Yukon-Tanana Terrane Research Articles

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Jurassic location of the Yukon-Tanana Terrane from paleomagnetism of the folded Mississippian Tatlmain Batholith and Ragged Stock

Summary The extensive Yukon-Tanana terrane of the northern Canadian Cordillera has been considered controversially to be part of the allochthonous ‘Baja B.C.’ microcontinent or of the para-autochthonous North American cratonic margin during the Mesozoic. Paleomagnetic methods have isolated a very-stable Early Jurassic thermochemical remanent remagnetization in the terrane's felsic Tatlmain batholith and mafic Ragged stock after correction for: 1) rotation from northeast-plunging anticlinal deformation; 2) northerly dipping tectonic tilt of the host rocks; and, 3) northwestward regional translation on the adjacent Tintina transcurrent fault zone. The resulting 196 ± 6 Ma Tatlmain and Ragged paleopoles are 64.9° N, 44.8° E (A95 = 5.9°) and 64.2° N, 58.5° E (A95 = 7.7°), respectively. The YTT paleopoles support para-autochthonous tectonic models that have the YTT: 1) accreting to North America by the Early Jurassic; 2) undergoing non-significant orogen-perpendicular shortening by mid-Early Cretaceous from thrust-faulting; and, then 3) undergoing significant orogen-parallel northward translation of ∼500 km from mid-Early Cretaceous to the Eocene. In contrast, the paleopoles for Stikinia and Quesnellia of the Intermontane Belt show progressive northwestward translation relative to North America by ∼1000 km and a rotation of ∼55° cw since mid-Early Cretaceous. We speculate that ∼500 km of the northward translation is related to dextral motion on the Tintina and Northern Rocky Mountain Trench fault in British Columbia, and that the clockwise rotation is related to upper crustal tectonics in both Yukon and southern British Columbia.

Structural evolution of a crustal-scale shear zone through a decreasing temperature regime: The Yukon River shear zone, Yukon-Tanana terrane, Northern Cordillera

Structural evolution of a crustal-scale shear zone through a decreasing temperature regime: The Yukon River shear zone, Yukon-Tanana terrane, Northern Cordillera

A petrochronological approach for the detrital record: Tracking mm-sized eclogite clasts in the northern Canadian Cordillera

Sutures recording the accretion history of the Canadian Cordillera terranes are poorly preserved. The Whitehorse trough syn-orogenic basin formed during early Mesozoic terrane accretion at the western margin of Laurentia and contains a ∼300 m thick horizon that includes eclogite clasts possibly sourced from a suture zone. By applying petrochronological micro-analytical techniques to the mm-diameter eclogite clasts, including thermobarometry and in situ rutile thermochronology, as well as detrital zircon geochronology and thermal diffusion modeling, we constrain a source-to-sink path for the clasts. The eclogite clasts likely reached peak metamorphic conditions of 2.2–2.9 GPa and ≥800 °C, cooled through Pb closure in rutile during Early Jurassic at ≥610 °C and were deposited into the basin by latest Pliensbachian/earliest Toarcian. This history implies minimum mean cooling and exhumation rates on the order of ∼38 °C/myr and ∼4.1 km/myr, respectively, consistent with rates reported for subduction-related eclogite worldwide. We suggest the most likely source for the clasts is the suture between the Yukon–Tanana and Stikinia terranes, involving a latest Triassic collision, followed by rapid Early Jurassic exhumation of the lower plate Yukon–Tanana terrane, either by buoyant extrusion or in a plate boundary zone metamorphic core complex. Our study demonstrates that micro-analytical techniques used for petrochronology can be applied to very small lithic clasts in the sedimentary record towards the tectonic reconstruction of accretionary orogens.

