21.4 Western Canada during the Mesozoic
The Mesozoic extends over 187 million years from the beginning of the Triassic (252 Ma) to the end of the Cretaceous (65 Ma). It was a particularly important period for the geology of western Canada. During this time, several continental collisions occurred along the west coast, resulting in the formation of the Rocky Mountains and the accretion (addition) of much of the land mass of British Columbia, and continuing deposition within the WCSB.
Terrane Accretion in British Columbia and Yukon
Continued subduction along the western edge of North America carried a number of continental terranes toward the coast, with the first collisions taking place in the early part of the Triassic, as the Quesnel, Cache Creek, and Stikine Terranes combined to form the Intermontane Superterrane, so named because it forms the interior plateau of British Columbia, between the Rockies to the east and the Coast Range to the west (Figure 21.13).
Approximately 100 million years later, another pair of terranes — Alexander and Wrangellia — collided to form most of Vancouver Island and Haida Gwaii, plus a significant part of Alaska. During the Cenozoic, additional terranes (the Outboard terranes) were added to the western edge of North America. An overview of the accreted terranes of B.C., Yukon, and Alaska is given in Figure 21.14.
During the Jurassic, the Intermontane Superterrane acted like a giant bulldozer, pushing, folding, and thrusting the existing Proterozoic and Paleozoic west coast sediments eastward and upward to form the Rocky Mountains (Figure 21.15). The same process continued into the Cretaceous as the Insular Superterrane collided with North America and pushed the Intermontane Superterrane farther east. Folding in Rocky Mountain rocks, like that shown in Figures 21.16 and 21.17, is one of the results of this process.
Thrusting is another important process in the formation of fold-belt mountains, as described in Chapter 12. During plate convergence, entire sheets of sedimentary rock are slowly pushed over top of other sheets, resulting in situations where older rocks lie on top of younger ones. One of the best known examples of this is at Mt. Yamnuska, near Exshaw, Alberta (Figure 21.18), where the older Cambrian rocks were pushed east by a total of 40 km, over top of younger rocks.
In the area near the U.S. border, within B.C., Alberta, and Montana, a sheet of Paleozoic rocks has been thrust about 80 km east over top of Cretaceous rocks along the Lewis Fault (Figure 21.19).
Not only did the subduction of oceanic crust beneath North America during the Mesozoic deliver geologically exotic terranes to the western edge of the continent, it also resulted in massive amounts of volcanism along the boundary (Figure 21.13). The upper-crustal magma chambers that fed those now-eroded volcanoes slowly cooled into granitic and dioritic stocks, and those stocks gradually coalesced into batholiths that extend from the southwest corn