Evolution of Western North America
The following brief synopsis of the the geologic evolution of the western half of North America is perhaps the best way to start an examination of the physiographic regions of the western United States (Figure 105).
At depth, ancient crystalline basement rocks of Precambrian age underlie most of the United States. Crust that had assembled into the stable cratonic shield in Precambrian became foreland basins where sediments accumulated across much of Midwest and Great Plains. At the same time, sediments spread accumulated in the Pacific Ocean margin basin (Figure 105A). In the early Paleozoic Era, shallow seas transgressed and retreated several times onto the stable continental foreland. Thick packages of sedimentary rock, mostly marine limestones, accumulated across much of what is now the western United States.
By Mid Paleozoic time the western foreland margin began to undergo tectonic changes with a mountain-building event called the Antler Orogeny (Figure 105B). The Antler orogeny impacted the impacted the continental margin region beginning in Late Devonian and continuing into early Pennsylvanian time. The orogeny impacted what is now the Nevada region, but the effects may have been more widespread.
Over time the Nevadan Orogeny transitioned into the Sevier Orogeny, a mountain-building period that affected the entire Cordilleran region (Canada to Mexico), lasting from 140 million to 50 million years ago. The orogeny was produced by the subduction of the Farallon Plate beneath North America. Tectonic compression and sub-crustal heating pushed up thrust-faulted mountain belts in what was previously the foreland shelf and continental margin basin regions (western Utah and Wyoming regions). The weight of the ranges and the sediments they contributed worked to push down the mid continent region, allowing the Western Interior Seaway to flood across the continent (Figure 105E, see also Figure 68).
By Pennsylvanian time, tectonism impacted the foreland region, producing the ancestral Rocky Mountains (Figure 105C). Along the West Coast, sediments continued to accumulate along the continental margin and shallow warm seas flooded onto the continental platform. Small island arcs formed along westward-dipping subduction zones. some of these small island arcs were eventually ripped apart and accreted as terranes onto the Pacific continental margin.
The breakup of supercontinent Pangaea in Mesozoic time changed western North America from a passive to an active continental margin. Subduction changed from west-dipping
to east-dipping, and volcanic arcs began forming along greater Cordilleran mountain belt)( Figure 105D). The Nevadan Orogeny in Mid- to Late-Jurassic time (180 to 140 million years ago) resulted in the formation of the Cordilleran volcanic arc which included the ancestral Sierra Nevada and Peninsula Ranges of California.
The mountain building continued to migrate eastward during the Laramide Orogeny. The Laramide Orogeny occurred in a series of pulses of uplift and volcanism that impacted the greater Rocky Mountain Region between about 80 million to 35 million years ago (Figure 105F). Laramide mountain building overlapped the Sevier mountain building further west. Current thought is that North America overrode the Farallon Plate at a rapid pace and a low angle, disrupting subduction or moving it further east. Sediments from Laramide ranges gradually filled in the Western Interior Seaway basin.
In late Cenozoic time, North America began to override the spreading center that formed the boundary between the Farallon Plate and the Pacific Plate (Figure 105G). As the spreading center as moved eastward under North America, crustal compression changed to crustal extension, resulting in the opening of formation of the Great Basin and Basin and Range structural regions. Sub-crustal heating resulted in the regional rise of the entire region including the Rocky Mountains, Colorado Plateau, and western Great Plains.
Fig. 105. Generalized geologic evolution of western United States from Late Precambrian time (1 billion years) to the present.