CHARACTERIZING SPATIOTEMPORAL VARIATIONS IN EXHUMATION ACROSS THE ST. ELIAS SYNTAXIS REGION USING MULTI-METHOD THERMOCHRONOLOGY

The border region between southeastern Alaska and southwestern Yukon is marked by the extreme topography of the St. Elias Mountains, the world’s tallest coastal mountain range with peaks rising up to ~6,000 m asl. This high topography is coincident with the indenting corner of the Yakutat microplate where it collides with the North American plate. Here, the plate boundary forms a ~30 degree syntaxial bend where dextral strike-slip motion along the Fairweather Fault transitions to convergence and subduction-collision-related deformation. The St. Elias syntaxis region supports extensive glaciers due to its high latitude and precipitation rates, which obscure many of the structures that accommodate strain from the Yakutat subduction-collision. Recent detrital thermochronology studies suggest that an area of localized rapid exhumation exists beneath the Seward and Hubbard Ice Fields at the St. Elias syntaxis (Enkelmann et al., 2009, 2010; Falkowski et al., 2014; Grabowski et al., 2013). To further explore this area, we use a multi-thermochronometer approach on 43 bedrock samples from the St. Elias syntaxis and along transects to the north and east, across the Denali Fault. Our goals are to 1) determine if bedrock thermochronology can be used to constrain the spatial extent of previously identified rapid exhumation, 2) evaluate differential exhumation across large-scale structures over time, and 3) explore how glacial erosion affects regional exhumation. We present 125 new thermochronometric ages including apatite and zircon (U-Th)/He (AHe and ZHe) and apatite and zircon fission-track (AFT and ZFT) ages. The youngest ages (<6 Ma AHe; <15 Ma AFT; <40 ZHe; <60 ZFT) are confined within the Seward and Hubbard Glacier catchments and sharply transition to older ages (>15 Ma AHe; >70 Ma AFT; >100 Ma ZHe and ZFT) north and east of the Hubbard Glacier and the postulated Connector Fault, which may transfer strain from the Fairweather Fault to the Denali Fault. The youngest AHe ages likely reflect exhumation dominated by Pliocene–Recent glacial erosion while age variations in the higher-temperature thermochronometric systems may record differential exhumation along structures as a response to changes in the tectonic style and geometry of Cenozoic terrane accretion.