Yukon-Tanana Terrane Research Articles

Kilometre-scale folding in the Teslin zone, northern Canadian Cordillera, and its tectonic implications for the accretion of the Yukon-Tanana terrane to North America

Kilometre-scale folding in the Teslin zone, northern Canadian Cordillera, and its tectonic implications for the accretion of the Yukon-Tanana terrane to North America

Paleozoic and Mesozoic high-pressure metamorphism at the margin of ancestral North America in central Yukon

Eclogite and blueschist in the Yukon-Tanana and Slide Mountain terranes in the Yukon and Alaska preserve a partial record of middle Paleozoic, late Paleozoic, and early Mesozoic subduction and subsequent unroofing. Near Faro, eclogites are commonly mylonitized and partially retrogressed. The analysis of Fe-Mg exchange between garnet and clinopyroxene suggests temperatures ( T ) in the 400–500 °C range for eclogite, and jadeite (∼40 mol%) + quartz suggest minimum pressures ( P ) of 11–13 kilobar at that temperature range; clinopyroxene-garnet-epidote-quartz suggest pressure near 15 kilobar ( P H 2 O = P S ). Eclogite near Last Peak, which contains brown hornblende, late-stage oligoclase, and biotite, yielded P-T estimates up to 625 °C at a minimum pressure of 14 kilobar. Eclogite near Ross River yielded temperatures near 525 °C and a minimum pressure near 13 kilobar. The previously proposed Permian-Triassic age of high-pressure tectonism for the entire Yukon-Tanana terrane in the Yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities. In this study, isotopic 40 Ar/ 39 Ar analysis of white mica from eclogite in the Simpson range yielded a date of 344 ± 1 Ma. Eclogite and blueschist from Ross River yielded white mica 40 Ar/ 39 Ar dates of 267 ± 3 Ma and 273 ± 3 Ma, respectively. White mica in eclogite from Faro was dated at 260 ± 3 Ma. White mica in two eclogites from Stewart Lake, about 2 km apart, yielded 40 Ar/ 39 Ar dates of 228 ± 1 Ma and 346 ± 3 Ma. White mica in Last Peak eclogite, for which a concordant U-Pb zircon date is 269 Ma, yielded an integrated date of 236 ± 1 Ma. Overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others. The cooling ages indicate significant plate convergence as early as Carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone. When this is considered together with the range of ages of other high-pressure occurrences, the North American Cordillera is seen to be characterized by multiepisodic and diachronous high-pressure metamorphism.

Detrital zircon geochronology and regional correlation of metasedimentary rocks in the Coast Mountains, southeastern Alaska

U-Pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the Coast Mountains in southeastern Alaska. These rocks belong to the Port Houghton assemblage, which consists of upper Paleozoic pelitic and psammitic schist, metaconglomerate, metabasalt, and marble. The Port Houghton assemblage rests unconformably(?) on metamorphosed and deformed mid-Paleozoic arc-type volcanics (Endicott Arm assemblage), which gradationally overlie upper Proterozoic(?) - lower Paleozoic continental margin strata (Tracy Arm assemblage). Three main clusters of ages are present: 330-365 Ma (19 grains), 1710-2000 Ma (27 grains), and 2450-2680 Ma (6 grains). Additional grains are approximately 2334, 2364, and 3324 Ma. Comparison of these ages with detrital zircon ages in other Cordilleran assemblages supports previous interpretations that metasedimentary rocks in the Coast Mountains (i) form a southwestern continuation of the Yukon-Tanana terrane of eastern Alaska and Yukon, (ii) are not correlative with strata of the Alexander terrane, and (iii) contain detritus that was probably shed from cratonal rocks in the Canadian Shield to the east. Several scenarios exist to explain the occurrence of these continental margin rocks west (outboard) of arc-type and ocean-floor assemblages such as the Stikine, Cache Creek, Quesnel, and Slide Mountain terranes.

