Sheath folds formed by drag induced by rotation of rigid inclusions in viscous simple shear flow: nature and experiment

Abstract

Natural examples of fold patterns associated with rotated boudins occur in the Ossa–Morena Zone (S Portugal) and triggered the present study. In ductile marbles with embedded competent mafic boudins, the folds seem to have originated by rotation of these bodies and show differential development, from gentle deflections to sheath folds. This suggests a dependence of the structure type on the distance of the metamorphic layering to the rigid body. The experimental work presented here was performed with rotating inclusions in bulk simple shear and introduces a new variable, the distance of the marker layer from the rigid inclusion (d), expressed in terms of E, the ratio between the spacing between marker layers (D) and the greater principal dimension of the inclusion (a). Contrary to previous experimental work in the literature, we used planar marker layers without concomitant boudinage. Our results show new processes of development of sheath folds from planar layers, and also that some non-cylindrical layer deflections are transient and, thus, that sheath folds do not originate from them. When d is small and the inclusion starts with its longest axis normal to the shear plane, sheath folds develop by drag pull processes on originally planar marker layers. Transient non-cylindrical deflections develop on originally planar marker layers when the distance between the marker layers is smaller than the longest principal dimension of the rigid particle.The monoclinic symmetry of the drag sheath fold patterns associated with rotating rigid inclusions indicate non-coaxial flow and can be used as a shear sense criterion (quarter structure).