Thermal Conductivity Profile in the Nankai Accretionary Prism at IODP NanTroSEIZE Site C0002: Estimations From High-Pressure Experiments Using Input Site Sediments

Depth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3,262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below similar to 1,100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144-518 mbsf at Site C0012 up to a maximum effective pressure of similar to 50 MPa, corresponding to depths greater than similar to 4 km below seafloor. We obtained an empirical relation between thermal conductivity lambda(Bulk) in W m(-1) K(-1)and fractional porosity phi for the Nankai Trough accretionary prism as lambda(Bulk) = exp(-1.09 phi + 0.977). Based on porosity data measured using core/cuttings samples and data derived fromPwave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to similar to 3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples. Plain Language Summary Depth profiles of sediment thermal conductivity are required for understanding the thermal structure and earthquake occurrences in active seismogenic zones such as the Nankai Trough, SW Japan. The depth profile in Nankai Trough accretionary prism, however, is not available because sediment drill core samples from great depths are hard to be obtained. We collected six core samples from shallower sediments that overlay the incoming subducting oceanic basement at Nankai Trough Drill Site C0012 by IODP, which can be considered to have the same solid grain components as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure. We pressurized the core samples to simulate deeper sediments in accretionary prism by reducing sample porosity. As the result, we obtained an empirical relation between thermal conductivity and porosity for the Nankai Trough sediments and then estimated thermal conductivity profiles down to similar to 3 km below seafloor based on porosity profiles in the Nankai Trough accretionary prism.