ABSTRACT
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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