Allochthons in the Cassiar Mountains of northern British Columbia contain assemblages belonging to two distinct Canadian Cordilleran terranes, Yukon–Tanana (YTT) and Quesnellia. These assemblages, of pre-Late Devonian, Devonian–Mississippian, Pennsylvanian–Permian, and Early Jurassic age, occur in intrusive and depositional, as well as structural, contact with each other. The allochthons are gently dipping thrust panels, interrupted by the mid-Cretaceous Cassiar Batholith. A key element for correlation across the batholith is the Mississippian and older pericratonic Dorsey Complex. New Devonian–Mississippian U–Pb ages for deformed plutons within it document an igneous suite like those in type Yukon–Tanana exposures farther north. Other characteristics of the Dorsey Complex that ally it with YTT are orthoquartzites and grits, and amphibolite bodies with transitional mid-ocean ridge basalt (MORB) to ocean-island basalt (OIB) petrochemical signatures. Unconformities, deformed clasts in the late Paleozoic sequences, and a shared mid-Permian intrusive suite show that later arcs onlapped the mid-Paleozoic and older YTT assemblage. The Early Jurassic intrusive suite cuts all major contacts and fabrics except the terrane-bounding fault between the Slide Mountain and combined YTT–Quesnel terranes. It represents a northern continuation of a plutonic belt that extends the length of the Mesozoic Quesnel magmatic arc. These relationships carry important implications for Cordilleran terrane history and the tectonic evolution of the North American margin. At least some of the major terranes were not unrelated entities prior to their accretion to the continent, but a system of superimposed and interconnected arcs that developed over a protracted time interval, with complex and evolving paleogeographic configurations much like the modern western Pacific province.