International Geologiical Congress - Oslo 2008

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STT-09 New concepts in global tectonics

 

Suruga Bay: Origin of one of the world's deepest bays

 

Masaaki Hanada, Institute of Oceanic Research and Development, Tpkai University (Japan)
Michihei Hoshino, Emeritus Professor of Tokai University (Japan)
 

 

Suruga Bay is located at the southern extremity of the Fossa Magna, a major tectonic zone that crosses through the center of the Japanese archipelago, and the Izu-Bonin submarine ridge continues on toward the south. The depth of the mouth of the bay is 2,500m and it is the third deepest bay in the world after the Gulf of Aden in the Indian Ocean (5,360m at the mouth) and the Gulf of California on the Pacific Coast of Mexico (3,700m at the mouth).
The base of the Izu-Bonin Arc consists of an ophiolite layer and a crustal compositions layer made up of granitic and metamorphic rock Vp=6.0 km/sec. It has uplifted blocks corresponding to a rift shoulder on east and west sides of the apex of the 6.0km/sec layer and, in the surface layer.
Crustal cross-section in the east-west direction based on explosive seismic survey in the northern part of Suruga Bay indicate that the depth of the 6.0km/sec layer is 10km below the ground surface. Benthic foraminifera discovered in the Early Miocene marine strata distributed along the Fossa Magna indicate that they all inhabited a deep sea environment from the Pacific Ocean side to the Japan Sea side. (There are known to be species among them that indicate a depth below the carbonate compensation depth.)
The Gulf of California off Mexico is one of the world's deepest bays and base depth of the basement, like Suruga Bay, is 10-11 km. The Gulf of California is also located at the northern extremity of an ocean ridge just as Suruga Bay is located at the northern extremity of the Izu-Bonin ridge. The Mid-Atlantic Ridge is one of the major oceanic ridges of the world and it is a widely known fact that a rift valley is developing at its crest, which continues on as far as the rift valley developing on shore in Iceland. In Iceland, the depth of the base of the rift valley covered by a lava layer is thought to be 10 km. There is known to be much continental rock of the late Proterozoic Era in the Mid-Atlantic Ridge. The Vp=6.0km/sec layer in the Rockall Ridge, a branch of this ridge, consists of Grenvillean rock.
Submarine ridges currently observed on the earth are late Proterozoic geosynclinal-orogenic belts and the rift belts on the submarine ridges are the descendents of rift belts formed at the crest of Grenvillian orogenic belts. The Gulf of Aden is the deepest bay in the world and it is also located on an extension of the Carlsberg Ridge. The bottom of the three bays indicated above are places of advancing crustal solidification due to multiple disturbances during the Proterozoic Era and are areas left behind by the upthrust of the earth's crust during the Paleozoic Era and later. The upthrust formations observed on both sides of the bay are primarily rift shoulder upthrust during the Cretaceous Period and later and the furrows of these bays are a part of the penetrating aulacogene.

 

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