Seismic and gravity constraints on the crustal architecture of the Intermontane terranes, central Yukon

In 2004, two seismic reflection lines were shot across the Mesozoic Whitehorse trough and adjacent terranes. Three-dimensional first-arrival tomographic inversion is used to constrain lithology to 800–1200 m depth, and surface structures are extrapolated into the middle crust using the coincident reflection data. In the Yukon–Tanana terrane, the metasedimentary Snowcap assemblage is characterized by velocities of 4.5–5.5 km/s, while in Quesnellia, velocities of 5.0–6.0 km/s occur at 500 m depth, and probably represent igneous rocks of the Tatchun batholith. Across the Whitehorse trough, velocities >4.0 km/s correspond to clastic rocks of the Jurassic Laberge and Triassic Lewes River groups; velocities <4.0 km/s probably present the clastic Jurassic to Cretaceous Tantalus Formation. Several near-surface units with velocities of 2.0–3.0 km/s are identified; some correlate well with volcanic rocks of the Upper Cretaceous Carmacks Group, but others could be attributable to alluvial deposits or faulting. The Big Salmon fault is interpreted to dip southwest, implying that rocks of the Yukon–Tanana terrane extend beneath Quesnellia. Stikinia and Quesnellia underlie up to 5–8 km of Triassic to Early Cretaceous sedimentary strata, and appear to be a single allochthon within an 18–20 km deep synform above the Yukon–Tanana terrane, which we name the Northern Intermontane synform. In general, reflection geometries in the upper crust are complex, but are consistent with large-scale imbricate structures that have been dissected into numerous blocks by displacement along moderately to steeply dipping strike-slip faults, which may be part of a crustal-scale flower structure extending to the base of the crust.

A Pangean rim of fire: Reviewing the Triassic of western Laurentia

A synthesis of U-Pb detrital zircon data from the Triassic of western Laurentia is placed in the context of paleogeographic models for Pangea. We find that an emerging body of evidence supports the hypothesis that a Triassic subduction zone and continental magmatic arc system fringed western Laurentia starting in the southwestern United States, continuing northward along the Cordillera, including arc rocks of the Quesnel and Yukon-Tanana terranes, and extending further into the Arctic region. In the context of this geodynamic setting, the western interior basins of North America would have formed, probably by subduction dynamics, as a collage of backarc and retroarc foreland basins. The convergent tectonic model for western Laurentia is consistent with paleogeographic reconstructions that show a subduction zone encircling Pangea, called the Pangean rim of fire.

U–Pb and Hf isotopic analyses of detrital zircons from the Taku terrane, southeast Alaska

The Taku terrane consists of metamorphosed Carboniferous through Triassic marine clastic strata, volcanic rocks, and limestone that occur along the western margin of the Coast Mountains in southeastern Alaska. These rocks are juxtaposed along mid-Cretaceous thrust faults over Jura-Cretaceous basinal strata of the Gravina belt to the west and beneath Proterozoic through Carboniferous metamorphic rocks of the Yukon–Tanana terrane to the east. This paper presents U–Pb ages and Hf isotope analyses of detrital zircons from the Taku terrane, and compares these values with information from the adjacent Wrangellia, Alexander, and northern and southern portions of the Yukon–Tanana terrane (YTTn and YTTs). These comparisons suggest that (i) Carboniferous strata of the Taku terrane were shed mainly from mid-Paleozoic igneous rocks of YTTs, (ii) Permian strata of the Taku terrane were shed from mid-Paleozoic igneous rocks and intraformational Lower Permian volcanic rocks of YTTs as well as Upper Permian volcanic rocks exposed in YTTn, and (iii) Triassic sandstones were shed from mid-Paleozoic igneous rocks of YTTs, whereas conglomerates were shed mainly from mid-Paleozoic arc rocks in YTTn. Hf isotope analyses of Paleozoic zircons record increasing continental input during Silurian–Devonian and Permian phases of magmatism. Similarities in isotopic characteristics, combined with stratigraphic and geochemical information presented by previous workers, suggest that strata of the Taku terrane accumulated on (and partly as lateral equivalents of) rocks of YTTs, and that the combined assemblages formed outboard or along strike of YTTn.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska

Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian–Devonian Endicott Arm assemblage, and Mississippian–Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Proterozoic (ca. 2.0–1.6, 1.2–0.9 Ga) and Archean (2.7–2.5 Ga) zircons that yield typical cratonal eHf( t ) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and eHf( t ) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician–Devonian ages and eHf( t ) values, and also include early Mississippian zircons with highly evolved eHf( t ) signatures. Comparison of these age-Hf patterns with data from nearby assemblages suggests the following: (1) Results from YTTs are similar to (or compatible with) available data from rocks of the Yukon-Tanana terrane in eastern Alaska, Yukon, and northern British Columbia (YTTn) and pericratonic strata in east-central Alaska (NAa). (2) YTTs contains abundant Late Ordovician–Early Devonian magmatism that is not recorded in YTTn and NAa. (3) The eHf( t ) values from YTTs display two excursions from juvenile to evolved eHf( t ) values, which are interpreted to record cycles of crustal thinning and then thickening within a convergent margin system. (4) Available data from both YTTs and YTTn support Neoproterozoic(?)–early Paleozoic positions along the northern Cordilleran margin. (5) The Late Ordovician–Early Devonian magmatic record of the southern Alexander terrane is very similar to that of YTTs, which raises the possibility that these assemblages evolved in the same convergent margin system along the northern (Alexander) and northwestern (YTT) margins of Laurentia. These results support a tectonic model in which: (1) YTTs formed outboard of (or northward along strike of) YTTn and NAa along the northern Cordilleran margin during Neoproterozoic(?)–early Paleozoic time; (2) initial subduction-related magmatism during Late Ordovician to Early Devonian time records a progression from crustal thinning to crustal thickening, and is preserved only in YTTs; (3) a second phase of magmatism records Middle–Late Devonian crustal thinning followed by early Mississippian crustal thickening; (4) YTTs and YTTn evolved as an intra-oceanic arc outboard of the Slide Mountain ocean basin during Carboniferous–Permian time and were accreted to the continental margin during Triassic time; and (5) YTTs is interpreted to have been displaced ∼1000 km southward, from an original position outboard of YTTn/NAa to its present position outboard of the Stikine terrane, along a sinistral fault system of Late Jurassic–Early Cretaceous age.

Ambient Redox and Hydrothermal Environment of the Wolverine Volcanogenic Massive Sulfide Deposit, Yukon: Insights from Lithofacies and Lithogeochemistry of Mississippian Host Shales

The Wolverine volcanogenic massive sulfide (VMS) deposit is a polymetallic, felsic-siliciclastic deposit hosted by ~352 to 347 Ma volcanic and sedimentary rocks of the Yukon-Tanana terrane, Yukon, Canada. Shales are located at various stratigraphic levels and in various mineralized zones within the Wolverine deposit area (e.g., Fisher, Puck, and Sable zones). Shales present along the mineralized horizon near the immediate hanging wall and footwall were deposited under anoxic conditions (e.g., low Mn, anoxic V-Cr-Mo-U systematics), whereas in the stratigraphically deeper footwall and uppermost hanging wall, the redox signatures imply deposition under suboxic to oxic conditions. The Mo-U systematics of the shales suggest that during the time of sulfide mineralization, the ambient basin was periodically euxinic with aqueous H2S present in the water column that this H2S contributed to the sulfur budget of the deposit. Furthermore, the Mo-U and Corg-Ni systematics favor deposition in a restricted basin (i.e., nutrient trap) where restriction of the water column led to H2S formation via sulfate reduction associated with excess organic carbon preservation. There is also evidence of a progressive shift upward in the stratigraphy to more oxygenated conditions in the uppermost hanging wall. The shift from euxinic to oxic-suboxic conditions is consistent with regional tectonic models that indicate a change from rifting during Wolverine deposit formation, where the basin was partially restricted with minimal circulation (i.e., restricted depocenter), to an incipient back-arc basin accompanied by extension and likely ingress of oxygenated seawater. The rare earth element and Y (REY) systematics in Wolverine shales illustrate that proximal to the mineralized horizon, shales have higher Y/Ho (>27), and Ce/Ce*<<1 and negative Ce anomalies indicative of oxygenated seawater. The Ce/Ce* values in the Wolverine shales have an inverse correlation with P2O5 content and suggest partial control by detrital apatite. It is envisioned that apatite formed in the upper, oxygenated portion of the water column, inherited the REE signature of oxygenated seawater (i.e., Ce/Ce*<<1), and was subsequently deposited into deeper waters as detrital grains. The Ce/Ce* and Y/Ho also correlate with CO2 and carbonate content of the shales. Moreover, the shales that have the strongest REY signature of oxygenated seawater also coincide with the strongest euxinic signatures. This paradox can be reconciled by enhanced deposition of apatite coincident with deposit formation, coupled with a late hydrothermal overprint on the shales from low-temperature, CO2-rich (oxygenated?) hydrothermal fluids (i.e., high Y/Ho and Ce/Ce*<<1) in a vent-proximal environment. This model is consistent with the geology, stratigraphy, and hydrothermal alteration in the immediate footwall and hanging wall of the deposit. Prospective shales in the Wolverine basin, and similar sediment-rich hydrothermal basins globally, should exhibit evidence for deposition under anoxic conditions (i.e., Mn <1,000 ppm; high V-Mo-U, UEF, MoEF), hydrothermal Fe-Al-Mn systematics (i.e., high Fe/Al), and evidence for hydrothermal alteration (i.e., high CIW values, high molar sericite and chlorite). Identification of samples having these features is useful in targeting prospective VMS environments in shale-dominated successions and potential targets within shale-rich basins.

