Yukon Tanana Research Articles

Thermobarometric constraints on the structural evolution of the Coast Mountains batholith, central southeastern Alaska

Thermobarometric data from amphibolite-facies metamorphic rocks west of the Coast Mountains batholith provide important constraints on the structural evolution of the mid-Cretaceous Sumdum–Fanshaw fault system and Late Cretaceous – Paleocene Le Conte Bay shear zone in central southeastern Alaska. Ductile structures that make up the Sumdum–Fanshaw fault system record the east-directed underthrusting of the Alexander terrane and Gravina belt beneath the Ruth assemblage (Yukon–Tanana terrane) and Taku terrane. These structures are truncated to the east by the Le Conte Bay shear zone. Temperature and pressure estimates calculated from the garnet–biotite geothermometer and garnet–rutile–ilmenite–plagioclase–quartz geobarometer suggest juxtaposition of the Gravina belt and Yukon–Tanana terrane at relatively deep levels (>7 kbar) during mid-Cretaceous time. Rocks west of the Le Conte Bay shear zone yield thermobarometric estimates of 465–890 ± 50 °C and 7.1–11.8 ± 1 kbar (1 kbar = 100 MPa). Late Cretaceous and Paleocene metamorphism associated with the Le Conte Bay shear zone reflects synkinematic emplacement of tonalitic intrusions along the western margin of the Coast Mountains batholith. Thermobarometric results from samples adjacent to the tonalite bodies record uplift and retrogression and suggest tonalite emplacement at 7.5–7.7 ± 1 kbar. An eastward increase in thermobarometric estimates observed in Thomas and Le Conte bays is inferred to record uplift and east-side-up tilting of rocks west of and within the Le Conte Bay shear zone during Late Cretaceous and Paleocene time. Rocks within the Le Conte Bay shear zone were apparently rapidly (1.5–2 mm/a) uplifted to shallow crustal levels prior to mid-Eocene time. Thermobarometric results for the Petersburg region are similar to those previously reported along the western flank of the northern Coast Mountains batholith.

Geology and U–Pb geochronology of the Klondike District, west-central Yukon Territory

Geological mapping and U–Pb geochronology of the Klondike District provide new information on the nature and evolution of the Yukon–Tanana terrane (YTT) in western Yukon. The area is underlain by a sequence of thrust panels of regional extent. A continuously mappable sequence of interlayered metasedimentary and metavolcanic rocks is intruded by a variety of deformed metaplutonic rocks within two of these thrust sheets. Layering in the metasediments and metavolcanics is considered to be at least in part transposed stratigraphy. Small bodies of greenstone and altered ultramafic rocks thought to be part of the Slide Mountain terrane occur discontinuously along the thrust faults.U–Pb age determinations indicate that the uppermost thrust panel (assemblage I), which underlies much of the Klondike District, consists largely of metamorphosed, mid-Permian felsic plutonic, subvolcanic, and tuffaceous rocks. Beneath assemblage I is a second thrust panel (assemblage II), also of large areal extent, of mid-Paleozoic or older metasedimentary and mafic and felsic metavolcanic rocks, intruded by a large body of latest Devonian – Early Mississippian granitic augen orthogneiss. U–Pb analyses of zircon from the orthogneiss reflect both lead loss and a significant inherited zircon component. A third structural unit (assemblage III), which consists mainly of carbonaceous schist and phyllite, crops out in the northern part and along the southwestern edge of the study area, where it underlies both assemblages I and II.The earliest stage of deformation and metamorphism that affected the area (F1) produced the pervasive recrystallization fabric characteristic of all of the metamorphic rocks in assemblages I, II, and III, and occurred between mid-Permian and Late Triassic time. Thrust faulting, presumed to be northerly or northeasterly directed, postdates Late Triassic but predates mid-Cretaceous. The second phase of deformation (F2) was either synchronous with or later than thrust faulting. Monazite ages for the augen orthogneiss indicate that at least local metamorphism and (or) deformation lasted until Early Cretaceous time.Close similarities between composition, U–Pb ages, as well as timing and style of deformation, documented in the Klondike District and observed elsewhere in the YTT in southeastern Yukon and east-central Alaska suggest that much of the YTT either evolved as a single entity or else shared a very similar history.

