Detrital thermochronology of the Alaska Range: Exhumation during Cenozoic subduction and translation

Abstract:

Detrital thermochronology of modern river sands from the Alaska Range quantifies the timing, extent, and magnitude of regional exhumation during Cenozoic subduction and translation along the southern Alaska margin. This detrital approach provides an effective way to quantify regional exhumation in remote areas like Alaska, where sampling bedrock and acquiring exhumation data for large areas requires tremendous effort. We present apatite and zircon fission track (FT) and zircon U/Pb ages from 13 catchments that span 450 km east to west along the Alaska Range. Zircon U/Pb ages indicate a dominant igneous first-cycle source that comprises >90% of most age distributions, even in areas where igneous outcrop comprises <10% of the catchment area. In addition, zircon FT age peaks are equivalent to zircon U/Pb age peaks in most cases, suggesting contemporaneous pluton cooling from 800°C through 240°C. On the other hand, apatite FT ages that record cooling through 110°C are younger and illuminate exhumation during mountain-building. We deconvolved detrital apatite FT age distributions into constituent age peaks. Overall, the young apatite FT age peaks are commonly 9-4 Ma and of high proportion in the central Alaska Range, as well as on the western and eastern ends of the range. In more detail, catchments draining both Mt. Foraker and the eastern Alaska Range have dominant 4 Ma age peaks in contrast to 9-6 Ma age peaks near Denali (Mt. McKinley) and in the western Alaska Range. These data suggest a large but discontinuous region of Late Miocene-Pliocene exhumation during growth of the Alaska Range, with exhumation influenced by local structures. In addition, most catchments display a 25-20 Ma peak that indicates regional exhumation concurrent with the onset of Yakutat flat slab subduction along the southern Alaska margin. Finally, some catchments display a 50-45 Ma peak that suggests localized exhumation during and after spreading ridge subduction along the southern Alaska margin. To estimate age-elevation relationships in each catchment, we employ recent Bayesian inverse modeling of cooling ages and hypsometries. We demonstrate the utility of this technique for the Ruth and Kahiltna catchments draining Denali, where new detrital data, as well as a previous bedrock vertical transect, both suggest a 7-6 Ma onset of rapid erosion.