Geological Map North of OKUSHIRI ISLAND

Marine Geological Map “North of Okushiri Island” has been compiled based on interpretation of seismic profiles and correlation of seismic stratigraphic units with the published stratigraphic data of surrounding areas. The map area covers the northern offshore area of the Okushirito Island in the northern part of the Japan Sea to the west of the Oshima Peninsula, Hokkaido. The area is border zone between the Japan Basin underlain by the Early Miocene oceanic crust and the Okushirito Island and the Oshima Peninsula composed of the pre-Miocene continental crust. This map area is divided into the Japan Basin, the Okushiri Ridge, and western slope of the Oshima Peninsula based on the characteristics of geologic structure.

Sedimentary covers in the map are divided into Units M, P, and Q in ascending order. Unit M is Miocene to Early Pliocene and Unit P is Pliocene sediments. They cover the Japan Basin, the Okushiri Ridge and western slope of the Oshima Peninsula. Unit Q is Quaternary sediments which is distributed mainly in basins of this map. The ages of these units are inferred by correlation with stratigraphic data of Site 796, ODP Leg127 and geological map adjacent to the north of this map.

The Japan Basin in this map is 3,500–3,700 m below sea level (bsl) and is underlain by the basement inferred to be Early Miocene oceanic crust and Neogene and younger sediments of 2–3 sec thick (two-way travel time). The depth of the basement is 6.0 to 7.0 sec which increases to the south. Several mounds are observed on the basement that is covered with sediments in the northern part in this map. Unit M covers the basement and is characterized by weak or faint internal parallel reflections. Its thickness increases where the basement depth increases, thus the depth of the top of Unit M is nearly horizontal except for the uplifts or subsidence zones. Unit P conformably covers Unit M and has nearly parallel continuous reflections. In the southern part of this map, N–S to NNE–SSW trending reverse faults dipping east and hanging wall anticlines have developed in the Japan Basin.

Okushiri Ridge elongated in the N–S direction is composed of several hanging wall anticlines bounded by west-dipping reverse faults trending in N–S, NNE–SSW and NNW–SSE directions and reaches the Okushirito Island at the southern part of this map. These faults have been active mainly in the Quaternary period. The Motsutamisaki-oki Fault truncates the ridge in the NE–SW direction and extends into the Japan Basin. To the south of the Motsutamisaki-oki Fault, the Okushiri Ridge is depressed, but the ridge continues to the south decreasing its depth and reaches to the Okushirito Island. The western slope of the Okushirito Island is very steep about 3,000 m high. No continuous reflections are observed under the seafloor of the slope, and the basin floor to the west of this slope is filled by sediments of chaotic pattern with scattered hammocks, suggesting that sediments of the steep slope have been removed by large-scale landslides and deposited on the basin floor. The 1993 Hokkaido Nansei-oki earthquake (M=7.8) occurred under the Okushiri Ridge in this map area. The aftershock area of the 1993 earthquake extended along the ridge for about 150 km and its southern part reached the western slope of the Okushirito Island.

The western slope of the Oshima Peninsula is composed of the Shiribeshi Trough and its eastern slope, the Okushiri Strait and the Okushiri Basin from north to south. The Shiribeshi Trough is an N–S elongated basin along the eastern boundary fault of the Okushiri Ridge. The seafloor of the trough about 3,300 m bsl. is covered with horizontal sediments that have wedge-shaped profile thickening westward. The eastern slope of the trough is covered with Unit M, P and Q and is highly disrupted by landslides. No reflections are observed on the steep and smooth slope of about 30 km wide and 3,000 m high to the west of Cape Motta, suggesting that the sediments above a certain horizon have thoroughly slid down. The seafloor of the Okushiri Strait is underlain by well stratified sediments of the units younger than Miocene. These sediments are deformed by N–S trended west-dipping reverse faults and anticlines above the faults. To the south of the strait, sediments thicken to the Okushiri Basin. Two east-dipping reverse faults developed in the basin of this map.

Most of reverse faults in this map have been active in the Quaternary period. The 1993 Hokkaido Nansei-oki earthquake occurred by the rupture of the reverse faults along the Okushiri Ridge in this map.

Several volcanic mounds are aligned in NW-SE direction from the northern slope of the Okushirito Island to the northern part of the Okushiri Basin. Unit M onlaps against the volcanic mounds, suggesting that volcanos are Miocene in age. In the southeastern corner of this map, part of shelf and slope are inferred to be underlain by Quaternary volcanics.