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.

Late Cretaceous to Early Tertiary granitic plutons associated with W skarn or Sn greisen-skarn occur interspersed in a belt 70 × 200-km-long just northeast of Fairbanks, Alaska. All plutons intrude the late Precambrian–early Paleozoic Yukon-Tanana terrane and are similar in major-element compositions (dominantly granodiorite to monzogranite), initial Sr isotopic ratios (0.710 to 0.719), and Pb isotopic signatures ( 206 Pb/ 204 Pb = 19.17 to 19.37). Biotite compositions and opaque mineral abundances indicate both types of plutons crystallized along a buffered path intermediate between nickel–nickel oxide and quartz-magnetite-fayalite. Both suites contain multiple igneous units, with younger, usually equigranular, units spatially related to mineralized zones. Isotopic, trace-element, and mineralogical data suggest an “I-type,” “ilmenite-series” classification for both pluton suites. Because the W and Sn plutons appear to represent magmas with similar origins and source materials, differences in observed metallogeny are thought to be related to differences in environment of crystallization and vapor loss. Such differences include: age (102 to 87 Ma for W plutons, 73 to 50 Ma for Sn plutons), crystallization pressure (1 to 2 kbar for W plutons, <0.5 kbar for Sn plutons), vapor loss history (late for the W plutons and early + late for the Sn plutons), and fluorine trends (decreasing F with increasing differentiation for the W plutons and increasing F for the Sn plutons). Differences in confining pressure (depth) and vapor loss history are associated with differences in age: the younger (Sn) plutons are shallower, and the older (W) plutons are deeper. Trace-element patterns (e.g., Rb, B, Be, W, Sn, Li) are similar for least differentiated units of both pluton types, increasing modestly with increasing differentiation for the W plutons and increasing strongly for the Sn plutons. Data are most compatible with 80 to 95 percent fractionation (crystal-liquid) followed by vapor loss for the W plutons and 80 to 90 percnt fractionation (crystal-liquid) for the Sn plutons, with early vapor loss followed by (liquid-liquid?) “ultrafractionation.” Ultrafractionation and subsequent ore element enrichment occurs in the Sn plutons by early vapor loss and subsequent F enrichment in the residual magma. The data suggest that metallogeny differences for W vs. Sn plutons in our study area are not a function of differences in initial metal contents of the magmas but are more likely due to differences in magmatic evolution.

Luminescence techniques can provide ages for deposits undatable by routine geochronometric techniques (e.g., 14 C, K-Ar, fission track). Two classes of events can be dated by luminescence methods: (I) growth of a mineral or its last cooling, and (II) the last exposure to sunlight. Within the past few years, significant advances in procedures, technology, and understanding of the thermoluminescence (TL) behavior of minerals have been made that place luminescence dating techniques on the verge of widespread application to Quaternary deposits. Most progress has come from studies of known-age material deposited under known conditions. Within class I, both distal and proximal tephra deposits have been dated, using TL techniques originally developed for pottery dating. Within class II, loess, buried soils, and waterlaid silts have been successfully dated. Means have been demonstrated for isolating and controlling several major sources of error, such as the type of TL instability known as anomalous fading, as well as the effects of uncertainty about the degree of zeroing of the luminescence signal in certain depositional environments. In particular, because of different sensitivities to light of the TL of quartz and feldspars, feldspars have been shown to be the preferred component for dating most unheated sediments. Of the competing TL methods for dating the last exposure to sunlight, the partial bleach (R-Gamma or R-Beta) technique, when properly applied, has been shown to yield the best results in general. Nevertheless, in future dating studies of unheated sediments, this laborious method may be displaced by a novel technique that uses laser light, rather than heat, to stimulate the luminescence that is a measure of the past ionizing radiation absorbed dose. This new optical (OSL) method of dating promises to be simple, sensitive, and speedy.

Eclogitic rocks occur in a restricted area, some 13 mi north of Fairbanks, as conformable bands and lenses intercalated with amphibolite, impure marble, and pelitic schist. The structural style of the eclogite-bearing terrane is characterized by northwest-trending, isoclinal recumbent folds that have been deformed by open or overturned folding along northeast-trending axes. Crystalline schist masses south of the eclogite-bearing terrane show only the northeast-trending folds and contain mineral assemblages of the greenschist facies. Mineral assemblages from the crystalline schists, which are intimately associated with the eclogitic rocks, are of the lower amphibolite facies. No basic igneous analogs were found for the eclogitic rocks, and their bulk compositions are very different from those reported for other eclogites. On the ACF plot, the calcite-rich variants appear to have been derived from marls, and the calcite-free varieties are compositionally similar to subgraywackes rather than mafic igneous rocks. The eclogitic rocks and associated amphibolites are characterized by the following essential mineral assemblages: (a) garnet-clinopyroxene (± calcite, quartz, sphene); (b) garnet-clinopyroxene-amphibole (± calcite, quartz, sphene); and (c) garnet-amphibole (± calcite, quartz, plagioclase, epidote, rutile). Mica and plagioclase feldspar also occur in some variants. The garnets in these eclogites are compositionally similar to those from “group C” eclogites as defined by Coleman and others (1965), and many plot within the field defined by the above authors for garnets from eclogites within blueschist terrane. The eclogitic clinopyroxenes are true omphacites, averaging 27 percent of the jadeite component. When shown on White’s (1964) diagram, these clinopyroxenes also plot in a field defined by other clinopyroxenes from group C eclogites. The group C affinity of these eclogitic rocks is also reinforced by the garnet-pyroxene tie-line relations in the MgO-CaO-FeO system, and even more conclusively by K D data on garnet-clinopyroxene pairs. Based on experimental data from various authors, and thermodynamic considerations, these eclogitic mineral assemblages were probably crystallized at temperatures of 540° to 590°C at 5.5 to 7.5 kb. K 40 /Ar 40 mica and hornblende dates indicate that earlier deformation of the eclogite-bearing terrane occurred in Paleozoic time, while the later event was of Cretaceous age.