U–Pb, Rb–Sr, and K–Ar isotopic constraints for ductile deformation and related metamorphism in the Teslin suture zone, Yukon–Tanana terrane, south-central Yukon

The Teslin suture zone (TSZ) comprises a portion of the Yukon–Tanana terrane (YT), in the Yukon, formed by steeply dipping layering and L–S tectonite foliation. The TSZ forms the fundamental tectonic boundary between rocks deposited along the ancient margin of North America and allochthonous terranes to the west. TSZ tectonites evolved during initial penetrative dip-slip deformation (Dds) and later dextral strike-slip shear (Dss) along steep, 1–3 km wide shear zones. Several workers have speculated that the TSZ and related YT heterogeneous ductile deformation and associated metamorphism are Devonian to Mississippian in age and related to the intrusion of a similar-age orthogneiss throughout the Yukon and Alaska. However, recent structural and metamorphic studies of the TSZ provide evidence contradicting this view. New isotopic evidence, presented herein, indicates that TSZ dynamothermal metamorphism was cooled by Early Jurassic time, that it cannot be related to Devonian–Mississippian and Permian granitic intrusion, and that it predates Cretaceous plutonism.U–Pb zircon dating of peraluminous orthogneiss constrains primary peraluminous granite crystallization at 355 ± 25 Ma. Three Rb–Sr whole rock + muscovite and three K–Ar muscovite cooling dates of rocks containing Dds and Dss fabrics place a younger age limit of 182–213 Ma (latest Triassic to Early Jurassic) on Dds/Dss deformation. In addition, three Rb–Sr whole-rock + muscovite isochrons and one K–Ar date on biotite indicate peraluminous orthogneisses in the eastern portion of the study area were affected by a mid-Cretaceous thermal event. These data, togemer with structural and metamorphic relationships reported elsewhere, are summarized in pressure–temperature–time–displacement diagrams illustrating the evolution of TSZ and adjacent rocks from Devonian to Late Cretaceous time. Tectonites within the TSZ can be differentiated from peraluminous orthogneiss east of the d'Abbadie fault on the basis of their respective cooling histories.

A model for terrane accretion: Yukon‐Tanana and Slide Mountain Terranes, northwest North America

To date, many North American Cordilleran geologists employing the concept of suspect terranes have emphasized differences in lithology, metamorphism, styles of deformation, and timing of these “distinct” tectonic units, arguing that each evolved separately from its neighbors. This practice of focusing on dissimilarity, and nomenclature used in terrane analysis, may be detrimental to our understanding of the evolutionary relationships between terranes. I present a model in which the Yukon‐Tanana (YT) and Slide Mountain (SM) terranes, which do exhibit certain lithotectonic disparities, are correlated within a unifying tectonic framework in which each terrane represents a different crustal level of the same late Paleozoic‐early Mesozoic magmatic arc and west dipping subduction complex formed during plate convergence outboard of western North America. The SM represents the upper crustal level of the arc and accretionary complex, and the heterogeneously metamorphosed and ductilely deformed YT represents the deeper crustal level. The Teslin suture zone (TSZ), the high‐P/T portion of the YT, formed the décollement between the YT‐SM composite terrane hanging wall and the descending North American plate; TSZ structural relations thus provide insight into the kinematic interaction of this composite terrane with North America prior to its late‐stage emplacement onto North America. The present‐day “collage” of suspect terranes clearly comprises a multitude of structurally dismembered and dissimilar lithotectonic assemblages. Nonetheless, it is possible to see order in the chaos, to reassemble the pieces in a tectonically meaningful fashion. The YT‐TSZ‐SM association in Yukon may serve as a model for tectonically coherent continental margin expansion.

