In Cape Breton Island and southwestern Newfoundland, the Appalachian orogen is extremely narrow; the rocks were strongly deformed and underwent high-grade Barrovian-type metamorphism in Silurian time, and the orogen-parallel Silurian thrusts are dominantly west-vergent, antithetic to the westward subduction. This area is bounded by two transverse dextral wrench faults, the Canso fault to the southwest and the Gunflap Hills fault to the northeast. These tectono-stratigraphic, structural, and metamorphic features are explained by Silurian collision between the St. Lawrence promontory on the Laurentian margin and the Cabot promontory on the Avalon margin during the final closure of the Iapetus ocean. The west-vergent antithetic thrusting is interpreted to be related to tectonic wedging, while the two transverse wrench faults accommodated differential movement from promontories to reentrants during the collision. Promontory-promontory collision should be a common process and probably played a significant role in shaping the tectonic framework of many orogenic belts.

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