IODP Sites C0006 and C0007 (accretionary prism toe) [ Screaton et al ., ] and from IODP Site C0008 (footwall of the megasplay fault) [ Kimura et al ., ; Conin et al ., ; Strasser et al ., ] is not obvious, when the V p / V s ratios (Figure ) and Poisson's ratios (Figure ) are compared between overconsolidated (prism toe, megasplay footwall) and normally consolidated (incoming plate, megasplay hanging wall) settings.…”

“…The varying V p data for the different tectonic settings are probably due to structural (microfabric) and/or mineralogical differences and different initial porosities. Prism toe sediments are affected by overconsolidation because of erosion of the original overburden as well as tectonic deformation [e.g., Kinoshita et al ., ; Screaton et al ., ; Tsuji et al ., ], while samples from the footwall and the hanging wall of the megasplay fault represent a normally sedimented slope cover without significant tectonic overprinting [ Expedition 316 Scientists , ; Kinoshita et al ., ]. Lateral tectonic loading as well as overconsolidation at the accretionary prism toe decrease the porosity in comparison to samples of the megasplay fault area or the incoming plate from similar depth (Table ) and enhance the grain‐grain contacts of the sediments [ Stoll , ; Erickson and Jarrard , ; Gettemy and Tobin , ; Hashimoto et al ., ; Raimbourg et al ., ].…”

Hydrocarbon seepage and its sources at mud volcanoes of the Kumano forearc basin, Nankai Trough subduction zone

“…A megasplay, which apparently intersects the Nankai accretionary prism between the currently formed accretionary prism and the older accreted belt [Moore et al, 2007], branches upward from the plate-boundary interface at a depth of about 10 kmbsf and about 50-55 km landward to the deformation front [Gulick et al, 2010;Strasser et al, 2009]. The OOSSs are believed to branch from the up-dip limit of the seismogenic zone, and thus, to potentially serve as migration pathway for rapid fluid ascent to the seafloor [Park et al, 2002;Tobin and Saffer, 2009], but indications for deep-sourced fluids are missing yet [e.g., Screaton et al, 2009].…”

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“…In the case of the shallow fault zone, these materials may in fact host SSEs if the events propagate to the trench (e.g., Wallace et al, 2016 To achieve this goal, characterization of the incoming stratigraphy and upper oceanic basement rocks, together with the shallow, most active strand of the frontal thrust system, is essential. This characterization involves a combination of coring, downhole measurements, and logging at proposed Sites HSM-13B, HSM-08A, and HSM-18A, followed by a strategy of carefully coordinated sampling and postexpedition laboratory analyses (e.g., Screaton et al, 2009;Underwood et al, 2010). Proposed Site HSM-13B (or HSM-05A) targets the entire sediment package on top of the Hikurangi Plateau and, if drilling conditions allow, will penetrate into the top of the basaltic lava and volcaniclastic sequence.…”