Structural and kinematic evolution of the Yukon-Tanana upland tectonites, east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly

The Yukon-Tanana terrane, the largest tectonostratigraphic terrane in the northern North American Cordillera, is polygenetic and not a single terrane. Lineated and foliated (L-S) tectonites, which characterize the Yukon- Tanana terrane, record multiple deformations and formed at different times. We document the polyphase history recorded by L-S tectonites within the Yukon-Tanana upland, east-central Alaska. These upland tectonites compose a heterogeneous assemblage of deformed igneous and metamorphic rocks that form the Alaskan part of what has been called the Yukon-Tanana composite terrane. We build on previous kinematic data and establish the three-dimensional architecture of the upland tectonites through kinematic and structural analysis of more than 250 oriented samples, including quartz c-axis fabric analysis of 39 samples. Through this study we distinguish allochthonous tectonites from parautochthonous tectonites within the Yukon-Tanana upland. The upland tectonites define a regionally coherent stacking order: from bottom to top, they are lower plate North American parautochthonous attenuated continental margin; continentally derived marginal-basin strata; and upper plate ocean-basin and island-arc rocks, including some continental basement rocks. We delineate three major deformation events in time, space, and structural level across the upland from the United States–Canada border to Fairbanks, Alaska: (1) pre-Early Jurassic (>212 Ma) northeast-directed, apparent margin-normal contraction that affected oceanic rocks; (2) late Early to early Middle Jurassic (>188–185 Ma) northwest-directed, apparent margin-parallel contraction and imbrication that resulted in juxtaposition of the allochthonous tectonites with parautochthonous continental rocks; and (3) Early Cretaceous (135–110 Ma) southeast-directed crustal extension that resulted in exposure of the structurally deepest, parautochthonous continental rocks. The oldest event represents deformation within a west-dipping (present coordinates) Permian-Triassic subduction zone. The second event records Early to Middle Jurassic collision of the arc and subduction complex with North American crust, and the third event reflects mid-Cretaceous southeast-directed crustal extension. Events one and two can be recognized and correlated through southern Yukon, even though this region was affected by mid-Cretaceous dextral shear along steep northwest-striking faults. Our data support a model of crustal assembly originally proposed by D. Tempelman-Kluit in which previously deformed allochthonous rocks were thrust over parautochthonous rocks of the attenuated North American margin in Middle Jurassic time. Approximately 50 m.y. after tectonic accretion, east-central Alaska was dissected by crustal extension, exposing overthrust parautochthonous strata.

The Slide Mountain Terrane and the structural evolution of the Finlayson Lake Fault Zone, southeastern Yukon

The Finlayson Lake Fault Zone forms a fundamental, but little studied, tectonic boundary between strata of autochthonous North America and the accreted Slide Mountain and Yukon–Tanana terranes in southeastern Yukon. A structural and petrologic study was undertaken to examine the depositional environment of the Slide Mountain Terrane, its tectono-thermal evolution in the fault zone, and its relationship with the Yukon–Tanana Terrane. The Slide Mountain and Yukon–Tanana terranes are divisible into units dominated by metavolcanic and metasedimentary rocks. Field observations and whole-rock geochemistry indicate that Slide Mountain greenstone is ocean-floor basalt deposited in a deep submarine basin with a proximal terrigenous sediment influx. Either a marginal- or ocean-basin setting is supported by the data. Slide Mountain greenstone is thrust northeastward over metasedimentary rocks of Slide Mountain Terrane and southwestward over rocks of the Yukon–Tanana Terrane. Regional metamorphic grade ranges from subgreenschist to greenschist facies. Pressure–temperature estimates for the subgreenschist–greenschist facies transition are 270–310 °C and 2.1–3.6 kbar (1 kbar = 100 MPa), based on assumed geothermal gradients and the reaction isograd Pmp + Chl = Act + Ep + H2O. Metamorphic peak postdates motion along the westernmost reverse fault that juxtaposes the Slide Mountain and Yukon–Tanana terranes. We interpret the Finlayson Lake Fault Zone as a northeasterly directed thrust sequence disrupted by synmetamorphic back thrusts. The back thrusting may be the consequence of shortening in the upper crust, or larger scale processes such as "tectonic wedging" of Yukon–Tanana Terrane under Slide Mountain Terrane.

Petrofacies and Provenance of a Late Cretaceous Suture Zone Thrust-Top Basin, Cantwell Basin, Central Alaska Range

