NORTH AMERICA/Northern Cordillera 43 distinctive record of Neoproterozoic and Early Palaeozoic arc magmatism and orogeny; suggestions have been made for its origin in regions as far apart today as eastern Australia and the Barents Sea, with the latter currently favoured Northern Cordilleran Mountain Building The final accretion of terranes to the former continental margin started in the Early Jurassic and was associated with the subduction of oceanic lithosphere beneath the western North American Plate, accompanying arc magmatism, and the collapse of back-arc basins By the Late Cretaceous, almost the entire region was above sea-level For much of its length, the northern Cordillera features two tracts of Mesozoic and Palaeogene high-grade metamorphic rocks that are loci of the greatest amounts of burial, crustal thickening, differential uplift, and erosion and/or tectonic exhumation The metamorphic rocks are located in internal parts of the Rocky Mountains and Pacific Mountains Systems and are flanked on both sides by far less metamorphosed rocks in much of the remainder of the Cordillera (Appendix A) Some metamorphic rocks in Intermontane Alaska and Yukon (in the Ruby, Seward, and Yukon–Tanana terranes) are older Starting in the latest Early Jurassic ( 185 Ma), terranes (Cache Creek, Quesnel, Stikine, and Slide Mountain) in the Intermontane Plateau System in Canada were thrust towards the continent and deeply buried former continental margin rocks that are exposed today in internal parts of the Rocky Mountains System Metamorphism along the southern Arctic Alaska Terrane and in parts of Intermontane Alaska similarly appears to be related to Middle Jurassic– Early Cretaceous thrusting of oceanic rocks and arc terranes (Angayucham, Koyukuk, Nyac, and Togiak) on to Arctic Alaska Beginning in the mid-Cretaceous ($95 Ma), the terranes (Alexander, Wrangellia) near the present coast tectonically overlapped the previously accreted terranes, mainly preserved in the Intermontane Plateau System, along a boundary located in the internal part of the Pacific Mountains System Compressional structures in the remainder of the northern Cordillera can be directly related to those in the two tracts of high-grade metamorphic rocks (Figure 4) External parts of the Rocky Mountains System contain Cretaceous and Paleocene thrust faults that carried miogeoclinal and older deposits for distances of up to 200 km over the continental platform In the Pacific Mountains System, Late Cretaceous to Holocene folds and thrust faults mostly verge oceanward In the Intermontane Plateau System in Canada, oceanward-verging folds and thrust faults are Jurassic, whereas those directed towards the continental interior are mainly Cretaceous Convergent deformation is contemporaneous with uplift, erosion, and deposition in marine to non-marine basins that are both internal and external to the Cordillera (Figure 4) In western Alaska, Cretaceous and Palaeogene convergence between the North American and Eurasian Plates may have driven the 40–60 counterclockwise rotation of south-western Alaska Rotation, combined with dextral displacement on major faults in Intermontane Alaska, created the curvilinear grain of the Alaskan Cordillera and its expanded western termination Late Cretaceous, Tertiary, and Holocene dextral strike-slip faults lace the northern Cordillera and apparently reflect oblique subduction and partial coupling of the western North American Plate margin to the northward-moving Pacific Plate and to its precursor, the Kula Plate, which had been entirely subducted by $37 Ma (Figure 4) The total amount of displacement on these faults, as determined from geological features offset across them, approaches 1000 km In the north, there are displacement amounts of 425 km on the Tintina Fault and up to 400 km on the Denali Fault, and, in the south, about 300 km on combined Fraser and Yalakom Faults A major unresolved discrepancy exists between the above numbers and those arising from palaeomagnetic studies on Late Cretaceous rocks ( 85 Ma), which suggest dextral displacements of as much as 3500 km for parts of the western Cordillera In addition to the above, a sparse record of Early Cretaceous sinistral, Early to Middle Jurassic dextral, and Triassic–Jurassic sinistral strike-slip faulting was possibly responsible for the fragmentation, displacement, and duplication of arc terranes in the collage prior to their final accretion and further disruption by dextral faulting In the Pacific Mountains System, a Middle Jurassic–Early Cretaceous island arc founded on Wrangellia and Alexander terranes was apparently displaced southward in Early Cretaceous time There is about 800 km of overlap between it and the northern end of the contemporary continental arc in the southern Intermontane Plateau and internal Rocky Mountains Systems (Figure 4) In addition, Early Mesozoic oroclinal bending, or left-lateral strike-slip faulting, has been invoked to account for apparent duplication of a Quesnel–Stikine Arc and enclosure of the associated Cache Creek Accretionary Complex (Figure 3) Palaeogene (58–40 Ma) normal faults are widespread south of latitude 55 N in the Intermontane System and in parts of the Pacific and Rocky Mountains Systems Many faults are oriented north–northeasterly or north-easterly, at angles to the prevailing