Heat flow and fluid flow regime in the western Nankai accretionary prism

Abstract

The thermal structure of the western Nankai subduction zone has been studied through surface heat flow measurements with conventional geothermal probes and estimation of heat flow from depths of gas hydrate bottom simulating reflectors. Temperature and thermal conductivity measurements at ODP Site 808, located at the toe of the Nankai accretionary prism, added important information on any variation in heat flow with depth and thus on thermal effects of pore fluid flow in the prism. Surface heat flow is generally higher than 130 mW/m2 on the floor of the Nankai Trough. Such high heat flow cannot be explained simply by conductive cooling of the subducting Shikoku Basin lithosphere. Recent closely spaced probe measurements revealed that the heat flow on the trough floor shows local variation, with particularly high values near the deformation front. The heat flow distribution suggests that warm fluid flows seaward possibly along the décollement and locally rises toward the seafloor along some channels, resulting in both the regional and local high heat flow anomalies. Contrary to this inference, no active localized fluid flow was detected at Site 808. The temperatures measured at six depths down to 347 m below seafloor (mbsf) just below the frontal thrust can be well explained by conductive heat flow of 125–130 mW/m2. No geochemical anomaly associated with present-day pore fluid movement was observed in cores recovered from the frontal thrust or the décollement. On the other hand, the high heat flow at Site 808 requires some heat source below 350 mbsf. There is a possibility that the fluid flow in the Nankai accretionary prism is localized and transient, and thus was not detected at this single drill site.