Abstract
Heat flow measurements were conducted at four sites in the Nankai accretionary prism southeast of the Kii Peninsula, around the area where the megasplay fault reaches the surface, in conjunction with long-term monitoring of bottom water temperature at nearby stations. Analysis of the obtained data showed that variations in bottom water temperature seriously affect surface heat flow measurements in the areas with water depths of less than about 2,800 m. This effect can reach up to 20% to 30% and may have significantly contributed to a large scatter in the heat flow values previously measured in the study area. The temperature records were also used to determine heat flows from sediment temperature profiles disturbed by bottom water temperature variations. Results of measurements at sites deeper than 2,800 m indicate that the regional heat flow, corrected for surface disturbances including the influence of bathymetric relief, is about 65 mW/m2, which is consistent with the value calculated using subduction thermal models. Local high heat flow values were obtained in the vicinity of the tips of the branches of the splay fault, suggesting advective heat transport by upward pore fluid flow along the faults.
Highlights
Heat flow measurements were conducted at four sites in the Nankai accretionary prism southeast of the Kii Peninsula, around the area where the megasplay fault reaches the surface, in conjunction with long-term monitoring of bottom water temperature at nearby stations
In the Nankai subduction zone, where the Philippine Sea plate is subducting beneath southwest Japan, extensive heat flow measurements have been made around the Nankai Trough, mainly on the floor of the trough and in the accretionary prism that has developed landward of the trough (e.g., Kinoshita and Yamano 1995)
Long-term bottom water temperature (BWT) records showed that the BWT variation is large enough to have a significant influence on heat flow measurements with surface probes to water depths of at least 2,755 m (WT3), while the influence is small at 3,340 m (WT2)
Summary
Heat flow measurements were conducted at four sites in the Nankai accretionary prism southeast of the Kii Peninsula, around the area where the megasplay fault reaches the surface, in conjunction with long-term monitoring of bottom water temperature at nearby stations. Temporal variation in the bottom water temperature (BWT) causes a transient effect on the temperature distribution in sediments, resulting in disturbance of heat flow measurements with surface probes. In the off-Kumano area, nonlinear temperature profiles were often measured at stations with water depths of less than 2,100 m, indicating BWT variations with significant amplitudes (Hamamoto et al 2005).
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