Evolution of the Escarpments located in the Back-arc Basins in the Southern Waters of Japan - Mechanical Boundary Between Sinking Back-arc Basins and Buoyant Active Island Arc -

Abstract

Swath bathymetric data, single channel seismic reflection profiles, magnetic and gravity anomalies in the northern part of the Philippine Sea were obtained through comprehensive surveys conducted by Hydrographic Department of Japan.The surveyed area covers Izu-Ogasawara Arc/ Trench system, Shikoku and Oki-no-Tori-Sima (Parece Vela) back-arc basins, northern to middle Kyushu-Palau Ridge, Daito Ridge Regions and a part of the northern Philippine (West Philippine) Basin. Our survey revealed that the eastern margin of the Shikoku Basin is marked by a steep escarpment with 500-800m relative height. This predominant escarpment is named Kinan Escarpment and it extends north-south for about 500km in length along the eastern part of the Shikoku Basin. A large escarpment (Oki-Daito Escarpment, tentative name) trending in a WNW-ESE direction, roughly parallel to the Oki-Daito Ridge was also recognized by our recent survey in the northern part of the Philippine Basin.The maximum relative height reaches about 1000 meters. We propose that the Kinan Escarpment is a large normal fault that was formed due to the post-spreading deformation of the back-arc basin oceanic crust along the mechanical boundary between buoyant active island arc and isostatically sinking inactive back-arc basin. The Oki-Daito Escarpment was probably formed in as imilar manner by this mechanism.The location of these large normal faults indicates that the vertical displacement concentrates along a zone of possible structural weakness which is characterized by an abrupt change in the trend of the topographic fabric and magnetic lineations of the seafloor in the trenchward side of back-arc basins.