3:00 PM - 3:15 PM
[SCG52-06] Heat flow distribution on the outer rise of the Japan Trench: Indication of fluid circulation in the oceanic crust
★Invited Papers
Keywords:heat flow, Japan Trench, outer rise, oceanic crust, fluid circulation, heat transport
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. The distribution of the high values does not extend beyond about 150 km from the trench axis, indicating that the anomaly is related to deformation of the Pacific plate associated with subduction. The broad high heat flow zone 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.
Overlapping the broad anomaly, local variations at a scale of several km were detected through concentrated measurements along lines perpendicular to the trench. The local anomalies may reflect heterogeneity in fracture development: fractures with higher permeability and/or deeper extent would pump up more heat. The most detailed heat flow distribution obtained along a line normal to the trench around 39oN shows a prominent sawtooth-like variation at a scale of 3 to 5 km. It suggests that fractured zones with higher permeability have developed at intervals of several km. We made closely-spaced measurements along two N-S lines intersecting the 39oN trench-normal (E-W) line as well. The heat flow profiles along the two N-S lines both show significant variation at a scale of several km, a similar feature to that along the E-W line. The heat flow distribution in this area indicates that fracture development process and the permeability structure in the oceanic crust on the outer rise are highly heterogeneous, including substantial variations in the trench-parallel direction.
We further conducted heat flow measurements on the outer rise of the westernmost part of the Kuril Trench, adjacent to the Japan Trench. Most of the obtained values are normal for the seafloor age, in contrast to the high average heat flow on the Japan Trench outer rise. Nearly normal heat flow off the Kuril Trench may result from lower average permeability in less fractured crust, which suppresses development of fluid circulation. 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.
Overlapping the broad anomaly, local variations at a scale of several km were detected through concentrated measurements along lines perpendicular to the trench. The local anomalies may reflect heterogeneity in fracture development: fractures with higher permeability and/or deeper extent would pump up more heat. The most detailed heat flow distribution obtained along a line normal to the trench around 39oN shows a prominent sawtooth-like variation at a scale of 3 to 5 km. It suggests that fractured zones with higher permeability have developed at intervals of several km. We made closely-spaced measurements along two N-S lines intersecting the 39oN trench-normal (E-W) line as well. The heat flow profiles along the two N-S lines both show significant variation at a scale of several km, a similar feature to that along the E-W line. The heat flow distribution in this area indicates that fracture development process and the permeability structure in the oceanic crust on the outer rise are highly heterogeneous, including substantial variations in the trench-parallel direction.
We further conducted heat flow measurements on the outer rise of the westernmost part of the Kuril Trench, adjacent to the Japan Trench. Most of the obtained values are normal for the seafloor age, in contrast to the high average heat flow on the Japan Trench outer rise. Nearly normal heat flow off the Kuril Trench may result from lower average permeability in less fractured crust, which suppresses development of fluid circulation. 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.