Abstract:
[1] We report detailed thermal measurements undertaken during Integrated Ocean Drilling Program (IODP)
Expeditions 315 and 316 along the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) transect
offshore the Kii Peninsula, Japan. Geothermal objectives included determining thermophysical rock properties
of the cored material and characterizing the background thermal regime along this transect. New thermal
conductivity measurements made with a divided bar are reported and supplement shipboard full‐ and
half‐space thermal conductivity measurements for a total of 938 thermal conductivity measurements. Thermal
conductivity varies between about 1.0 W m⁻¹ K⁻¹ near the seafloor to 1.6 W m⁻¹ K⁻¹ at a depth of
1 km. Thermal conductivity generally increases with depth and correlates with variations in porosity
and lithology. Thermal gradients along the transect are characterized by 48 sediment temperature measurements
from 6 sites. Thermal corrections for the effects of bathymetric relief and sedimentation improve the
confidence with which the heat flow values can be interpreted. Heat flow generally decreases with landward
distance from the deformation front and varies from 70 mW m⁻² just landward of the deformation
front to 54 mW m⁻² at sites characterizing the outer fore‐arc high and to 57 mW m⁻² at the Kumano Basin
Site. IODP heat flow measurements are significantly lower than nearby seafloor heat flow measurements.
This difference is most likely due to variations in bottom water temperature that have a large effect on
values of seafloor heat flow. Thus the heat flow of the Nankai accretionary prism is lower than previously
thought. We present thermal models of subduction along this transect and explore the impact of the initial
geotherm. Conductive plate cooling based on the age of subducting seafloor (20 Myr) under predicts the
observed heat flow. We find a good fit to the data using a geotherm appropriate for 10 Myr seafloor. The
extra heat is interpreted in terms of back‐arc thermal environments.
