Microstructures and sense of shear in the Brevard Zone, southern Appalachians
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The Brevard Zone, which separates the Blue Ridge and Inner Piedmont geologic provinces in the southern Appalachians, is a major structural feature with a multiple deformation history. Microstructures in oriented thin sections from rocks in the Brevard Zone in Tugaloo, Whetstone and Tamassee quadrangles, South Carolina, Rosman quadrangle, North Carolina, and in the sheared Ben Hill Granite in Atlanta, Georgia, indicate that there were at least two early ductile deformations and a later, locally developed, brittle deformation. The oldest recognizable microstructures arc a prominent foliation (S₁), quartz ribbons and garnets. The age of these features and the sense of shear during their formation is unknown. The remainder of the observed microstructures are categorized into groups A, B, C and D on the basis of orientation, overprinting relationships and direction of motion as indicated by sense of shear criteria present. Group A is the oldest of these microstructures and group D is the youngest. Group A features consists of northwest-verging, tight to isoclinal F₂ folds, a weakly developed, axial planar foliation (S₂), and scattered F₃ folds, coaxial with F₂. The F₁ folds of Roper and Dunn (1973) are not observed due to later deformation. Group A microstructures are ductile features which formed during a west- to northwest-directed thrusting motion. Group B features include type II s-c mylonites, c-surfaces, scattered folds and garnet pressure shadows. The orientation of these features indicates that they formed during a period of dextral strike-slip shearing with a possible thrust component. Group C contains an extensional crenulation cleavage (ECC) which is relatively younger than features in groups A and B. The orientation of ECC is incompatible with dextral motion, thus they suggest a change in the direction of bulk motion in the Brevard Zone, the direction of which is unknown. Along strike a notable change in deformation conditions occurred during the ductile deformation(s) which formed features in groups A, B and C. This change is reflected in highly recrystallized quartz textures in Tugaloo, relative to partially recrystallized textures in Rosman quadrangle. Retrograde metamorphism postdates the formation of features in groups A, B and C. Group D contains the youngest microstructures which formed during a localized brittle deformation. Brecciation is visible in thin section and outcrop, however no sense of shear direction can be determined. Drag folds and faults are present in several outcrops but their geometry is highly variable. The bulk motion during brittle deformation is unknown. Sense of shear criteria in group A are compatible with tectonic models for both the Taconic and Alleghanian orogenies in the southern Appalachians. However, group A probably formed in the Taconic because the most intense ductile deformation has been reported for this time period. Microstructures in group B and C are found in the sheared, Permian Ben Hill Granite in Atlanta and thus are Alleghanian in age. Rocks containing group B and C in the northeastern study areas cannot be radiometrically dated with confidence, however, their orientations, deformation conditions and sense of shear is similar to group B and C in the Ben Hill Granite indicating that they are also Alleghanian in age. The dextral strike-slip motion indicated by groups B is compatible with the results of previous workers (Reed and Bryant, 1964; Bobyarchick, 1983) elsewhere along the Brevard Zone who have also demonstrated Alleghanian dextral motion. Thus the results of this study confirm an episode of ductile, dextral strike-slip motion in the Brevard Zone during the Alleghanian. Group C and D may also be Alleghanian or they may be the result of a separate and more recent deformation, possibly related to the Triassic opening of the present-day Atlantic Ocean.