Results of ocean bottom survey in the area connecting Sikoku Basin with West-Mariana Basin.

Abstract

The Hydrographic Department, JAPAN conducted multi narrow beam bathymetnc, seismic reflection, magnetics and gravity survey in the north-eastern part of the Philippine Sea. The survey area lies southern part of Sikoku Basin which adjoins to the northern part of the West Mariana Basin. The survey area includes the middle part of the Kyusyu-Palau Ridge extending NNW-SSE on the west and the southern most end of the Nisi-Sitito Ridge extending NNW-SSE on the east. The steep gradient of the east side of the Kyusyu-Palau Ridge slopes down to the Sikoku Basin. We regard this steep slope, forming alignment with an azimuth of 160°, is fault escarpment and it represents remnant cliff of rifted island arc that was formed immediately after the opening of the Sikoku Basin. Remarkable alignments formed by minor ridges and troughs with NNW-SSE trend were found in the western half of the southern Sikoku basin. The magnetic anomalies in the basin are somewhat irregular because of their small amplitude(less than 200 nT), but NNW-SSE linear trends were recognizable. This suggests that the basin was formed by the process of sea floor spreading under an extensional stress perpendicular to the trend. It is reported that the Sikoku Basin has a lineated feature trending roughly NNW-SSE, while the West Mariana Basin has a NE-SW trend. We could not find major topographic feature, such as transform fault dividing the southern part of the Sikoku Basin and the northern part of the West Mariana Basin, however, slight geomorphogical discontinuities running roughly NNE-SSW direction were recognized.They may show the transitional zone between the two basins.