The Late Cretaceous to early Tertiary Great Tonalite Sill of southeast Alaska and British Columbia is a very long (∼1,000 km) and thin (<25 km), orogen-parallel, composite batholith, which lies between the Insular superter- rane (including the Alexander and Wrangellia terranes) and the Intermontane superterrane (including the Stikine and Cache Creek terranes). The batholith is delineated by many steep, sheet-like plutons, which are dominated by northwest- southeast-striking concordant fabrics with steep lineations that formed during deformation in a country-rock shear zone prior to the complete crystallization of the magmas. Deformation in this shear zone is dominated by northeast- southwest-directed contraction orthogonal to the orogenic strike, associated with a component of northeast over southwest, high-angle shear. The steep, multiple-dike-like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike-wedging mechanisms along the deep-reaching shear zone. The remarkable narrowness of the Great Tonalite Sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length. Such a shear zone of Late Cretaceous to early Tertiary age, lying along 800 km of the possible boundary between the Insular and Intermontane superterranes, implies that it may represent the actual boundary between them. If this hypothesis is correct, it implies that the large-scale tectonic regime during emplacement of the Great Tonalite Sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data.