UPLIFT AND EXHUMATION OF THE CHUGACH-PRINCE WILLIAM TERRANE, ALASKA, REVEALED THROUGH VARIABLE ANNEALING OF FISSION TRACKS IN DETRITAL ZIRCON

The Chugach-Prince William (CPW) terrane is a Mesozoic-Tertiary accretionary complex exposed for ~2000 km in southern Alaska. Pervasive prehnite-pumpellyite metamorphism presents unique challenges for thermochronolgy because many of these rocks lack primary datable phases. We use variable thermal resetting of fission tracks in detrital zircon from sandstones of the CPW terrane to understand the time-temperature history of these low-grade rocks. On Kodiak Island, analysis of samples from the Maastrictian Kodiak Formation and the Paleocene Ghost Rocks, shows they experienced a similar thermal history, and they record two distinct cooling events: one at c. 60 Ma and one at c. 47 Ma: the latter population is more robust as it can only be comprised of fully annealed zircon. The fraction of fully annealed grains in the younger population may suggest hotter Tmax for the Kodiak Formation. Other studies have shown that these units have vitrinite values (%Ro = 2.95 to 4.0) and fluid inclusions that suggest Tmax of 250-300°C and depths of c. 10 km. The more outboard Eocene Sitkalidak Formation has not seen thermal annealing of zircon, and the populations of these grains reveal details of sediment provenance. In Prince William Sound, about 375 km to the east, in and around Bainbridge Island, we dated 14 samples from the Orca Group that show young ZFT reset ages between 30-40 Ma. Given the mineral assemblage and vitrinite values from these samples (%Ro = 2.9-4.0), these rocks also experienced Tmax between 250-300°C and depths of c. 10 km. In the main Chugach Range, some 260 km to the east of PWS, we use detrital zircon from modern rivers confined to the CPW bedrock as a proxy for the thermal evolution of this part of the terrane. These data show that most of these rocks have young ZFT reset ages between 20 and 35 Ma. While low-grade metamorphism is common here as well, locally these rocks experienced amphibolite-grade metamorphism. We ask whether there is a spatial and temporal pattern of exhumation in the CPW related to translation, accretion, and collision. These three locations show progressive younger reset ages from west to east. This progression has implications for the locus of unroofing and the volume of material that must have fed adjacent basins.