Abstract

In its broadest part the North American Cordilleran system displays a heterogeneity of structural type and of geologic history which is unusual among the large mountain systems. There are four major regional structural types and superimposed on them are younger, different types of deformation.

The first of the four original regional types of Cordilleran deformation is that of the western Paleozoic-Mesozoic volcanic geosyncline. The belt described forms a huge arc, "Arc of the Pacific Northwest".

The second regional type is a sedimentary geosyncline and comprises all of the northern Rockies, the western most part of the central Rockies and continues on the west side of the Colorado Plateau into southernmost Nevada and southeastern California.

The two remaining structural types of the Cordilleran system are non-geosynclinal. The first of these is the Colorado Plateau and the second is the non-geosynclinal Rockies.

Superimposed on the four original or primary regional types of Cordilleran structures are later elements of different character. The most widespread is basin and range structure. Other types of super-imposed structures are the Cenozoic folds of the Pacific Coast Ranges, the Tertiary basaltic plateau eruptions of the Columbia-Snake River region and the young, broad downwarps such as the Gulf of California.