THERMOCHRONOLOGIC CONSTRAINTS ON MIOCENE TOPOGRAPHIC DEVELOPMENT OF THE CENTRAL ALASKA RANGE SOUTH OF THE DENALI FAULT WITHIN THE MCKINLEY RESTRAINING BEND

The central Alaska Range is located along the right-lateral Denali fault system (DFS), ~500 km inboard of the active southern Alaska plate boundary. The central Alaska Range, host to Denali (aka. Mt. McKinley), lies dominantly south of the well-defined McKinley restraining bend in the DFS. Samples from the southwestern flank of Denali were collected over ~4 km of vertical relief. Previously reported apatite fission track (AFT) ages range from ~16 to ~4 Ma, with a well-defined inflection at ~6 Ma. This inflection, plus track length data, supported by inverse thermal models constrains the onset of significant cooling (exhumation and uplift) in the Late-Miocene. Models from higher elevation samples indicate the onset of a ~10 Ma cooling (exhumation) event. Increasing AFT ages with distance south of the DFS reflect decreasing exhumation, with age breaks across faults. Apatite (U-Th)/He (AHe) ages from Denali display some single grain age variation, but minimum AHe ages decrease with elevation from ~7 to ~2.5 Ma, consistent with AFT data. The AHe age trend possibly suggests that exhumation rate decreases following initial rapid Late Miocene exhumation, then increases. AHe ages proximal (≤1 km) to either side of the DF at Peters Pass are similar (ca. 1.5 to 4 Ma). These similar AHe ages close to the DF likely reflect the active fault zone and elevated geotherm, rather than differences in exhumation history. At the Chedolothna Glacier, SW of Peters Pass, samples just south of the DF have older AHe ages. Strain, as a result of southern Alaska plate boundary processes, notably initial flat slab subduction and then collision of Yakutat terrane is transferred inland, causing anticlockwise rotation of the southern Alaska block. Strain is partitioned along the DFS as well as onto thrust faults, and decreasing slip rates west along the DF toward the restraining bend likely reflect greater thrust accommodation south of the DF. Higher temperature thermochronometers (⁴⁰Ar/³⁹Ar) indicate faster exhumation in the Alaska Range started ca. 25 Ma, likely due to initial Yakutat interaction. Lower-temperature thermochronometers (AFT, AHe) chronicle episodes of faster exhumation within the ~25 Ma envelope. Initiation of these exhumation episodes correlates temporally with apparent Pacific plate motion change.