Yoshifumi Kawada

Hydrothermal heat mining in an incoming oceanic plate due to aquifer thickening: Explaining the high heat flow anomaly observed around the Japan Trench

To explain the origin of a high heat flow anomaly observed within 150 km seaward of the Japan Trench, we construct a thermal model for an oceanic plate prior to subduction that includes the effect of hydrothermal circulation within a high-permeability aquifer in its uppermost part. The model includes the effects of aquifer thickening, which is expected to occur near subduction zones where plate bending prior to subduction causes fracturing and faulting within the oceanic plate. Using typical parameter values for the Japan Trench, we find that hydrothermal circulation in the thickening aquifer mines heat from the underlying basement and can account for the observed high heat flow anomaly. The ratio of heat supply below the aquifer as a result of aquifer thickening to the inverse of the thermal resistance of the sediment layer is a control parameter for the system. As long as the aquifer permeability is higher than ∼10−13 m2, a typical value for the uppermost part of the oceanic plate, variations in other details of the hydrothermal circulation such as the exact value of the aquifer permeability and the size of the convection cells do not significantly change model results. Despite its strong influence on seafloor heat flow seaward of the trench, this hydrothermal heat mining does not affect significantly the thermal structure of the subducted oceanic plate. This finding indicates that surface heat flow anomaly around the trench may not correspond to temperature anomaly within the subducted oceanic plate and the megathrust seismogenic zone.

Multiple‐scale hydrothermal circulation in 135 Ma oceanic crust of the Japan Trench outer rise: Numerical models constrained with heat flow observations

Anomalous high heat flow is observed within 150 km seaward of the trench axis at the Japan Trench offshore of Sanriku, where the old Pacific Plate (∼135 Ma) is subducting. Individual heat flow values range between 42 and 114 mW m−2, with an average of ∼70 mW m−2. These values are higher than those expected from the seafloor age based on thermal models of the oceanic plate, i.e., ∼50 mW m−2. The heat flow exhibits spatial variations at multiple scales: regional high average heat flow (∼100 km) and smaller-scale heat flow peaks (∼1 km). We found that hydrothermal mining of heat from depth due to gradual thickening of an aquifer in the oceanic crust toward the trench axis can yield elevated heat flow of the spatial scale of ∼100 km. Topographic effects combined with hydrothermal circulation may account for the observed smaller-scale heat flow variations. Hydrothermal circulation in high-permeability faults may result in heat flow peaks of a subkilometer spatial scale. Volcanic intrusions are unlikely to be a major source of heat flow variations at any scale because of limited occurrence of young volcanoes in the study area. Hydrothermal heat transport may work at various scales on outer rises of other subduction zones as well, since fractures and faults have been well developed due to bending of the incoming plate.