Regional variations in deformation mechanisms and structural styles in the central Appalachian orogenic belt

Abstract

Orogenic belts, such as the central Appalachian fold and thrust belt, consist of distinct tectonic provinces characterized by different structural styles as a result of deformation under different physical conditions. A quantitative study of deformation mechanisms was undertaken for quartzites and sandstones of the central Appalachian orogenic belt in order to understand the controls of different physical parameters on the relative importance of these mechanisms and on the geometry and mechanics of formation of structures in the different provinces. In the Blue Ridge province, the dominant deformation mechanisms are pressure solution, dislocation creep, and extension fracturing, with some accompanying grain boundary sliding and recrystallization. The proportion of dislocation creep to pressure solution strain increases both with temperature and deviatoric stress. An axial plane slaty cleavage, which contains the maximum and intermediate principal directions of the strain ellipsoid, maintains a uniform orientation in both the Blue Ridge and the Great Valley provinces. Folds in the Blue Ridge province have a similar geometry, formed by progressive flattening of parallel folds due to internal deformation. Ductile deformation zones on the overturned limbs of folds acted as precursors for the propagation of thrust faults in the late stages of folding. In the Valley and Ridge and Massanutten synclinorium, important deformation mechanisms in the sandstone units include pressure solution, plastic deformation, cataclasis, brecciation, extension fracturing, and slip along minor faults and bedding planes. Major folds are strongly disharmonic and show variations between concentric and angular geometries. They are associated with thrust ramps that propagated through progressively developing folds. In the late stages of folding, brittle deformation zones characterized by high intensities of extension fractures and cataclastic bands formed on the steep forelimbs. These features are interpreted to be the result of layer-parallel extension and the propagation of splay faults through these limbs. Cleavage developed during layer parallel shortening prior to the onset of folding and was rotated to its present fanned geometry during folding. On the basis of the results of this study, the central Appalachian orogenic belt can be subdivided into two deformational provinces dominated by different deformation mechanisms and characterized by different geometries and mechanics of formation of the structures. For orthoquartzites, the tectonite front which separates the tectonites of the hinterland from the non-tectonites of the foreland is located along the Blue Ridge front. For carbonates and argillaceous sandstones, the tectonite front is located along the North Mountain fault.