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Abstract: 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. [...]

Cape Kiber on the Arctic coast of Chukotka, northeast Russia, consists of a granite intruding Devonian (and older?) strata in the core of a large southeast trending anticline. These strata are structurally overlain by Carboniferous and younger strata. AU-Pb age of 351.4 ±5.6 (2σ)Ma shows that the granite is Early Carboniferous in age. A large granite cobble extracted from a Carboniferous conglomerate produced a Late Devonian or Early Carboniferous U-Pb age of ~355-361 Ma. Also, a deformed and altered granitic dyke yielded an age of 363.7 ± 5.7 (2cr) Ma. Major and trace elements suggest a syn-collisional (orogenic) setting. The granite's (biotite) Ar release spectrum is reset. The granitic dyke also shows a disturbed Ar-Ar whole-rock spectrum implying an Early Cretaceous age (~122-130 Ma) for closure of the Ar system. We interpret this as due to widespread greenschist metamorphism accompanying regional deformation of the Anyuy-Chukotka Fold Belt that accompanied closure of the South Anyuy Ocean. Regionally, this event predates deposition of Aptian and Albian strata and the eruption of Okhotsk--Chukotsk magmatic rocks. An Ar-Ar (biotite) plateau age of 96.4 [+ or -]1.0 (2σ) Ma from a mildly deformed, lamprophyre dyke reflects intrusion in a setting of regional extension. Its deformation reflects a younger tectonic event. The record of Devonian--Carboniferous magmatism and early Carboniferous unroofing is younger and less complex than that of Arctic Alaska. However, evidence for Early Devonian (Caledonian) or Late Devonian (Ellesmerian) deformation could have been masked by intense Mesozoic deformation. Outcrop data and geochronology support and refine regional interpretations of Early Cretaceous deformation and mineral growth accompanying accretion of Chukotka to north Asia, followed by regional extension and subsequent convergent deformation. Resume : Le cap Kiber sur la cote arctique de Chukotka, dans le nord-est de la Russie, est forme d'un granite qui penetre des strates datant du Devonien (et plus anciennes?) dans le creur d'un grand anticlinal a pendage vers le sud-est. Ces strates sont recouvertes de maniere structurale par des strates datant du Carbonifere et plus recentes. Un age, determine par U-Pb, de 351,4 ± 5,6 (2σ) Ma montre que le granite date du Carbonifere precoce. Un gros caillou de granite, extrait d'un conglomerat datant du Carbonifere, a donne un age U-Pb de Devonien tardif ou de Carbonifere precoce de ~355-361 Ma. De plus, un dyke de granite deforme et altere a donne un age de 363.7 ± 5.7 (2σ) Ma. Les elements majeurs et en trace suggerent un environnement de collision synchrone (orogenique). Le spectre du relachement d'Ar du granite (biotite) est reinitialise. Le dyke de granite montre aussi un spectre Ar-Ar perturbe de la roche entiere, ce qui signifierait un age Cretace precoce (environ 122-130 Ma) pour la fermeture du systeme Ar. Nous interpretons cela comme etant du a un metamorphisme au facies des schistes verts etendu qui accompagne la deformation regionale de la zone de plissements Anyuy-Chukotka, laquelle accompagne la fermeture de l'ocean Anyuy sud. A l'echelle regionale, cet evenement a eu lieu avant la deposition des strates aptiennes et albiennes et l'eruption des roches magmatiques d'Okhotsk-Chukotsk. Un age plateau de 96,4 ± 1,0 (2σ) Ma, determine par Ar-Ar (sur une biotite) d'un dyke de lamprophyre legerement deforme, correspond a une intrusion dans un environnement d'extension regionale. Sa deformation est le reflet d'un evenement tectonique plus jeune. L'enregistrement de magmatisme au Devonien-Carbonifere et le retrait du toit au Carbonifere precoce est plus recent et moins complexe que celui de l'Alaska arctique. Toutefois, les evidences pour une deformation au Devonien precoce (Caledonien) ou au Devonien tardif (Ellesmerien) pourraient avoir ete masquees par une deformation intense au Mesozoique. Les donnees provenant des affleurements et de la geochronologie supportent et peaufinent les interpretations regionales d'une deformation et d'une croissance mineralogique au Cretace precoce qui accompagnaient l'accretion de Chukotka a l'Asie septentrionale; cette accretion a ete suivie d'une extension regionale et d'une deformation subsequente de convergence. [Traduit par le Redaction], Introduction As part of the Canada-Russia Science and Technology Exchange Agreement, co-operative field projects were undertaken to clarify the distribution and correlation of circum-Arctic Phanerozoic strata. One project, in 1989, [...]

The plate- boundary conditions of the Mesozoic North American Cordillera remain poorly constrained, but most studies support large (>800 km) southward motion of the Insular and Intermontane superterranes during Jurassic– Cretaceous time. An implicit feature in these models of large coastwise displacements is the presence of one or more continental- scale sinistral strike- slip faults that could have dismembered and displaced terrane fragments southward along the western margin of North America prior to the onset of mid- Cretaceous shortening and dextral strike- slip faulting. In this study, we documented a system of sinistral intra- arc shear zones within the Insular superterrane that may have accommodated large southward motion. Employment of a new large-n igneous zircon U-Pb method more than doubled the precision of measurements obtained by laser ablation– inductively coupled plasma– mass spectrometry (from ~1% to 0.5%) and allowed us to demonstrate the close temporal- spatial relationship between magmatism and deformation by dating comagmatic crosscutting phases. Crystallization ages of pre-, syn-, and postkinematic intrusions show that the intra- arc shear zones record an Early Cretaceous phase of sinistral oblique convergence that terminated between 107 and 101 Ma. Shear zone cessation coincided with: (1) collapse of the Gravina basin, (2) development of a single voluminous arc that stitched the Insular and Intermontane superterranes together, and (3) initiation of east- west contractional deformation throughout the Coast Mountains. We interpret these concurrent tectono- magmatic events to mark a shift in plate kinematics from a sinistral- oblique system involving separate terranes and intervening ocean basins to a strongly convergent two- plate margin involving a single oceanic plate and the newly assembled western margin of North America. [ABSTRACT FROM AUTHOR]