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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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. 40 Ar/ 39 Ar dating to detrital muscovite from strata of the Neogene Tanana Basin (sink for the paleo-Nenana River), located along the northern front of the Alaska Range (Figs. 1 and 3), and to detrital muscovite from modern river sediment (proxy for basinwide source; Fig. 1). In addition, we applied 40 Ar/ 39 Ar dating to GEOSPHERE

Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

“…The northeastern Pacific plate margin is presently characterized by northwestward convergence along the southern Alaskan margin and shows a west to east transition from typical subduction to flat-slab subduction to strike-slip tectonics (e.g., Eberhart-Phillips et al, 2006;Haeussler, 2008). Subduction of the Yakutat microplate, interpreted to be anomalously thick and buoyant oceanic crust (e.g., an oceanic plateau), is responsible for Oligocene-Holocene volcanism and deformation in eastern Alaska (Plafker and Berg, 1994;Pavlis et al, 2004;Benowitz et al, 2011Benowitz et al, , 2014Benowitz et al, , 2019Worthington et al, 2012). Recent geophysical studies beneath south-central Alaska reveal an eastward increase in the angle of the subducted Yakutat slab, from ~6° south of the Talkeetna Mountains, and ~11° to 16° below the volcanoes of the Wrangell Mountains, with a projected depth of 80 km to the top of the slab (Bauer et al, 2014;Pavlis et al, 2019).…”

Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

“…1 and 2B) (Hoffman and Armstrong, 2006;Finzel et al, 2011). This is primarily based on the modern position of the Alaska Range over the subducted portion of the flat-slab, limited Miocene Talkeetna Mountain AHe bedrock cooling ages (e.g., Arkle et al, 2013), and enhanced sediment accumulation rates and sediment delivery from bedrock sources exhumed above the flat-slab region (Cook Inlet;Finzel et al, 2011Finzel et al, , 2016Tanana Basin;Benowitz et al, 2019).…”

Stitch in the ditch: Nutzotin Mountains (Alaska) fluvial strata and a dike record ca. 117–114 Ma accretion of Wrangellia with western North America and initiation of the Totschunda fault

“…Although a fundamental and global tectonic process, details of this process remain poorly understood. Well-exposed geologic records of past suturing events await detailed investigation using modern analytical techniques (e.g., Finzel et al, 2011;Benowitz et al, 2014Benowitz et al, , 2019Orme et al, 2015). Ancient suture zones crop out across Earth's continents in regional structural, sedimentary, and petrologic trends that extend hundreds to thousands of kilometers along strike.…”