Abstract
The Summit Valley plutonic complex (SVPC) is a multi-phase
intrusive body of Late Jurassic age. Crystallization age for a
zircon of 150±1 Ma and a cooling age for a hornblende of 144±1 Ma
indicate a protracted period of high temperature. The SVPC is
irregularly shaped, covers <15 km2, and is believed
to be in the western limb of a post-Nevadan syncline, with an
easterly dip estimated at about 45°. The SVPC penetrated the
Orleans Fault and intruded and contact metamorphosed the upper and
lower plates of the thrust. The Orleans Fault is a major tectonic
boundary separating the Western Paleozoic and Triassic Belt
(upper) from the Western Jurassic Belt (lower). Ductile shear
zones at the contact of the plutonic complex and in the interior
suggest that the plutonic complex was sheared either during or
soon after intrusion; movement along the Orleans Fault may have
been the cause.
Igneous rocks range from ultramafic to dioritic composition, with
the most abundant rock type being hornblende gabbro. Many of the
shear zone rocks are in granulite facies containing neoblasts of
clinopyroxene; amphibolites are also found, and locally, shear
zones are retrograded to greenschist facies (both statically and
dynamically). Exposed rocks of the lower plate in contact with the
plutonic complex include the pebbly mudstone of the LRO which is
metamorphosed to biotite hornfels within the contact aureole; the
slatey mudstone of the Galice Fm is in fault contact with the
plutonic complex and only exhibits greenschist facies assemblages;
an intrusive contact has not been found. Upper plate rocks of the
RCT are primarily serpentinized harzburgite which is found in
screens and pendants within the plutonic complex; there is a
pronounced contact aureole in RCT rocks at the southeastern margin
of the plutonic complex.
Ductile shear zones found within and at the edge of the plutonic
complex vary in thickness from centimeters to tens of meters.
Dikes are found throughout the plutonic complex and in the country
rocks, generally measure from 6cm to 60cm, and occasionally are
seen to cross-cut ductile shear zones. Vein-filled fractures
cross-cut both ductile shear zones and dikes and are considered to
be the latest of the three types of structures.
Several shear zones in the SVPC were analysed to determine sense
of shear. The data from the shear zones and from veins, many with
slickenfibers to indicate slip sense, were used to determine the
paleostress field of the deformation which caused them. These
structures were analysed graphically with the use of stereographic
projection plots, and mathematically with the use of two computer
programs for stress inversion, Hardcastle's (based on Reches'
method) and Lisle's method (called ROMSA). Dikes in the SVPC and
in the adjacent LRO were also used to constrain sigma3.
After rotation to remove post-Nevadan dip, the stress field during
deformation of the SVPC was found to have sigma1 almost vertical,
and sigma2 and sigma3 subhorizontal and switching places with each
other in positions trending approximately 180° and 270°. Stress
ratio (Phi) values for fractures were low (about 0.22) suggesting
that sigma2 and sigma3 were very close in magnitude of stress and
may appear to flip with each other. Direction of transport of
material in the hanging wall was west-northwest, with some
indication that this progressively changed through time to a more
north-northwesterly direction.
The Nevadan Orogeny is believed to have been a compressive event
with crustal slices telescoped beneath each other, possibly
achieving tens of kilometers of crustal shortening along the
Orleans Fault alone. Such a tectonic regime would have required
that sigma1 was horizontal, with the intermediate stress also
horizontal and a minimum stress that was vertical. Tectonic burial
may have been responsible for the stress field configuration seen
in the SVPC if the vertical stress became the maximum stress
through underthrusting to greater and greater depths.
Griesau, N.E., 1992. A kinematic study of the Summit Valley
Plutonic Complex, Klamath Mountains, California. Unpublished MSc.
thesis, State University of New York at Albany. 134 pp., +xii
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 1992 G75
thesis (scanned text) - 13.8MB pdf
file
Plate 1 - Shear zone map of the Summit
Valley Plutonic Complex, Del Norte Co., California
uncoloured geological map (scale 1:12.240)
1.5MB pdf file
Plate 2 - Summit Valley Plutonic Complex
outcrop map
uncoloured outcrop location map (scale
1:12.240) 1.4MB pdf file
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