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Geophysical and geological surveys were carried out in the 1999-2000 austral summer season for the TH99 cruise aboard the R/V Hakurei-maru. The survey includes the abyssal basin and the continental rise area of the Cooperation Sea (CS) and Prydz Bay (PB), offshore of Mac Robertson Land and Princess Elizabeth Land, Antarctica. In the northern abyssal plain of the CS, a mantle-like sequence (G) appears at a depth of 10 s two-way travel time (TWT), or approximately 13 km. The sequence disappears south of 64°S, in the southern part of the CS. These observations suggest that the southern part of the CS and PB is underlain by intermediate type crust that is characteristic of oceanic and continental material. The deep structure of most of the PB continental shelf is not clear due to strong seafloor multiples. More than 7 km of thick sedimentary sequences in the northwest area of PB are confirmed by the MCS survey. Very thick sedimentary sequences, interpreted as prerift, rift and drift sediment can be observed under thick pelagic sediment. Geological samples, which were recovered in the CS and PB, offer important evidence on the recent history of drift sediments caused by iceberg floes. Two cores reached 0.78 Ma (Bruhnes-Matsuyama (B-M) boundary) in magneto-stratigraphic measurements, and they gave distinct age determinations for recent sediments.

Geophysical and geological surveys of the Japan National Oil Corporation (JNOC) TH97 cruise on the R/V Hakurei-maru, in the Pacific margin of the northern part of the Antarctic Peninsula, were carried out in the 1997-98 austral summer season. The cruise was devoted to the western, northern and eastern continental margins of the northern part of the Antarctic Peninsula. The seismic data are interpreted together with the data which were collected in this area during JNOC TH80, 88, 96 and PetroBras and British Antarctic Survey cruises. The area north of the Antarctic Peninsula is interpreted as the triple junction area among the Antarctic-Scotia-(former) Phoenix plates. The seismic data on the continental shelf of the northern part of the Antarctic Peninsula show distributed highly deformed sedimentary basins and volcanic features. Kr-Ar dating of fresh andesite recovered from a minor ridge south of Elephant Island gave a 7 to 8 Ma, Late Miocene age. It suggests the Late Miocene tectonic event which outlined the tectonic framework in the area. The deformation trend is interpreted to be controlled by the regional trend of sinistral transtensional movement between the Antarctic and Scotia plates.

Geophysical and geological surveys of the JNOC TH96 cruise on the R/V HAKUREI-MARU, in the Pacific margin of the northern Antarctic Peninsula, were carried out in the 1996-97 austral summer season. The early and later halves of the cruise were devoted to a continental margin survey of the western and northern margins of the northern Antarctic Peninsula, respectively. The seismic data are interpreted together with the data which were aquired in this area during JNOC TH80, 88 and PetroBras and British Antarctic Survey cruises. The geophysical data in the southwestern margin show typical passive continental margin character with abyssal plain-continental slope-continental shelf from sea to land. The thickness of the sedimentary sequence in the abyssal plain is about 3 s (twt). The development of the continental rise between abyssal plain and continental slope is very poor, compared with more mature passive continental margins. The area north of the Antarctic Peninsula is interpreted as the triple junction area among the Antarctic-Scotia-(former) Phoenix plates. The seismic data in the area show distributed highly deformed sedimentary basins and volcanic features. The deformation trend is interpreted to be controlled by the regional trend of simstral trans-tensional movement between the Antarctic and Scotia plates.