Heat flow distribution on the floor of Nankai Trough off Kumano and implications for the geothermal regime of subducting sediments

Abstract

During three research cruises (KY05-14, KY07-01 and KY08-01) using JASMTEC R/V KAIYO, we obtained 13 heat flow data with piston core samples across the Nankai Trough and the Kashinosaki Knoll (KK). Subbottom temperatures were measured by precise thermometers attached to the piston corer. Thermal conductivity, color reflectance and vane shear strength were measured for the cored samples onboard KAIYO. Surface heat flow in the Nankai Trough north of KK ranges from 90- 135 mW/m2 . With the sedimentation effects considered, basal heat flux beneath the Nankai Trough off Kumano is roughly esti- mated as 100-130 mW/m2 , most likely as 120-130 mW/m2 . In the Shikoku Basin south of KK, heat flow ranges 120-147 mW/m2 , (average~135 mW/m2 ), significantly higher than the theoretical value (110 mW/m2 ) estimated from the age of the Shikoku Basin. Possible cause for the heat flow anomaly is either the postspreading volcanism at KK, thermal disturbance by tectonic compression at KK, or subseafloor fluid migration. More intensive study, including scientific drilling is necessary to estimate the accurate thermal structure of the incoming Shikoku Basin.