Riedel shear: a mechanism for crenulation cleavage

Abstract

Drag folds (1) from the microfaulting of inclined markers distinguish crenulation cleavage from other cleavage types. Cleavage, as Riedel (R) shears nucleates obliquely to planar anisotropy of foliated rocks and at various angles (related to α) to the shear zone boundaries. Folds (2) with axes parallel to the instantaneous λ 1 at high angle and partway with R-shears are formed in the XZ plane of strain ellipsoid. The angle between folds and cleavage is changed in the progressive deformation. The largest antithetic (counterclockwise) R-shears rotation and largest opening of initial angle between folds and fractures up to 170° is achieved in the progressive transtension (α < 1). This is not geometrically possible in the simple shear and transpression but forward rotation of the R-shears of excess length is prevented by the constant or diminishing thickness of the shear zone. The crenulation folds (3) amplify the drag folds of the inclined faulted marker and nucleate with axial planes parallel to the cleavage at higher strains. The cleavage propagates further as P-shears, parallel to flanks of tightened crenulation folds. The R-shears are in higher order folds (4) reoriented into the axial planar position by pure shear in the YZ plane of the first incremental deformational ellipsoid, and by the shear induced by flexural slip along the layer boundaries. The stretching of the fold limbs and the non coaxial ductile deformation of microlithons in their slip perpendicular to fold axis deform former drag and crenulation folds to a characteristic pattern. Striation and/or faint lineation preserved in the cleavage plane trace the shear directions. The shear sense is indicated by the cleavage-folds transection and by the asymmetrical structures in the plane cut perpendicularly to clevage and parallel to striations.