The P–T path of eclogites in the St. Cyr klippe, Yukon, Canada: Permian metamorphism of a coherent high-pressure unit in an accreted terrane of the North American Cordillera

The St. Cyr klippe, part of the Yukon–Tanana terrane in Yukon, Canada, preserves fresh and retrogressed eclogite hosted by quartzofeldspathic schists. Petrology, mineral chemistry and compositional zoning combined with isochemical phase equilibrium (pseudosection) modeling shows that eclogites followed a clockwise pressure–temperature path. An amphibolite facies, pre-eclogite stage (pre-Stage I) preceded early eclogite-facies conditions of about 16 kbar and 510 °C (Stage I) and the peak pressure (Stage II), which reached up to 20 kbar and 670 °C. The peak observed and modeled eclogite-facies assemblage is garnet, omphacite, Na-rich amphibole, phengite, quartz and rutile. Pseudosections were modeled for varying Fe 3+ contents, and Fe 3+ = 20 % of total Fe is found to best match the pyrope and grossular content observed in garnet. Retrogression back to amphibolite-facies conditions (Stage III) followed a clockwise path as temperature decreased during decompression without passing through the chlorite stability field. U-Pb SIMS (secondary ion mass spectrometry) dating and trace-element analysis of zircon shows that the protolith of the eclogites formed within the Yukon–Tanana terrane during early arc activity, between 364 and 380 Ma. The rocks were then subducted to mantle depths and metamorphosed to eclogite-facies conditions during the Late Permian, between 267 and 271 Ma. These medium-temperature eclogites are part of a coherent high-pressure (HP) terrane that was tectonically eroded from the Yukon-Tanana composite arc in a subduction zone environment.

Determining the provenance of Triassic sedimentary rocks in northeastern British Columbia and western Alberta using detrital zircon geochronology, with implications for regional tectonics

Triassic rocks of the Western Canada Sedimentary Basin (WCSB) have previously been interpreted as being deposited on the passive margin of North America. Recent detrital zircon provenance studies on equivalent Triassic rocks in the Yukon have suggested that these rocks were in part derived from the pericratonic Yukon–Tanana terrane and were deposited in a foreland basin related to the Late Permian Klondike orogeny. Detrital zircons within a number of samples collected from Triassic sediments of the WCSB throughout northeastern British Columbia and western Alberta suggest that the bulk of the sediment was derived from recycled sediments of the miogeocline along western North America, with a smaller but significant proportion coming from the Innuitian orogenic wedge in the Arctic and from local plutonic and volcanic rocks. There is also evidence of sediment being derived from the Yukon–Tanana terrane, supporting the model of terrane accretion occurring prior to the Triassic. The age distribution of detrital zircons from the WCSB in British Columbia is similar to those of the Selwyn and Earn sub-basins in the Yukon and is in agreement with previous observations that sediment deposited along the margin of North America during the Triassic was derived from similar source areas. Together these findings support the model of deposition within a foreland basin, similar to the one inferred in the Yukon. Only a small proportion of zircon derived from the Yukon–Tanana terrane is present within Triassic strata in northeastern British Columbia, which may be due to post-Triassic erosion of the rocks containing these zircons.

