CRUSTAL ARCHITECTURE AND DEFORMATIONAL HISTORY OF THE BACKBONE OF ALASKA: THE BROOKS RANGE OROGENIC BELT

Seismic reflection and refraction data collected along the north-south Trans-Alaska Crustal Transect (TACT) in northern Alaska reveal that the Moho descends southward from about 32 km under the North Slope to nearly 50 km near the crest of the Brooks Range, and then ascends again to about 32 km under the Koyukuk basin south of the Range. The crustal section under the Brooks Range is marked by a wedged-shape zone of high reflectivity whose base climbs smoothly from a depth of over 30 km in the southern Brooks Range to about 10 km in the northern foothills and which probably marks the basal detachment for north-directed Brooks Range deformation. Seismic reflectors and boundaries in this zone are interlacing and inclined southward at mid-crustal levels, probably because of small- to large-scale duplexes in this region. At shallow levels (<5 km), reflections are gently inclined southward under the North Slope, but become hummocky under the Brooks Range, possibly due to the presence of antiformal stacks of Paleozoic carbonate rocks. The deformational history of the Brooks Range in outcrop is dominated by north-directed, thin-skinned deformation, emplacement of far-traveled allochthons, and the presence of high-pressure metamorphic assemblages in the southern Brooks Range, all caused by arc-continent collision in the Late Jurassic and Early Cretaceous. Structures produced by this deformation are only weakly evident in the seismic data, however, and instead are dominated by younger structural features. For example, the basement-involved Mt. Doonerak antiform near the crest of the range is a crustal-scale duplex that fission-track data indicate was active in the Early Tertiary and Neogene, and the range-front triangle zone in the North Slope folds Late Cretaceous strata and yields Early Tertiary fission-track cooling ages. These and other observations lead to an interpretation whereby the hinterland of the Early Cretaceous collisional crustal section was thinned by regional brittle to ductile extensional tectonism in the mid-Cretaceous and then was redeformed by north-directed shortening, probably related to north-directed, Rocky Mountains-like shallow-slab subduction emanating from southern Alaska in the Early Tertiary. Shortening was reactivated in the Neogene, probably by terrane collision in southern Alaska.