ABSTRACT The lower Cantwell Formation, in the Cantwell basin, provides an excellent opportunity to study the petrofacies and provenance of siliciclastic deposits from an ancient thrust-top basin. The Late Cretaceous Cantwell basin formed on a series of thrust sheets along a suture zone between the allochthonous Wrangellia composite terrane and the North American continental margin (i.e., the Yukon-Tanana terrane). Analysis of the lower Cantwell Formation provides an understanding of sediment transport in the interior of an ancient suture zone. Sandstone and conglomerate compositional data indicate two main source areas for the lower Cantwell Formation: (1) uplifted thrust sheets, located south of the basin, associated with development of the suture zone, and (2) a regional anticlinorium, locat d north of the basin, along the North American continental margin. In the northwestern Cantwell basin, petrofacies, southward paleocurrent directions, and alluvial lithofacies record reactivation of a regional anticlinorium located in the metamorphic Yukon-Tanana terrane. Lithic types in this part of the basin are dominated by quartz-mica tectonite, mica schist, metamorphic chert, and other quartz-rich metamorphic and sedimentary varieties (Q48F2L50; Lv0Lm54Ls46). In the northeastern and southern parts of the basin, sediment was derived from volcanic and sedimentary strata exposed in the hanging walls of thrust faults that define the southern margin of the basin. Source-diagnostic conglomerate clasts, spatial variations in sandstone detrital modes, northward paleocurrent indicators, and progressively rotated strata record syndepositional uplift of local thrust sheets along the suture zone. The dominant sandstone grain types in these parts of the basin are plagioclase grains and lithic fragments of argillite, radiolarian-bearing mudstone, and metabasalt (Q44F28L28; Lv2Lm23Ls75). Sandstone compositions documented for the Cantwell basin may be of thrust-top basins that form along suture zones. The two main petrofacies of the lower Cantwell Formation consist of a petrofacies rich in sedimentary and volcanic detritus associated with uplifted oceanic rocks along areas of the basin proximal to the suture zone, and a petrofacies characterized by quartz-rich metamorphic and sedimentary detritus along areas of the basin proximal to the deformed continental margin. Compositional data from this study, combined with previous petrologic studies of similar basins, show that sandstones from suture zone thrust-top basins are more feldspathic than sandstones from typical foreland basins. These compositional differences reflect a complex assemblage of accreted terran s, which include relict volcanic arcs, batholiths, oceanic sedimentary deposits, and ophiolitic melanges, that are accreted to continental margins along suture zones in collisional-accretionary orogenic belts.

Mississippian assembly of the Nisutlin assemblage: evidence from primary contact relationships and Mississippian magmatism in the Teslin tectonic zone, part of the Yukon–Tanana terrane of south-central Yukon

Metamorphosed and ductilely deformed sedimentary, plutonic, and volcanic rocks of the Nisutlin and Anvil assemblages make up the Yukon–Tanana terrane in the Teslin tectonic zone study area. The Nisutlin assemblage consists of siliceous schist–quartzite and graphitic phyllite that share a primary depositional contact, and Early Mississippian tonalite to quartz diorite that intrudes the siliceous schist–quartzite and possibly the graphitic phyllite. The Anvil assemblage includes metagabbro and mafic schist–greenstone that share an intrusive contact relationship. Tonalite to quartz diorite of the Nisutlin assemblage is characterized by minor zircon inheritance with an average Proterozoic age, εNd(350 Ma) values of −2.5 to −6.2, and Nd model ages of 1.50–1.79 Ga. These data suggest that the magmatic bodies have inherited a component of continentally derived material. Primary contact relationships and age data indicate that the Nisutlin assemblage had formed by Mississippian time, and regional correlations show that this assemblage makes up a large part of the Yukon–Tanana terrane of southern Yukon. Assembly of the Nisutlin assemblage by Mississippian time indicates that it did not form as a late Paleozoic and early Mesozoic subduction melange, and it suggests that its tectonic fabrics did not result from the progressive growth of a Permo-Triassic subduction complex. We suggest that the Nisutlin assemblage was part of a crustal block that lay outboard of North America in Mississippian time, and that it lay in the hanging-wall plate of a Permo-Triassic subduction zone as a relatively coherent assemblage, rather than forming within the zone as a subduction complex.

New geophysical data from the northern Cordillera: preliminary interpretations and implications for the tectonics and deep geology

