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doi:10.2204/iodp.pr.315.2008

Abstract

During Integrated Ocean Drilling Program Expedition 315, coring at two planned riser drilling sites was conducted. For Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 2, 3.5 km riser drilling is planned at Site C0001. We cored at this site to 458 m core depth below seafloor (CSF; IODP Method A: core expansion lengths overlap [are not scaled]) and cut 59 cores (31 with the hydraulic piston coring system [HPCS], 2 with the extended shoe coring system [ESCS], and 26 with the rotary core barrel [RCB]) from five holes covering a slope basin (Unit I) and the top 250 m of the underlying accretionary prism (Unit II). The slope basin is composed mainly of Quaternary to late Pliocene silty clay and clayey silt with intercalations of volcanic ash. The boundary between Units I and II, identified at 207.17 m CSF, is an unconformity characterized by a thick sand layer. Unit II is composed of more consolidated mud-dominated sediments of late Pliocene to late Miocene age. Structural style and stress state vary widely across a highly deformed zone at 220 m CSF. A normal fault indicating northeast–southeast extension is dominant above this zone; however, a few thrust faults dipping at 50° were encountered just above the deformed zone. These thrust faults are consistent with the northwest–southeast shortening subparallel to the direction of plate convergence. On the other hand, many thrust and strike-slip faults as well as a normal fault are found below the 220 m CSF deformed zone. The geometry and kinematics of planar structures display great variation. Fault plane solutions computed from normal and thrust faults are consistent with northeast–southwest extension and northwest–southeast shortening, respectively. A total of 48 whole-round samples were taken for interstitial geochemistry. Obtained data show meaningful trends for most elements, and potential contamination of drilling fluid is taken into consideration; however, changing trends do not necessarily correspond to unit boundaries. Methane and ethane concentrations and their ratio (C₁/C₂) decrease with depth to 100 m CSF and remain constant to the base of Unit I. The increase of methane concentrations and C₁/C₂ ratios in Unit II indicate the contribution of biogenic methane. Total organic carbon and calcium carbonate decrease monotonously to the base of Unit I and remain low throughout Unit II. Physical properties also show a clear break at the boundary between Units I and II. Porosity decreases downhole within each unit; however, there is a gap across the unit boundary. Thermal conductivity is almost constant throughout Unit I and decreases with depth in Unit II. Downhole temperature was measured with the advanced piston corer temperature tool (APCT3) at seven depths to 170.98 m CSF and yielded a generally linear downhole temperature increase, with a gradient of 0.042°C/m.

For NanTroSEIZE Stage 3, 6 km riser drilling is planned at Site C0002. We drilled to 1057 m CSF, cored the intervals from seafloor to 204 m CSF and 475 to 1057 m CSF, and cut 86 cores (18 with the HPCS, 2 with the ESCS, and 66 with the RCB) from three holes. We penetrated the basal unconformity of the Kumano forearc basin at ~936 m CSF and cored another 120 m into the accretionary prism. The forearc basin sequence was divided into two units based on lithofacies; these units corresponded to Units II and III defined by LWD, respectively. Both units are dominated by mud and mudstone; however, the Unit I contains more sand and silt intercalation and has a much faster sedimentation rate. The age ranges from Quaternary to late Miocene. Underlying accretionary prism materials contain more lithified and deformed sediments. Only one nannofossil event, of late Miocene age, was determined for Unit IV; hence, no significant gap was detected across the unconformity. Faults and shear zones are clustered at certain depths around 700, 920–950, and 1000–1050 m CSF. Three deformation phases were recognized by fault analyses. The earliest phase is a thrust fault (and possibly a strike-slip fault) and exhibits northwest–southeast shortening. Two phases of normal faulting occurred subsequent to thrusting. The first is recorded in shear zones and indicates northeast–southwest extension. The second is recorded in normal faults and indicates north–south extension, consistent with the present stress direction acquired from LWD results. A total of 31 whole-round samples were taken for interstitial water analyses. Changes in concentration for most elements seem to be controlled by unit boundaries. A downward increase of ethane and concomitant decrease of C₁/C₂ ratios in Unit IV suggest some contribution of thermogenic hydrocarbons. Physical properties show complex trends with depth. Downhole temperature was measured at eight depths to 159.0 m CSF and showed an almost linear downhole increase with a gradient of 0.043°C/m, identical to that found at Site C0001.

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