The Josephine Peridotite is a large ultramafic complex exposed in northern California and southern Oregon and represents the mantle portion of the Late Jurassic Josephine ophiolite. This unit has been subjected to deformation over a broad range of physical conditions. Deformation at temperatures below ~550°C invariably appears to have been accompanied by serpentinization. At relatively low temperatures, the dominant mode of deformation apparently was cataclasis. The products of such deformation are referred to as incohesive serpentinites, since the primary cohesion was greatly reduced during the extensive fracturing associated with cataclasis. Despite abundant indications of cataclasis, these rocks typically have planar anisotropies, and occasionally composite planar fabrics.
Deformation and concomitant serpentinization near the upper limit of antigorite stability (based on oxygen isotope data and microstructural observations) resulted in the formation of serpentinite mylonites which are strongly foliatied, typically lineated mylonitic rocks with microstructures very similar to those commonly found in quartzofeldspathic mylonites. Of particular interest are the shear band foliations and porphyroclast systems observed in these rocks.
Several deformation styles have been inferred to have resulted from deformation above ~550°C. These include: (a) distributed flow – which probably occurred at asthenospheric conditions, (b) localized olivine plasticity within shear zones with extensive recovery – which is interpreted to have occurred at high temperature lithospheric conditions, (c) localized olivine plasticity associated with some cataclasis producing strongly foliated peridotite mylonites – which is interpreted to have occurred near the lower limits of olivine plasticity, (d) and extensive cataclasis of peridotite which is interpreted to have occurred below the lower limits of olivine plasticity, but above the upper limit of antigorite stability. Serpentinite mylonites and subordinate peridotite mylonites occur within an extensive, originally subhorizontal shear zone which occurs approximately one kilometer beneath the base of the crustal sequence. This structure is interpreted to represent an extensional detachment shear zone which formed from amagmatic lithospheric extension during periods of low magma supply, and is considered to have accommodated the previously determined rotations of the overlying crustal sequence.
Norrell, G.T., 1989. Structural geology of the Josephine
Peridotite, northern California: implications for structural
processes at slow spreading centers. Unpublished PhD dissertation,
State University of New York at Albany. 233pp., +xiv;+Appendices
38pp., 4 folded plates (maps)
University at Albany Science Library call number: SCIENCE MIC Film QE 40 Z899 1989 N67
Copies of this PhD dissertation can be ordered from Proquest UMI
Front matter (title, table of contents, abstract, acknowledgements) - 0.4MB pdf file
Photo pages in dissertation (colour and greyscale photos with captions): - 12.1MB pdf file
Plate A - Structural
data from the Josephine Peridotite - Gasquet 7.5' quadrangle
(uncoloured geological structure map; 1:24,000)
Plate B - Structural data from the Josephine Peridotite - High Plateau 7.5' quadrangle (uncoloured geological structure map; 1:24,000)
Plate C - Structural data from the Josephine Peridotite - Klamath Glen 7.5' quadrangle (uncoloured geological structure map; 1:24,000)
Plate D - Structural data from the Josephine Peridotite - Cant Hook 7.5' quadrangle (uncoloured geological structure map; 1:24,000)
Appx A - Explanation, key, and index map for Structural data maps of the Josephine Peridotite
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