Neogene strata of the Tanana basin provide a long-term record of a northward-propagating, transpressional foreland-basin system related to regional shortening of the central Alaska Range and strike-slip displacement on the Denali fault system. These strata are ∼2 km thick and have been deformed and exhumed in thrust faults that form the foothills on the north side of the Alaska Range. The lower part of the sedimentary package, the Usibelli Group, consists of 800 m of mainly Miocene strata that were deposited in fluvial, lacustrine, and peat bog environments of the foredeep depozone of the foreland-basin system. Compositional data from conglomerate and sandstone, as well as recycled Upper Cretaceous palynomorphs, indicate that the Miocene foreland-basin system was supplied increasing amounts of sediment from lithologies currently exposed in thrust sheets located south of the basin. The upper part of the sedimentary package, the Nenana Gravel, consists of 1200 m of mainly Pliocene strata that were deposited in alluvial-fan and braidplain environments in the wedge-top depozone of the foreland-basin system. Compositional data from conglomerate and sandstone, as well as 40 Ar/ 39 Ar dating of detrital feldspars in sandstone and from granitic clasts in conglomerate, indicate that lithologies exposed in the central Alaska Range provided most of the detritus to the Pliocene foreland-basin system. 40 Ar/ 39 Ar dates from detrital feldspar grains also show that two main suites of plutons contributed sediment to the Nenana Gravel. Detrital feldspars with an average age of 56 Ma are interpreted to have been derived from the McKinley sequence of plutons located south of the Denali fault. Detrital feldspars with an average age of 34 Ma are interpreted to have been derived from plutons located north of the Denali fault. Plutons located south of the Denali fault provided detritus for the lower part of the Nenana Gravel, whereas plutons located north of the Denali fault began to contribute sediment during deposition of the upper part of the Nenana Gravel. This age distribution documented in detrital feldspars of the Nenana Gravel is interpreted as representing a progressive northward exhumation of plutons that were located south of the Pliocene Tanana basin. In contrast to previous studies, we interpret the Usibelli Group and Nenana Gravel to represent a continuum in the evolution of a transpressional foreland basin that began during Miocene time on the north side of the Alaska Range.

Geomorphic data from rivers in the Tanana foreland basin and northern foothills of the Alaska Range indicate that this is an actively deforming landscape. The Tanana basin is an alluvial and swampy lowland of ∼22,000 km 2 located in south-central Alaska between the northern flank of the Alaska Range and the Yukon-Tanana uplands. The major axial drainage of the basin is the Tanana river, which is fed by large transverse braided rivers flowing northward out of the Alaska Range. To better define active structures and the neotectonic configuration of the basin, we have constructed a series of longitudinal stream profiles along the major rivers of the Tanana basin. Stream profiles along with changes in channel morphologies delineate four main areas of active deformation. (1) In the western part of the basin, major rivers in the Kantishna Hills area have stream profiles and changes in channel morphologies that indicate that the northeast-trending Kantishna Hills anticlinorium is an active structure. All longitudinal stream profiles in this area exhibit convexity, suggesting tectonic perturbation, as they cross the trend of this 85-km-long structure. In addition, the channel of the McKinley River clearly becomes entrenched as it flows around the southwestern nose of the Kantishna Hills anticlinorium suggesting that the structure may be propagating southwestward. Our geomorphic data from this area are consistent with well-documented seismicity along the southwestern part of the Kantishna Hills. (2) In the central part of the basin, the Nenana River area, changes in channel morphology, stream profile perturbations, and uplifted Pliocene-Pleistocene erosional surfaces coincide with a series of east-trending anticlines. We interpret these folds as part of an active Neogene thrust belt that forms the foothills of the north-central Alaska Range. This active thrust belt is propagating northward and deforming the proximal part of the Tanana foreland basin. North of the topographic front of the foothills, stream profiles indicate active subsidence of the basin. (3) In the eastern part of the Tanana basin, the Delta River area, stream profiles and channel morphologies delineate active deformation along the strike-slip Denali fault and the Granite Mountain/Donnelly Dome thrust fault system. (4) In the northern part of the Tanana basin, the Fairbanks area, stream profiles and channel morphologies delineate northeast-trending active structures that coincide with known seismic zones. These structures are most likely related to block rotation between the Denali and Tintina fault systems along northeast-trending sinistral strike-slip faults. An interesting result of our analysis of the Fairbanks area is the hypothesis that the Tanana River has been forced to abandon its previous channels due to progressive uplift along an active northeast-trending structure. This forced migration has resulted in a series of watergaps, with the modern Tanana River having been deflected around the southwestern culmination of this structure. Interactions between fluvial systems and active structures of the Tanana basin provide a surface record of regional transpressional deformation. This deformation is accommodated by strain partitioning between strike-slip faults like the Denali fault, an active thrust belt along the northern flank of the Alaska Range, and rotation of crustal blocks between the Denali and Tintina fault systems.