ALBERT

Description: This paper is a geological and topographical description of the sea bottom off northeastern Hokkaido district, stretching Soya Misaki to Shiretoko Misaki. The continental shelf off Soya Misaki to Notoro Misaki region shows broad width and its outer edge is fairly deep ranging 169 to 230 m. The sea area off the coast of Notaro Misaki to Shiretoko Misaki has narrow shelf and the depth of edge ranges 135 to 150 m. There is Kitami YamatoTai (bank) off Notoro Misaki, and continental slope is developed around this bank. From the results of continuous seismic profiling, subsurface strata are divided into several formations; A, B, C, D and E descending order. The E Formation which is the lowest acoustically opaque layer is named the basement in the investigated area. The authors concluded the process of geotectonic development in the investigated area as follows． The D Formation had deposited in the continental shelf off Soya Misaki to Notoro Misaki excluding northern part of outer front of continental shelf area. Then coastal area off Yamausu to Saroma Ko (lake) and Monbetsu Tai in adjacent to outer front zone had been upheaved. As a result of the movement, a part of this area became island which was eroded. During this tectonic movement，flat zone of the middle of shelf had been continuously subsided down so that CL Formation had deposited there. After deposition of the CL Formation the sedimentary basins reduced their areas. During the Cu Stage, the areas have subsided and the basins have gradually enlarged when the Cu Formation has overlapped the CL Formation, as well as the B Formation to Cu Formation. The A Formation has buried the depression where the B Formation has down warped. The central flat zone was filled with much sediments consisting of several formation, and outer margin of shelf and coastal zone were connected by sedimentation. The Kitami Yamato Tai was uplifted during deposition of the D Formation for’the tectonic movement. The CL, CM, Cu, and B Formations were overlapping each of their under posed layer and these formations became thinning out to the top of bank. After the B Formation deposited or during the AL Formation was depositing, tectonic movement with fault was occurred. The result of that continental shelf and the bank have separated each other. After all, present form of Kitami Yamato bank was shaped in this stage. When the lowering of sea level at the glacial age, the continental shelf margin was formed and the top of Kitami Yamato Tai was suffered from wave erosion and continuously tilting movement occurred which has resulted western incline on the northern part and east incline on the southern part of bank.

Description: Published

Description: Published

Description: Published

Description: Published

Description: Published

Description: The Hydrographic Department of Japan has conducted hydrographic, geological and geophysical surveys in the southwestern Okinawa Trough and adjacent area in 1984. The purpose of this survey is to collect the basic information of Japanese continental shelves and to publish the results in bathymetric chart, geological structure, geomagnetic and gravitational anomaly maps. The surveyed area in this report includes southwestern region of Tunghai Shelf, Okinawa Trough and Nansei Syoto (Ryukyu Island Arc). Okinawa Trough is a back-arc basin behind Ryukyu Island Arc. Whole survey area is included in Eastern China Sea as a Marginal Sea between Asian Continent and Ryukyu Island Arc. It was reported that the Okinawa Trough probably represents a feature of the primary stage of back arc spreading related to subduction of Philippine Sea plate along the Nansei Syoto Trench. High heat flow values, fresh igneous rock intrusions and remarkable graben structures have been observed at the center of the Trough. These evidences have been considered as active back arc spreading occurring in this place. We confirmed that the graben formed en-echelon pattern in the trough axis. We also found a conspicuous small knoll (tentative name is Yaeyama central knoll) at the central graben in this survey area. That knoll have been consisted of two pyroxene andesite. According to the bottom photograph, there is biological colony by Glassy Sponges, Sea Anemones and other benthos at this Yaeyama central knoll. Warm water probably exhales here because of existence of this colony and photographic flickering phenomena. It is supported that the crust of southwestern Okinawa Trough has continental character, because fresh andesitic rocks formed central part of this trough ares. Gravitational data and previous seismic refraction data support this assumption that thin continental crust exists under this surveyed area. The results from interpretation of seismic profiles, sedimentary pattern showing at the central graben formed by rapid subsidence of basement at the axis zone of trough. We could not determined in the present-day’s actual spreading of Okinawa Trough in our data.

