Subduction of Daiiti-Kasima Seamount into the landward slope of the Japan Trench.

Abstract

Daiiti-Kasima Seamount is a 3,000-4,000 m high flat-topped seamount, and lies in the axis of the Japan Trench, located 220 kilometers east of Tokyo. In 1977, the Hydrographic Department of Japan conducted a survey over the seamount and adjacent waters. The average spacing of the tracks of the survey was 2 nautical miles, and it was done by the survey ship “SHOYO" as part of “The Basic Map of the Sea Project. " It fairly revealed topography, geological structures, geomagnetic features and gravity anomalies of the seamount. Studies by Mogi and Nishizawa (1980) had provided a hypothesis that a breakdown had taken place, and the seamount was divided into two parts by a large NNE-SSW trending fault, while the western half of the seamount had subducted into the Japan Trench. In 1983, the Hydrographic Department of Japan commissioned a new survey vessel “TAKUYO” (2,600 tons) in which a set of Sea Beam System, a single and multi-channel seismic profiler, a sub-bottom profiler, magnetometers, a gravimeter and a set of integrated positioning system were equipped. In December 1983, the Department carried out a detailed survey of Daiiti-Kasima Seamount by “TAKUYO” to confirm the subduction of the seamount. The survey was done along ten track lines parallel to the trench with a 1.5 nautical mile average spacing, and eight track lines perpendicular to the trench with 1 nautical mile average spacing. A multi-beam bathymetric survey with Sea Beam System has provided detailed data of the seamount, and bathymetric charts on a scale of 1/40,000 with 50-meter-interval contours were drawn. In addition, single-channel seismic reflection, sub-bottom profile, geomagnetic total intensity and gravity were recorded along each track. A multi-channel seismic reflection survey was performed along a WNW-ESE trending track line crossing the western half of the seamount. Previous researches could not provide clear evidence to show that the l ,600 m high steep slope running straight across the central part of the seamount is a fault scarp, but the multi-beam bathymetry confirmed the fault scarp characteristics of the slope. It also documented a lot of linear structures, which were considered to be faults, running parallel to the trench on the seamount. These faults indicate that the seamount had been affected by tension in the oceanic plate along the outer slope of the trench. The subducted western half of the seamount was clearly shown beneath the landward slope of the trench on the records of both the single-channel and multi-channel seismic reflection surveys. An asymmetrical steep V shaped depression, stretching parallel to the trench, was found along the edge of the landward slope of the trench. Horizontal sediments were absent in the bottom of the depression. The sub-bottom profiler record, taken at a small-scale pond sediment on the landward slope of the trench, shows a nearly horizontal deposit on the surface and other underlying sediment layers dipping about 2 degrees toward the depression, below some 10 to 20 meters under the surface. It means that the lower part of the landward trench slope has been tilting toward the depression. From these facts, the depression was considered to be formed by tectonic erosion. These results (provided by the detailed survey of “TAKUYO”） confirm the subduction of the Daiiti-Kasima Seamount into the landward slope of the Japan Trench.