In the Preston Peak area, Klamath Mountains, California, a regional thrust fault separates metasedimentary rocks of the Late Jurassic Galice Formation from an overlying plate of older ophiolitic rocks. The ophiolite consists of a basal sheet of ultramafic tectonite overlain and intruded by a heterogeneous mafic complex that in turn is overlain by metabasaltic and metasedimentary rocks.

Field relations indicate that the ophiolite is polygenetic, with a major temporal hiatus separating the tectonitic ultramafic rocks and the associated mafic rocks. Mineral assemblages and textures in the ultramafic rocks suggest high-temperature recrystallization and penetrative deformation. In contrast, diabase and diabase breccia, the most abundant constituents of the mafic complex, are nonschistose rocks metamorphosed to lower greenschist facies. Contacts between ultramafic rocks and rocks of the mafic complex are fault contacts, intrusive contacts, or both. Mafic rocks occur in the ultramafic rocks as diabase dikes with chilled margins and as tectonic inclusions. Piecemeal growth of the ophiolite is also indicated by minor features: scarce jackstraw-textured talc-olivine rocks in tectonitic peridotite, cognate xenoliths of gabbro and olivine clinopyroxenite in diabase, and scattered dikes of intermediate composition in both ultramafic and mafic rocks.

Field aspects of the ophiolite appear more compatible with a primitive island-arc setting than with a spreading oceanic ridge or marginal-basin model. The temporal relations between the ultramafic and mafic rocks, the presence of pyroclastic breccias, and the character of associated epiclastic rocks support this hypothesis. On the basis of this interpretation, the tectonic history of this segment of the Klamath Mountains during late Paleozoic to Jurassic time was dominated by island-arc genesis and westward extensional rifting. The ultimate collapse of this system occurred during the Late Jurassic (Nevadan orogeny) when the Galice Formation (Jurassic island arc and associated sedimentary basin) was thrust beneath the Preston Peak ophiolite, a Permian-Triassic remnant arc.

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