BRITTLE-DUCTILE SHEAR ZONES IN PRE-DEFORMED ROCKS

Orogenic belts are commonly characterized by sequences of deformation episodes contributing to the final structural morphology and fabrics documented in rocks. Studies of localized shear zones affecting pre-deformed gneisses indicate that the final fabrics can reflect the total strain cumulated over several deformation phases. The greenschist facies shear zones have progressively modified pre-existing fabrics in Biotite gneisses, exposed in the Piedmont Province, Southern Appalachians. The gneisses are dominated by sub-horizontal L- fabrics with varying intensity, which is progressively modified to S-fabric towards the center of the ductile shear zones. Displacements within these zones seem to be dominated by simple shear and reveal no evidence for significant pure shear components. Present studies investigate the role of flattening strain operating on the initial L-fabrics causing the formation of S-fabrics at the shear zones. High-resolution grid mapping, at the scale of 1:10 document the reorientation patterns of the pre-existing fabric and allow analysis of the behavior of the shear zones and how they accommodate pre-existing rheological anisotropy as they progress. Microstructural studies and magnetic fabric analysis along cross sections of those shear zones are used for determining mineral/ metamorphic reactions and to study fabric reorientation mechanisms taking place within the country rocks. A numerical model gives an additional evaluation of the mechanism for the modification of pre-existing fabric. In continuation of individual shear zones, ductile deformation is repeatedly replaced by segments of purely brittle deformation, where displacement is concentrated on discrete slip planes. The change in character of these shear zones appears to be related to variable rheological properties within the country rock which is expressed by differences in magnitude and orientation of the pre-existing fabric/ magnetic anisotropy. Implications of this study might be highly relevant for the deformational behavior of major fault zones and their seismicity.