NEOTECTONICS OF THE ALASKA RANGE FORELAND BASIN, SOUTH-CENTRAL ALASKA

Geomorphologic, stratigraphic, seismological, and palynological data indicate that the Neogene foreland basin of the Alaska Range is an actively deforming landscape and has been since the Miocene. The strata of the Neogene foreland basin consist of the Miocene Usibelli Group (~600m thick), the Pliocene Nenana Gravel (~1000m thick), and unnamed Quaternary deposits. Evidence of active deformation is derived from digital topographic data and Landsat 7 imagery that document the interaction between fluvial systems and basement-cored folds in the foreland basin. For example, in the distal part of the foreland basin, the Goldstream anticline is a plunging anticline whose western nose is buried by modern deposits of the foreland basin. The abrupt deflection of the Tanana River around the nose of this anticline and the documentation of several wind gaps along the western end of the anticline suggest that the Tanana River has had to abandon its channel several times due to recent uplift along this fold. In the proximal part of the foreland basin, similar relationships can be shown between the McKinley River and the Kantishna Hills anticline. Seismicity data from both the Goldstream and Kantishna Hills anticlines also document active deformation.

Evidence for Miocene and Pliocene deformation in the foreland basin comes from stratigraphic data. Documentation of abrupt changes in paleodrainage, local lacustrine deposits, regional changes in stratigraphy, and recycled palynomorphs indicate deformation coeval with sedimentation. Reconstruction of Miocene depositional systems indicates that the foreland basin was filled axially by a west-southwestward prograding deltaic system. The Pliocene foreland basin was characterized by northward prograding, transverse braided streams systems.