Microstructural and crystallographic fabric analysis of stretched-pebble conglomerates in central Vermont
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Microstructures and quartz crystallographic fabrics of conglomerate lenses from the Tyson Formation, which composes the basal portion of the authochthonous Paleozoic cover sequence in central Vermont, were analyzed to investigate the complex deformational history associated with a polydefonned area. Microstructural observations and crystallographic fabric techniques were used for kinematic analysis, and to relate the development of microstructures and lattice preferred orientations to the local and regional structural settings. Additionally, the study compares the development of microstructures and lattice preferred orientations, and investigates strain heterogeneity that develops within deformed conglomerate.
Recrystallization textures within quartz-rich pebbles suggest dynamic recrystallization via subgrain rotation and grain boundary migration. Dynamically recrystallized quartz grains have a grain shape preferred orientation, which is parallel to a series of intracrystalline strain features including undulatory extinction, deformation bands, and subgrains. These features are believed to be a manifestation of shearing along a conjugate set of intracrystalline shear planes that appear as deformation lamellae (Brace, 1955). Plastic deformation along these shear planes has developed a grain shape preferred orientation that forms at high angles (45Â° to 60Â°) to the foliation plane, rather than parallel to the XY plane of the incremental strain ellipsoid.
Although the Tyson Formation contains more than one tectonic fabric, lattice preferred orientations within quartz-rich domains appear to have formed in response to the second Taconian deformational event of Stanley and Ratcliffe (1985). The crystallographic fabrics show considerable variation on many scales. The distribution of the fabrics relative to local structures show no consistent or predictable relationship.
Fabric variation between the matrix and pebbles is believed to be a manifestation of strain heterogeneity. Shear strain within the matrix is accommodated by grain boundary sliding, therefore only coaxial portions of the strain are recorded by the matrix quartz lattice preferred orientation. Furthermore, strain partitioning, and possibly the lack of extensive recrystallization, has precluded the development of strong lattice preferred orientations in the matrix. Within quartz pebbles, conflicting kinematic indicators, microstructures, and c-axis fabrics indicate strain path partitioning due to rheological variability within a flowing rock mass, which deformed under predominantly coaxial conditions. The asymmetric crystal fabrics in this study are approximately 50% top-to-the- west and 50% top-to-the-east. Additionally, porphyroclasts and mica fish locally suggest either non-coaxial or coaxial deformation, as well as conflicting shear senses.
Deformation probably occurred within the constrictional field, and was locally accommodated by shearing. Kinematic indicators such as mica fish, asymmetric porphyroclast tails, and c-axis fabrics suggest that partitioning occurred not only between pebbles, but within individual pebbles as well.
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