Graduate Thesis Or Dissertation
 

Structure and petrography of the schist of Skookum Gulch, Callahan-Yreka area, eastern Klamath Mountains, northern California

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gb19fb30j

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  • The schist of Skookum Gulch (SSG) is an informal name applied to a fault-bounded melange composed mainly of schistose metamorphic rocks and less abundant sedimentary and igneous rocks located in the eastern Klamath Mountains of northern California. The SSG features outcrops of lawsonite+sodic amphibole blueschist and epidote+sodic amphibole rocks transitional to the greenschist facies. Isotopic dating indicates that the schist was metamorphosed during the Ordovician. The SSG is the oldest known Paleozoic blueschist-bearing melange in California and one of the oldest preserved blueschist terranes in North America. Tonalitic rocks associated with the schist have Early Cambrian ages and are among the oldest rocks yet dated within the Klamath Mountains. Field relations indicate that the schist of Skookum Gulch is a complex tectonic melange composed of metavolcanic, carbonate, and metasedimentary blocks and lenses of diverse sizes and shapes dispersed without apparent stratigraphic coherency in a sheared matrix of clastic to pelitic schist, metavolcanic schist, and discontinuous thin lenses of marble. Rocks of the matrix have been metamorphosed to chlorite-grade greenschist facies, whereas the blocks have been metamorphosed under a variety of pressure-temperature conditions. Some blocks have been feebly metamorphosed and retain features of the original protolith material; others have been thoroughly recrystallized under blueschist, transitional, and greenschist facies conditions. Blueschist blocks within the schistose matrix reveal six deformation events, (D1-D6): four are folding events, and at least two are ductile and brittle shear deformations. One period of metamorphism under blueschist-facies conditions is recorded in the blueschist blocks. The blocks lack evidence of prograde, greenschist-facies overprinting. Schistose rocks of the matrix are less deformed than the blueschist blocks. Matrix schists show at least two phases of folding. The predominant foliation is the result of tranposition of an early foliation or compositional layering. Other deformations include kink folding, ductile shearing, and brittle fracturing. The polydeformed tectonic blocks are hypothesized to have been incorporated into the melange matrix along a system of faults and rotated into a preferred alignment with the pervasive foliation of the matrix during D3. Feebly deformed and metamorphosed blocks such as chert, marble, and tonalite were incorporated prior to the time of brittle shearing. More is now known about the relative timing of these events. Blueschist-facies and greenschist-facies metamorphism occurred during the Late Ordovician to Early Silurian. Clasts of matrix components and tonalitic plutonic rocks of the SSG melange have been found in exposures of Late Silurian conglomerate beds in nearby units providing evidence that the SSG was uplifted and eroded by the Late Silurian. Minerals within the schist were thermally reset by Devonian metamorphism, possibly related to metamorphism and juxtaposition of the Duzel Phyllite and amphibolite of the Central Metamorphic Belt. However, the SGG does not show direct evidence of Devonian deformation.
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