The crustal thickness of the Canadian Cordillera, which may have reached 50–65 km during the Cretaceous, is now only 32–38 km along its length. In the south, postorogenic extension during the Eocene resulted in the formation of core complexes and crustal-scale extensional shear zones, whereas in the north evidence for postorogenic extension is either limited to relatively minor basin formation in a transtensional environment or has not been recognized. Using new images of Lithoprobe seismic reflection line 2a in northern British Columbia, which has been previously interpreted in terms of terrane accretion, I make an alternative interpretation of crustal-scale postaccretion extension in the middle and upper crust. As early as the late Permian to Early Triassic, the Quesnellia and Stikinia arc terranes were accreted to ancestral North America along a crustal ramp, which is imaged in the lower crust. An extensional shear zone, which extends from close to the surface to 20 km depth, is identified from the geometry of reflections in the overlying structural basin and a lateral change in seismic velocity. The oceanic Cache Creek terrane, which is <4 km thick in the vicinity of line 2a, was preserved, because it was imbricated with its overlap assemblage and dropped into the 10-km-deep structural half-graben, which may correspond approximately to the combined extent of the Whitehorse trough and the Cache Creek terrane in southern Yukon and northern British Columbia.

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