Topography and geological structure in the bottom of the Suruga Trough : A geological consideration of the subduction zone near the collisional plate boundary.

The topographic features of the Suruga Trough can be divided into five deep sea basins, four gorges and a fan-frontal slope. In a longitudinal topographic section of the trough floor shows a step-like topography with prominent three steps. The lower, middle and upper steps correspond to the southern, middle and northern parts of the Suruga Trough, respectively.In the southern part (water depth is 2900-3700m), the characteristics of a convergent boundary are essentially similar to those in the Nankai Trough. The subducting Philippine Sea Plate (PHS) is at least broken to three slabs by the ENE-WSW reverse faults with dextral strike-slip motion. The slabs dip 6° to 10° west. The trough floor gently dips landward, and the main channel shows a reversed sigmoidal meandering pattern. The trough-fill sedimentary sequence is characterized by wedge-type shape and seaward thinning-out trend.In the middle (1750-2900m) and the northern (0-1750m) parts, the accretionary prisms are in direct contact with the subducting plate (PHS), and PHS dips 23° or more west. The trough-fill sedimentary sequence don't show a wedge-type shape, but it is like a slope-basin sequence which filled up a depression on the accretionary prism. This sequence is more intensely deformed in the northern part than the middle part. The main channel shows the sigmoidal migration pattern. In these parts, the trough floor dips east or south-east as clearly shown in a cross section of the trough.These structural features of the Suruga Trough has been formed by the collision of the Tanzawa block at the late Miocene and of the Izu block at the middle Pleistocene to the Honshu Island. At the late Miocene, the bending of the paleo-Nankai-Suruga Trough also formed, and this time corresponds with the time that the plate motion of the PHS changed north to northeast.