In this paper we analyze recently acquired geophysical data from the northern Cordillera and their relation to the mapped geology. A prominent gravity high (> −45 mGal (1 Gal = 1 cm/s2)) coincides with a magnetic low and an aseismic region in west-central Yukon where the underlying geology is dominated by quartzo-feldspathic rocks having moderate densities. Extension (~15%), magmatic underplating, and accretion of the anomalous region onto oceanic crust are three possible explanations.Magnetic, gravity, and seismicity data all show significant differences in the physical state of the crust on either side of the Tintina Fault and, together with geological data indicating large offset, suggest it was once a major crustal-scale strike-slip fault. The new gravity data also delineate an arcuate zone of steep gradients (up to 1.4 mGal/km) in the miogeocline, which may correlate with a west-dipping Proterozoic basement ramp mapped on deep seismic sections farther to the north and a transition from thin (east) to thick sediment cover (west). Seismicity data show that current tectonic activity is concentrated along the Pacific – North America plate margin in southwestern Yukon and adjacent Alaska and, although there is a marked decrease in activity inland of this margin, notable concentrations occur along the Denali Fault System and in the eastern miogeocline. There is a distinct absence of earthquakes in parts of the Selwyn Basin and in the northern Yukon–Tanana Terrane. Limited field studies suggest activity is confined to the upper 10–15 km of the crust.

Lower-crustal deformation during terrane dispersion along strike-slip faults

After accretion, terranes caught between obliquely converging plates are often displaced several hundreds of kilometers. How this motion is accommodated at depth is not well known. Spurred by our Trans-Alaskan Crustal Transect (TACT) seismic studies of the Yukon-Tanana terrane, Alaska, I propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin-parallel translation. The locations of these ductile regions are dependent on temperature, composition and strain rate. For geotherms based on moderate surface heat flow (60–90 mWm −2) and reasonable geological strain rates (100% in 3 Ma), typical lower-crustal rocks can deform ductilely. The resultant subhorizontal layered fabric should be detectable with seismic techniques. Two terranes that have experienced hundreds of kilometers of margin-parallel translation, the Yukon-Tanana terrane sampled by our seismic refraction/wide-angle reflection profiles and the Salinian block (California) sampled by both seismic refraction and reflection, have reflective lower crust. Both of these terranes have moderate to high surface heat flows, have relatively thin crust, and are interpreted seismically to be underlain by a quartzo-feldspathic lower crust; conditions favorable for ductile deformation in the lower crust. Interpretations of lower-crustal layering for these two terranes is based on an increase in lower-crustal reflectivity. This layering is attributed to tectonic banding consistent with ductile deformation.

Crustal velocity structure of the northern Yukon-Tanana upland, central Alaska: Results from TACT refraction/wide-angle reflection data

The Fairbanks North seismic refraction/ wide-angle reflection profile, collected by the U.S. Geological Survey Trans-Alaska Crustal Transect (TACT) project in 1987, crosses the complex region between the Yukon-Tanana and Ruby terranes in interior Alaska. This region is occupied by numerous small terranes elongated in a northeast-southwest direction. These seismic data reveal a crustal velocity structure that is divided into three upper-crustal and at least two middle- to lower-crustal domains. The upper-crustal domains are delineated by two steeply dipping low-velocity anomalies that are interpreted as signatures of the Victoria Creek fault, and the Beaver Creek fault or a fault buried by the Beaver Creek fault. This tripartite upper crust extends to 8-10 km depth where a subhorizontal interface undercuts the northern and central domains. Beneath the northern domain, this interface is interpreted as the southeastwardly dipping boundary between the Tozina and Ruby terranes. The continuation of this interface beneath the central domain suggests that it may represent the detachment or basal thrust for thin-skinned tectonic amalgamation of the terranes caught between the Yukon-Tanana and Ruby terranes. The lower crust and Moho reflection exhibit differences from north to south that define at least two lower-crustal domains, interpreted as the Yukon-Tanana and Ruby terranes. Finally, the crustal thickness along the profile is nearly uniform and ranges from 31 to 34 km. Our data suggest that after initial thin-skinned amalgamation of the various terranes, this region experienced thick-skinned tectonic reorganization via strike-slip faulting. This interpretation supports a model in which at least one strand of the Tintina fault exists in this important region of Alaska.

Nd- and Sr-Isotope Evidence for Proterozoic and Paleozoic Crustal Evolution in the Brooks Range, Northern Alaska

