Marine geology of the Aleutian Abyssal Plain |
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| Authors: | Edwin L Hamilton |
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| Institution: | Naval Undersea Center, San Diego, Calif. U.S.A. |
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| Abstract: | Five expeditions (1965–1970) across parts of the Aleutian Abyssal Plain and adjacent areas in the Gulf of Alaska, and results of the Deep Sea Drilling Project, provide new information for the geologic history of the region which forms restrictive limits on models of plate tectonics. In general: (1) the Eocene-Oligocene, turbidite Aleutian Abyssal Plain was deposited from channelized turbidity currents from the north or northeast; (2) the plain is bounded on the south by the northern ridges of the Surveyor Fracture Zone, and is isolated from the Tufts Abyssal Plain; (3) turbidites were deposited from many buried channels and smaller surficial channels, but mainly from four great channels: Seamap, Sagittarius, Aquarius, and Taurus.The channels are depositional features; accumulation of sediments causes the channels to lie, topographically, along low ridges, with channels above distal portions of their levees. Western levees are higher and broader than eastern levees. Levee heights decrease from 30–100 m in the north to 15–25 m in the south.Rates of deposition and thicknesses of pelagic sediments in the northwest are 3 to 4 times greater than in the southeast. The data indicate the pelagics were deposited near the margin of the Pacific, at or near present locations. Thus, little or no northward plate motion is indicated.Turbidite thicknesses decrease from about 400–800 m in the north to about 200 m in the south. Turbidite thicknesses in the east-central plain are greater than in the Alaskan Abyssal Plain (formed since the Miocene), the northern Tufts Abyssal Plain, or the Sohm Abyssal Plain in the North Atlantic.Faulting and flexure of the oceanic crust seaward of the Aleutian Trench have strongly affected the channels. Seamap Channel has its high point midway along its course. The other three major channels are uplifted and faulted in the north.Required volumes of off-scraped sediments, undisturbed turbidites in the Aleutian Trench floor, and paleoclimatology also argue for little northward plate movement.The total evidence indicates that the turbidite Aleutian Abyssal Plain was formed in the Eocene-Oliogocene at, or near, its present position, and that the sediment source was probably Alaska. Cretaceous flysch of the Alaska Peninsula continental terrace was a possible source.The evidence does not require, but does not exclude, plate tectonics hypotheses. The evidence apparently excludes those continuous spreading models which cannot explain deposition of an Eocene-Oligocene turbidite plain over the magnetic bight, or which require an active, subducting, paleogene Aleutian Trench. Plate movements to the north over small distances cannot be excluded. The evidence is consistent with concepts of discontinuous sea-floor spreading with episodic subduction, or discontinuous, relative plate motion in this area. Two models are outlined which are consistent with the regional evidence: (1) a model with discontinuous relative plate motion and episodic subduction (a variation of one published by Hayes and Pitman, 1970); or (2) a no-plate-motion, or very-little-motion, model with long periods of inter-plate inactivity without subduction. |
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