Description: Published

Description: Published

Description: The investigated area includes the continental shelf and continental slope off the coast from Kuji to the Nemuro Peninsula. The survey was carried out by continuous seismic profiler and echo sounder, spacing normally two nautical miles. The acoustical strata are divided into the As, Bs, Cs, Ds, and Es Formations in descending order off the northern part of Sanriku District and southern coast of Hokkaido ; each formation shows relationship of disconformity to lower formation. On the other hand, the AK, BK, CK, DK, Upper EK and Lower EK Formations are recognized off Tokachi, Kushiro and Nemuro Districts. Each formation superposes lower formation with disconformity. The disconformities between the Upper and Lower EK Formation, and the CK and DK Formations are remarkable. The ages of acoustical strata were estimated as follows ; Sanriku-Hidaka Districts As ... Quaternary Bs ... Pliocene and late Miocene Cs ...late Miocene Ds…middle Miocene Es ... early Miocene and pre-Neogene Kushiro District Ak ... Quaternary Bk ... same as above Ck ...Pliocene Dk ...same as above Ek ... Miocene EK. (lower) ... pre-Neogene General strike of the geological structures in the area is divided into three groups, namely N-S (NNE-SSW to NNW-SSE), N W-SE (WNW-ESE), NE-SW (ENE-WSW). The first predominates on the continental shelf and the marginal plateau off the northern part of Sanriku and Oshima Districts. The second predominates on the area of the continental shelf in the environs of the southern part of Hidaka Range and the deep sea terrace including Hiroo spur. The third predominates on the continental shelf and slope area off Kushiro and Nemuro bordered by the Kushiro submarine Canyon. The authors assumed the process of development of geological structures as follows ; In the continental shelf area off the northern part of Sanriku District, there occured a crustal movement accompanied with uplifting after the deposition of the Ds Formation and a movement occurred after the deposition of Cs Formation again. A crustal movement of folding accompanied with uplifting after the deposition of the Cs and lower part of Bs Formations an effect on the area of the outer edge of marginal plateau along the Oshima Peninsula. In the area off Hidaka District along the west side of Hidaka Range, there was a crustal movement of uplift accompanied with folding after the deposition of the Cs Formation. Consequently,the Hidaka Trough area became a synclinal sedimentary basin where the Bs and As Formations successively deposited. On the other hand, in the area located eastward of the Hidaka Range, there was a conspicuous crustal movement after the deposition of the Lower EK Formation, then the Hiroo spur off Tokachi District and the folding of the continental shelf area of Kushiro and Nemuro Districts were originally formed. After the deposition of the Dk Formation in the area between them, the crustal movement occurred again to form the original shape of the Kushiro Submarine Canyon and the deep sea terraces. The Ck, Bk, and Ak Formations successively deposited in the area of the deep sea terraces accompanying with some crustal movement.

Description: Published

Description: Published

Description: Daiiti-Kasima Seamount is a 3,000-4,000 m high flat-topped seamount, and lies in the axis of the Japan Trench, located 220 kilometers east of Tokyo. In 1977, the Hydrographic Department of Japan conducted a survey over the seamount and adjacent waters. The average spacing of the tracks of the survey was 2 nautical miles, and it was done by the survey ship “SHOYO" as part of “The Basic Map of the Sea Project. " It fairly revealed topography, geological structures, geomagnetic features and gravity anomalies of the seamount. Studies by Mogi and Nishizawa (1980) had provided a hypothesis that a breakdown had taken place, and the seamount was divided into two parts by a large NNE-SSW trending fault, while the western half of the seamount had subducted into the Japan Trench. In 1983, the Hydrographic Department of Japan commissioned a new survey vessel “TAKUYO” (2,600 tons) in which a set of Sea Beam System, a single and multi-channel seismic profiler, a sub-bottom profiler, magnetometers, a gravimeter and a set of integrated positioning system were equipped. In December 1983, the Department carried out a detailed survey of Daiiti-Kasima Seamount by “TAKUYO” to confirm the subduction of the seamount. The survey was done along ten track lines parallel to the trench with a 1.5 nautical mile average spacing, and eight track lines perpendicular to the trench with 1 nautical mile average spacing. A multi-beam bathymetric survey with Sea Beam System has provided detailed data of the seamount, and bathymetric charts on a scale of 1/40,000 with 50-meter-interval contours were drawn. In addition, single-channel seismic reflection, sub-bottom profile, geomagnetic total intensity and gravity were recorded along each track. A multi-channel seismic reflection survey was performed along a WNW-ESE trending track line crossing the western half of the seamount. Previous researches could not provide clear evidence to show that the l ,600 m high steep slope running straight across the central part of the seamount is a fault scarp, but the multi-beam bathymetry confirmed the fault scarp characteristics of the slope. It also documented a lot of linear structures, which were considered to be faults, running parallel to the trench on the seamount. These faults indicate that the seamount had been affected by tension in the oceanic plate along the outer slope of the trench. The subducted western half of the seamount was clearly shown beneath the landward slope of the trench on the records of both the single-channel and multi-channel seismic reflection surveys. An asymmetrical steep V shaped depression, stretching parallel to the trench, was found along the edge of the landward slope of the trench. Horizontal sediments were absent in the bottom of the depression. The sub-bottom profiler record, taken at a small-scale pond sediment on the landward slope of the trench, shows a nearly horizontal deposit on the surface and other underlying sediment layers dipping about 2 degrees toward the depression, below some 10 to 20 meters under the surface. It means that the lower part of the landward trench slope has been tilting toward the depression. From these facts, the depression was considered to be formed by tectonic erosion. These results (provided by the detailed survey of “TAKUYO”） confirm the subduction of the Daiiti-Kasima Seamount into the landward slope of the Japan Trench.

Description: Published