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doi:10.2204/iodp.proc.314315316.121.2009 Expedition 315 summary1J. Ashi, S. Lallemant, H. Masago, and the Expedition 315 Scientists2AbstractDuring Integrated Ocean Drilling Program (IODP) 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 was originally planned at Site C0001. We cored at this site to 458 m core depth below seafloor (CSF) and cut 60 cores—32 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 located immediately below a thick sand layer. Unit II is composed of mud-dominated sediments of late Pliocene to late Miocene age. Structural style and inferred stress state vary widely across a deformed zone between 220 and 230 m CSF. Normal faults indicating northeast–southwest extension are 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 a northwest–southeast shortening subparallel to the direction of plate convergence. On the other hand, many thrust and strike-slip faults and a normal fault are found below the 220 m CSF deformed zone. The geometry and kinematics of planar structures display great variation. Kinematic 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 water 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. In situ 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 0.044°C/m, resulting in a heat flow of 47 mW/m2. For NanTroSEIZE Stage 3, 6 km riser drilling is planned at Site C0002. We drilled to 1057 m CSF, cored the intervals from 0 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 ~922 m CSF and cored another 135 m into the accretionary prism. The forearc basin sequence was divided into two units based on lithofacies. The boundary core between Units I and II was not recovered; hence, we applied the logging unit boundary defined by logging-while-drilling (LWD) during IODP Expedition 314 for this boundary. All units are dominated by mud and mudstone; however, Units I and II contain more sand and silt intercalation and have a much faster sedimentation rate. The age ranges from Quaternary to late Miocene. Underlying accretionary prism materials contain moderately more lithified and much more deformed sediments. Biostratigraphic data show that the transition from Pliocene to late Miocene strata occurs as a biostratigraphic gap around 922 m CSF, ~15 m above the lithostratigraphic unit boundary defined during Expedition 314 based on LWD data. 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 northwest–southeast shortening sequence with thrust faults (and possibly strike-slip faults). Two phases of normal faulting occurred subsequent to thrusting. The first is recorded in shear zones in Unit III and the uppermost part of Unit IV 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 downhole 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. In situ 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, nearly identical to that found at Site C0001 but corresponding to a lower heat flow of 39 mW/m2. Principal results of Expedition 315 include the following:
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