Active Structures in the Foothills of the North-Central Alaska Range

The northern foothills of the Alaska Range are situated at the apex of the northward bend of the Denali Fault in central Alaska. Despite the proximity of the northern foothills to the Denali Fault and historic large-magnitude earthquakes, the tectonic framework of this region has not been well-studied. To assess the active structures of the region, I mapped the sequence of Quaternary fluvial terraces, performed differential GPS transects across a number of these terrace treads, analyzed landforms and drainage basins, and developed cross-sections along the Nenana River, which bisects the foothills belt. The mapping and GPS transects show evidence for displacement of terraces by both folds and faults. Geomorphic analyses indicate deformation and uplift over the entire foothills belt. A distinct pattern of east-trending folds and faults exists in both the bedrock and the younger geomorphic features. This corresponds with a northward decrease in relief to the abrupt mountain front that forms the boundary between the foothills and the Tanana basin to the north. These patterns indicate that the northern foothills is an active fold-and-thrust belt that is prograding northward into the Tanana basin. The basal detachment beneath this fold-and-thrust belt likely begins at the Denali fault in the south, and shallows northwards toward the mountain front. The faults that are recognized at the surface are backthrusts from south-dipping splays of the basal detachment. The activity of these structures implies that this region is a likely seismogenic source area with a significant potential impact on military facilities and important transportation corridors that are nearby.