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
thesis (scanned
text) - 25.1MB pdf file
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