Geophysical features of the Oki-no-Tori-Sima basin in the Philippine Sea and their tectonic implications.
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AbstractHydrographic Department of Japan (JHD) has been conducting geophysical surveys by S/V TAKUYO under the Continental Shelf Surveys Project since 1983 in the southern waters of Japan. The surveyed area has covered southeastern part of the East China Sea, northern Philippine Sea and westernmost of North Pacific Basin. JHD obtained SeaBeam swath batymetric data, single channel seismic reflection profiles, magnetic and gravity data in the northern part of the Oki-no”tori-shima (Parece Vela or West Mariana) Basin north of 18° N and west of 140。E from 1992 to 1994. Central part of the Oki-no-tori-shima Basin is characterized by the N-S trending chain of the depressions forming en-echelon alignment. These depressions are diamond-shaped and bordered by the steep escarpments with 1000～1300 meters maximum relative height. These fault escarpments extend northeastward and southwestward from the depressions into the surrounding basin floor and they gradually fade out away from the axial area. It is noted that these escarpments have S-shaped curved trend and their geometry seems to be symmetric about the depressions. Minor ridges and troughs trending orthogonal to the S-shaped curved escarpment are recognized. We consider these depressions and escarpments as topographic expression of extinct spreading axes and fracture zones respectively. Western province of the basin floor and basement is extremely rugged and characterized by minor ridges and troughs trending N-S direction. Although magnetic anomalies of the basin are very weak，magnetic lineations trending parallel to the topographic trend are recognizable in the central and western part of the basin. Based on updated geomorphological features and magnetic anomalies revealed by our survey, together with previously published data including results of drilling, we propose that the evolution of Oki-no-tori-shima Basin has following four stages. (1) rifting of the paleo Kyushu-Palau Ridge and separation of the ridge from Mariana arc, (2) seafloor spreading in an E-W direction, (3) counter-clockwise rotation of spreading axes accompanying their segmentation and formation of S-shaped fracture zones due to the change of spreading direction (4) possibly post spreading deformation and volcanism in the basin Our proposed spreading model of the Oki-no・tori-shima Basin is similar to that of the Shikoku Basin. Spreading axes of both basins have been segmented and gradually rotated counter clockwise in a later phase of the basin evolution after cessation of relatively uniform spreading nearly in an E-W direction. But present morphology of extinct spreading axes and fracture zones in the central province of the two basins show quite different features.
JournalReport of Hydrographic and Oceanographic Researches