HR: 1340h
AN: T23C-1534    [Abstracts]
TI: Detrital zircon fission track analysis reveals the thermotectonic history of ice-covered rocks of the Chugach-St. Elias orogen, SE-Alaska
AU: * Enkelmann, E
EM: eve205@lehigh.edu
AF: Earth and Environmental Science Department, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015, United States
AU: Garver, J I
EM: garverj@union.edu
AF: Department of Geology, Union College, Olin Building 807 Union St., Schenectady, NY 12308, United States
AU: Pavlis, T L
EM: pavlis@geo.utep.edu
AF: Department of Geological Sciences, University of Texas at El Paso, Geology 405, 500 West University Blvd, El Paso, TX 79968, United States
AU: Bruhn, R L
EM: rlbruhn@mines.utah.edu
AF: Department of Geology and Geophysics, University of Utah, 1471 Federal Way, Salt Lake City, UT 84102, United States
AU: Chapman, J B
EM: jbchapman@miners.utep.edu
AF: Department of Geological Sciences, University of Texas at El Paso, Geology 405, 500 West University Blvd, El Paso, TX 79968, United States
AB: Investigating the exhumation history of the Chugach-St. Elias orogen (SE Alaska) using low-temperature thermochronometers is challenged by significant ice cover. Assuming exhumation drove cooling, cooling ages increase with elevation in an orogenic belt, and as such the youngest ages occur in valley bottoms. Cooling and exhumation rates are expected to be very high in the Chugach-St. Elias orogen due to efficient glacial erosion and the most intense erosion occurs under the major ice fields. To study the cooling history of rapidly exhuming rocks underneath this ice cover, we analyzed detrital zircon fission track (DZFT) ages of Recent sand samples from modern rivers that drain the central Bagley Ice field and smaller glaciers draining north (Chitina valley) and south (Pacific) of the mountain range. A distinct advantage of DZFT is that it allows one to sample a landscape regardless of accessibility. The youngest ZFT component populations of samples north and south of the Bagley Ice field record a Late Miocene (5-13 Ma) cooling of the orogen. The pattern of cooling ages shows symmetry across the orogen predates the earliest record of the collision of the Yakutat terrane with Alaska. This result contrasts with the asymmetric cooling pattern displayed by low- temperature thermochronological ages (AFT and AHe) of the exposed bedrock within the range. Apatite FT and U- Th/He ages of bedrock samples south of the Bagley Ice field record the syn-collisional (<5 Ma) fast exhumation whereas apatite ages to the north reveal more heterogeneous exhumation and vary widely from Miocene to Eocene. The bedrock samples from throughout the orogenic belt thus display predominantly the effects of the recent climatic situation of the mountain range with very high precipitation on the south, seaward side versus a more arid north side. Our ZFT results from the northern drainages highlight the relative sense and timing of two important fault zones, both accommodate south-side-up exhumation. The Steward Creek fault zone, located north of the Bagley Ice field, limits the Late Miocene exhumation, whereby samples north of it yielded age populations that are Late Eocene to Cretaceous (30-120 Ma) or older. The Border Ranges fault zone, located farther north, limits the Late Eocene cooling and exhumation of the low-P and high-T Chugach Metamorphic Complex that is inferred to have formed during Eocene ridge subduction. This study provides the first insights on the exhumation history of the Chugach- St. Elias orogen between the time of Eocene ridge subduction and full collision of the Yakutat terrane with North America in the latest Miocene.
DE: 1100 GEOCHRONOLOGY
DE: 1140 Thermochronology
DE: 8104 Continental margins: convergent
DE: 8175 Tectonics and landscape evolution
DE: 9350 North America
SC: Tectonophysics [T]
MN: 2007 Fall Meeting