Abstract

Metamorphosed strata of the Ritter Range pendant record structures formed during the emplacement of granitic plutons at upper crustal levels (<8 km) in the Late Cretaceous Sierra Nevada magmatic arc. Paleozoic to Middle Jurassic strata of the pendant have been penetratively shortened 30%–50%, forming slates, phyllites, and schists with bedding-inclined cleavage and associated linear structures. The cleavage, secondary cleavages, and folds associated with these cleavages have orientations and geometries consistent with dextral transpression between the Farallon and North American plates. Microstructural relations show that cleavage development accompanied greenschist to amphibolite facies metamorphism. Metamorphic hornblende, muscovite, and biotite in the wall rocks show weak to strong preferred orientation and have 40Ar/39Ar plateau ages ranging from about 85 to 80 Ma. Hornblende, and possibly muscovite, are expected to retain 40Ar at or near estimated peak metamorphic temperatures of ∼500 °C, thus their 40Ar/39Ar ages closely correspond to the time of cleavage formation. Granitic plutons adjacent to the Ritter Range pendant have yielded U-Pb and other ages that cluster from 91 to 82 Ma. We conclude that bedding-inclined cleavage and metamorphism in the Ritter Range wall rocks developed in a dextral transpressive strain regime penecontemporaneously with emplacement of adjacent granitic plutons.

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