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A structural study of a metamorphic sole beneath the Josephine ophiolite, western Klamath terrane, California-Oregon

By
Gregory D. Harper
Gregory D. Harper
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Kristin Grady
Kristin Grady
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John Wakabayashi
John Wakabayashi
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Published:
January 01, 1990

The 162-Ma Josephine ophiolite was emplaced over an active mafic batholith (Chetco River complex) along the Madstone thrust in southwestern Oregon during the Nevadan orogeny, beginning at ∼155 Ma. Strongly deformed amphibolite and minor quartzite occur between the ophiolite and the batholith and are interpreted to make up a metamorphic sole formed during thrusting. Retrograde metamorphism is ubiquitous, and amphibolite has been locally converted to greenschist-facies mafic phyllonite adjacent to the Madstone thrust. Pegmatite dikes locally cut the amphibolite but are also penetratively deformed, indicating syntectonic intrusion. A geochronologic study (Harper and others, 1989) indicates cooling from ∼450°C at 153 Ma, intrusion of the pegmatite at 150 Ma, and cooling to ∼350°C at 146 Ma. Geobarometry, using amphibole composition and phengite content of muscovite, indicates relatively low P/T metamorphism. The lower contact of the amphibolite sole with the Chetco River complex, as described by previous workers, is intrusive and syntectonic with deformation of the amphibolite sole.

In the hanging wall of the Madstone thrust, 20 to 40 m of high-T serpentinite mylonite occurs along the base of the Josephine Peridotite. The serpentinite apparently formed during ophiolite emplacement because it is structurally concordant with the underlying amphibolite and phyllonite. In addition, the serpentinite locally shows metasomatism, which probably resulted from interaction with fluids derived from the amphibolite sole.

The amphibolite shows two generations of folds having fold hinges parallel to a NNE-stretching lineation. These structures, along with grain-size reduction and asymmetric fabrics, indicate that the amphibolites are mylonites formed by progressive simple shear. The lineations and sense-of-shear criteria for the amphibolite and serpentinite mylonite indicate thrusting of the Josephine ophiolite toward the north-northeast, over the Chetco River complex. Continued north-northeast thrusting during greenschist retrograde metamorphism is indicated by lineations and microstructures in phyllonites and a pegmatite dike. A minimum displacement of 12 km is inferred from the outcrop pattern of the Madstone thrust.

The metamorphic sole and regional geologic setting of the Josephine ophiolite are distinct from other ophiolites. There is no inverted gradient, maximum temperatures were lower, syntectonic magmas were intruded into both the metamorphic sole and the ophiolite, and the ophiolite was thrust over an active magmatic arc rather than a continental margin. In addition, the ophiolite and overlying Galice Formation were thrust beneath the North American continent by >40 km along the roof thrust (Orleans fault) and regionally metamorphosed to low grade.

Geochronologic and structural studies indicate that the basal Madstone thrust and the roof thrust were both active at 150 ± 1 Ma, but the thrusting direction along the roof thrust appears to have been west or northwest. The cause and tectonic significance of nearly orthogonal thrusting directions between the basal and roof thrusts of the ophiolite is enigmatic. One possibility is that thrusting occurred during sinistral oblique subduction, and the Josephine thrust sheet was effectively decoupled along the roof thrust due to high pore-fluid pressures in the Galice Formation.

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GSA Special Papers

Paleozoic and Early Mesozoic Paleogeographic Relations; Sierra Nevada, Klamath Mountains, and Related Terranes

David S. Harwood
David S. Harwood
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M. Meghan Miller
M. Meghan Miller
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Geological Society of America
Volume
255
ISBN print:
9780813722559
Publication date:
January 01, 1990

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