ABSTRACT
The Galice Formation is a thick, turbiditic flysch sequence that
depositionally
overlies the Josephine ophiolite in the western Klamath Mountains
of northwestern
California and southwestern Oregon. The Josephine ophiolite and
Galice
have been interpreted as the basement and sedimentary cover of a
Late Jurassic
back-arc basin that opened proximal to the continental margin.
During the
Nevadan orogeny (ca. 151-147 Ma), the ophiolite and overlying
Galice sediments
were thrust eastward beneath the continental margin along the
regional
Orleans fault. Two distinct Nevadan deformational phases are
recognized
in the Galice – an initial D1 or "main-phase" deformation and a
later brittle
deformation dominated by thrusting.
D1 involved development of tight to isoclinal northwest-vergent F1
folds which possess an axial-planar S1 cleavage. S1 ranges from a
weak
pressure-solution cleavage in the northernmost Galice, to a strong
slaty
cleavage or schistosity in the southern Galice. S1 is ubiquitously
parallel
to bedding in the absence of recognizable F1 fold closures.
Bedding is
commonly transposed into parallelism with cleavage in southern
exposures
of the Galice.
Syntectonic fibers in pressure shadows of pyrite grains reflect a
north-south
gradient in Nevadan penetrative strain. The fibers are straight
and parallel
to the trace of slaty cleavage, suggesting that cleavage formed
during
a stage of pure flattening. Percent shortening normal to Sl
determined
from fibers in the northern Galice (SW Oregon) is approximately
33%. However,
fibers from slates in northern California record an average of 70%
shortening
normal to S1. A large increase in the intensity of S1 towards the
south
also suggests a gradient in finite strain.
Main-phase Nevadan deformation was followed by an episode of
brittle
shearing. Nevadan thrusts reveal top-to-northwest sense to
displacement,
similar to vergence observed from F1 folds. A second generation of
folds,
F2, appear to be genetically related to the thrusts. Nevadan
thrusts clearly
overprint main-phase structures (F1, S1). In the northernmost
Galice, angular
F1 folds are thrusted-out parallel to bedding in one of the fold
limbs.
Further to the south, F1 folds are more typically truncated by
thrusts
that are parallel to S1 cleavage planes. Small-scale thrust
systems in
thinly-bedded rocks have produced peculiar bedding truncations or
"pseudocross-bedding".
These small thrusts locally truncate the S1 cleavage.
Fundamental conclusions obtained in this study are:
1) The Galice Formation is tightly folded on both outcrop and
regional
scales.
2) There is a north-to-south increase in Nevadan regional
metamorphic
grade and penetrative strain. This gradient may reflect a small
southward
component of underthrusting along the Orleans fault.
3) Nevadan vergence from folds and thrusts in the Galice is
dominantly
northwestward.
4) Most Nevadan thrusting in the Galice took place after the
main-phase
penetrative deformation and peak metamorphism. This
ductile-to-brittle
transition may have resulted from continued underthrusting during
regional
uplift.
Jones, F.R., 1988. Structural geology of the northern Galice
Formation,
western Klamath Mountains, Oregon and California.
Unpublished MSc.
thesis, State University of New York at Albany. 211 pp., +x; 1
folded plate
(map)
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1988 J65
Plate 1 - Structural
map and cross-sections of the Galice Formation east of Kerby,
Oregon
coloured geological map (scale 1:18,526), and
cross-sections (scale 1:9,263) 7.3MB pdf file
Return to MS Theses completed in the
Geological
Sciences Program, University at Albany