THE CRUSTAL STRUCTURE OF THE BROOKS RANGE OROGENIC BELT—THE BACKBONE OF ALASKA

The Brooks Range is characterized by north-directed, thin-skinned deformation, emplacement of far-traveled allochthons including ophiolite at high structural levels, high-pressure metamorphic assemblages in the southern Brooks Range, and syntectonic foreland basin deposits in the northern foothills. These features have been interpreted as the products of north-directed arc-continent collision in the latest Jurassic and Neocomian. Seismic reflection and refraction data from the north-south Trans-Alaska Crustal Transect (TACT) generally support this interpretation and show that 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. The Moho in these data descends southward from about 32 km under the North Slope to nearly 50 km near the crest of the Brooks Range, but then ascends again to about 32 km under the metamorphic hinterland. This latter area is dominated by late, down-to-the south extensional fabrics that yield 103-96 Ma Ar/Ar ages. The basement-involved Mt. Doonerak antiform near the crest of the range is revealed to be a major crustal-scale duplex that fission-track data indicate was active at 70-65 Ma and was reactivated at 25 Ma, and the range-front triangle zone in the North Slope foreland basin folds strata at least as young as latest Cretaceous and yields 60-45 Ma fission-track cooling ages. These observations are best accounted for by a multistage deformational model that includes north-directed arc-continent collision in the Jurassic and Neocomian, regional extension in the hinterland in the Albian-Cenomanian, and retro-arc thrusting in the early Tertiary to Neogene. The southward-deepening Moho that was established in the Neocomian is interpreted to have been modified by thinning of the crust in the hinterland due to regional tectonic extension in the middle Cretaceous, followed by renewed contraction superposed on the older thrust belt in the early Tertiary. Early Tertiary shortening was probably related to north-directed flat slab subduction in southern Alaska, whereas shortening in the Neogene in mainly northeastern Alaska may represent a far field effect of terrane collision and accretion in southern Alaska.