A river runs through it both ways across time: 40Ar/39Ar detrital and bedrock muscovite geochronology constraints on the Neogene paleodrainage history of the Nenana River system, Alaska Range

Abstract

The Alaska Range, the topographic signature of the Denali fault, has an unusual physiography, with the Nenana River sourced from the south side of the divide and traversing along the range front some distance before heading north across the mountain range. Previous researchers suggested that a change from south-flowing to north-flowing drainage occurred at ca. 6 Ma, or early-middle Miocene, during initial phases of Alaska Range uplift. We applied 40Ar/39Ar dating of detrital micas from modern river sediment (proxy for basinwide source) and strata of the Neogene Tanana Basin (sink for the paleo–Nenana River), located along the northern front of the Alaska Range, to further investigate this hypothesis. In addition, we acquired 40Ar/39Ar muscovite ages from bedrock for additional source constraints and compared our results to regional geochronology data sets and geological mapping. During the earliest Miocene, the paleo–Nenana River likely flowed south. By the early Miocene, the paleo–Nenana River flowed to the north. During the middle Miocene, drainage reorganization continued, suggesting a variable history of rock uplift in the Alaska Range. By the late Miocene, sediment recycling occurred as the southern extent of the Tanana Basin was uplifted and eroded. The modern Nenana River near Cantwell has a muscovite age signature different than the Tanana Basin strata, implying continued drainage reorganization after the deposition of the Pliocene Nenana Gravel. In summary, the Nenana River drainage changed direction to north-flowing by ca. 18 Ma, driven by tectonism. Drainage reorganization continues today, demonstrating that strike-slip fault transpressive orogens can have complex paleodrainage histories.