Devonian subduction-related magmatism and sedimentation in the Ulungarat Basin of Arctic Alaska

Middle Paleozoic rocks of the continental divide region of the northeastern Brooks Range, Alaska, have long served as a key constraint on the tectonic history of the Arctic Alaska terrane and greater circum-Arctic region. Specifically, a postcollisional, rift basin interpretation for the Middle to Upper Devonian Ulungarat Formation constrains models for the hypothesized Early Devonian Romanzof orogeny. New field and analytical data from the continental divide region indicate that regional orogenesis and arc magmatism in the Arctic Alaska terrane persisted through the Late Devonian. Our geologic mapping demonstrates that the Ulungarat Formation and unconformably overlying Kekiktuk Conglomerate reside on a distinct Brookian (Mesozoic to Cenozoic) thrust sheet that is intermediate in structural position between the parautochthonous North Slope subterrane and potentially far-traveled Endicott Mountains allochthon. A second newly identified thrust sheet in the same intermediate structural position contains two key new units that are defined herein: the Middle to Upper Devonian Double Mountain Formation, which consists of interbedded siliciclastic and bimodal volcanic rocks, and the Lower Mississippian Chandalar Formation, which occupies a similar stratigraphic position to the Kekiktuk Conglomerate. Based on new biostratigraphic, geochemical, and geochronological results, the Ulungarat Formation and clastic-dominated portion of the Double Mountain Formation are equivalent in age, highlighting a more complex tectonic setting for the proposed “Ulungarat Basin.” New U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) zircon dates from volcanic and plutonic rocks of the Double Mountain Formation yield consistent ages of 375.14 Ma to 372.97 Ma, and new geochemical data from these volcanic rocks record arc signatures. The Kekiktuk Conglomerate and Chandalar Formation rest unconformably on different stratigraphic levels of the Ulungarat and Double Mountain Formations, the latter of which also contains local deformational fabrics not observed in the overlying Mississippian units. Together, these data indicate that the Middle to Upper Devonian Ulungarat and Double Mountain Formations were generated in a convergent tectonic setting and no longer provide a Middle Devonian maximum age constraint on the Romanzof orogeny of the Arctic Alaska terrane. Rather, we suggest that Early Devonian Romanzof contraction in Arctic Alaska was simply a phase of the broader Ellesmerian orogeny that culminated in the amalgamation of the Arctic Alaska terrane, which ultimately shed synorogenic detritus onto northern Laurentia.