VORTICITY FROM KINEMATIC INDICATORS AND EXPOSITION OF REGIONAL SHEAR IN HIGH STRAIN ZONES OF THE CENTRAL AND SOUTHERN APPALACHIAN MOUNTAINS

As modern kinematic vorticity indicators were developed in other orogenic belts, these structures became valuable tools to unravel displacement histories of crystalline rocks in the central and southern Appalachian mountains. Displacement derived from quantitative strain had already been utilized in select areas, such as those studied by Ernst Cloos in the Blue Ridge, but those measures were best applied in rocks with relatively simple strain histories. Nonetheless, the results of strain analysis from classic markers and cleavage, lineation, and boudinage provided critical support for displacement histories particularly in thrust sheets of the Blue Ridge province. In the complex Paleozoic tectonic core of the orogen, however, protoliths with suitable classic strain indicators are relatively rare, and displacements along high strain zones were often a matter of inference because piercing points from stratigraphic markers are absent or elusive. Mesoscopic fold asymmetries and hinge line orientations similarly conveyed elements of movement sense. Perhaps the single-most important development in the use of kinematic indicators in this orogen was asymmetric shear bands in phyllosilicate-rich mylonites in the early 1980s. The tectonite produced by spaced, anastomosing shear bands were “mica button” or “fish-scale” textures, terms already in use before the arrival of shear band kinematics. Importantly, shear bands were visibly superimposed on older structures by strain partitioning so the bands could be resolved into a time sequence. The first extensive application of shear bands in orogen parallel shear zones here was the Brevard fault zone. Remarkably, the sense derived from shear bands in the Brevard fault zone was consistently dextral and sub-horizontal for hundreds of km. These results revised the apparent displacement history for this shear zone. Subsequently, careful analysis of kinematic indicators in the eastern Piedmont fault system, elsewhere in the Piedmont, and along strike of the Brevard fault zone into northern Virginia established the existence of late Alleghanian dextral shear across the crystalline core. Recognition of lateral movement in the central and southern Appalachian mountains from kinematic indicators was one of the key progressions of regional analysis in the 20th century.