This paper describes topographic and geologic features of the northern part of the Sagami Trough and surrounding areas, off central Japan, and discusses the role of the Philippine Sea plate in the development of the trough's topography and fault systems. In the study area, the Okinoyama Bank Chain, Miura Basin Chain, and several spurs are aligned NW-SE parallel to the axis of the Sagami Trough. We examine the lithological compositions of rock and piston-core samples collected from the R/V Hyper Dolphin during Japan Agency for Marine-Earth Science and Technology dives 906 and 907, and rock samples dredged during Ocean Research Institute of the University of Tokyo cruise KT88. These samples represent mainly volcaniclastic rocks derived from the volcanoes of the Izu arc and subordinate amounts of terrigenous sediment interpreted as derived from the Sagami and Sakawa Rivers.
The topographic and geologic data define five fault systems in the eastern part of the northern Sagami Trough. We interpret the Okinoyama Bank Chain as an elevated outer-ridge belt developed at the landward slope of the plate boundary, which is represented by NW-SE trending right-lateral reverse faults (F1). At the northeast side of the Okinoyama Bank Chain, NW-SE trending normal faults (F2) formed and the Miura Basin Chain developed. The cutting relationship between these fault sets suggests that the collision of the Izu arc with the Honshu arc imposed a right-lateral shear stress field on the Okinoyama Bank and Miura Basin Chains. This shear stress may have led to clockwise rotation of these tectonic blocks formation of a set of NE-SW trending left-lateral reverse faults (F3). The initiation of the Izu block collision is interpreted as a turning point after which the northward motion of the Philippine Sea plate abruptly changed to NW and the Sagami Bay area came under a NW-SE compressional stress field, which resulted in the development of E-W trending right-lateral faults (F4) in the narrow shelf off the Miura Peninsula. A number of spurs subsequently developed along these faults. Finally, NNE-SSW trending right-lateral normal faults (F5) developed under an E-W extensional stress field, which caused the formation of N-S trending topographic depressions such as Tokyo Canyon and submarine highs such as Okinoyama and Oiso Spur.
