THERMAL EVOLUTION OF FLYSCH OF THE CHUGACH-PRINCE WILLIAM TERRANES, EASTERN PRINCE WILLIAM SOUND, ALASKA

Flysch of Chugach-Prince William composite terrane represents an accretionary complex that formed along the western margin of North America during subduction of oceanic lithosphere in the Campanian to Eocene. The terrane was intruded by near-trench plutons of the Sanak-Baranof belt ascribed to ridge-trench interaction, but the location of these rocks at the time of ridge interaction is under debate. Detrital zircons from two transects (Richardson highway, Cordova) record the thermal evolution and exhumation history of the accretionary wedge. Rocks in the rearward part of the belt (Valdez Group) are fully reset, but record two thermal events after deposition. These rocks attained a higher Tmax (c. 300-350°C) and have a common cooling age of ~38 Ma and several samples have a second population at ~51 Ma. Rocks in the outboard part of the belt (Orca Group) have reset, partially reset, and possibly un-reset zircon grains. These rocks experienced a cooler Tmax (c. 250°C) and show much greater grain-age variation. Orca zircon have a common cooling age of ~49 Ma, but also populations at ~79 Ma and older. Samples of the Orca collected east of the NE-SW-trending Rude River fault have a young age population at ~31 Ma, with track-lengths that indicate a thermal discontinuity across the fault. U/Pb ages of zircon grains from the Orca Group indicate the time between deposition and intrusion by the near-trench Sheep Bay Pluton (part of Sanak-Baranof belt) was likely 5 to 8 Ma or less. The older thermal event in both transects (c. 50 Ma) was pervasive, was responsible for widespread thermal resetting across Prince William Sound, and likely corresponds to cooling following passage of the slab window: the 54 Ma Sheep Bay granite (U/Pb date) is also related to the Sanak-Baranof slab window. The ~38 Ma event likely corresponds to a thermal anomaly related to plutons of the Eshamy Suite (also newly dated), which may be related to development of the Alaska Orocline.