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Abstract
The Canadian Cordillera is composed of three major allochothonous tectonic assemblages: the Insular Superterrane to the west, the Intermontane Superterrane to the east, and the intervening Coast Plutonic Complex. Both the Insular and Intermontane Superterranes are suspected of having undergone several phases of major coastwise displacements when the Coast Plutonic Complex was undergoing large-scale magmatic accretion and crustal recycling.
The Coast Plutonic Complex is divided into two parts, a pre-Late Cretaceous belt consisting of medium to high-pressure batholiths that were emplaced during crustal scale sinistral transpression and southwards displacement of the Insular Superterrane. The second portion is composed of Late Cretaceous to Early Eocene batholiths. The western side of these younger batholiths is defined by the 1200 km long Coast shear zone. Detailed study near 54°N shows the Coast Shear zone underwent dextral transpressive deformation beginning at ∼83 Ma and continuing until ∼59 Ma. Paleomagnetic data indicate this structure could record as much as 1500-km northward displacement of the Insular Superterrane with respect to the Intermontane Superterrane in this time interval.
Following this phase of major dextral transpressive deformation convergence between the Kula Plate and North America became more oblique causing extension across the Eocene magmatic arc. Exhumation during the Early Eocene of the already hot core of the batholith led to decompression melting in the mantle and lower crust and the emplacement of some of the largest batholiths in the Coast Plutonic Complex. Tectonic and erosional denudation of the upper crust led to the exhumation of the batholith and the termination of calc-alkaline magmatism within the batholith by ∼48 Ma.
