Riverside Mountains

The Riverside Mountains are one of several ranges that constitute the Maria Fold and Thrust Belt (MFTB). The Maria Fold and Thrust Belt underwent generally thick-skinned (involving basement rocks) north–south-trending crustal shortening in the Cretaceous. The structures of the MFTB are exposed by to later generally east–west-trending large-scale crustal extension in the Miocene, through what is known to geologists as the Colorado River Extensional Corridor. This north–south shortening is anomalous, as crustal shortening in the rest of the North American Cordillera is oriented generally east–west because of the generally east–west compression that was due to the subduction of the Farallon plate under western North America. Also unlike the rest of the North American Cordillera, deformation in the Maria Fold and Thrust Belt involved rocks of the North American Craton, most notably the Grand Canyon sequence of sedimentary rocks.

The Riverside Mountains contain rocks from both the lower and upper plates of a large detachment fault and metamorphic core complex system as a result of the Miocene-age extension. The lower plate consists of a stack of metamorphosed units, which comprise Jurassic metavolcanics, the metamorphosed Grand Canyon sequence, the metamorphosed McCoy Mountains Formation and related Mesozoic rocks, and the Precambrian basement. The upper plate of the detachment fault consists of a small sedimentary basin containing Tertiary-age syntectonic (deposited during tectonic activity) volcanic units, conglomerates, and other sedimentary rocks.