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
Return to MS Theses completed in the Geological
Sciences Program, University at Albany