奄美海台衝突域における変動地形 The Tectonic Landform in the Continental Slope Deformed by the Collision of the Amami Plateau
The Amami Plateau is a remnant arc situated in the northwestern part of the Philippine Sea Plate. The convergent rate of the Philippine Sea Plate relative to the Ryukyu Arc is 5.0 to 7.0cm/yr in a N50° W direction (Seno <I>et al</I>., 1993). The morphology of the continental slope of the collision area between the Amami Plateau and the Ryukyu Arc is characterized by a wide variety of tectonic landform.<BR>Since 1983 HDJ (Hydorographic Department of Japan) has conducted geophysical surveys by S/ V TAKUYO and S/ V MEIYO under the Continental Shelf Surveys Project and the Earthquake Prediction Project in the adjacent water of Japan. The area surveyed by these projects includes the northern part of Ryukyu forearc and the northwestern part of the Philippine Sea Plate. HDJ obtained SeaBeam swath bathymetric data, single channel seismic reflection profiles, and magnetic and gravity data in the collision area. These surveys reveal the process of the formation of the tectonic landforms caused by the collision between the plateau and the arc.<BR>The continental slope of the collision area is characterized by two tectonic landforms, ridges and minor ridges. Ridges are distributed on the topographic high including the Kikai Island, which are NE-SW trending straight ridges and valleys. The north and south ends of the topographic high are consistent with these of the Amami Plateau. Minor ridges are distributed on the gentle continental slope in the east of the topographic high with the ridges. These minor ridges are divided into two groups by the strike of the ridge axis, which are NE-SW and NW-SE trending ridges. The NE-SW trending ridges are distributed on three blocks in the gentle slope bordered by NW-SE trending ridges and valleys. A subducted seamount, which is estimated from the magnetic anomaly, lies below the central block, and the Kikai Seamount borders on the northeastern block on the trench.<BR>The collision between the Amami Plateau and the arc results in large tectonic effects to the Ryukyu Arc. In the collision area the continental slope is uplifted in two difference processes correspondent with two tectonic landforms, one is uplift caused by the buoyant subduction of the plateau, the other is uplift caused by the subduction of seamounts on the plateau.
地學雜誌 106(4), 567-579, 1997-08-25
Tokyo Geographical Society