NEVADAN OROGENY

a mountain-building event in the Sierra Nevada region of eastern California, believed to have taken place in the latest Jurassic time (about 144 million years ago). The term now is generally expanded for numerous orogenic pulses in the western portion of the Cordilleran Geosyncline of western North America that range in age from Late Jurassic to Early Cretaceous (from about 163 million to 97.5 million years ago). Evidence for the early phases of the Nevadan orogeny consists of Late Jurassic igneous intrusions and folding and thrust faulting on the western slope of the Sierra Nevada and in the Klamath Mountains of northern California. Medial and late phases of the Nevadan orogeny are represented by the formation of vast batholiths (igneous bodies greater than about 104 square km [40 square miles] in area) of Early Cretaceous age in southern California, the Sierra Nevada, in Idaho, and in British Columbia. Folding and faulting of strata in western Nevada also may be attributed to the Nevadan orogeny. The Nevadan orogeny may have been the result of underthrusting of the western portion of the North American Plate by oceanic crust, along a former oceanic trench located in the western portion of the Cordilleran Geosyncline.