Deformation conditions and kinematic vorticity within the footwall shear zone of the Wildhorse detachment system, Pioneer metamorphic core complex, Idaho

Abstract

Detailed analysis of numerous sequential fabrics and microstructures preserved in the footwall shear zones of detachment fault systems can be used to clarify the spatial relationships and timing of deformation related to emplacement and exhumation of metamorphic core complexes. To quantify the kinematics during deformation, we conducted microstructural analyses along two ∼300 m transects across mylonitic sections of the footwall of the Eocene Wildhorse Detachment system within the Pioneer metamorphic core complex. These results enable comparison of deformation at different structural levels during exhumation and between monomineralic and polymineralic lithologies.In both transects, mylonitic rocks preserve normal-sense asymmetric structures such as recrystallized quartz oblique foliation, S/C–C′ structures, and asymmetric feldspar porphyroclasts. Quartz microstructures, CPO patterns, and kinematic vorticity analyses consistently indicate simple shear-dominated deformation, which is predicted for late stage, shallow-level deformation within a rolling hinge detachment system exhuming a metamorphic core complex. Variations in c-axis patterns and active slip systems are associated with temperature, influenced by structural level, and grain boundary processes related to lithology. To determine the degree of non-coaxiality and minimize limitations and uncertainties with kinematic vorticity methods, we used two methods to evaluate flow vorticity. Kinematic vorticity ranges from 0.55 to 0.95 for the oblique quartz foliation (δ/β) and C′-type shear bands (C′-c) methods. Quartz recrystallized grain size ranges from 10 to 37 μm, indicating maximum differential stresses of 80–110 MPa, which is consistent with the footwall shear zones of numerous metamorphic core complex detachment systems.