Dredge samples from the Chukchi Borderland: Implications for paleogeographic reconstruction and tectonic evolution of the Amerasia Basin of the Arctic

The Chukchi Borderland is a large bathymetric high that extends from the Alaskan Chukchi Shelf into the Amerasia Basin of the Arctic Ocean. Widely interpreted to be underlain by continental crust, the Chukchi Borderland has played a pivotal role in plate reconstructions of the Arctic, despite the fact that its geologic nature, origin and evolution are largely unknown. We present new lithologic descriptions, along with U-Pb, 40Ar/39Ar, and apatite fission track analyses, for rocks dredged from the Chukchi Borderland. These new results provide constraints on the Chukchi Borderland9s crustal architecture, Paleozoic evolution, and pre-Cretaceous paleogeographic position prior to the formation of the Amerasia Basin. Based on the location and nature of dredged rocks, the Chukchi Borderland comprises at least two distinct terranes, juxtaposed by the proposed Chukchi Borderland fault zone. Early Paleozoic units recovered from Northwind Ridge in the eastern Chukchi Borderland consist of low-grade metasedimentary rocks with a Laurentian detrital zircon signature that is statistically indistinguishable from that of Cambrian age units of the Franklinian passive margin exposed on Ellesmere Island in the Canadian Arctic Archipelago. In contrast, high-grade gneisses and amphibolites dredged from the Chukchi Plateau in the central Chukchi Borderland yield U-Pb and 40Ar/39Ar ages that suggest an affinity with rock units of the Pearya terrane of Arctic Canada. U-Pb and 40Ar/39Ar ages reveal a Paleozoic tectonic history beginning in Early Cambrian to Early Ordovician time with subduction/arc-related high-T metamorphism. Intrusive relationships indicate that Late Ordovician to Silurian orthogneisses intruded the high-T units within an arc setting. Devonian exhumation of these high-grade units is recorded by U-Pb dating of sphene and 40Ar/39Ar thermochronology of micas and potassium feldspar. The exhumation of the high-grade rocks was coeval with the deposition of coarse-grained sediments bearing detrital zircon compositions similar to Silurian foreland basin sediments deposited in Pearya, as well as with greenschist facies metamorphism of metasedimentary rocks dredged from Northwind Ridge. The new data suggest paleogeographic restoration of the Chukchi Borderland to a position adjacent to Ellesmere and Axel Heiberg islands prior to Amerasia Basin rifting. This pre-rift position supports and refines the widely cited rotational models for opening of the Canada Basin.