Hydrothermal Heat Transport within an Abruptly Formed Permeable Slot: An Application to Heat Flow Peaks Observed at the Japan Trench Offshore of Sanriku

Abstract:

Anomalous heat flow has been observed within 150 km seaward of the trench axis at the Japan Trench offshore of Sanriku, northeast Japan [Yamano et al., 2008, 2014]. The average heat flow is ~70 mW/m² (50 mW/m² is expected for the plate age), and small-scale (2 to 3 km) variations, whose peak value is >100 mW/m², are embedded. In the area of high heat flow, a high V_p/V_s layer in the oceanic crust has been detected [Fujie et al., 2013]. The layer thickness is increased toward the trench axis. Fujie et al.[2013] interpreted this layer as a result of fracturing due to bending of the subducting plate. This layer may work as an aquifer in which heat is transported by fluid circulation.

In a previous study [Kawada et al., 2014], we constructed a model for aquifer thickening in order to explain the observed anomalous heat flow: a 500-m-thick aquifer 150 km seaward of the trench axis is gradually thickened to 3000 m toward the trench axis. We found that hydrothermal circulation within the thickening aquifer mines heat beneath its underlying part, and heat flow is elevated accordingly. Although the model can explain the observed high heat flow in an average sense, the origin of the small-scale heat flow variations remains to be solved.

This study expands the previous model by incorporating abrupt thickening of the aquifer to account for the small-scale heat flow. Typically, we consider a situation in which a permeable slot of 2000 m wide and 3000 m deep suddenly appears. This model results in a high heat flow peak of 100 mW/m² above the permeable slot, which persists over hundreds of thousand years. Heat outside the slot is transported horizontally toward the slot by thermal conduction and then is transported vertically due to hydrothermal circulation within the slot. Because the volume of the permeable slot is smaller than its surrounding region, high heat flow above the slot lasts for a long time.