There are three longitudinal divisions of the western cordillera: the Coast Range belt along the Pacific margin, the Nevadan belt next inland, and the Laramide belt, farthest inland. A fourth tectonic division is superposed on parts of the Nevadan and Laramide belts: the Basin-Range system. Ancestral to these Mesozoic and Cenozoic mountain systems was a late

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Precambrian and Paleozoic geosyncline with marginal eugeosyncline and inward miogeosyncline divisions. The Antler orogenic belt of central Nevada evolved in Mississippian and Pennsylvanian times, and thenceforth clearly separated the eugeosyncline from the miogeosyncline. The Ancestral Rockies as range-size uplifts in Colorado, Utah, Arizona, and New Mexico also came into being during Pennsylvanian time.

Deformational and igneous phases in the eugeosyncline after the Antler orogeny continued until the climactic phase in mid-Cretaceous time. Then, most of the voluminous batholiths were emplaced. With the batholiths and the deformed and metamorphosed eugeosynclinal sediments, the Nevadan belt had been brought into being.

In Alaska, the Nevadan belt makes up the Alaska and associate ranges from the Yukon Valley to Cook Inlet. In British Columbia, the Interior Plateaus, the Coast Range, and the island ranges comprise the Nevadan belt. In the western United States it extends from western Nevada through the Sierra Nevada of California to the Coast Ranges. In fact, the basement of the Coast Ranges is a Nevadan complex. The core of the Nevadan belt extends the length of the peninsula of Baja California.

The miogeosyncline and an adjacent belt of the shelf were deformed in Cretaceous and early Tertiary times to form the Laramide Rockies. In Alaska the Brooks Range, as a geanticlinal uplift, and the fold system of the Yukon Valley constitute the Laramide Rockies. The Canadian Rockies and Foothill or Disturbed belt extend the Rockies into Montana. In the western United States two divisions of the Rockies occur: the ranges caused by deformation of the miogeosyncline, and the ranges created by deformation of the shelf. A complex system of folds and thrusts characterizes the miogeosyncline division, whereas individual elongated uplifts with intermontane basins characterize the shelf. The writer resolves the folds and thrusts of the miogeosyncline into gravitational glide structures margina to several primary uplifts, and into shallow compressional structures between the uplifts, but other geologists do not share this view.

The Laramide Rockies extend into central Mexico, principally as a fold system.

The Cenozoic Era in the western cordillera was a time of extensive volcanism and tensional faulting. The two are so closely related temporally and spatially that they must be related genetically. The faulting created the Basin-Range system and the Rocky Mountain trench. Most of the Basin-Range province is laden with volcanics, and among the numerous volcanic fields is the great Sierra Madre Occidental of Mexico and the basaltic extrusions of the Cascades and the Columbia River Plateau. The Pacific margin of Mexico and the United States is being deformed by major strike-slip faults, with the Pacific block moving northwest.

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