SUPERPOSED COAXIAL OROGENS IN THE BROOKS RANGE FOLD-AND-THRUST BELT, ALASKA

Similar to fold-and-thrust belts worldwide, the Brooks Range includes (1) a passive-roof duplex at the deformation front (2) an axial zone with imbricate thrusts, and (3) a hinterland composed of ductiley deformed metamorphic rocks. It is unconventional, however, in that timing of contractional deformation is Tertiary in the frontal zone and dominantly Jurassic and Early Cretaceous in the axial zone and hinterland, although structures are everywhere mostly coaxial and north-vergent. As a result, kinematic models calling for deformation in the Jurassic and Cretaceous do not explain the young age of frontal structures, whereas those calling for deformation in the Tertiary do not explain Mesozoic deformation in other parts of the Brooks Range. A regional network of seismic reflection data, geologic field studies, and fission-track analyses shows that the Brooks Range consists of superposed contractional orogens. The first orogen was a north-directed arc-continent collisional zone that was active from 140-120 Ma. Because it was subduction-related and involved a thin (1-4 km thick) stratigraphic section, it produced a thin-skinned deformational wedge characterized by far-traveled allochthons and relatively low structural relief. A foredeep was developed in front of the orogen, but filled with only a thin sequence of syntectonic deposits. After an important phase of regional extension in the hinterland that resulted in deposition of a thick foreland basin sequence in the mid-Cretaceous, the second contractional orogen was formed by retroarc thrusting at about 60 Ma. Deformation involved a stratigraphic-structural section 5-10 km thick, was basement-involved in the hinterland, and produced a thrust belt with relatively high structural relief and low amounts of shortening that terminated in a frontal triangle zone in foreland basin deposits. Younger orogenic episodes at 45, 35, and 25 Ma had similar structural style but were areally restricted to the eastern Brooks Range. All deformations utilized the same mechanically incompetent stratigraphic units (Kayak Shale, Kingak Shale) as sites of thrust detachment.