Crustal Formation at Depth During Continental Collision
The rapid (1–2 mm/yr) and substantial (over 10 km) Tertiary exhumation of the Coast Crystalline Complex near Prince Rupert, British Columbia led to the exposure of rocks formed at crustal depths of 15-20 km and greater. Abundant sills, mostly tonalite in composition, delivered a thermal pulse which locally raised temperatures to the low-pressure granulite facies within the intruded section. The thermal pulse and exhumation are recorded by the metamorphic mineral assemblages and textures, and by isotopic cooling ages. The net result of the thermal and deformation events is a sequence more than 10 km thick of interlayered migmatite, tonalite sills, ductile high strain zones, sillimanite- and garnet-rich rocks (stronalite), other metasedimentary rocks, and amphibolite. Foliations in all units dip gently except where cut by near vertical ductile shear zones, within which foliations are sub-parallel to the shear zone boundaries. During uplift, strain was concentrated in the ductile shear zones. If the section had not been exhumed, seismic reflection profiling would show shallow dipping reflectors in the deep crust due to the contrast in density and fabric of the layers.