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.

ABSTRACT Asymmetric anticlines with steeptooverturned forelimbs areacommon element of fold-and-thrust belts, but typically their origin must be interpreted on the basis of incomplete knowledge of their geometry and kinematics. Many such folds are interpreted to be fault-propagation folds, but their known characteristics fit as well or better with interpretation as a detachment fold or a thrust-truncated example of either fold type. A fault-propagation fold forms by propagation of a ramp tip, so a ramp on which displacement decreases upward to a tip is consistent with this fold type. Fault-propagation fold models generally assume that hinges migrate with respect to the rock, especially in synclines, and that limbs, especially backlimbs, do not rotate with fold growth. A detachment fold forms above a décollement that may have a fixed tip or a propagating tip or that may extend beyond the limits of the fold. Nonparallel thickening in the anticlinal core and lack of a ramp are characteristic of a detachment fold. Detachment-fold models assume either fixed or migrating hinges and either fixed or rotating limbs, although rotating limbs and at least a fixed anticlinal hinge seem best supported by natural examples. Truncation and displacement of a preexisting fold by a thrust fault modifies fold geometry and makes it more difficult to determine a fold’s origin. A ramp results from truncationof the forelimbof an existing anticline, so a ramp does not, in itself, rule out a detachment-fold origin. An anticlinal forelimb in the hanging wall may be steepened either by displacement over a convex-upward bend in the underlying thrust or by the thrust being folded into an antiform. Thrust truncation of an existing anticlinal fore-limb may result in a footwall syncline, but most fault-propagation fold models require either an abandoned ramp tip or significant strain within the forelimb to account for a footwall syncline. Origin as a detachment fold is possible if a footwall syncline is present, especially if an abandoned ramp tip is absent, or if the anticlinal core is internally thickened. The structural style and mechanical stratigraphy of a region may provide additional useful information for determining fold origin. Knowledge about a specific fold may be insufficient to determine its origin, but other, less ambiguous examples in an area may indicate the most likely possibilities to consider. Detachment folds are likely where a competent unit overlies a much less competent unit, and fault-propagationfoldsmaybe more likely in evenly layered rocks that have relatively high competency but weak layer interfaces. The originof asymmetric map-scale folds in the northeastern Brooks Range of Alaska is difficult to determine on the basis of their geometry alone, especially because most of them have been truncated by thrust faults. The presence of thickened anticlinal cores, the absence of ramp tips, the presence of remnant uncut detachment folds, a mechanical stratigraphy characterized by a competent unit over an incompetent unit, and a transition from unbroken detachment folds to thrust-truncated asymmetric folds, together suggest that the thrust-truncated folds originated mainly as detachment folds rather than as fault-propagation folds.

Abstract This chapter describes the geology of northern Alaska, the largest geologic region of the state of Alaska. Lying entirely north of the Arctic Circle, this region covers an area of almost 400,000 km 2 and includes all or part of 36 1:250,000 scale quadrangles (Fig. 1). Northern Alaska is bordered to the west and north by the Chukchi and Beaufort seas, to the east by the Canadian border, and to the south by the Yukon Flats and Koyukuk basin. Geologically, it is notable because it encompasses the most extensive area of coherent stratigraphy in the state, and it contains the Brooks Range, the structural continuation in Alaska of the Rocky Mountain system. Northern Alaska also contains the largest oil field in North America at Prudhoe Bay, the world's second-largest zinclead- silver deposit (Red Dog), important copper-zinc resources, and about one-third of the potential coal resources of the United States (Kirschner, this volume; Magoon, this volume; Nokleberg and others, this volume, Chapter 10; Wahrhaftig and others, this volume).

Abstract Southwest Alaska lies between the Yukon-Koyukuk province to the north, and the Alaska Peninsula to the south (Wahrhaftig, this volume). It includes the southwestern Alaska Range, the Kuskokwim Mountains, the Ahklun Mountains, the Bristol Bay Lowland, and the Minchumina and Holitna basins. It is an area of approximately 175,000 km 2 , and, with the exception of the rugged southwestern Alaska Range and Ahklun Mountains, consists mostly of low rolling hills. The oldest rocks in the region are metamorphic rocks with Early Proterozoic protolith ages that occur as isolated exposures in the central Kuskokwim Mountains, and in fault contact with Mesozoic accretionary rocks of the Bristol Bay region. Precambrian metamorphic rocks also occur in the northern Kuskokwim Mountains and serve as depositional basement for Paleozoic shelf deposits. A nearly continuous sequence of Paleozoic continental margin rocks underlies much of the southwestern Alaska Range and northern Kuskokwim Mountains. The most extensive unit in southwest Alaska is the predominantly Upper Cretaceous Kuskokwim Group, which, in large part, rests unconformably on older rocks of the region. Volcanic rocks of Mesozoic age are common in the Bristol Bay region, and volcanic and plutonic rocks of latest Cretaceous and earliest Tertiary age are common throughout southwest Alaska. Two major northeast-trending faults are known to traverse southwest Alaska, the Denali-Farewell fault system to the south, and the Iditarod-Nixon Fork fault to the north. Latest Cretaceous and Tertiary right-lateral offsets of less than 150 km characterize both faults. The Susulatna lineament (or Poorman fault), north of the Iditarod-Nixon Fork