CAUSES OF ALONG-STRIKE VARIATION IN THE BROOKS RANGE OROGEN OF NORTHERN ALASKA

The Brooks Range is a north-vergent fold-and-thrust belt that extends E-W from the Yukon across northern Alaska to the Chukchi Sea. The orogen has a generally linear orientation across central Alaska but displays a large uplifted salient that straddles the Alaska-Yukon border to the east and an abrupt reentrant and change to a NW trend where it connects with the Wrangel-Herald Arch in the west. The orogen is the product of two episodes of deformation, including (1) south-directed subduction and collision of the continental Arctic Alaska terrane with the Koyukuk arc terrane in the Early Cretaceous, and (2) retroarc thrusting that emanated northward from subduction in southern Alaska in the Early Cenozoic. The Early Cretaceous deformational belt is a thin-skinned, large-magnitude thrust belt with a blueschist-bearing metamorphic hinterland, whereas the Cenozoic belt consists of deep-seated, lower displacement thrusts in the Brooks Range and a northerly belt of detachment folds that advanced beyond the Early Cretaceous deformation front.

Along-strike variation in the two orogenic belts differ because of the types of basement involved in their deformation and changes in the style of tectonism. The Early Cretaceous belt developed during subduction of the northern margin of North America prior to rifting of the Arctic Alaska terrane away from this margin. A linear pattern of deformation occurs in the eastern and central areas due to the presence of Neoproterozoic to middle Paleozoic slope facies and overlying Upper Paleozoic and Lower Mesozoic platform facies strata that were deposited along the North American craton margin. To the west, the basement includes thick Baltica-derived continental crust of the Chukchi platform, which may have produced the deflection of trends to the NW. Alternatively, Early Cretaceous deformation in Chukotka may have been caused by a westward change to Andean-style retroarc thrusting. In contrast, the more northerly propagation of Cenozoic shortening resulted in involvement of a rifted fragment of the craton-margin crust of North America in the northeastern Brooks Range. This produced higher uplift than where underlain by deformed slope deposits to the west. Farther west, the Cenozoic deformation terminates against the thick continental crust of the Chukchi platform.