室戸沖南海トラフ付加体前縁部の高熱流量と地下深部の圧力・浸透率の関係

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The Nankai Trough is a convergent plate boundary where the Shikoku Basin subducts beneath southwest Japan. Turbidite and hemipelagic sediments on the trough floor form an accretionary prism landward of the trough axis through a process of sediment compaction. Near the toe of the accretionary prism, a series of frontal thrusts have formed above the decollement, which is a major slipping plane on the plate boundary. In 2002, we discovered an extremely high heat flow at the second frontal thrust near the toe of the Nankai Trough off Muroto using ROV KAIKO during the KR02-10 research cruise of the Japan Agency for Marine-Earth and Technology (JAMSTEC). The background heat flow value was 160 mW/m2 and high values of 250-280 mW/m2 were obtained within 50 m above the second frontal thrust. The heat flow gradually decreases landward from this location. A simple numerical calculation was carried out to test the hypothesis that the heat flow anomaly is caused by an interstitial fluid flow through the decollement/frontal thrust system as a permeable channel. Two parameters—permeability within the fault zone (Kch) and excess pore pressure (ΔP)—at the landward boundary of the decollement were adjusted to produce the best-fit combination to explain the observed heat flow anomaly. The simulation result of 500 combination patterns indicates that Kch and ΔP cannot be determined independently. Instead, only their product (Kch × ΔP) can be estimated. The best-fit result, simulating the highest heat flow of 280 mW/m2 was achieved where the product Kch × ΔP is 8 × 1015, with a surrounding permeability of 10-17 m2. Based on the estimation of excess pore pressure in the decollement of less than 4 MPa, the most appropriate value of channel permeability is estimated to be higher than 2 × 10-15 m2. Our numerical result is generally consistent with the previously estimated fracture permeability, i.e. three orders of magnitude higher than that of ambient sediment. Thus, we attribute the surface heat flow anomaly to the influence of the fluid flow through the permeable channel along the frontal thrust.