GRAVITY AND MAGNETIC ANOMALY MAPS OF NORTH OF OKUSHIRI ISLAND

Geophysical surveys of the North of Okushirito Island were carried out by R/V Hakurei-maru in 1994, 1995, and 1996. Gravity and total geomagnetic force data were compiled into three maps (free-air gravity anomaly, Bouguer gravity anomaly, and magnetic anomaly maps). In the northern offshore of Okushirito Island, extensive positive higher magnetic anomaly trending is observed. A series of Miocene volcanic mounds aligned in a NW–SE orientation, distributed to the northern offshore of Okushirito Island, suggest a potential relationship with the extensive positive magnetic anomaly. The positive magnetic anomaly extends continuously toward the Motsuta Spur and the Okushiri Ridge, forming a continuous belt of positive magnetic anomalies. Bathymetric highs such as the Okushiri Ridge, a rise to its west, the deepening of the Shiribeshi Trough, and the Motsuta Spur are clearly expressed in the free-air gravity anomaly map, but not in the Bouguer gravity anomaly map. These gravity anomalies are inferred to be primarily attributed to variations in seafloor bathymetry. Along the Okushiri Ridge and the steep escarpment trending southwest from Okushirito Island, a sharp gradient in Bouguer gravity anomalies is evident, which is interpreted as reflecting the boundary between oceanic and continental crust. The Okushiri Ridge is interpreted as an uplifted structure formed by tectonic inversion during the Quaternary. It is likely that compressional deformation along the oceanic-continental crust boundary played a key role in its formation.