MIOCENE EXHUMATION OF THE SOUTHERN TALKEETNA MOUNTAINS, SOUTH CENTRAL ALASKA, BASED ON APATITE (U-TH)/HE THERMOCHRONOLOGY

The Talkeetna Mountains, bounded by the Chugach Mountains to the south and the Alaska Range to the north, are in a key location to study the effects of Cenozoic shallow subduction of the Yakutat microplate in southern Alaska. Apatite fission-track data from the Chugach Mountains suggest that uplift/exhumation coincided with the collision and underthrusting of the Yakutat microplate about 16-22 Ma. AFT data along the Castle Mountain fault at the southern boundary of the Talkeetna Mountains suggest cooling about 10 Ma. Farther north, in the Denali area of the Alaska Range, AFT data suggest rapid cooling about 5-6 Ma. To address the exhumation timing of the region between the Chugach Mountains and the Alaska Range, apatite (U-Th)/He ages were determined for granite and granodiorite samples collected along a generally north-south transect at elevations ranging from approximately 600 to about 2000 m. Most samples south of the Kashwitna River are 15-20 Ma and show slight increase in age with increasing elevation, suggesting rapid cooling of this area 15-20 Ma. Farther north, (U-Th)/He ages are 60 to 73 Ma indicating that there is considerable complexity and spatial variability in the timing of the Talkeetna mountains. The early Miocene (U-Th)/He ages suggest that the most recent exhumation event of the southern Talkeetna Mountains occurred 10-15 My prior to uplift/exhumation of the Denali area farther north, perhaps associated with the early stages of Yakutat microplate subduction and prior to development of shallow subduction beneath the Talkeetna area.