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1 School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK
2 Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148, USA
3 Department of Geology, Western Washington University, Bellingham, Washington 98225, USA
4 University of California–Santa Cruz, Santa Cruz, California 95060,USA, and U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA
5 Department of Geological Sciences, University of California–Santa Barbara, Santa Barbara, California 93106, USA
6 Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, 61 Route 9W, Palisades, New York 10964, USA
The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma.
Key Words: Alaska • terrane accretion • subduction • tectonic erosion • collision
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