40AR/39AR DATING OF DETRITAL MICAS FROM THE PALEO AND MODERN TANANA BASIN, ALASKA: PROVENANCE AND PALEODRAINAGE HISTORY CONSTRAINTS ON SPATIALLY VARIABLE NEOGENE OROGENESIS IN THE ALASKA RANGE

The Alaska Range has an unusual physiography with two rivers (the Nenana and the Delta) sourced from the south side of the divide which traverse along the range front some distance before heading north, cutting through the mountain range. Previous researchers suggested that the change from south-flowing to north-flowing rivers occurred during initial Alaska Range uplift (~6 Ma). We applied ⁴⁰Ar/³⁹Ar dating of detrital micas from modern river sediment (proxy for basin-wide source) and sandstone 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 dated select bedrock samples for additional source constraints and compared our results to regional geochronology data sets and geological mapping.

During the deposition of the late Oligocene-early Miocene Healy Creek Formation the paleo-Nenana River likely flowed south, based on the dominance of muscovite from the Yukon-Tanana Highlands to the north (21%; ~153 Ma to ~120 Ma) and limited Talkeetna Arc aged muscovite to the south (3%;~201 Ma to ~153 Ma). By the time of the deposition of the early Miocene Sanctuary Formation, the source includes significant mica from the south, with one population unique to the Alaska Range (5%; ~37 Ma) and another possibly from the Talkeetna Arc (43%; ~201 Ma to ~153 Ma). The mid Miocene Suntrana formation had a large contribution of Triassic aged muscovite grains (13%; ~250 Ma to ~200 Ma) indicating a variable history of rock uplift in the Alaska Range. During the deposition of the Plio-Quaternary Nenana Gravels, there is a large population of muscovite grains sourced from the Yukon-Tanana Highlands (28%) implying sediment recycling as the southern extent of the Healy Creek Formation was uplifted and eroded. The modern Nenana River near Cantwell has an 87% contribution of muscovite grains from the Alaska Range and no ~201 Ma to ~153 Ma aged grains, implying continued drainage reorganization after the deposition of the Nenana Gravels. This supports the Broad Pass Fault being active during the Quaternary. In summary, the Alaska Range was being unroofed by the early Miocene, the Nenana River drainage changed direction by this time period to northward flowing, and drainage reorganization continues today.