CONSTRAINTS ON EXHUMATION OF THE WESTERN CHUGACH MOUNTAINS (ALASKA) BASED ON ZIRCON FISSION-TRACK ANALYSIS OF MODERN GLACIAL OUTWASH

The collision between the Yakutat microplate and the North American plate in south-central Alaska has had a more profound effect on deformation in southern Alaska than any other geologic event in the late Tertiary. Shallow subduction of the Yakutat microplate began ~25 Ma and has caused underplating below the western Chugach Mountains, which form a syntaxial region where the mountains and fault systems curve from northwest to west and southwest trends; this area may be the locus of accretionary complex exhumation associated with Yakutat subduction. The heavily glaciated and towering peaks of the Chugach Mountains make sampling difficult. However, glacial outwash provides samples from the regions traversed by the glacier. Outwash samples were collected from the toes of the Knik, Matanuska, and Tazlina glaciers for detrital zircon fission-track (ZFT) analysis. The Knik and Matanuska glaciers effectively sample the northwest slopes of the Chugach Mountains, whereas the Tazlina Glacier samples the eastern part of the Chugach Mountain syntaxial region. A peak-fitting routine (Binomfit) was used to evaluate ZFT age distributions and peaks. The Knik Glacier yields ZFT ages of 22.2 to 142.5 Ma with peak ages of 40.1, 53.7, and 85.5 Ma (n=100). The Matanuska Glacier yields similar age peaks at 36.2, 54.3, and 68.0 Ma (n=58), with ages ranging from 21.7 to 87.0 Ma. These peaks are younger than the depositional age of the Cretaceous Valdez Group rocks, which underlie all three glaciers, but are too old to record exhumation related to Yakutat subduction. Assuming a typical geothermal gradient of 25°C/km and a closure temperature of 240°C, ZFT ages constrain exhumation to less than 9-10 km since initiation of Yakutat subduction. Preliminary ZFT ages from the Tazlina Glacier range from 0.1 to 198.6 Ma with peak ages at 3.1, 28.2, and 122.0 Ma (n=45). The young ZFT ages and young peak age from Tazlina outwash may indicate very rapid and focused exhumation in this eastern area associated with transpression in the accretionary complex above the locked megathrust. Alternatively, these young ages may be related to young volcanic activity associated with the Wrangell volcanoes to the east.