Tectonic evolution of the Teslin zone and the western Cassiar terrane, northern Canadian cordillera.

摘要

The Teslin zone in south-central Yukon Territory has previously been described as a discrete zone with a steep foliation unique to the zone. It includes the Anvil assemblage and the narrowest portion of the Yukon-Tanana terrane (the Nisutlin assemblage), and is defined to the east and to the west by northerly trending, post-accretionary faults. The zone has been interpreted as a lithospheric suture or a crustal-scale transpression zone, and as the root zone of klippen lying on continental margin rocks of the North American craton (“North America”) to the east.; Detailed structural analysis has invalidated the earlier interpretations. F₁–F₂ transposition folds (and associated penetrative S₁–S₂ axial plane fabrics), and pre-F₃, pre- to syn-peak metamorphic shear fabrics affected the Anvil and Nisutlin assemblages and North America both inside and outside of the Teslin zone. They are interpreted as having formed during ductile thrusting (obduction), conceivably between the middle Early Permian and late Middle Triassic, of the two assemblages onto North America. During obduction, North America experienced Barrovian-type, prograde amphibolite facies metamorphism.; The steep transposition foliation (defined by transposed compositional layering and S₁–S₂) in the Teslin zone, in contrast to adjacent rocks to the east, coincides with the steep limb of a regional F₃ structure, the Grizzly synform. This fold has a shallow limb in the easternmost part of the zone and immediately east of the zone. North American rocks pass under the eastern Teslin zone and outcrop to the west of the Nisudin and Anvil assemblages. This geometry precludes the possibility of the zone being the root zone of the klippen. The basal thrust separates mafic eclogite-bearing and probable ophiolitic rocks of the Anvil assemblage in its hanging wall from continental margin rocks in its footwall, and can be defined as a lithospheric suture. However, there is no indication that this thrust is part of a reworked subduction complex since evidence of high-P metamorphism during thrust-related transposition is absent in any of the Anvil, Nisutlin, or North American rocks.; Southwest-northeast-directed, ductile to brittle extension in the mid-Cretaceous resulted in uplift of the relatively high-grade North American footwall rocks. This event, closely associated in time with calcic metasomatism, is interpreted as recording crustal extension in the Omineca belt of southern Yukon Territory.