Exposures of Proterozoic crust in Alaska are rare; however, isotopic data from younger rocks derived from unexposed basement suggest that Proterozoic (or older) crust may underlie large areas of northern Alaska. Late Proterozoic orthogneisses, Late Proterozoic to Cambrian metasedimentary rocks, and Devonian granitic orthogneisses in the western Brooks Range have Nd-and Sr-isotope compositions consistent with derivation, at least in part, from Middle to Early Proterozoic crust. Proterozoic metasedimentary rocks have calculated Nd crustal residence ages of 2.0 Ga and are intruded by Late Proterozoic metagranites with somewhat more radiogenic initial isotopic compositions than the metasedimentary rocks at the time of intrusion. Initial isotopic compositions of Devonian metagranitic rocks range from . The least radiogenic values are compatible with nearly pure crustal melts of a source with isotopic composition similar to what exposed Proterozoic crust, as sampled by the metasedimentary rocks and Late Proterozoic metagranites, would have had at 370 Ma. The range of observed isotopic compositions may be explained by mixing mantle-derived magmas, or melts of juvenile crust, with melts of Proterozoic crust in varying proportions. Nd isotope systematics of the Late Proterozoic metasedimentary rocks are identical to reported values from Early Proterozoic metaplutonic rocks in the Idono Complex and Kilbuck terrane in SW Alaska, and to clastic sedimentary rocks from the Belt Supergroup in Montana. Isotopic compositions of sampled metagranitic and metasedimentary rocks are not compatible with derivation exclusively from crust with the age and isotope characteristics of the Yukon-Tanana terrane. Very limited isotopic data from other continental domains within North America leave open the possibility of affinity with crust exposed in NW Canada, the Canadian Arctic Islands, or the NW Canadian Cordillera.

A fluid inclusion and stable isotope study of Au quartz veins in the Klondike District, Yukon Territory, Canada; a section through a mesothermal vein system

Gold-bearing quartz veins in the Klondike district of Yukon Territory are of earliest Cretaceous age and are hosted by Permian, middle to upper greenschist facies, metasedimentary and meta-igneous lithologies belonging to the Yukon-Tanana terrane. The veins occur in brittle, extensional structures, which may have developed during extensional deformation during uplift of the Klondike region in the Late Jurassic to Early to middle Cretaceous, following thrust imbrication in the Early Jurassic. The quartz veins are massive and unbanded, with localized carbonate flooding and/or pyritization of the wall rocks.Fluid inclusion studies have delineated significant, regional P-T-X trends in primary inclusion fluids from the Au quartz veins. The fluids have low salinities (<6 wt % NaCl equiv), but variable CO 2 concentrations, decreasing from approximately 0.13 to 0.18 mole fraction CO 2 in the southeastern Klondike, through variable concentrations in the central Klondike (X (sub CO 2 ) < 0.01 to [asymp] 1.00), to dominantly aqueous fluids (X (sub CO 2 ) < 0.01) in veins from the northwest portion of the region. Estimated trapping pressures decrease from 2,300 + or - 700 bars to between 300 to 900 bars, from southeast to northwest. Estimated fluid temperatures range from 300 degrees to 350 degrees C in central and southern lodes, to 200 degrees to 250 degrees C in the northwest Klondike. The P-X trends are interpreted as evidence for (1) effervescence of CO 2 from the mineralizing fluid, most likely in response to regional uplift during the life of the hydrothermal system, and (2) that the Au quartz vein system represents a section through a mesothermal vein system with deeper structural levels exposed in the southeastern Klondike. Convection of the hydrothermal fluid may have been driven by an elevated geothermal gradient resulting from the rapid uplift.The calculated isotopic composition of the Au quartz vein fluids (delta 18 O fluid 8.3 + or - 1.2ppmSMOW, delta D fluid [asymp] -150 + or - 18ppmSMOW) indicates that the mineralizing fluid was meteoric in origin but underwent considerable isotopic evolution prior to mineralization. Additionally, the delta D fluid values of extracted inclusion fluids (-166 + or - 11ppm) and from hydrothermal muscovites (-174 + or - 3ppm) from the Sheba vein system are in good agreement and indicate that the deuterium signature reflects that of the primary mesothermal fluid.

Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana Terrane, eastern Alaska Range, Alaska

