FORMATION OF NEW CONTINENTAL CRUST IN WESTERN BRITISH COLUMBIA DURING TRANSPRESSION AND TRANSTENSION, BASED ON DATA FROM THE 1993-2000 ACCRETE CD PROJECT

Crustal growth in the Coast Mountains of British Columbia, along the leading edge of the Canadian Cordillera, was the result of processes associated with horizontal flow of material during transpression and subsequent transtension, and the vertical accretion of mantle derived melts. From 85 to 58 Ma, as exotic terranes were translated northward during transpression, the crust was thickened to about 55 km, and melt that originated from a mix of mantle-derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones. When the orogen extended, between 58 and 50 Ma, there was large-scale decompression melting in the mantle and dehydration melting in the lower crust. Voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones. By 50 Ma, the final crustal thickness of the new continental crust was about 34 km. Comparison of seismic data with other crustal sections suggests that the crust-forming processes identified in western British Columbia have general applicability to models for the formation of continental crust.