Much of the North American Cordillera is a tectonic collage of allochthonous crustal fragments accreted to the western margin of the craton from middle Paleozoic time onward. This

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feature is obscured by the three, major, through-going, late Mesozoic-early Tertiary elements of the Cordillera, namely (1) the eastern fold and thrust belt, formed largely from the displaced Proterozoic to Jurassic sedimentary prism (or miogeocline) that once fringed the craton, (2) the western accretionary prisms or subduction complexes, and (3) extensive granitic rocks. These impose a superficial simplicity on this 8,000-km-long fold belt. Its intrinsic complexity is reflected by the segmented nature of the Cordillera.

The Alaskan-western Yukon, Canadian-northwestern United States, California-Coloradan, and Mexican segments each have distinctive geologic characteristics different from those of adjoining segments. In part, these differences are due to features formed late in Cordilleran history, such as pervasive, dextral strike-slip faults in Alaska and Canada, and rotations and extensions in the conterminous United States. Other, far more fundamental, differences emerge from stratigraphic analysis of rocks west of the fold and thrust belt.

The western rocks can be divided into more than 40 terranes, each with distinctive laterally persistent stratigraphy which is different from those of neighboring terranes, and each is commonly separated from neighboring terranes by major faults. Each terrane consists of one or more tectonostratigraphic assemblages, interpreted mainly as deposits of volcanic arcs and ocean basins but including some rocks of probable continental origin. In addition, many terranes have paleontologic and paleomagnetic records different from those of coeval, colatitudinal deposits on the craton, so that the western Cordillera is interpreted as a collage of small crustal fragments accreted in different ways and times to the western margin of North America.

The Alaska-western Yukon segment is largely composed of small terranes, many of southerly derivation, each of which apparently remained a discrete entity until accretion late in Mesozoic time. The Canadian-northwestern United States segment is dominated by two large composite terranes made up of smaller fragments, again mainly derived from the south, that coalesced prior to accretion. The inner terrane accreted to the miogeocline in the Jurassic and the outer to the inner in the Cretaceous. Boundaries of the two composite terranes with one another and the miogeocline coincide with the two metamorphic, granitic, and deformational welts that dominate the Canadian Cordillera. Because the times of development of these welts also are the times of accretion, the welts are interpreted as bei g due largely to collisions of crustal fragments, rather than subduction of oceanic crust with accompanying upper plate magmatism. The California-Coloradan segment reflects successive accretions of small discrete fragments from middle Paleozoic time onward, together with possible removal of terranes, some of which subsequently possibly lodged farther north in the Cordillera. Recent studies in Mexico suggest that it, too, is made up of allochthonous fragments.

From the foregoing, it should be obvious that only the most rigorous and detailed geologic studies will enable us to understand the evolution of such a long-lived mountain belt as the Cordillera. This is only to be expected from the rapidly changing, complex patterns of movement known from recent plate movements.

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