A belt of volcanogenic massive sulfide deposits extends for over 150 km along the southern margin of the Yukon-Tanana terrane of the eastern Alaska Range. Located north of the Denali fault, the Yukon-Tanana terrane forms a major basement unit in east-central Alaska. The volcanogenic massive sulfide deposits are primarily in the Jarvis Creek Glacier subterrane, one of the highest structural-stratigraphic levels of the Yukon-Tanana terrane. The Jarvis Creek Glacier subterrane consists of a volcanogenic massive sulfide-bearing metavolcanic rock member and a metasedimentary rock member. The volcanogenic massive sulfide deposits and enclosing metavolcanic and metasedimentary schists exhibit two periods of regional metamorphism and penetrative deformation: an older, Early Cretaceous, amphibolite facies event and a younger, mid-Cretaceous lower greenschist facies event.From northwest to southeast, the major volcanogenic massive sulfide districts are the Bonnifield, Trident Glacier, and Delta. The Bonnifield district contains massive sulfide deposits in metavolcanic and metasedimentary rocks that are correlated with the Jarvis Creek Glacier subterrane. The 25-km-long, northwest-trending Trident Glacier district contains several occurrences consisting of massive pyrrhotite and lesser pyrite, minor chalcopyrite with or without sphalerite, and galena and arsenopyrite in south-dipping massive sulfide pods, lenses, and stringers that parallel the foliation of the enclosing schists. Host-rock protoliths were mainly andesite, dacite, rhyodacite tuffs and flows, shale, and limestone, with lesser quartz-rich keratophyre flows and tuffs, volcanic graywacke, and siltstone.The Delta district contains at least 35 deposits in an 800-km 2 area. Thirty-one of the deposits define four mineral trends, 5 to 32 km long, that are subparallel to the west-northwest-striking, southwest-dipping structures and lithologies. The deposits consist of layers and zones containing varying amounts of massive to disseminated pyrite and pyrrhotite, and lesser amounts of chalcopyrite, galena, sphalerite, and arsenopyrite. The larger deposits range between 1.0 and 12.4 million metric tons with combined grades of 5.6 percent total metal (Cu, Pb, Zn), and Ag and Au contents of 56 and 1.9 g/metric ton, respectively. Hostrock protoliths are similar to those found in the Trident Glacier district, but some of the deposits are spatially associated with diorite or gabbro sills that exhibit lower greenschist facies metamorphism and deformation.Sulfur isotope values of 26 pyrrhotite separates from the Trident Glacier district occurrences average 7.7 per mil, the mean value of seven pyrite separates is 8.1 per mil. Forty-one Delta district pyrite separates average 5.9 per mil and ranges of values of the four northwest-trending trends overlap. The relative consistency of sulfur isotope values of iron-bearing sulfides in the Trident Glacier district, and the apparent lithologic relationship of isotope values in the Delta district, may reflect original-rock sulfur isotope values. Alternatively, the 34 S-enriched values may have resulted from the addition of reduced seawater sulfate to the hydrothermal fluid. The sulfide accumulations are interpreted as kuroko-like deposits that formed along a Devonian continental margin igneous arc. Deeper level portions of the arc are exposed in structurally lower levels of the Yukon-Tanana terrane to the north and contain metamorphosed granodiorite and granite plutons of Devonian to Mississippian age. U-Pb isotope data from zircons from metavolcanic rocks hosting the sulfide bodies indicate an age of about 370 Ma. The 207 Pb/ 204 Pb ratios for sulfides from the three districts are similar and indicate lead was in part derived from first-cycle sediments that were eroded largely from Precambrian crystalline rocks. The 207 Pb/ 204 Pb ratios from feldspars in Devonian metavolcanic rocks are similar to those in sulfides and indicate incorporation of radiogenic Precambrian lead into the Devonian magmas. A Precambrian crystalline source for detritus in metasedimentary rocks is indicated by locally abundant quartzite containing approximately 2.3 Ga zircons which were derived from the Hudsonian-age block of the Canadian Shield in southern British Columbia.Cretaceous greenschist facies retrogressive metamorphism and intense penetrative deformation is indicated by reequilibration of U-Pb isotope ratios, metamorphic Rb-Sr mineral isochrons, and K-Ar biotite and muscovite ages of 110 to 115 Ma. The data indicate that the mid-Cretaceous retrogressive metamorphism and associated intense deformation occurred during the accretion of the Yukon-Tanana terrane to Alaska.

The Sylvester Allochthon: upper Paleozoic marginal-basin and island-arc terranes in northern British Columbia

