南海トラフ付加体の温度構造と間隙流体による熱輸送

Abstract

The thermal structure of the Nankai subduction zone has been investigated through conventional surface heat flow measurements and estimations of heat flow from the depths of gas hydrate BSRs (bottom simulating reflectors). In the middle and western parts of the Nankai Trough, heat flow on the trough floor is generally higher than that expected from the age of the subducting Shikoku Basin lithosphere, while it is consistent with the age in the eastern part. This high heat flow anomaly might be attributed to advective heat transfer by pore fluid flows from deeper parts of the accretionary prism along the decollement zone, but the amount of pore fluid expelled from the accreted sediments may not be enough to produce the observed anomaly. The deficit in the amount of pore fluid compared to the thermal anomalies is a problem common to other accretionary prisms. Local heat flow anomalies around the deformation front off Shikoku and around thrust faults off the Tokai district indicate the existence of channelized fluid flows along fault zones.A detailed heat flow survey was conducted on the Nankai prism off Shikoku in 1999. Over 60 new heat flow values were obtained along a 80km long line perpendicular to the trough axis, which greatly improved the knowledge of the surface heat flow distribution on this accetionary prism. The heat flow profile landward of the deformation front is generally smooth and seems to correlate well with the thickness of the prism. Local high heat flow anomalies were observed in the vicinity of the deformation front and may correspond to thrust faults. This excellent set of data, combined with BSR heat flow data, should be able to give better constraints for the thermal structure of the accretionary prism and the seismogenic zone.More direct evidence of heat transfer by fluid flows has been obtained in the Barbados and Cascadia accretionary prisms. In ODP drill holes within 15km of the deformation front of the Barbados prism, anomalies in temperature and pore water chemistry were detected around the decollement and thrust faults, indicating localized fluid flows. Some of the anomalies must have been produced by episodic flows. The heat flow around the deformation front in the Barbados prism considerably varies along the arc, which is similar to the observation in the Nankai Trough. A marked increase in temperature in the vicinity of a thrust fault was detected during a long-term monitoring experiment in a drill hole in the Cascadi accretionary prism, and is believed to represent an effect of an episodic flow event.