2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 23
Presentation Time: 8:00 AM-12:00 PM


DRAUT, Amy, USGS Pacific Sciences Center, University of California, Santa Cruz / US Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060, CLIFT, Peter D., Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell-Kniffen Geoscience Complex, Baton Rouge, LA 70803, RIOUX, Matthew, Geological Sciences, Univ of California, Santa Barbara, CA 93106, PAVLIS, Terry, Dept. of Geology and Geophysics, Univ of New Orleans, New Orleans, LA 70148, KELEMEN, Peter, Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964 and DEBARI, Susan, Geology Department, Western Washington University, Bellingham, WA 98225, adraut@usgs.gov

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. The volcanic and volcaniclastic rocks of the Talkeetna Volcanic Formation show close similarities with the eruptive products found around active arcs in the western Pacific. They allow the paleography of the arc and its temporal evolution to be reconstructed. There is no evidence of major rifting or collisional events and the arc appearsnot to involve much continental recycling in petrogenesis. Most rocks were deposited very proximal to arc volcanic centers, with no evidence for trench or trench slope deposits of Early Jurassic age. A small back-arc center appears to have been preserved north of the main arc outcrop in the Oshetna Valley of the Talkeetna Mountains. Around 180 Ma magmatism migrated northward from the main arc outcrop along the Border Ranges Fault into the region of the Talkeetna Mountains. 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 coincides 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 switch 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 at ca. 160 Ma.