Sheath folds as discriminators of bulk strain type

Nested elliptical closures defining "eye-folds" represent classic (y-z) cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks. Systematic analysis of 1425 such eye-folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer- (R-yz) to the inner-most (R-y ' z ') elliptical "rings" of individual sheaths. The variation in overall aspect ratios from outer to inner rings is defined as R ' (where R '= R-yz/R-y ' z ') and may display a relative increase or decrease in ellipticity to define 'cats-eye' (R ' < 1) or 'bulls-eye' (R' > 1) fold patterns respectively. Layer thicknesses may also be measured along the y axis (parallel to the axial surface) (t(y)) and at 90 degrees to this along the z axis (t(z)) to define the ratio of T-yz. Sheath folds generated during broadly simple shear deformation (k congruent to 1) display (mean) R-yz 4.61, T-yz 3.31 and cats-eye-folds (R ' 0.69). Sheath folds generated during general shear also display cats-eye-folds with identical mean R ' 0.69 values, but greater thickness variations and elliptical ratios (T-yz 4.35, R-yz 5.76). Thus, within both simple- and general shear-dominated deformations, the overall variation in layer thickness (T-yz) and ellipticity of eye-folds (R-yz) increases with increasing deformation and a greater component of pure shear, whilst the R, value remains constant and reflects original fold patterns. Sheath folds formed during constrictional (k > 1) deformation display markedly lower aspect ratios (R-yz 2.42) and thickness variations (T-yz 2.94), together with distinctive bulls-eye-folds (R ' 1.23). These empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types, and allow sheath folds to act as both effective (> 95% consistent) and robust discriminators of bulk strain type. (c) 2006 Elsevier Ltd. All rights reserved.