BASEMENT-INVOLVED DEFORMATION OF THE NORTHEASTERN BROOKS RANGE FOOTHILLS AND ARCTIC COASTAL PLAIN, ALASKA – FAR-FIELD OROGENIC RESPONSE TO SOUTHERN ALASKAN COLLISIONAL TECTONICS

The foothills of the northeastern Brooks Range (NEBR), northern Alaska, and part of the Arctic coastal plain, are underlain by Paleogene and Neogene basement-involved folds and thrusts. Like the Laramide province of the central Rocky Mountains in the conterminous United States, the basement-involved structures on Alaska's North Slope are superimposed on the foreland of a thin-skinned fold-thrust belt. Unlike the classic Laramide province, the NEBR basement-involved structures have quite a consistent orientation, generally parallel to the overall orogen.

The pre-Mississippian metasedimentary basement of the NEBR is overlain by a Mississippian through Triassic passive margin sequence, a Jurassic and Lower Cretaceous rift-related sequence, and a Lower Cretaceous to Neogene foreland basin.

The western part of the NEBR foothills are underlain by a north-driven basement wedge. In some places, the basement wedge overrides upper Paleozoic and Mesozoic strata, so that the wedge point impinges on Tertiary foreland basin strata. Secondary splays from the floor thrust of the basement wedge produce local folding. In the eastern part of the NEBR (within the Arctic National Wildlife Refuge), the foothills and part of the Arctic coastal plain are likely underlain by a north-vergent basement duplex, a more complex structural configuration than that to the west. The basement-involved structures clearly deform pre-existing Tertiary thin-skinned thrusts on some transects. In other locations, thin-skinned thrusting at the deformation front apparently is linked to young basement-involved thrusting directly to the south, with no evidence for older thrusting.

The Tertiary basement-involved shortening in the NEBR foreland stands in marked contrast to thin-skinned Tertiary shortening in the central and western Brooks Range foothills. Previous workers have suggested that the young basement-involved structures of the NEBR are driven by Neogene to Holocene plate convergence and terrane collision in southern Alaska, 900 km to the south. If so, the basement-involved structures of the NEBR represent the tip of an orogenic wedge of crustal or lithospheric scale that provides a kinematic connection between terrane collision at the southern margin of Alaska and emergent thrusting at the northern margin of Alaska.