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AbstractThe Hydrographic Department of Japan carried out the survey of submarine topography by P.D.R., and of geological structure by continuous seismic profiler (air-gun system) from off the Shima Peninsula to off the Izu Peninsula in July to October, 1974. The survey is based on the project "Basic Map of the Sea on Continental Shelf". The survey ship “Meiyo”conducted survey of 7600 nautical miles, and position fixes were made by Loran C system. Results from the analyses on the data obtained are as follows ; 1. Submarine Topography The physiography of the sea area extending from the Shima Peninsula to Cape Omeezaki is classified as continental shelf, continental slope, deep-sea terraces and knoll chains, and each of them distributes in parallel with the direction of NE-SW. There exist many submarine valleys from on the shelf break to the continental slope. They disappear on the deep-sea terrace at the base of the continental slope． The deep-sea terrace off the Shima Peninsula develops remarkably in the depth from 1600 to 2000 m. One which develops from off Cape Irago to off the mouth of Tenryu River becomes narrow depression in the depth from 700 to 1500 m. The depression off the mouth of Tenryu River correspond to the upper reaches of the Tenryu Submarine Valley. The knoll chains exist between the outer edge of the deep-sea terraces and the Trough, and the width of which varies from 40 to 60 km. Two chains and small troughs between them run along the Nankai Trough and they form en echelon structure. The Tenryu Submarine Valley reaches from offing of Tenryu River to the Nankai Trough, descending through the knoll chains area. Both east and west slopes of the Suruga Trough are considerably steep. On the east slope the configuration is simple, but on the west slope there distributes a submarine basin and bank. 2. Geological Structure The strata in the area were classified acoustically as Ae, Be, Ce and De formations in the descending order. The main trends of the geological structure are NE-SW or ENE-WSW, which are represented by the Nankai Trough, the knoll chains area and the deep-sea terraces, and N-S by the Suruga Trough. The author estimates the history of the development of geological structure as follows ; After the deposition of De formation, uplift and subsidence with NE-SW trend faults occurred from off the Shima Peninsula to off Cape Omaezaki. The subsided area became the prototype of the Nankai Trough. After the deposition of Ce formation, landside part of the knoll chains lifted up with the NNE-SSW trend faults which cut upheaval axis. By this movement,the base of the continental slope off the Shima Peninsula became subsidence area which was the prototype of the Kumano Trough, and the lower reaches of the Tenryu Submarine Valley was formed. The Nankai Trough continued subsiding, but the elevation of the knoll chain and ridge around the Trough, which is consisted of De and Ce formations, kept Be formation from depositing there. During and after the deposition of Be formation, the upper stream of the Tenryu Submarine Valley was formed by the graben-like faults showing NE-SW trend at the base of the continental slope off the mouth of Tenryu River, and other submarine valleys on the continental slope were also formed in this time. Later, sediment of Ae formation was supplied through the submarine valleys from the landside and deposited in the Kumano Trough, the upper of the Tenryu Submarine Valley and the Nankai Trough. In the Suruga Bay after the deposition of Ce formation, this area became subsidence area with the N-S direction because of the down warping. This movement continued after the deposition of Be formation, and the prototype of the Suruga Trough was formed. Consequently, the Suruga Trough connected to the Nankai Trough. After that event, the sediment which was mainly supplied along the main trend of the Suruga Trough formed Ae formation in the Suruga Trough and the Nankai Trough.
JournalReport of Hydrographic and Oceanographic Researches