THE BROOKS RANGE AND THE CANADIAN ROCKIES—A STRUCTURAL COMPARISON

The Canadian Rockies (CR) fold-thrust belt and its hinterland exemplify a classic tectonic style, in which crustal-scale hinterland shortening is linked to large-scale thin-skinned foreland thrusting, and significant younger crustal extension is superimposed on the hinterland. The Brooks Range (BR) in northern Alaska, historically viewed as the northwest continuation of the CR, shares many characteristics with the CR, but there are also important differences.

In both the BR and the CR, outboard accretionary tectonics drove major phases of shortening that began in the Jurassic and climaxed in the Cretaceous. In the southern CR, shortening continued into the Paleocene and was supplanted in the Eocene by crustal extension. In the BR hinterland, dominantly Neocomian to Aptian contraction gave way to dominantly Albian crustal extension; seismic reflection and refraction evidence suggest that, like the Eocene extension in the southern Canadian hinterland, this BR extensional phase involved the entire crust, not just the upper part of an overthickened orogenic stack. Younger episodes of Tertiary thrusting in the BR were apparently driven by strongly convergent plate motions and terrane collision in southern Alaska. In the northeastern BR, prominent Neogene deformation includes large-scale basement-involved thrusting beneath the Beaufort coastal plain. By contrast, the lack of Neogene shortening in the CR is likely related to weakly convergent to margin-parallel transform plate interactions along the western edge of North America at these latitudes.

The foreland for the CR was the western North American miogeocline, whereas the BR foreland was a large rifted continental fragment, the Arctic Alaska terrane. Distal continental-margin terranes with affinities to the autochthons are thoroughly involved in both belts, and are overthrusted by arc-related terranes in the hinterland.

Tectonic wedging and frontal triangle zones, structural concepts born in the southern CR, are prominently expressed in the BR. Both orogens feature carbonate-dominated antiformal duplexes in forward structural positions; these host large gas fields and some oil fields in the CR, but remain unproven gas prospects in the BR.