W. C. McClelland | USGS Science

USGS2013

Laurentian origin for the North Slope of Alaska: Implications for the tectonic evolution of the Arctic

The composite Arctic Alaska–Chukotka terrane plays a central role in tectonic reconstructions of the Arctic. An exotic, non-Laurentian origin of Arctic Alaska–Chukotka has been proposed based on paleobiogeographic faunal affinities and various geochronological constraints from the southwestern portions of the terrane. Here, we report early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope subterrane of Arctic Alaska, as well as new Neoproterozoic–Cambrian detrital zircon geochronological data, which are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern subterranes of Arctic Alaska–Chukotka. The North Slope subterrane is accordingly interpreted as allochthonous with respect to northwestern Laurentia, but it most likely originated farther east along the Canadian Arctic or Atlantic margins. These data demonstrate that construction of the composite Arctic Alaska–Chukotka terrane resulted from juxtaposition of the exotic southwestern fragments of the terrane against the northern margin of Laurentia during protracted Devonian(?)–Carboniferous tectonism.

Lithosphere↗

USGS1991

U–Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith

U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.

Alaska↗