An orogenic wedge model for diachronous deformation, metamorphism, and exhumation in the hinterland of the northern Canadian Cordillera

Development of amphibolite-facies transposition fabrics in the northern Canadian Cordilleran hinterland occurred diachronously in the Permian–Triassic, Early Jurassic, Middle Jurassic to Early Cretaceous, and Early to mid-Cretaceous. Rocks tectonized in the Permian–Triassic and Early Jurassic were exhumed in the Early Jurassic, while rocks immediately to the northeast (toward the foreland) were not buried and heated until the Middle Jurassic to mid-Cretaceous. Early Jurassic to mid-Cretaceous emplacement of the Yukon-Tanana terrane on the North American continental margin, together with the imbrication of parautochthonous rocks, formed a foreland-propagating orogenic wedge. Cooler rocks in front of the wedge were progressively buried and metamorphosed to amphibolite facies from the Jurassic to mid-Cretaceous as they were underthrust into a spatially and temporally transient distributed ductile shear zone near the base of the overriding wedge. Rocks previously incorporated into this zone were displaced upward and exhumed through the combined effects of renewed underplating at depth and compensating extensional and erosional denudation above to maintain a critically tapered wedge. Extensional exhumation of the metamorphic hinterland in the mid-Cretaceous marked the collapse and end of orogen-perpendicular wedge dynamics in operation since the Early Jurassic. Rocks incorporated into the midcrustal shear zone in the Middle Jurassic to mid-Cretaceous were exhumed in the mid-Cretaceous along southeast-directed (orogen-parallel) extensional faults from beneath a supracrustal “lid” tectonized in the Permian–Triassic and Early Jurassic. Like the Himalayan orogen and eastern Alps, orogen-parallel extension developed in an orthogonal plate-convergent setting, simultaneous with, and bounded by, orogen-parallel strike-slip faulting that facilitated northwestward lateral extrusion of rocks normal to the direction of convergence.

Contrasting geochemistry of orogenic gold deposits in Yukon, Canada and Otago, New Zealand

The Yukon-Tanana Terrane (YTT) of western Yukon Territory in NW Canada and Otago Schist belt (OSB) of South Island, New Zealand share similar geological evolutionary histories as convergent orogenic belts. Both belts host orogenic gold deposits of mainly Jurassic to Early Cretaceous age. Jurassic mineralization in the YTT occurred during convergent orogenesis and stacking of previously-metamorphosed (Palaeozoic) greenschist-amphibolite facies metasediments, metavolcanic rocks, and metagranitoids. Early Cretaceous OSB mineralization occurred in the latter stages of terrane accretion of un-metamorphosed turbidites with minor basaltic rocks. Metamorphism of the OSB turbidites mobilised background levels of Au (0.6–1.3 ppb), As (2–20 ppm), Sb (0.1–1 ppm), and W (&lt; 10 ppm), primarily under greenschist to lower amphibolite facies conditions when diagenetic pyrite (Au c . 0.5–2 ppm; As c . 500–10 000 ppm) transformed to pyrrhotite on a regional scale. In contrast, the previously-metamorphosed YTT rocks had generally low background As contents (1–2 ppm) apart from some As-rich quartzites (up to 100 ppm As). Consequently, there was less As available for orogenic mobilisation, and YTT Au deposits generally have lower concentrations of this pathfinder element compared to the OSB. YTT host rocks, especially metagranitoids, have anomalous levels of Mo (10–300 ppm), and many orogenic deposits contain elevated Mo, locally including molybdenite. OSB turbidites have elevated Mo (2–200 ppm), along with elevated Au and As, in diagenetic pyrite, but this Mo became largely dispersed through the metamorphic pile as metamorphic grade increased and pyrite transformed to pyrrhotite. OSB orogenic deposits have only marginally elevated Mo ( c . 1 ppm), no molybdenite, and accessory scheelite in these deposits is distinctly Mo-poor. Only minor mobilisation of base metals occurred in these orogenic belts, and orogenic Au deposits contain sparse base metal sulphides. Orogenic deposits in the YTT and OSB differ in that Au (and other associated elements) in many of the orogenic deposits in the YTT was remobilised from relatively local sources (e.g. pre-existing Cu-Mo-Au porphyry or volcanogenic sulphide mineralization) whereas Au in the OSB was mobilised from larger volumes of homogeneous rock at depth.

Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front

In situ sensitive high-resolution ion microprobe monazite geochronology and garnet isopleth thermobarometry reveal a previously unrecognized Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphic event in the eastern part of the Yukon-Tanana terrane (Finlayson Lake district, southeast Yukon) in the northern Canadian Cordillera. Intersection of garnet end-member compositional isopleths applied to single-stage, growth-zoned garnet records progressive garnet growth from 550 °C and 6.1–6.6 kbar to 600 °C and 7.5 kbar. Monazite textures, chemical zoning, and in situ U-Pb ages record a single protracted episode of monazite growth from ca. 169 to 142 Ma coeval with the development of transposition fabrics and the late stages of garnet growth. This event post-dates widespread Early Jurassic exhumation of Yukon-Tanana terrane rocks west of the Tintina fault in west-central Yukon, which were previously ductily deformed and metamorphosed in the Permo-Triassic. The lack of evidence for Permo-Triassic ductile deformation and high-grade metamorphism within the Finlayson Lake district, and its position east of the Permian arc center and west of Permian blueschists and eclogites, suggests this eastern part of the terrane occupied the cool forearc at this time. These data indicate younger, more protracted mid-crustal orogenesis in the northern Cordillera than was previously recognized, with deformation and metamorphism migrating toward the foreland and downwards in the Middle Jurassic to Early Cretaceous, in part contemporaneous with and analogous to that in the southeastern Canadian Cordillera.

Two flysch belts having distinctly different provenance suggest no stratigraphic link between the Wrangellia composite terrane and the paleo-Alaskan margin

The provenance of Jurassic to Cretaceous flysch along the northern boundary of the allochthonous Wrangellia composite terrane, exposed from the Lake Clark region of southwest Alaska to the Nutzotin Mountains in eastern Alaska, suggests that the flysch can be divided into two belts having different sources. On the north, the Kahiltna flysch and Kuskokwim Group overlie and were derived from the Farwell and Yukon-Tanana terranes, as well as smaller related terranes that were part of the paleo-Alaskan margin. Paleocurrent indicators for these two units suggest that they derived sediment from the north and west. Sandstones are predominantly lithic wacke that contain abundant quartz grains, lithic rock fragments, and detrital mica, which suggest that these rocks were derived from recycled orogen and arc sources. Conglomerates contain limestone clasts that have fossils matching terranes that made up the paleo-Alaskan margin. In contrast, flysch units on the south overlie and were derived from the Wrangellia composite terrane. Paleocurrent indicators for these units suggest that they derived sediment from the south. Sandstones are predominantly feldspathic wackes that contain abundant plagioclase grains and volcanic rock fragments, which suggest these rocks were derived from an arc. Clast compositions in conglomerate south of the boundary match rock types of the Wrangellia composite terrane.

Thermochronology of the Yukon-Tanana Terrane, West-Central Yukon: Evidence for Jurassic Extension and Exhumation in the Northern Canadian Cordillera

Within the McQuesten area of the central Yukon region of the Canadian Cordillera, the >100-km-long Willow Lake fault juxtaposes two crustal domains: (i) on the northeast, the Reid Lakes complex, an unmetamorphosed and predominantly undeformed Devono-Mississippian assemblage of plutonic and volcanic rocks; and (ii) on the southwest, typical Yukon-Tanana terrane (YTT) with deformed greenschist to amphibolite facies metamorphosed pre–late Devonian to late Permian rocks. In this study we investigate the tectonic history of these two crustal domains and present sensitive high-resolution ion microprobe U-Pb ages that delineate middle to late Paleozoic arc magmatism and fit key units into the YTT’s regional lithotectonic framework. Notably, magmatic zircons possess ca. 193 Ma overgrowths, and patches of recrystallized zircon with low Th/U ratios occur in samples proximal to the fault. Mississippian (ca. 349−325 Ma) biotite and hornblende 40Ar/39Ar cooling ages of the Reid Lakes complex suggest cooling immediately following volcanism and batholith emplacement. Plutonic samples near the fault trace, exhibiting weak to high strain, yield younger, ca. 193−191 Ma biotite 40Ar/39Ar ages. Zircon (U-Th)/He ages from the magmatic complex reflect ca. 216−214 Ma cooling through the uppermost crust of structurally higher volcanogenic rocks, followed by ca. 159–130 Ma cooling of the deeper-seated plutonic phase. Across the Willow Lake fault, YTT metamorphic rocks yielded ca. 197−165 Ma biotite and hornblende 40Ar/39Ar ages and ca. 113−90 Ma zircon (U-Th)/He ages. The marked step in cooling ages across the fault and the juxtaposition of the deformed rocks against undeformed rocks strongly indicates the Willow Lake fault is an extensional structure with top-down-to-the-northeast displacement, which exhumed the middle crust commencing in the early Jurassic. Aishihik suite magmatic rocks of the Stikinia/Quesnellia arc in the McQuesten area yielded a zircon age of ca. 205 Ma and 205−196 Ma40Ar/39Ar cooling ages. These ages and the presence of magmatic epidote suggest that the Aishihik suite cooled rapidly and was exhumed from midcrustal levels, further illustrating the significance of the Jurassic event that exhumed the central belt of YTT rocks. A similar tectonothermal evolution in the nearby Stewart River area, along strike to the northwest from McQuesten, was attributed to transtensional exhumation and cooling during slab rollback, leading to unroofing of the outboard regions of the terrane. We suggest that early Jurassic extensional exhumation of the YTT domain was mechanically linked to the Teslin fault system and more widespread that previously considered, likely accommodating crustal thinning from the Yukon-Tanana upland of eastern Alaska to the Glenlyon region of central Yukon. Parts of this system may have been episodically reactivated and overprinted, perhaps even as part of the Eocene Tintina fault, masking the early history.

