FISSION-TRACK DATING OF RESET DETRITAL ZIRCON FROM THE VALDEZ GROUP (THOMPSON PASS) AND ORCA GROUP (CORDOVA): IMPLICATIONS FOR THE THERMAL EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE, ALASKA

The Chugach-Prince William (CPW) composite terrane is a Mesozoic-Tertiary accretionary complex that is well exposed for ~2200 km in southern Alaska and is inferred to be one of the thickest accretionary complexes in the world. Detrital zircon in flysch of the CPW record the thermal evolution and exhumation history of the accretionary wedge. Samples of the Campanian-Maastrichtian Valdez Group and the Paleocene-Eocene Orca Group, of the Chugach terrane and the Prince William terrane, respectively, were analyzed using detrital zircon fission track techniques to understand the thermochronology of the rocks. Six sandstones from the Valdez Group, six sandstones from the Orca Group, and one sample of the Sheep Bay Granite were collected, dated, and track lengths were analyzed. In the Valdez Group, the six samples from along the Richardson highway (most from Worthington Glacier-Thompson Pass) have FT grain ages much younger than inferred age of deposition: they have a common young cooling age of ~38 Ma and a memory of grains that are no younger than ~51 Ma, which likely contain partially annealed grains. Track lengths in the Valdez Group are bimodal in samples that have a significant population of partially annealed, retentive grains but unimodal in fully reset samples. In the more outboard Orca Group, the six samples (Sheep Bay to Cordova) have a common young cooling age of ~32 Ma and significant populations that are ~49 Ma and older. The ~52 Ma Sheep Bay granite has a ZFT cooling age of ~37 Ma. Ages and track-length distributions vary on either side of the NE-SW-trending Rude River fault. To the east of the Rude River fault, in the Orca Group, track lengths are long and unimodal, suggesting full resetting of grains. To the west of the Rude River fault, track lengths are bimodal with an abundance of shortened tracks, indicating a significant degree of partial annealing. All samples from both the Valdez and the Orca have a memory of cooling at c. 50 Ma that is likely related to an excursion in the regional geothermal gradient related to granitic intrusions of the Sanak-Baranof belt driven by passage of the slab window and appears to lag that event by only several Myr. Younger cooling ages record a seaward younging of exhumation of the orogenic wedge in the Late Eocene (~38 to 32 Ma).