Thermal conductivity profile in the Nankai accretionary prism at IODP
NanTroSEIZE Site C0002: estimations from high-pressure experiments using
input site sediments
Abstract
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, a borehole penetrated to a depth of 3262.5 meters below
seafloor (mbsf) at site C0002. Because core samples obtained from below
~1100 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 in-coming subducting oceanic
basement at NanTroSEIZE 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 ~50
MPa, corresponding to depths greater than ~4 kmbsf. We
obtained an empirical relation between thermal conductivity and
fractional porosity for the Nankai Trough accretionary prism as =
exp(-1.09φ+0.977). Based on porosity data measured using core/cuttings
samples and data derived from P-wave velocity logs, we estimate two
consistent and complete thermal conductivity profiles down to
~3 kmbsf in the Nankai Trough accretionary prism. These
profiles are consistent with the existing thermal conductivity data
measured using limited core samples.