Extremely high heat flow anomaly in the middle part of the Nankai Trough

Detailed heat flow measurements were carried out in the central part of the Nankai Trough off eastern Shikoku, where the fossil spreading center of the Shikoku Basin is subducting beneath the southwest Japan arc. The quality of temperature gradient values was examined for all the existing and new heat flow data and the average of new thermal conductivity data was used at all the stations. Reliable heat flow on the floor of the trough was found to be very high and rather uniform, with an average of about 200 ± 20 mW/m2. Taking account of the effect of recent rapid sedimentation on the surface heat flow, the heat flow from the deeper part is estimated to be even higher by 20–30%, almost twice as high as the value expected from the age of the subducting Shikoku Basin, about 15 m.y. Such a large heat flow anomaly cannot be attributed solely to advective heat transfer by pore fluid flows associated with subduction and accretion of the sediments brought into the trough, suggesting that the underlying Shikoku Basin lithosphere is anomalously warm in the study area. A possible cause of the anomaly is reheating of the lithosphere by post-spreading thermal activities along the fossil spreading center, which produced the Kinan Seamount Chain and continued until about 7 Ma. However, such reheating even at 6 Ma cannot produce a heat flow high enough to explain the observed values. A steep landward decrease in the heat flow is observed at the toe of the accretionary prism. The scale of this decrease indicates that its origin is not deep seated, and thus a part of the anomalous heat flow on the trough floor may result from some processes in relatively shallow layers. The thermal structure of the Shikoku Basin is a critical boundary condition for thermal models of the Nankai subduction zone and its seismogenic zone for large thrust earthquakes. More heat flow measurements need to be made around the study area in order to clarify the extent and origin of the high heat flow anomaly. [Copyright &y& Elsevier]