Crustal deformation and regional metamorphism across a terrane boundary, Coast Plutonic Complex, British Columbia

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Abstract
The Coast Plutonic Complex between Prince Rupert and Terrace, British Columbia, developed in two stages between mid‐Cretaceous and mid‐Eocene time. The early stage (>100–70 Ma) involved crustal thickening as the amalgamated Alexander and Wrangellia terranes were emplaced against the Stikine terrane to the east. This thickening resulted from tectonic stacking of crustal slabs, lubricated by intrusion of melt generated at the base of the thickened crust. Emplacement of westward directed thrust slabs along the western margin of the orogen was accompanied by intrusion of a high pressure epidote‐bearing tonalite pluton and associated sills. Thrusting resulted in inversion of the metamorphic sequence as supracrustal rocks buried early in the collision were tectonically exhumed. Pluton emplacement also accompanied eastward directed thrusting of high temperature gneisses over low‐grade rocks on the eastern margin of the orogen as the tectonic welt was backthrust over Stikinia. The earliest recognized events in the core of the orogen involve pervasive ductile deformation accompanied by emplacement of tonalite sills between about 85 and 50 Ma. Uplift of the orogen core between 60 and 48 Ma coincided with and was facilitated by emplacement of large volumes of tonalitic magma, accompanied by anatexis and the development of ductile shear zones. One of these shear zones, the Work Channel lineament, presently separates the core of the orogen from the schists to the west. The main orogenic cycle ended by 48 Ma when the rocks cooled rapidly through the biotite and hornblende Ar blocking temperatures. The time and space associations of deformation, metamorphism, and plutonic intrusion imply that substantial differential movement occurred across zones which contained fluid during deformation of the Coast Plutonic Complex. In the region of westward directed thrusting and inverted metamorphism, melts generated in the tectonically thickened lower crust and fluids released during metamorphic recrystallization of supracrustal rocks, which were rapidly buried during tectonic thickening, acted to reduce the strength of the crust. In the core zone of the complex, injections of tonalite magma into the lower crust induced pervasive anatexis. The weakened core zone yielded along both low angle and steep shear zones that were further lubricated by melt as the relatively buoyant hot crust rose rapidly in response to continued compression and underthrusting.