The Sylvester Allochthon is a composite klippe of upper Paleozoic ophiolitic, island-arc, and pericratonic assemblages, which rests directly on the Cassiar terrane, a displaced sliver of Ancestral North America. Each tectonic assemblage occurs at a distinct and consistent structural level within the allochthon. They are assigned, respectively, to the Slide Mountain, Harper Ranch, and Yukon–Tanana terranes. The Sylvester Allochthon provides a view of the structural relationships between these terranes prior to Early Cretaceous – early Tertiary strike-slip dismemberment, as well as possible sedimentological links to late Paleozoic North America. Slide Mountain Terrane assemblages, designated divisions I and II, form the lowest structural panels. Chert – quartz sandstones are interbedded with Lower Mississippian deep-water sediments in division I and ocean-floor basalts and deep-water sediments in division II. They are similar in age and character to sandstones in the autochthonous Earn Group. Division II assemblages represent atypical oceanic crust and upper mantle assemblages. Continuous basalt–sedimentary sequences, well dated by conodont faunas, span Early Mississippian to mid-Permian time. Feeders for the basalts are sills rather than sheeted dyke swarms, suggesting very slow spreading and high(?) sedimentation rates in a marginal-basin setting. These supracrustal sequences are thrust-imbricated with ultramafite–gabbro panels. Division II is in part overlain by a Triassic siliciclastic and limy sedimentary sequence, which resembles the basal Takla Group, Slocan Group, and autochthonous Triassic units. Division III occupies the highest structural levels in the allochthon. With one exception, thrust sheets within it consist of Pennsylvanian to Upper Permian mixed calc-alkaline volcanic and plutonic rocks, chert, tuff, and limestone, assigned to the Harper Ranch Terrane. One panel, assigned to the Yukon–Tanana Terrane, consists of an Early Mississippian quartz diorite pluton with Precambrian inheritance that intrudes older volcanogenic sediments, pyroclastics, limestone, and siliciclastic sediments. Preferred pre-Mesozoic restoration of these terrane elements shows a Harper Ranch arc, built partly on pericratonic Yukon–Tanana and partly on primitive oceanic basement (division III), which is separated from North America by the Slide Mountain marginal basin (divisions I and II).

Mid‐Cretaceous extensional tectonics of the Yukon‐Tanana Terrane, Trans‐Alaska Crustal Transect (TACT), east‐central Alaska

Mid‐Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon‐Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end‐member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid‐Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity spreading.

Cache Creek ocean: Closure or enclosure?

Research Article| February 01, 1993 Cache Creek ocean: Closure or enclosure? JoAnne Nelson; JoAnne Nelson 1British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, 553 Superior Street, Victoria, British Columbia V8V 1X4, Canada Search for other works by this author on: GSW Google Scholar Mitch Mihalynuk Mitch Mihalynuk 1British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, 553 Superior Street, Victoria, British Columbia V8V 1X4, Canada Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (2): 173–176. https://doi.org/10.1130/0091-7613(1993)021<0173:CCOCOE>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JoAnne Nelson, Mitch Mihalynuk; Cache Creek ocean: Closure or enclosure?. Geology 1993;; 21 (2): 173–176. doi: https://doi.org/10.1130/0091-7613(1993)021<0173:CCOCOE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Exotic Tethyan faunas within the Cache Creek terrane contrast markedly with faunas and lithologic associations in the adjacent Quesnel and Stikine terranes. In northern British Columbia and southeast Yukon, all three terranes are enveloped in the north by pericontinental rocks of the Yukon-Tanana terrane, a geometry that imposes severe constraints on terrane assembly models for the northern Canadian Cordillera. Our solution to the problem invokes a northern join between the Stikinia and Quesnellia arcs through the Yukon-Tanana terrane, forming an orocline that encloses the Cache Creek terrane. This model involves (1) collision of a linear oceanic plateau at the cusp between Quesnellia and Stikinia, (2) anticlockwise rotation of Stikinia about an axis in the Yukon-Tanana terrane, (3) simultaneous enclosure of the Cache Creek ocean, and (4) emplacement of Quesnellia onto the margin of ancestral North America and the Cache Creek terrane onto Stikinia during final closure of the orocline. Early Mesozoic Paleomagnetic declinations in Stikinia are permissive of the large anticlockwise rotations predicted by the model. Similar large-scale rotations and ocean-basin enclosure are common features in the southwest Pacific. This model accounts for Paleozoic and younger linkages between Yukon-Tanana and both northern Stikinia and Quesnellia, the striking similarity between Triassic-Jurassic arcs east and west of the Cache Creek terrane, and the profound early Mesozoic deformational event in the Yukon-Tanana terrane. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contractional nature of Devonian-Mississippian Antler tectonism along the North American continental margin

Regional tectonic analysis suggests that the entire Cordilleran continental margin, from Nevada to the Yukon, underwent contractional orogeny involving emplacement of deep basinal, submarine-fan, and mafic volcanic strata over autochthonous continental margin strata during Late Devonian to Mississippian time (Antler orogeny). Displaced parautochthonous strata are present from Nevada to east-central British Columbia, and perhaps within the Yukon-Tanana terrane of Yukon and eastern Alaska. Manifestations of the orogenic event in the northern Cordillera are preserved primarily by normal faults and related coarse clastic strata of the Selwyn basin. We propose that these features formed due to flexural extension during foreland deformation, rather than mid-Paleozoic rifting or transtension.

