Crustal implications of bedrock geology along the Trans‐Alaska Crustal Transect (TACT) in the Brooks Range, northern Alaska

Geologic mapping of the Trans‐Alaska Crustal Transect (TACT) project along the Dalton Highway in northern Alaska indicates that the Endicott Mountains allochthon and the Hammond terrane compose a combined allochthon that was thrust northward at least 90 km in the Early Cretaceous. The basal thrust of the combined allochthon climbs up section in the hanging wall from a ductile shear zone in the south through lower Paleozoic rocks of the Hammond terrane and into Upper Devonian rocks of the Endicott Mountains allochthon at the Mount Doonerak antiform, culminating in Early Cretaceous shale in the northern foothills of the Brooks Range. Footwall rocks north of the Mount Doonerak antiform are everywhere parautochthonous Permian and Triassic shale of the North Slope terrane rather than Jurassic and Lower Cretaceous strata of the Colville Basin as shown in most other tectonic models of the central Brooks Range. Stratigraphic and structural relations suggest that this thrust was the basal detachment for Early Cretaceous deformation. Younger structures, such as the Tertiary Mount Doonerak antiform, deform the Early Cretaceous structures and are cored by thrusts that root at a depth of about 10 to 30 km along a deeper detachment than the Early Cretaceous detachment. The Brooks Range, therefore, exposes (1) an Early Cretaceous thin‐skinned deformational belt developed during arc‐continent collision and (2) a mainly Tertiary thick‐skinned orogen that is probabty the northward continuation of the Rocky Mountains orogenic belt. A down‐to‐the‐south zone of both ductile and brittle normal faulting along the southern margin of the Brooks Range probably formed in the mid‐Cretaceous by extensional exhumation of the Early Cretaceous contractional deformation.