16:30 〜 16:45
[SCG55-11] 日本海溝・千島海溝海側における熱流量分布の特徴：海洋地殻内の流体循環による熱輸送
キーワード：熱流量、海洋地殻、流体循環、熱輸送、日本海溝、千島海溝
Anomalous heat flow values, higher than that expected for the seafloor age of the incoming Pacific plate, are pervasively distributed 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. 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. Local variations may have arisen from heterogeneous development of fractures: higher surface heat flow is observed at sites where the crust has been fractured down to deeper part.
On the seaward slope of the Japan Trench, where normal faults are well developed, heat flow data has been rather sparse compared to dense data on the outer rise. We conducted heat flow measurements around faults with large surface displacement in the upper part of the seaward slope, on research cruises in 2015, 2016, and 2020. At around 39oN, heat flow values obtained at sites near faults are, on average, lower than the values on the outer rise, indicating that fluid circulation pattern in the oceanic crust may have changed associated with development of normal faults. Local heat flow variations were found in the vicinities of fault escarpments, possibly related to focused fluid flow along faults.
We also conducted heat flow measurements on the outer rise of the westernmost part of the Kuril Trench, adjacent to the Japan Trench, in 2018 and 2020. 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. Local high heat flow anomalies were detected at two sites. One of them, at around 70 km from the trench axis, is rather broad, extending at least 5 km along a line perpendicular to the trench. We should make more measurements to delineate the shapes of local anomalies off the Kuril Trench for comparison with those off the Japan Trench.
These characteristics in the heat flow distribution provide information on fracturing process and fluid flow pattern in the oceanic crust. Nearly normal heat flow off the Kuril Trench may result from lower average permeability in less fractured crust, which suppresses development of large-scale 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. More detailed survey of the local anomalies would allow us to discuss heterogeneity in the oceanic crust and its regional variation along the trench.
On the seaward slope of the Japan Trench, where normal faults are well developed, heat flow data has been rather sparse compared to dense data on the outer rise. We conducted heat flow measurements around faults with large surface displacement in the upper part of the seaward slope, on research cruises in 2015, 2016, and 2020. At around 39oN, heat flow values obtained at sites near faults are, on average, lower than the values on the outer rise, indicating that fluid circulation pattern in the oceanic crust may have changed associated with development of normal faults. Local heat flow variations were found in the vicinities of fault escarpments, possibly related to focused fluid flow along faults.
We also conducted heat flow measurements on the outer rise of the westernmost part of the Kuril Trench, adjacent to the Japan Trench, in 2018 and 2020. 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. Local high heat flow anomalies were detected at two sites. One of them, at around 70 km from the trench axis, is rather broad, extending at least 5 km along a line perpendicular to the trench. We should make more measurements to delineate the shapes of local anomalies off the Kuril Trench for comparison with those off the Japan Trench.
These characteristics in the heat flow distribution provide information on fracturing process and fluid flow pattern in the oceanic crust. Nearly normal heat flow off the Kuril Trench may result from lower average permeability in less fractured crust, which suppresses development of large-scale 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. More detailed survey of the local anomalies would allow us to discuss heterogeneity in the oceanic crust and its regional variation along the trench.