Structure and Geochemistry of the Cougar Creek Complex, Northeastern Oregon and West-Central Idaho
Type of Culminating Activity
Master of Science in Geology
C. J. Northrup
The Cougar Creek Complex (CCC) represents the mid-crustal levels of the Wallowa island arc terrane of the Blue Mountains Province of northeastern Oregon, west-central Idaho, and southeastern Washington. Variably metamorphosed and deformed igneous rocks that comprise the CCC are poorly understood in terms of their structural and petrologic evolution. Structural analyses of mylonitic rocks from the CCC provide an opportunity to: 1) constrain the kinematics and timing of mylonitic shearing within the mid-crustal levels of the Wallowa island arc terrane, and 2) document the deformational structures produced during general noncoaxial shear within quartz feldspar aggregates. Major and minor oxide and trace element analyses for intrusive rocks comprising the CCC provide: 1) a thorough geochemical characterization of the predominant lithologies within the CCC; and 2) clarification of the petrologic relationships between rock of the CCC and other late Paleozoic and early Mesozoic intrusive and extrusive rocks exposed throughout the Blue Mountains.
Structural analyses of mylonitic rocks from the CCC provide insight regarding their tectonic evolution. As well as previously documented sinistral movement, petrographic and field observations of shear sense indicators within mylonitic shear zones reveal a prominent dextral strike-slip component of history that has not been regionally recognized. Shear zones strike NE (020 to 090), are steeply to moderately inclined to the southeast and northwest, and range from a few centimeters to several meters in thickness. High-strain zones form an anastomosing network that is subparallel to dike orientations. Stretching lineations defined by aligned elongate mineral grains or polygranular aggregates are sub-horizontal, consistent with dominantly strike-slip structural transport. Kinematic indicators such as asymmetric porphyroclasts, shear bands, grain-shape preferred orientations, and asymmetric folds are common at microscopic and outcrop scale and provide evidence for both dextral and sinistral movement. Recrystallization textures of quartz and feldspar grains indicate greenschist to lower amphibolite facies temperature conditions during deformation. The presence of both dextral and sinistral shear may be explained by a combination of: 1) broadly coaxial bulk strain accommodated by local sinistral and dextral shearing; and/or 2) distinct episodes of movement with opposite senses of transport. Based on existing geochronologic data, mylonitic shearing is broadly constrained to have occurred between 246 Ma and 225 Ma. Dextral and sinistral movements within the CCC occurred during the structural evolution of the Wallowa arc prior to its accretion to North America. Thus, these mylonitic rocks are representative of the structural evolution of the Wallowa arc outboard of the continental margin, and possibly the product of oblique subduction across the convergent plate boundary associated with the Wallowa arc.
Well-developed porphyroclasts within six mylonite zones were utilized to determine bulk kinematic vorticity (Wk) using Rf/ɸ analysis. Knowing Wk reveals: 1) the position of the unstable eigenvector; and 2) the orientations of planes of the maximum angular shear strain rate associated with the general flow regime. A comparison of the orientations of SC' -Type extensional shear bands with the results of Rf/ɸ analyses indicate that SC' -Type shear bands initially form parallel to the direction of maximum shear strain rate, which, bisects the angle between the shear zone boundary and the inclined eigenvector. Given this relationship, SC' -Type shear bands provide a direct, semi-quantitative means of estimating Wk.
The mechanical and textural evolution of quartz-feldspar aggregates may be described in terms of rock strength and the volume proportion of a weaker matrix component, which controls bulk rheology. Previous criteria used to characterize the structural development of polyphase aggregates include: 1) the relative strengths of each mineral component; and 2) the volume proportion of the weakest mineral phase alone. However, progressive recrystallization produces an increase in the proportion of matrix material derived from the strong mineral component and combined with the original proportion of the weak phase, controls the mechanical and textural development of the aggregate.
Igneous rocks of the CCC were generated within an island arc setting and most likely crystallized from magmas derived from the partial fusion of the mantle wedge with variable contributions from the subducting oceanic crust. These intrusive rocks are low- K, island arc tholeiites with subordinate transitional to calc-alkaline affinities. Data suggests that the Wallowa island arc was dominantly tholeiitic but evolved petrologically, to the point that calc-alkaline magmas became more volumetrically significant. Rocks within the CCC are chemically similar and coeval with other late Paleozoic and early Mesozoic plutonic basement complexes in the region, suggesting that they represent portions of the same volcanic axis. In addition, basement rocks of the CCC are geochemically and chronologically similar to extrusive rocks throughout the Blue Mountains; thus, the CCC represents the intrusive equivalents to at least Early Permian and perhaps even Middle and Late Triassic extrusive assemblages in the region.
Kurz, Gene A., "Structure and Geochemistry of the Cougar Creek Complex, Northeastern Oregon and West-Central Idaho" (2001). Boise State University Theses and Dissertations. 406.