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Photograph of mylonitic and cataclastic rock in the Santa Rosa Mountains, southern California; pencil is about 6 inches (15 cm) long. These rocks occur in the lower plate of the eastern Peninsular Ranges Shear Zone, a zone of crustal movement and dislocation that occurs along the east margin of the San Jacinto and Santa Rosa Mountains and in various locations within and east of the Borrego Desert. The original rock type was plutonic granodiorite and tonalite that has been subjected to high-strain squeezing, stretching, and grinding of the mineral crystals to yield a rock whose original grain-to-grain relationships have been completely modified; such a high-strain rock is known as cataclasite or mylonite, depending on the brittle or ductile nature of the deformation style. The conspicuous streaking parallel to the pencil forms what geologists call a "lineation", in this case the streaking out of light-colored (felsic) and dark-colored (mafic) minerals, quartz and biotite, respectively. The streaking probably reflects a stretching-type stress applied to the mineral crystals after they had cooled from their plutonic magma. Photo by J.C. Matti, USGS, June, 1980.

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