Mar. 3, 2008 There is perhaps no better place on Earth to study the origin of continents than Alaska. Alaska is composed of fragments of crust that were transported northward on a plate-tectonic conveyor belt and scraped off against the North American continent as the conveyor belt (oceanic plates) descended beneath it.
These fragments included volcanic islands and plateaus that formed on the oceanic plates, parts of the continental shelf of North America, and even parts of the oceanic plates. During the last several hundred million years of Alaskan history, oceanic plates slid beneath North America in subduction zones, generating volcanoes inland above places where the descending plates dehydrated and generated melts in overlying mantle and crust.
At least twice, fragments of the oceanic plates became doubled up, and, in attempting to subduct, uplifted and compressed the Alaskan crust landward of the subduction zone. One instance was 40--60 million years ago when doubled oceanic plates uplifted and compressed the Pacific coastal region of Alaska. This compression was felt as far away as northern Alaska and Canada, where the Brooks Range and Canadian Rockies were uplifted.
The second instance was more recent, beginning only a few million years ago, when a piece of abandoned oceanic plate, known as the Yakutat terrane was underthrust by the Pacific plate and, together with the Pacific plate, entered the subduction zone. According to Fuis et al.'s* and other studies, it appears that the western half of this terrane is being subducted atop the Pacific plate at a very shallow angle, and this doubled thickness of oceanic crust has shut off volcanoes in the central part of Alaska that would normally accompany subduction.
The eastern half of the terrane is too buoyant to subduct and is being offscraped beneath southern Alaska, beneath the St. Elias Range, leading to some of the highest uplift rates in the world. The effects of this offscraping are being felt as far inland as the Brooks Range and Canadian Rockies - again. North of the St. Elias Range, the Pacific plate, freed of its piggyback Yakutat terrane, is sinking (or subducting) at a steep angle, and has torn away from the part of the Pacific plate to the west that is subducting at a shallow angle with the overlying Yakutat terrane.
The Great Alaskan earthquake, M 9.2, of 1964, nucleated near this tear and ruptured southwestward along the upper interface of the subducting Yakutat terrane.
*The research "Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting" by Gary S. Fuis et al., M/S 977, U.S. Geological Survey was published in the March issue of Geology, Pages 267-270.
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