FABRICS AND TEXTURES OF NATURALLY DEFORMED QUARTZ MYLONITES: A COMPARATIVE STUDY

Quartz mylonites provide excellent information on deformation conditions such as strain intensity, metamorphic grade, and strain rate as well as on shear sense and kinematic directions. Quartz is a sensitive mineral that exemplary records the variation of parameters controlling microstructural deformation mechanisms such as temperature, strain rate, differential stress and fluid activities. The crystallographic preferred orientations of quartz are conveniently quantified by methods of X-ray texture goniometry, which measures the spatial distribution of specific crystal lattices, by their diffraction of X-rays. The resulting distribution patterns reveal active slip systems, strain symmetries as well as asymmetries with macroscopic strain features indicating shear sense. The presented study compares quartz mylonites from three different regions: the Brevard fault zone (GA), the Winters Pass thrust (CA) and the Mischabel Fold (Switzerland). The individual areas are characterized by a dominant variation of the fabrics controlling parameters such as strain rate, fluids, or differential stress. The Brevard fault is one of the most dominant and controversial structures of the Southern Appalachian orogen. Quartz textures reflect a strain gradient across the shear zone indicating kinematics of a normal fault. The Winters Pass thrust of the Clark Mountains Thrust Complex (CA) is exposed at different crustal levels indicated by a south to north decrease in penetrative, distributed ductile strain and an increase in brittle faulting. Deformation regimes change from those representing higher to lower crustal levels but not as we would predict by increasing depth or temperature. Stable isotope studies suggest that fluid influence during thrusting may be a critical parameter in the development of the observed deformation regimes. In the western Swiss Alps, the basal thrust of the austro-alpine Dent Blanche nappe affects underlying Pennine units by forming a large, slightly south-vergent, recumbent fold system. Quartz fabrics analyses at the hinge zone, the normal and inverted limb indicate different deformation regimes operating during fold amplification, while quartz textures reveal persistent kinematic direction dominated by top to the south shear (alpine back-thrusting).