Introduction to the special issue of Canadian Journal of Earth Sciences: The Nechako NATMAP Project of the central Canadian Cordillera
The Canadian Cordillera in central British Columbia has seen the Mesozoic subduction of an oceanic terrane; the amalgamation of volcanic-arc terranes; continued intermittent Mesozoic compression and magmatism; and Tertiary wrenching, extension and magmatism. Except in its northernmost mountain ranges, the area is extensively cov- ered in glacial drift and thin veneers of Tertiary volcanic rocks. In 1994, a group of scientists and technologists be - lieved they could understand that cover, see through it, and discover the components of that collision and extensional orogen. They would apply modern techniques of isotopic and paleontological geochronology; lake-sediment, till, and plant geochemistry; detailed gravity, magnetic, radiometric, paleomagnetic, and electromagnetic surveys; and isotopic and trace element lithochemistry, as they conducted extensive bedrock and surficial mapping. This special issue summa- rizes a cross-section of the scientific contributions derived from that mapping conducted under the auspices of the Nechako NATMAP Project. It demonstrates the absolute necessity of applying modern isotopic and paleontologic geo- chronology to understand the Phanerozoic geology of the Cordillera. It emphasizes the necessity of detailed aeromag- netic surveys (500 m or less line spacing) in looking through covered terranes at anything more than 1 : 250 000 scale. And, it shows the immense utility of applying various geochemical techniques to solve geological problems and estab- lish baselines for future research and economic development. Bedrock and surficial mapping in the central Cordillera, using these and other techniques, have established the nature and timing of Mesozoic crustal growth, Tertiary crustal thinning, and the associated formation of mineral deposits.