2.2 Ga age of zircons in three occurrences of Upper Proterozoic clastic rocks of the northern Cassiar terrane, Yukon and British Columbia

Analyses of detrital zircons from three occurrences of Upper Proterozoic clastic rocks of the northern Cassiar terrane in Yukon and northern British Columbia yield a U–Pb age of 2224 ± 22 Ma. The zircons apparently belong to a single population similar in age to zircons in stratigraphically equivalent rocks of the southern Cassiar terrane and to zircons in rocks in the Yukon–Tanana terrane of Alaska. A source terrane or area of the required age and extent has not yet been identified.

Blueschist and eclogite in mylonitic allochthons, Ross River and Watson Lake areas, southeastern Yukon

Two new occurrences of eclogite associated with mylonitic rocks are documented. The first, near Ross River, is in a quartz-rich metasedimentary host that includes glaucophane schist. The host forms part of a belt of high-pressure rocks more than 50 km long and several kilometres wide between Ross River and Faro. In the second occurrence, north of Watson Lake, eclogite is associated with mafic and ultramafic rocks occurring in a klippe more than 100 km2 in area that rests on unmetamorphosed Triassic rocks of the North American miogeocline. Geothermobarometry shows that the eclogites were metamorphosed at 10–15 kb (1 kb = 100 MPa), between 470 and 750 °C. These peak conditions are comparable to those for other eclogites in the Yukon and adjacent Alaska. The age of metamorphism and mylonitization is Triassic or older.The high-pressure rocks occur at the leading edge of the most inboard accreted terrane, along its boundary with North American miogeoclinal strata, over a length of several hundred kilometres. Their extensive distribution makes these rocks regionally mappable units. This supports other evidence that both the Yukon–Tanana terrane and mylonite sheets obducted on North American foreland rocks are composed in part of trench and subduction-zone material.

Accretion tectonics and crustal structure in Alaska

The entire width of the North American Cordillera in Alaska is made up of “suspect terranes”. Pre-Late Cretaceous paleogeography is poorly constrained and the ultimate origins of the many fragments which make up the state are unclear. The Prince William and Chugach terranes accreted since Late Cretaceous time and represent the collapse of much of the northeast Pacific Ocean swept into what today is southern Alaska. Greater Wrangellia, a composite terrane now dispersed into fragments scattered from Idaho to southern Alaska, apparently accreted into Alaska in Late Cretaceous time crushing an enormous deep-marine flysch basin on its inboard side. Most of interior eastern Alaska is the Yukon Tanana terrane, a very large entirely fault-bounded metamorphic-plutonic assemblage covering thousands of square kilometers in Canada as well as Alaska. The original stratigraphy and relationship to North America of the Yukon-Tanana terrane are both obscure. A collapsed Mesozoic flysch basin, similar to the one inboard of Wrangellia, lies along the northern margin. Much of Arctic Alaska was apparently a vast expanse of upper Paleozoic to Early Mesozoic deep marine sediments and mafic volcanic and plutonic rocks now scattered widely as large telescoped sheets and Klippen thrust over the Ruby geanticline and the Brooks Range, and probably underlying the Yukon-Koyukuk basin and the Yukon flats. The Brooks Range itself is a stack of north vergent nappes, the telescoping of which began in Early Cretaceous time. Despite compelling evidence for thousands of kilometers of relative displacement between the accreted terranes, and large amounts of telescoping, translation, and rotation since accretion, the resulting new continental crust added to North America in Alaska carries few obvious signatures that allow application of currently popular simple plate tectonic models. Intraplate telescoping and strike-slip translations, delamination at mid-crustal levels, and large-scale lithospheric wedging were important processes in northern Cordilleran tectonic evolution.

