44 NORTH AMERICA/Northern Cordillera general grain of this part of the Cordillera, and some clearly are associated with movement on dextral strike-slip faults Normal faults bound graben and half-graben structures containing coeval clastic and volcanic rocks, and many, especially in south-eastern British Columbia and adjoining parts of Idaho and Montana, contributed to the tectonic exhumation of formerly deeply buried rocks in metamorphic core complexes, in some of which Palaeoproterozoic continental basement is exposed Economic Deposits Hydrocarbon deposits occur mainly in the Rocky Mountains System and include oil and gas originating from Palaeozoic and Early Mesozoic strata, and coal in foreland basins Coal was formerly exploited in Late Mesozoic and Early Tertiary basins in the western Cordillera, and oil and gas are known in rocks of the Pacific continental shelf Metallic mineral deposits occur widely Interior parts of the Rocky Mountains System contain widespread lead, zinc, and silver deposits, two of which, the former Sullivan Mine in south-eastern British Columbia and the active Red Dog mine in northwestern Alaska, are some of the largest known The Intermontane Plateau and Pacific Mountains Systems contain several large copper and molybdenum deposits, with associated golds in porphyry systems in arc-related plutons Placer gold deposits were of historical importance, but currently are less so Evolution of the Northern Cordillera The origins of the northern Cordillera extend back some 770 Ma to rifting events that marked initial stages in the break-up and dispersal of components of the Rodinian supercontinent By Early Cambrian time ($540 Ma), rifting had culminated in the seafloor spreading that opened the distant ancestor of the Pacific Ocean basin and created the Laurentian continent–ocean boundary that now is embedded in the Rocky Mountains System The intraplate continent–ocean boundary persisted until the Middle Devonian ($390 Ma), when it was succeeded by a convergent plate boundary that initially generated arc magmatic rocks within distal continental margin strata in a belt that extends from California to the Canadian Arctic Arc magmatism has continued until the present, but has varied greatly in character and volume From the Early Carboniferous until the Early Mesozoic (355–185 Ma), the period spanning amalgamation and eventual break-up of the supercontinent called Pangaea, the convergent plate boundary lay well offshore and featured island arc chains (Quesnel, Stikine) separated from the continental margin by back-arc basins (Slide Mountain) The region offshore western Pangaea probably was similar to the present western Pacific Ocean basin The accretion of arc, back-arc basin, and accretionary prism rocks and former continental margin fragments to the old continental margin started in the Jurassic at about 185 Ma The accretion records the initiation of Cordilleran mountain building and coincided with the break-up of Pangaea and the advance of the North American continent towards the trench on its western margin It resulted in the relocation of the continental margin some 500 km oceanward of its original position by the Late Cretaceous ($90 Ma), when the Cordillera probably resembled parts of the modern Andes During and following accretion, the collage and parts of the former continental margin were sliced and shuffled by strike-slip faults associated with oblique plate convergence, stretched and dismembered by extension and, in Alaska, apparently rotated counterclockwise Most of the major physiographical features of at least the Pacific Mountains System, and probably much of the remainder of the northern Cordillera, probably emerged in the later Neogene ( 12 Ma) Appendix A: Northern Cordilleran Physiographical Systems and Underlying Bedrock Geology The names Rocky Mountains, Intermontane Plateau, and Pacific Mountains Systems, used for the major physiographical divisions in Alaska, are approximately, but not completely, equivalent to the Eastern, Interior, and Western Systems used in British Columbia Features east and north of the Northern Cordillera: Interior Plains System, Arctic Coastal Plain, and Arctic Ocean Rocky Mountains System External part Some major physiographical units: Montana and Canadian Rocky Mountains, Mackenzie Mountains, northern Brooks Range Bedrock: (1) Archaean and Palaeoproterozoic continental basement; (2) Mesoproterozoic (locally Palaeoproterozoic) clastic, carbonate, and minor magmatic rock associated with rift-related basins; (3) Neoproterozoic clastic, minor carbonate, and mafic magmatic rocks formed during rifting of the Rodinian supercontinent; (4) Cambrian to Jurassic carbonate and shale deposited on platform, shelf, and slope of Laurentian continental margin; and