48 NORTH AMERICA/Southern Cordillera Southern Cordillera A W Snoke, University of Wyoming, Laramie, WY, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction The Spanish term ‘cordillera’ refers to a series of parallel ranges or chains of mountains The word was first applied in the western hemisphere to the mountain ranges of western South America, i.e., Las Cordilleras de los Andes In the western United States, the Rocky Mountains and Coast Ranges and mountains between are collectively called the Cordillera In Mexico, the cordillera includes the Sierra Madre Occidental, Del Sur, and Oriental systems, as well as the Sonoran Basin and Range Province and the Baja California peninsula (Figure 1) The focus in this article is on the southern half of the western North American Cordillera, including the mountain belt from $47 N to $16 N, an area that extends >4500 km Thus this region is referred to as the ‘Southern Cordillera’ Diverse physiographic provinces are included in the Southern Cordillera Where crossed by the Fortieth Parallel, it includes, from east to west, the Rocky Mountains, the Basin and Range Province, the Sierra Nevada, the Great Valley, and the Coast Ranges (Figure 1) Farther south, the Colorado Plateau is a prominent physiographic province between the Rocky Mountains and the Basin and Range Province, whereas in the north-western United States the Cenozoic volcanic provinces (Snake River Plain, Columbia Plateau, and Cascade Range) are prominent An important topographic feature in the Southern Cordillera is the tract of high elevation that constitutes the Rocky Mountains and adjacent Colorado Plateau and Great Plains Most of the 14 000-foot or greater mountain peaks in the conterminous United States are within the Southern Rocky Mountains, with the exception of Mt Whitney (14 494 ft; $4421 m) in the Sierra Nevada and Mt Shasta (14 162 ft; $4319 m) and Mt Rainier (14 410 ft; $4395 m) in the Cascade Range Crustal thickness in the Southern Cordillera is strikingly variable The Colorado Rocky Mountains and Sierra Nevada both exceed 50 km in thickness The thinnest continental crust is in the north-western and north-eastern parts of the Basin and Range Province ($25–20 km) and around the northern part of the Gulf of California (Salton Trough), where the crust of south-eastern California and south-western Arizona has been rifted The fundamental nature of the Southern Cordillera is a Mesozoic and Cenozoic history as a convergent margin The subduction of oceanic lithosphere along the Pacific margin and partial subduction of continental lithosphere in the interior of the orogen led to the development of accretionary complexes, magmatic belts, a mobile igneous and metamorphic core zone, and a foreland belt of thin- and thick-skinned contractional deformation In contrast, the Late Cenozoic history of this circum-Pacific orogen is characterized by partitioning of deformation and magmatism into discrete magmatotectonic zones related to ongoing subduction of oceanic lithosphere, evolution of the San Andreas transform-fault system, north-westward migration of the Mendocino triple junction, and intraplate continental lithosphere extension Precambrian Framework The nucleus of North America is the Precambrian craton, commonly referred to as ‘Laurentia’ This Precambrian craton consists of seven Archaean microcontinents welded together along Palaeoproterozoic collisional orogenic belts The Archaean Wyoming province is exposed in basement-involved, Laramide (Late Cretaceous and Early Palaeogene) uplifts of the Rocky Mountain foreland The Wyoming province is bordered by Proterozoic orogenic belts on its northern, eastern, and southern margins, whereas its western margin, which extends into north-eastern Nevada and under the Snake River Plain, is a rifted margin formed during Neoproterozoic and Early Cambrian breakup of the Late Mesoproterozoic supercontinent ‘Rodinia’ Along the southern margin of the Wyoming province, a Palaeoproterozoic geosuture (Cheyenne belt) is well exposed in the Medicine Bow Mountains of southeastern Wyoming, and is interpreted as a Palaeoproterozoic collisional zone between a rifted continental margin (Archaean Wyoming province and overlying Palaeoproterozoic rocks) and a Palaeoproterozoic oceanic supra-subduction complex This collisional orogenic event, the Medicine Bow Orogeny, developed during the interval 1.78–1.74 billion years ago (Ga) It was apparently the harbinger of a series of Palaeoproterozoic accretion events, which continued until $1.65 Ga This long history of lateral accretion of