STRUCTURAL STYLE AND TECTONIC CONTEXT OF BASEMENT-INVOLVED TERTIARY FORELAND DEFORMATION IN NORTHEASTERN ALASKA

The frontal part of the northeastern Brooks Range, northern Alaska, is characterized by Tertiary (45 Ma and younger) thrust-related basement-involved uplifts that include duplex and tectonic wedge structural styles; several prominent basement-involved structures are also concealed beneath the Arctic coastal plain to the north. These structures are superimposed on a Mesozoic orogen and appear to have been driven by plate convergence along the southern margin of Alaska, 1,000 km to the south. The acme of basement-involved folding and thrusting occurred during the late Oligocene to early Miocene, and present-day seismicity implies that the system might still be active beneath the coastal plain and Beaufort Sea. Like the Laramide province of the central Rocky Mountains in the conterminous United States and the modern Sierras Pampeanas of Argentina, the basement-involved structures on Alaska's North Slope deformed the foreland of a thin-skinned fold-thrust belt.

The Laramide and Sierras Pampeanas basement-involved foreland thrust provinces are widely recognized to be genetically linked to amagmatic flat-slab subduction. However, the 25- to 20-Ma climax of basement-involved deformation in the northeastern Brooks Range foreland cannot be correlated with the conventional flat-slab pattern of arc volcanism (typically a magmatic sweep followed by a magmatic lull). Terrane accretion in southern Alaska provides a possible tectonic driver for this pattern of thrusting, although the timing of accretion of the Yakutat terrane does not correlate well with the uplift history of the northeastern Brooks Bange. The present-day plate structure of southern Alaska consists of an abruptly creased (convex-upward) descending slab, which might act as a stress guide that drives crustal seismicity in central and northern Alaska. Possibly, this creased plate geometry propagated across southeastern Alaska in Neogene time and drove the Brooks Range foreland basement-involved deformation, 1,000 km north of the subduction zone. The basement-involved structures of the northeastern Brooks Range represent the tip of an orogenic wedge of crustal or lithospheric scale that provides a kinematic connection between plate convergence at the southern margin of Alaska and emergent thrusting at the northern margin of Alaska.