Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect

Abstract

The location of the Integrated Ocean Drilling Program's (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) was based on regional two-dimensional seismic reflection surveys carried out by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Final site locations were chosen based on a three-dimensional (3-D) seismic reflection survey acquired across the seaward margin of Kumano Basin and the accretionary prism from the basin to the deformation front. This survey covered a region 12 km wide (approximately parallel to the regional structural strike) and 56 km long (approximately perpendicular to the regional strike) and provided detailed images of the structure and seismic stratigraphy of the drill sites. Sites were drilled in the frontal thrust zone at the toe of the accretionary prism, the frontal region of the megasplay fault zone, and the forearc basin. The 3-D seismic data volume images a main frontal thrust at the prism toe with the hanging wall thrust at least 7.5 km seaward over the trench. This configuration is different from that in other parts of the Nankai prism. At the shallow end of the megasplay, the data images a complex thrust system that truncates older structures in the underlying accretionary prism and shows that the hanging wall block has overridden more than 1250 m of young slope sediments. At the forearc basin site, we interpret landward-dipping forearc basin strata onlapping older slope sediments, which in turn overlie an older part of the accretionary prism.

Lithologic control of frictional strength variations in subduction zone sediment inputs

“…The taper angle of the wedge at the Kumano transect is ~6°-16°; it is ~5°-8° at the Ashizuri transect and ~2°-9° at the Muroto transect (Taira et al, 1992;Kimura et al, 2007;Moore et al, 2009). Clearly, the narrow taper angle at the Muroto transect cannot be explained by elevated smectite, which is more abundant at the Ashizuri and Kumano transects.…”

“…These sites are part of three northwest-southeast-trending borehole transects aligned with Cape Ashizuri and Cape Muroto and through the Kumano Basin. These transects show significant along-strike variations in the accretionary prism taper angle (décollement dip plus surface slope), which is on average narrower along the Muroto transect (~2°-9°) compared to the Ashizuri and Kumano transects (~5°-8° and ~6°-16°, respectively) (Taira et al, 1992;Moore et al, , 2009Kimura et al, 2007). Seafloor surface heat flow also varies along strike; high values were measured near the Muroto transect (up to ~200 mW/m 2 ) compared to the Ashizuri (~100-130 mW/m 2 ) and Kumano transects (90-140 mW/m 2 ) (Yamano et al, , 2003Kinoshita et al, 2008;Harris et al, 2013a).…”

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“…In 2006, Japan and the United States conducted a joint 3-D seismic reflection survey over an ~11 km × 55 km area, acquired by PGS Geophysical, an industry service company (Moore et al, 2009). This 3-D data volume was the first deep-penetration, fully 3-D marine survey ever acquired for basic research purposes and has been used to (1) refine selection of drill sites and targets in the complex megasplay fault region, (2) define the 3-D regional structure and seismic Figure F1.…”

“…IL = in-line, XL = cross-line. stratigraphy, (3) analyze physical properties of the subsurface through seismic attribute studies, and (4) assess drilling safety (Moore et al, 2007(Moore et al, , 2009). These high-resolution 3-D data are being used in conjunction with physical property, petrophysical, and geophysical data obtained from core analyses and both wireline and logging-while-drilling (LWD) logging to allow extensive and highresolution integration of core, logs, and seismic data.…”

“…Interpreted seismic cross section of Kumano transect offshore and southeast of Kii Peninsula (modified from Moore et al, 2009;after Strasser et al, 2014). From the trench landward, the transect is separated into six morphotectonic domains: protothrust zone, frontal thrust zone, imbricate thrust zone, megasplay fault zone, Kumano Basin edge fault zone, and Kumano fore-arc basin.…”