日本列島周辺の海底地質 その１ 日本海溝及びその付近

Abstract

The topography and morphology of the Japan Trench and its vicinity are compiled in Figures 1 and 2, based on the results of submarine topographic and geological surveys, including multinarrow beam soundings. A typical feature of the Japan Trench and its vicinity from land to sea, consists of the continental shelf, the continental slope, the trench itself (trench landward slope, trench axis basin and trench seaward slope), and the marginal swell between the trench and the typical ocean floor. On the continental slope, generally on its outer margin, elevations of basement are often found. The structural basin on the back side of elevations is usually filled up with younger sediments and forms the deep-sea terrace. Benches are recognized on the trench landward slope. They repesent probably a hinge zone between whole uplifting of the Tohoku Arc and sinking of the lower part of the trench landward slope originated from the subduction around the trench axis since the late Miocene. Topographies of drainages, hollows and mounds caused by submarine landslides are developed along the foot of the trench slope, and in some places the trench-slope apron stretches from the foot of the trench landward slope to the trench axis. The trench axis consists of a series of the elongated basins arranged weakly in echelon. From the trench seaward slope to the marginal swell are found many narrow but long depressions, showing the horst and graben structure. These depressions develop in parallel with formation of the marginal swell under the tensile stress. The subducting angle of the oceanic plate beneath the Japan Trench is 14° at 110km west of the trench axis, based on the analysis of geomagnetic data.