GRAIN-SCALE DEFORMATION AND KINEMATICS ALONG A STRAIN GRADIENT, PIONEER METAMORPHIC CORE COMPLEX, IDAHO

Identification of microstructures and measurements of quartz c-axis define grain-scale mechanisms and conditions of deformation associated with extension-related detachment faulting and exhumation of lower crust in metamorphic core complexes. The low-angle detachment fault of the Pioneer metamorphic core complex separates lower ductile crust in the footwall from brittle upper crust in the hanging wall. The discontinuity of deformation fabrics with respect to the detachment provides information about strain gradients, kinematics, geometry, temperature and pressure conditions, and the duration and chronology of deformation. Samples were collected in the field from the mylonitic zone of the footwall. The transect is perpendicular to the detachment fault and parallel to vergence in order to observe strain gradients. The dominant foliation is concentrated near (220, 030), and lineation clusters around (20, 295), indicating a NW vergence. Slickenlines of brittle faults also record a NW vergence and match that of other core complexes in the North American Cordillera. A qualitative and quantitative microstructural analysis of quartz and feldspar will determine sense of shear and the characteristic behavior of detachment faulting in metamorphic core complexes. Quartz c-axis fabrics are indicators of the strain ellipse and progressive deformation of the rock, which could disclose the strain history of extensional exhumation. Because quartz is the major constituent of continental crust, its rheological behavior is largely responsible for crustal rheology. EBSD analysis enables measurements of quartz crystallographic orientation and determines active slip systems. The results of this study will determine characteristic rheology of minerals and rock types in the footwall and the sense of shear of the detachment.