Heat flow distribution on the Japan and Kuril trench outer rises: Implications for fluid circulation and heat transport processes in fractured oceanic crust

Abstract:

Anomalous heat flow values, higher than that expected for the seafloor age of the incoming Pacific plate, are pervasively observed on the outer rise of the Japan Trench. Overlapping the broad anomaly, local variations at a scale of several kilometers were detected through concentrated measurements along lines perpendicular to the trench. We recently conducted heat flow measurements on the seaward side of the westernmost part of the Kuril Trench, adjacent to the Japan Trench. Heat flow on the Kuril Trench outer rise was found to be generally normal for the seafloor age, in contrast to the high anomaly off the Japan Trench. The broad high heat flow zone seaward of the trench can be attributed to pore fluid circulation in a permeable layer developed through fracturing of the oceanic crust due to plate bending, which efficiently pumps up heat from deeper part of the crust. Two-dimensional numerical modeling of this process showed that heat transport by fluid circulation occurs only when the permeability is higher than some critical value. It indicates that normal heat flow off the Kuril Trench may result from the lower average permeability in less fractured crust than that off the Japan Trench. It is consistent with the observation that the anomaly in the seismic velocity structure is more significant off the Japan Trench than off the Kuril Trench. We also conducted numerical simulation of fluid flow and heat transport in the oceanic crust with a heterogeneous permeability structure for investigation of local heat flow variations found off the Japan Trench. A series of zones with high permeability were assumed to have developed at intervals of several kilometers, corresponding to well-fractured part of the crust. We found that fluid circulation in the high-permeability zones can yield large surface heat flow variations similar to the observed ones, though the extent of heat flow anomaly may be much broader than the width of underlying high-permeability zone. Detailed survey of surface heat flow distribution on the trench outer rise would provide information on fracturing process of the oceanic crust.