THE WESTERN CHUGACH MOUNTAINS AND NORTHERN PRINCE WILLIAM SOUND (ALASKA): LOCUS OF SUBDUCTION-RELATED EXHUMATION?

The western Chugach Mountains and Prince William Sound are located in the core of the southern Alaska syntaxis. This syntaxis is characterized by the curving of major mountain belts and fault systems from northwest trends in the eastern parts of southern Alaska to east-west and southwest trends farther north and west. Late Tertiary deformation in this area is mainly related to subduction of the relatively buoyant Yakutat microplate, which either directly or indirectly has had a more profound effect on deformation in southern and central Alaska than any other geologic event during this time. The Yakutat microplate began subducting ~25 Ma and is currently subducting at a shallow angle of about 6 degrees beneath Prince William Sound and much of southern Alaska. The Prince William Sound area is centered on a region where the interface between the North American and Yakutat/Pacific plates are locked to a depth of 20-30 km. If partial plate locking is extended to million year time scales such that some of the plate convergence produces permanent deformation above or north of the locked section, then rapid rock uplift and exhumation related to forearc convergence should be occurring across the western Chugach Mountains region. We report new apatite fission-track ages (AFT) and (U-Th)/He (AHe) from sea-level sites in the Prince William Sound fiords, which are surrounded by the high western Chugach Mountains. AFT ages generally decrease northward from ~35 to 11 Ma. AHe ages decrease northward from ~10 Ma to 3 Ma at the base of the highest mountains and largest glaciers of the western Chugach. The northward decrease of AFT ages and the young AHe ages relative to the mostly older AHe ages in the surrounding regions of the Chugach, Prince William Sound, and Kenai Mountains represent nested reset ages that record higher exhumation rates on the southern side of the Chugach Mountains and northwest most part of the Prince William Sound. These results suggest that the northwest Prince William Sound and highest part of the western Chugach Mountains may be the location of focused exhumation between the Border Ranges Fault backstop to the north and the region above the presently locked portion of the Yakutat – North American plate megathrust to the south.