The North Aleutian Basin is a large, Cenozoic sedimentary basin in the northern part of the central Alaska Peninsula and the southern Bering shelf. The gravity anomaly pattern, the geometry, and the structure of the basin suggest that it formed by downward flexure of the backarc lithosphere. Basin modeling suggests that the flexure was driven by the emplacement of surface and subsurface loads having densities comparable to those of oceanic crust and mantle rocks, at approximately the position of the present-day volcanic arc and forearc. We suggest that the inferred loads consist of tectonically thickened mafic crustal materials lying beneath the are and forearc of the central Alaska Peninsula. The crustal thickening may have occurred within a dominantly transpressional regime resulting from oblique convergence between the North American and Pacific plates during the Cenozoic.