A kinematic study of the Summit Valley Plutonic Complex, Klamath Mountains, California

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 \u3c15 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\u27s (based on Reches\u27 method) and Lisle\u27s 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