The Vulcan Peak alpine-type peridotite forms part of the Josephine ultramafic complex in the Klamath Mountains geologic province. The peridotite is a partly serpentinized highly deformed harzburgite-dunite complex, in which three episodes of high-temperature plastic deformation are recognized. The first deformation was the most intense and produced the dominant metamorphic foliation and scattered folds in crosscutting layers and in the foliation itself. The first deformation seems also to have produced an olivine fabric in which X is normal to the foliation. The second deformation superposed a similar and pervasive fabric on the first, in which X olivine is normal to a spotty weak subvertical north-striking foliation that crosscuts the first foliation. These olivine fabrics are analogous to fabrics produced experimentally by either gliding or syntectonic recrystallization at temperatures in the range 1000° to 1200°C. This temperature range agrees with the temperatures of formation calculated from the distribution of Mg and Fe in mineral pairs. The third deformation was characterized by a limited plasticity, in which deformation was restricted to scattered narrow northeast-striking subvertical plastic shear zones. The sense of movement on the shear zones is consistently down on the northwest. A homotactic olivine fabric is present in the shear zones, consisting of a strong Z-point maximum approximately parallel to the zone. This fabric suggests glide on the system {Ok1} [100], which has been produced experimentally in the temperature range 800° to 1000°C.

After the high-temperature and presumably deep-seated plastic deformation, the relatively cold peridotite was thrust, probably in post-Middle Jurassic time, northward against a complex of igneous and high-grade metamorphic rocks. Later, probably in Late Cretaceous or Tertiary time, the peridotite and the complex were thrust together westward against the low-grade Dothan Formation.

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