The Nevadan orogeny was a very short-lived event in the Late Jurassic that involved the deformation of a great variety of rock types and Paleozoic and Mesozoic terranes throughout the extent of the Sierra Nevada. The Nevadan structures show great variation in style but relatively constant orientations. These relations can be explained by considering the prior histories of the various terranes.
Slaty cleavages and tight folds are the characteristic main-phase structures in the western belt of Jurassic island-arc volcanic rocks and flysch-type sedimentary rocks. A strip of phyllites and greenschists along the eastern edge of the belt apparently represents similar Jurassic rocks that were deformed and metamorphosed at greater depths, probably during underthrusting of the western belt beneath the central belt. The central belt of Paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main-phase structures, from weak, spaced to crenulation cleavages in the south, where polyphase deformed rocks formed a structural basement, to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north, where most of the Paleozoic basement rocks lack penetrative pre-Nevadan fabrics. Eastward-directed thrust faulting apparently was important only in the northern part of the range, where main-phase deformation was most intense. The eastern belt of Jurassic and Triassic magmatic arc-volcanic and sedimentary rocks defines the core of a major synclinorium, and the rocks contain penetrative slaty cleavages and asymmetric, tight to isoclinal folds.
A late phase of Nevadan structures, consisting of northeast-trending cleavages and minor folds, also shows a marked variation in style, from relatively intensely developed in the north to very weakly developed in the south.
The regional extent and geometry of the Nevadan structures indicate that the Nevadan orogeny involved underthrusting of island-arc rocks on the west and significant crustal shortening in the central and eastern belts. These features suggest that the Nevadan orogeny resulted from the collision of the island arc (western belt) with an andean-type arc (eastern belt) situated at the western edge of North America.
- Geological Society of America