Late pleistocene sedimentation history of the Shirshov Ridge, Bering Sea

The analysis of the lithology, grain-size distribution, clay minerals, and geochemistry of Upper Pleistocene sediments from the submarine Shirshov Ridge (Bering Sea) showed that the main source area was the Yukon-Tanana terrane of Central Alaska. The sedimentary materials were transported by the Yukon River through Beringia up to the shelf break, where they were entrained by a strong northwestward-flowing sea current. The lithological data revealed several pulses of ice-rafted debris deposition, roughly synchronous with Heinrich events, and periods of weaker bottom-current intensity. Based on the geochemical results, we distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. The results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and glacial cycles in Alaska.

Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

Lithosphere-asthenosphere mixing in a transform-dominated late Paleozoic backarc basin: Implications for northern Cordilleran crustal growth and assembly

The Slide Mountain terrane is part of a North American Cordillera–long backarc basinal assemblage that developed between the ensialic arc terranes (Yukon-Tanana and affiliated pericratonic terranes) and the North American craton in the middle to late Paleozoic. The Slide Mountain basin started to open in the Late Devonian, and spreading continued through the late Paleozoic in an oblique (transform-dominated) manner such that the pericratonic terranes were translated into southerly latitudes. The basin closed, also in an oblique manner, by the Early Triassic, resulting in the reaccretion of the Yukon-Tanana terrane to the northwestern Laurentian margin. Both the opening and closing likely involved hundreds to possibly thousands of kilometers of intra-ocean and/or intra-arc strike-slip displacement, sinistral during the ocean's Late Devonian to mid-Permian opening and dextral during its Late Permian closing.

Eocene rotation of the Coast Plutonic Complex and Intermontane Belt: paleomagnetism of Eocene plutons along the Klondike Highway

Paleomagnetic results are reported for the ∼59 Ma Skagway, ∼54 Ma Fraser, ∼53 Ma Summit Lake, and ∼48 Ma Clifton felsic plutons of the eastern Coast Plutonic Complex (CPC) that outcrop along the south Klondike Highway in Alaska and British Columbia. Thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization (ChRM) directions for all 29 sites of normal, reversed, and mixed polarity. The ChRM resides in single or pseudosingle domain magnetite and (or) pyrrhotite that is shown to be primary by contact tests with the ∼47 Ma vertical White Pass mafic dikes. Paleopoles from six 56 to 50 Ma (mean 52 ± 2 Ma) Intermontane Belt – Yukon–Tanana terrane (IMB–YTT) units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 Ma (mean 52 ± 4 Ma) eastern CPC paleopoles. Both paleopole populations show nonsignificant poleward (northward) translation relative to North America (IMB–YTT, 3.7° ± 5.3°N; CPC, 4.3° ± 6.4°S; overall, 1.2° ± 4.9°S), indicating that northward translation of the accreted terranes ended by ∼58 Ma. Conversely, both populations show clockwise (CW) rotation that is either highly significant or substantial (IMB–YTT, 19.3° ± 10.5 °CW; CPC, 7.1° ± 16.1 °CW; overall 12.8° ± 10.9 °CW). The results are best explained by tectonic rotation from ∼50 to ∼45 Ma of the IMB–YTT as a thin-skin on top of North America during emplacement and co-incident rotation of the massive Eocene plutons of the eastern CPC along the North American margin.