Pre‐Mid‐Mesozoic tectonic evolution of the Yukon‐Tanana Terrane, Yukon and Alaska

Yukon‐Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid‐Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian‐Early Mississippian, mid‐Permian, and Late Triassic‐Early Jurassic time. From Late Devonian to mid‐Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid‐Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid‐Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent‐continent collision. Although the Triassic‐Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.

Structural and geochronologic relations along the western flank of the coast mountains batholith: Stikine river to Cape Fanshaw, central southeastern Alaska

Geologic and U-Pb geochronologic studies in central southeastern Alaska provide constraints on the mid-Cretaceous to mid-Tertiary deformation west of the Coast Mountains batholith. The NE-dipping Sumdum and Fanshaw faults record the W-directed emplacement of the Ruth assemblage (Yukon-Tanana terrane) and Taku terrane over the subjacent Gravina belt and Alexander terrane. Ductile fabrics of the Sumdum-Fanshaw fault system truncate early-formed foliation and thrust faults in Albian and older Gravina belt strata. U-Pb age data from syn- and post-tectonic plutons suggest that deformation was ongoing at 92.9 ± 3.0 Ma but had mostly ceased by 90 Ma. The Sumdum-Fanshaw fault system marks the tectonic boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes and lies within a thrust belt that extends from southern Alaska to northern Washington. Mid-Cretaceous structures are truncated to the east by NE-dipping ductile fabrics of the Coast shear zone. Sheet-like Paleocene tonalites were emplaced into and deformed within the shear zone. Undeformed mid-Eocene dikes cross-cut the tonalites. Kinematic relations in the Coast shear zone suggest a complex displacement history that includes both east-side-up (reverse) and west-side-up (normal) shear. The shear zone probably accommodated the collapse of overthickened crust developed during mid-Cretaceous shortening.

Thin, low‐velocity crust beneath the southern Yukon‐Tanana Terrane, east central Alaska: Results from Trans‐Alaska crustal transect refraction/wide‐angle reflection data

A seismic refraction/wide‐angle reflection survey for the Trans‐Alaska Crustal Transect program reveals a thin, reflective crust beneath the southern Yukon‐Tanana terrane (YTT) in east central Alaska. These data are the first detailed refraction survey of the southern YTT and compose a 130‐km‐long reversed profile along the Alaska and Richardson highways. Results from this study indicate that low‐velocity (≤ 6.4 km/s) rocks extend to approximately 27 km in depth. Based on these low velocities and an average Poisson's ratio of 0.23 determined for depths of ≤27 km, an overall silicic composition is interpreted for this portion of the crust beneath the Yukon‐Tanana terrane. From approximately 8 to 27 km depth the crust exhibits an increase in reflectivity. This middle to lower crustal reflectivity is modeled as alternating high‐ and low‐velocity lamellae with an average velocity of 6.1 km/s at 10 km depth to an average velocity of 6.4 km/s at 27 km depth. Beneath these reflective, low‐velocity rocks a 3‐ to 5‐km‐thick, 7.0 km/s basal crustal layer produces a prominent reflection that extends to offsets of up to 280 km. The crust‐mantle boundary, modeled at an average depth of 30 km, produces a variable PmP reflection, which may indicate lateral heterogeneity of this boundary, and a weak and emergent Pn refraction with a velocity of 8.2 km/s. We interpret the crustal section as follows: the low‐velocity rocks of the southern YTT extend from the surface to depths of approximately 10 km; underthrust Mesozoic flysch of the Kahiltna terrane, rocks of the Gravina arc, and basement of the Wrangellia(?) terrane extend from 10 to 27 km depth; a 3‐ to 5‐km‐thick layer of mantle‐derived mafic rocks, relic oceanic crust, or Wrangellia(?) terrane lower crust extends from 27 to approximately 30 km depth; a tectonically young Moho beneath the southern YTT is found at an average depth of 30 km; and it is underlain by a mantle that may be relatively cool and/or olivine rich. In this interpretation, the Yukon‐Tanana terrane is a thin‐skinned terrane. Our results indicate that tectonic, and possibly magmatic, underplating has played a significant role in crustal growth for central Alaska.