FOCUSED EXHUMATION IN THE SOUTHERN ALASKA SYNTAXIS

The western Chugach Mountains lie in a critical position that may link active margin-parallel deformation, where the interface between the North American Plate and Yakutat microplate are locked, to the southern Alaskan syntaxis. This syntaxis is characterized by arcuate fault systems and the highest topography of any accretionary prism in the world, whereupon accretionary wedge mass flux input may be focused and maintained by erosional mass flux output. We address mass flux and the exhumation history of the accretionary prism, with apatite fission-track (AFT) and apatite (U-Th)/He (AHe) ages from a N-S sea level transect in the Prince William Sound and compare these data with published data from the periphery ranges. New AFT and AHe ages from the center of Prince William Sound generally decrease northward from ~35 Ma to 11 Ma and from ~10 Ma to 3 Ma, respectively. The youngest ages are at the base of the highest mountains and largest glaciers of the western Chugach Mountains. Along the Border Ranges Fault Zone, north of the proposed syntaxial core, AFT ages are ~15-30 Ma and in the Talkeetna Mountains AHe ages are ~15-20 Ma; these ages may mark the beginning of Yakutat collision and subduction. AHe ages on the windward side of the Kenai and western Chugach Mountains are ~12-18 Ma, but on the leeward sides they range from ~20-40 Ma. These relations have been interpreted to reflect climatic control and faster exhumation on the windward sides of the ranges. Three samples from the Copper River/Cordova area are ~5-7 Ma and indicate more recent exhumation there. The northward decrease of AFT ages and the young AHe ages in the Prince William Sound, represent a nested pattern of reset ages that record higher exhumation rates on the southern side of the Chugach Mountains. Collectively, AFT and AHe ages from the Prince William Sound and surrounding regions decrease inward to form a bull's eye pattern of localized exhumation in the core of the syntaxis. These patterns suggest that partial plate locking and underplating, extended to million year time scales, has produced rapid exhumation above or north of the locked section and within the syntaxis. The arcuate structural boundaries, such as the Border Ranges Fault zone and Contact fault, may enhance and control localization of exhumed material related to forearc convergence.