MONOCLINIC FOLD STRUCTURES AS NATURAL SHEAR-SENSE INDICATORS: FIELD EXAMPLES FROM EASTERN NORTH AMERICA

Model makers showed 40 years ago that buckle folds originate as orthorhombic structures (with mirror planes parallel, respectively, to the profile section and axial plane) even if the compression direction is oblique to the compositional layering. During further oblique compression, the orthorhombic fold structures acquire S or Z geometry and effectively monoclinic symmetry, with a single mirror plane parallel to the profile section. This modeling result attests to the validity of the oldest shear-sense criterion in structural geology (see fold chapters in textbooks by Badgley, Billings, De Sitter and Hills), generally applicable also to monoclinic fold nappes and sheath folds that originated as orthorhombic structures. But only those cylindrical structures may be used as shear-sense indicators whose axis is parallel to a principal direction of imposed strain. This was recognized 45 years ago by W. F. Brace, while analyzing the finite strain of distorted ripple marks, and remains an important condition for the use of monoclinic folds as shear-sense indicators.

Cylindrical S or Z fold structures in repeatedly deformed metamorphic rocks are commonly pervaded by axial stretching lineations, effectively parallel to the principal maximum-extension direction of a large strain. Unless the lineations are components of axial-planar L-S shape fabrics, however, the lineation-producing strain may have commenced long before, and be partly unrelated to, the fold-forming strain. The latter scenario is most probable where the axial lineations are due to statistical alignment of stretched primary features such as relict sedimentary clasts, varioles, igneous megacrysts or mafic clots. Field examples will be presented from the Georgian Bay region (central Ontario) and the western Inner Piedmont in which monoclinic cylindrical folds with axial-planar L-S fabrics aid in the structural testing of mid-crustal flow hypotheses.