Jurassic accretion of Nisling terrane along the western margin of Stikinia, Coast Mountains, northwestern British Columbia

Abstract

A metamorphosed assemblage of continental margin-type strata, termed the Nisling terrane, exists as a narrow terrane sliver in the northern Coast Mountains of British Columbia. It could be a fragment rifted from North America and later accreted, or, alternatively, it could be allochthonous and far traveled. The Nisling terrane is tectonically bounded by younger, oceanic, arc-dominated terranes on both sides—the Alexander terrane to the west and meta-morphosed Paleozoic rocks of Stikinia to the east. The Nisling terrane was accreted to Stikinia along a sinistral transpressive ductile shear zone between 185 and 170 Ma, during or before the accretion of Stikinia to the ancestral margin of North America. Along this tectonic boundary, intense deformation was accompanied by lower amphibolite-facies metamorphism. Previous inferences by other workers that the Nisling-Stikinia accretion occurred in the Triassic are incorrect, as is the supposition that the Nisling terrane formed the depositional basement to Stikinia. Although the Nisling terrane exists for ∼1000 km along strike on the west side of Stikinia, its connection to either North America or another continent remains uncertain and tectonically puzzling. The middle Cretaceous collision of the Nisling-Stikinia terrane composite with the Alexander terrane overprinted the Jurassic deformational features and produced a complex belt of tectonic slivers involving all of these terrane fragments.