Date:2/29/2008

t 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.

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GSA TODAY Science Article

The mystery of the pre-Grand Canyon Colorado River - Results from the Muddy Creek Formation

Joel L. Pederson, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA.

The Colorado Rivers integration off the Colorado Plateau remains a classic mystery in geology, despite its pivotal role in cutting Grand Canyon and in the regions landscape evolution. The upper paleodrainage apparently reached the southern plateau during Miocene time (between 23 and five million years ago), and recent work supports the longstanding idea that the river was superimposed over the Kaibab uplift by this time. Once off the plateau, the lower river integrated with the Gulf of California by downstream basin spillover starting about five or six million years ago. An unknown link remains: the history of the river in the Grand Canyon region in Miocene time. One of the viable hypotheses put forward by previous workers - that the
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Contact: Ann Cairns acairns@geosociety.org 303-357-1056 Geological Society of America Source:Eurekalert

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