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Strong contrasts in the tectonic evolution of the three principal parts of the Intermontane System (Basin and Range, Colorado Plateaus, and High Lava Plains) are responsible for strong contrasts in crustal structure outlined elsewhere in this volume. Extremes in Cordilleran history are represented by the interior of the Colorado Plateaus and the Basin and Range province. The plateaus have enigmatically escaped strong deformation and magmatism since Precambrian time, whereas most parts of the Basin and Range have experienced repeated orogenesis, the youngest of which is continental magmatism and extensional deformation with dimensions and magnitudes that may not be exceeded anywhere in the world. Between these extremes lie the High Lava Plains, parts of which appear to be dominated by the passage of a single major pulse of magmatism and rifting that produced crust no older than late Cenozoic age. Numerous crustal-scale tectonic and magmatic events beginning in Archean time and extending into Cenozoic time are chronicled as events having a potential for molding crustal structure. They include protracted Archean and early Proterozoic south- or southwest-directed continental growth or cratonization by sparsely recorded processes of deformation, sedimentation, magmatism, and metamorphism, followed in middle Proterozoic time by epicratonal basin sedimentation and in the late Proterozoic by reshaping of the western continental margin by passive-margin rifting and foundering. Late Paleozoic and early Mesozoic continent-margin collisional events led to the obduction of the Roberts Mountains and Golconda allochthons and accretion of related terranes. A second event of continent-margin truncation and reshaping occurred in early Mesozoic time. It was followed during Mesozoic time by major east-directed subduction of oceanic lithosphere and associated accretions of volcanotectonic terranes to the west margin of the continent. Genetically related magmatism, metamorphism, and compressional tectonism reached far inboard of the continental margin; these were guided to some extent by preexisting tectonic features. During Cenozoic time, igneous activity of colossal magnitude swept through the Cordillera along regular paths without apparent regard for preexisting tectonic features. Coeval and subsequent extensional tectonism teamed with the magmatism to reshape the geophysical framework of much of the Basin and Range and High Lava Plains. Accordingly, it is a major challenge to recognize the fingerprint of ancient major crustal-scale events within the current geophysical framework.

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