BASEMENT-INVOLVED FORELAND DEFORMATION IN NORTHEASTERN ALASKA—STRUCTURAL STYLE, CORDILLERAN ANALOGS, AND TECTONIC DRIVERS

Tertiary shortening and uplift of the Brooks Range, northern Alaska, 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 youngest phase of Tertiary deformation is represented by 45 Ma and younger basement-involved folds and thrusts beneath the foothills of the northeastern Brooks Range and part of the Arctic coastal plain to the north. These structures are dominantly late Oligocene to early Miocene, and present-day seismicity implies that the system might still be active beneath the 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. Were these three regions of the Cordillera affected by similar tectonic drivers?

The Laramide and Sierras Pampeanas basement-involved foreland thrust provinces are widely recognized to be linked to amagmatic flat-slab subduction. However, the only reasonably well-documented flat-slab episode beneath southern Alaska was in Paleocene to early Eocene time, which corresponds temporally to the development of a fairly conventional thrust system in the central and western Brooks Range, in which basement-involved hinterland structures ramped up-section to the north to form a thin-skinned foreland fold-thrust belt. The Neogene 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). Collisional terrane accretion in southern Alaska provides a possible tectonic driver for this pattern of thrusting. In addition, the present-day plate structure of southern Alaska consists of an abruptly creased (convex-upward) descending slab, which might act as a stress guide, driving crustal seismicity in central and northern Alaska. If this creased plate geometry propagated across southeastern Alaska in Neogene time, it might have driven the foreland basement-involved deformation, 1,000 km north of the subduction zone.