Soil movements on permafrost slopes near Fairbanks, Alaska

Many north-facing slopes in the Yukon–Tanana Uplands of Alaska show signs of downhill movements in the form of hummocky surface and leaning trees. Measurements of movements and pore pressures were made at several sites in the Caribou–Poker Creek Research Watershed. It was found that, on slopes with inclinations near 30°, most of the movements occurred immediately after thaw when pore pressures were high.To evaluate the slope stability, the soil strength was measured by direct shear tests. The strength of the moss layer and tree roots was evaluated by performing tension tests on the roots and the moss–root complex. Results of stability analyses show that local failures involving individual wedges are likely. The displacements associated with a wedge slide would result in a hummock or step. The strength of the moss–root complex was found to be a significant factor in the stability of the wedge and of the step. Key words: moss, roots, permafrost, pore pressure, shear strength, slope stability, thaw.

Active faults of Alaska

Geologically young displacements have been observed along 24 faults in an area of Alaska of approximately 624,000 km 2. Active faults of southern Alaska include the Patton Bay and Hanning Bay reverse-slip faults, both reactivated in 1964. The Fairweather strike-slip fault experienced surface faulting in 1958 and possibly in 1899. Three normal-slip faults trending northerly were not reactivated in 1964; these are the Johnstone Bay fault, Ragged Mountain fault, and the Long Glacier fault. The Castle Mountain fault trends northeasterly and is marked by a low, south-facing escarpment. Active faults in the Alaska Range and adjacent areas include those of the Denali fault system, including the Totschunda fault and the McGinnis Glacier fault; inactive faults include the Hines Creek fault. Other active faults include the Donnelly Dome fault, the Granite Mountain fault, and the Healy Creek fault. The north front of the Alaska Range west of Donnelly Dome is a monocline with few associated surface faults. The Clearwater Lake fault, located along the Tanana River east of Delta Junction, appears to be active, although an origin of landforms due to faulting cannot be proven. Active faults in the Tanana—Kuskokwim Lowland and Yukon—Tanana Upland include the Minto, Shaw Creek, Champion Creek, and Tintina faults. The active Kaltag fault, located southwest of Tanana, does not appear to connect by means of an active trace with the Tintina fault, which is 300 km to the east. Active faults of the Kokrine—Hodzana Highlands include the Kanuti and Dall Mountain faults. Active faults west of the Kanuti include the Huslia and a fault along Shoestring Sand Dune, both in the Koyukuk Flats. The active Kobuk—Alatna Hills fault trends westerly south of the Brooks Range. No active faults were observed in the central and eastern Brooks Range, Arctic Foothills, or Arctic Coastal Plain in the area of study.

The Role of Transportation in the Fairbanks Area

The Yukon River with its numerous affluents dominates the great central plain of Alaska. The valley of the Yukon provides ready access to a territory which stretches from the Brooks Range on the north to the Alaska mountain system on the south, and penetrates southeastward across the border in a great crescent deep into the Canadian Klondike. This master river, 2,300 miles in length, is also the central transport artery of the interior. Of its many affluents, none is more centrally located than the Tanana-Chena, where Fairbanks has become the recognized trade and transportation hub. The city proper is located on the banks of the Chena River a few miles from its confluence with the Tanana, the urban area including a small por tion of the southern fringe of the Yukon Tanana Upland with its gold bearing gravels. Here, placer mining was responsible for the founding and early growth of Fairbanks. A spur of the upland extends to the bank of the Tanana some eight miles west of Fairbanks; known as Chena Ridge, this spur marks the westernmost spread of settlement along the floodplain. The right bank floodplain of the Tanana is relatively narrow because the stream has been deflected by the heavy deposition of tributary glacial streams from the Alaska Range to the south. Coriolis force, deflecting moving masses to the right in the northern hemisphere, also has contributed to this phenomenon. Settlement is concentrated on the floodplain and the south facing slope of the adjacent upland. South of the river, the plain has never been settled. A military reservation, Ladd Air Force Base Bombing and Gunnery Range, is now located on the plain and, together with the difficulties and expense associated with bridging the river, will negate civilian settlement in this direction in the foreseeable future.