Planar dispersion of folds in ductile shear zones and kinematic interpretation of fold-hinge girdles

Abstract

As indicated by theory and multiple case studies, hinge lines of minor folds in ductile shear zones are dispersed in a plane that is incompletely rotated toward mylonitic foliation. This yields a geometric relationship between the fold-hinge girdle and mylonitic foliation that can be used to determine sense of shear. Statistical study of published data from various shear zones and of new data from the Blue Ridge thrust complex reveals a small to moderate angle between the best-fit plane to hinge lines and mean mylonitic foliation. This angle is 4–9° in eight of 12 test cases. Moderate angles, 22–34°, are associated with nappes. Kinematic analyses of 12 fold-hinge girdles indicate the sense of shear otherwise determined for host shear zones. A theoretical model of contemporary fold rotation in simple shear closely simulates the geometric relationships described above for natural folds. Using this model, large shear strains ( γ ≥ 10) are implied by the 4–9° angle between the fold-hinge girdle and mylonitic foliation.