Structure and tectonics of the northern Sierra Nevada

Abstract

Recent detailed mapping suggests a new working hypothesis for the structure of the northern Sierra Nevada. We propose that pre-Cretaceous rocks are deformed by a series of eastward-directed over-thrusts modified by west-directed folds and faults. The highest and westernmost tectonic unit is the Jurassic Smartville complex. Structurally below it, there are imbricate thrust slices of Jurassic and older ophiolitic and oceanic sedimentary rocks in the Central belt and of Paleozoic to Jurassic sedimentary and volcanic rocks in the Eastern belt. The Paleozoic Feather River peridotite separates the Eastern and Central belts, and our hypothesis suggests that both of its margins may be important thrust faults. The east-directed faults and folds are deformed by northwest-trending, upright, and west-vergent folds and reverse faults that control the present outcrop pattern. These later structures have so modified the earlier east-directed structures that the latter have been recognized only recently. An important key to understanding the structure has been the recognition of ophiolitic complexes that can be correlated across major faults and that contain a pseudostratigraphy useful in determining local structures. The events recorded by both the east- and west-directed deformations occurred during Callovian through Kimmeridgian time and represent the Nevadan Orogeny. Our hypothesis of early east-directed overthrusts followed by west-directed back folding and faulting implies shortening and thickening of the crust during the Nevadan Orogeny and is consistent with the idea that the northern Sierra Nevada is the result of a crustal collisional process.