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Deformed Tertiary sedimentary rocks and geomorphic features within the northern foothills of the Alaska Range illustrate the development of a fold-and-thrust belt during the past ca. 3 m.y. The northern foothills form a northward-convex salient at the apex of the Alaska Range and the Denali fault. The neotectonic framework of this region has not previously been established despite the proximity of the northern foothills to the Denali fault, several historic large-magnitude earthquakes, and clear topographic evidence of Quaternary deformation. A distinct east-trending topographic grain corresponds with the orientation of folds and faults defined both by bedrock and geomorphic features. To characterize the active structures of the region, we interpreted previous geologic mapping, developed structural cross sections, analyzed topographic and stream profiles, mapped a sequence of Quaternary fluvial terraces, and surveyed several terrace treads. A northward topographic slope across the northern foothills coupled with the pattern of faulting and folding suggests the presence of an orogenic wedge overlying a south-dipping basal detachment. Mapping and surveying of the terraces document continued uplift, folding, and faulting during terrace formation. Geomorphic analyses demonstrate deformation and differential uplift over the entire foothills belt. Together, these data and interpretations indicate the northern foothills are an active fold-and-thrust belt that is propagating northward into the Tanana basin. The identification of this fold-and-thrust belt documents a significant contractional component to the late Cenozoic evolution of the Alaska Range in addition to the obvious strike-slip motion on the Denali fault. Also, the tectonic activity of these structures indicates that this region represents a potential seismic hazard for nearby military facilities and important transportation corridors.

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