Abstract |
- 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.
|
Additional Information |
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- For master (tiff) digital images of maps contained in this document contact scholarsarchive@oregonstate.edu
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Previous issue date: 1988-06-01
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