Remnants of a Late Triassic continental margin and ocean basin are scattered across central and southern Alaska. Little is known about the fundamental nature of the margin because most remnants have not been studied in detail and a protracted period of terrane accretion and margin-parallel translation has disrupted original stratigraphic and structural relationships. Three new conodont collections were recovered from a sequence of Upper Triassic calcareous sedimentary rocks in the central Alaska Range. One of the three localities is north of the Denali fault system in an area previously thought to be underlain by an uninterrupted sequence of metamorphic rocks of the parautochthonous Yukon-Tanana terrane. Structural relations in the immediate vicinity of this conodont locality indicate that mid-Cretaceous(?) thrust faulting imbricated Paleozoic metaigneous rocks with the Triassic sedimentary rocks. This may reflect a closer pre-Cretaceous relationship between the Yukon-Tanana terrane and Late Triassic shelf and slope deposits than previously appreciated. Reexamination of existing conodont collections from the central Alaska Range indicates that Upper Triassic marine slope and basin rocks range in age from at least as old as the late Carnian to the early middle Norian. The conodont assemblages typical of these rocks are generally cosmopolitan and do not define a distinct paleogeographic faunal realm. One collection, however, contains Epigondolella multidentata sensu Orchard 1991c , which appears to be restricted to western North American autochthonous rocks. Although paleogeographic relations cannot be determined with specificity, the present distribution of biofaces within the Upper Triassic sequence could not have been the result of simple accordion-style collapse of the Late Triassic margin.

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.

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.