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Neogene doming in the north-central Klamath Mountains, California, tilted the Rattlesnake Creek terrane, chiefly an ophiolitic mélange, exposing an oblique cross section through disrupted and metamorphosed oceanic crust and mantle. The deepest section of the tilted terrane, in the Kangaroo Mountain area near Seiad Valley, contains tectonic slices of ultramafic, mafic, and sedimentary rocks that were penetratively deformed and metamorphosed under upper-amphibolite- to granulite-facies conditions. This section, called the Seiad complex, is the ophiolitic basement of an accreted Mesozoic island arc, and its polygenetic history reflects the magmatic and tectonic processes that occur during island-arc construction and evolution.

The presence of metarodingite and metaserpentinite, and the concordance of structural elements and metamorphic grade among all units of the Seiad complex, indicate that initial tectonic disruption of the ophiolitic suite occurred in the upper crust and subsequent penetrative deformation and metamorphism occurred under high-temperature conditions in the deep crust. Crustal granulite-facies metamorphism is indicated by two-pyroxene metagabbroic bodies and two-pyroxene metasedimentary paragneiss. Geothermobarometric data from garnet amphibolite and granulite-facies metagabbro within the ophiolitic suite yielded pressure and temperature conditions of ~5–7 kb and ~650–750 °C. Geochemical data from samples of granulite, amphibolite, and leucotrondhjemite suggest a supra-subduction origin, although there is significant variation among the amphibolite samples, indicating multiple magma types.

Crosscutting, radiometrically dated plutons and the regional geologic context suggest that high-grade metamorphism and deformation of these disrupted ophiolitic rocks occurred in the Middle Jurassic (ca. 172–167 Ma). This time interval broadly corresponds with contraction along several regional thrust faults in the Klamath Mountains province and juxtaposition of the Rattlesnake Creek terrane with terranes to the east. A U-Pb zircon age of 152.7 ± 1.8 Ma on a sample of a crosscutting leucotrondhjemitic dike swarm and published 40Ar/39Ar hornblende age spectra of ca. 150 ± 2 Ma from amphibolite indicate that magmatism and an accompanying thermal flux continued to affect this region into the Late Jurassic.

Compared with the deep-crustal sections of the well-studied Kohistan and Tal-keetna arc complexes, the widespread mélange character of the Rattlesnake Creek terrane (including the Seiad complex) is unique. However, ophiolitic rocks, including mantle ultramafic rocks, are common components in the basal parts of these classic arc crustal sections. Hornblende gabbro/diorite and clinopyroxenite in the Seiad complex may be small-scale melt conduits that fed middle- and upper-crustal components of the arc, analogous to the relationship seen in Kohistan between deep-crustal ultramafic-mafic bodies and mid-crustal magma chambers.

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