PALEOZOIC FOLD-AND-THRUST BELTS IN ARCTIC ALASKA AND THE ADJACENT OFFSHORE REGION

Newly available 3D and recently reprocessed 2D seismic reflection data combined with zircon fission-track (ZFT), detrital zircon U-Pb (DZ), and lithologic data from well cores and cuttings provide an updated understanding of deformation in Paleozoic strata of Arctic Alaska and the adjacent offshore. Our new seismic interpretations of the Franklinian and Ellesmerian megasequences show that, while pre-Mississippian contractional deformation was ubiquitous, local shortening continued for much of the late Paleozoic. Late Paleozoic crustal extension on the North Slope is not extensive, unlike the clear Mississippian extension in the Chukchi shelf.

Paleozoic fold-and-thrust belts in the subsurface of Arctic Alaska and the adjacent Beaufort Sea shelf generally trend east-west (present-day coordinates), with a complex interplay of transport and vergence directions. South-directed belts are common in the North Slope east of the Colville River, the western Beaufort Sea shelf, and nearshore Point Barrow (Nuvuk) area. West of the Colville River, in the central National Petroleum Reserve in Alaska (NPR-A), north-directed contractional structures are present in Franklinian and Ellesmerian strata. DZ age spectra and lithology of pre-Mississippian rocks show variations that correspond with these belts. Quartz-rich sandstone with Proterozoic DZ ages are common in the south-directed belts, whereas chert-rich sandstone with Devonian DZ ages are common in the north-directed belts, suggesting differences in provenance. Though south-directed belts were active and deeply exhumed in the Devonian, the north-directed belts were both active in the Devonian and reactivated in the Mississippian and probably Pennsylvanian, as constrained by growth strata and distinct, though subtle, unconformities. Farther north in NPR-A, this north-directed thrust belt impinged on older south-directed structures, likely in Pennsylvanian–Permian time. The north-directed belt in part overlaps spatially with an area of Mississippian–Permian ZFT central ages.

These relationships indicate a more complex late Paleozoic tectonic history than previously recognized for the Arctic Alaska region. Further study is warranted to determine the affinity of these contractional belts to those of the Paleozoic circum-Arctic region.