STRAIN VARIATION IN KM-SCALE STRUCTURES WITH MAP PATTERNS OF SUBHORIZONTAL SHEATH FOLDS: EXAMPLES FROM THE WESTERN INNER PIEDMONT, SOUTHERN APPALACHIANS, AND THE GRENVILLE PROVINCE OF ONTARIO

Many gneiss complexes are replete with superimposed, subhorizontal, noncylindrical folds defined by litho-structural contacts and the main foliation. Typically, the short limbs of inclined or recumbent first-order folds have been strongly attenuated and converted into narrow high-strain zones or ductile thrusts. The map patterns of such large noncylindrical structures range from club-like to oval, and can be indistinguishable from those of km-scale sheath folds. Cobbold and Quinquis showed in simple-shear experiments that, under conditions of large quasi-homogeneous strain, sheath folds evolve by mechanically passive amplification of periclinal perturbations in coplanar marker surfaces. Other model makers demonstrated that, regardless of perturbation shape, finite-amplitude buckling of mechanically strong layers is affected by stress channeling, and results in quasi-cylindrical structures rather than sheath folds. Nonetheless, sheath-like structures can be produced, on various scales, by distortion of open to close buckle folds. The finite strain accumulated within distorted buckle folds must be heterogeneous, even if the deformation of the short limbs is spatially discontinuous. This is exemplified by well-exposed folds in (1) the Salem-Sunset area, western Inner Piedmont of the Carolinas and (2) the Georgian Bay-Muskoka region, Grenville Province of central Ontario. The km-scale folds to be discussed contain strong units of originally coarse-grained granitoids, and probably originated as quasi-cylindrical buckle folds. As expected, the accumulated strain of the strong units is markedly heterogeneous, and varies systematically between the hinge zone and limbs. Nonetheless, other workers attribute the structures in question to large-scale sheath folding, nappe emplacement and associated thrusting during extrusive channel flow of ductile rocks within the middle or lower crust.