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

Natural examples of fold patterns associated with rotated boudins occur in the Ossa-Morena Zone (S Portugal) and triggered the present study. In ducti le marbles with embedded competent mafic boudins, the folds seem to have or iginated by rotation of these, bodies and show differential development, fr om gentle deflections to sheath folds. This suggests a dependence of the st ructure type on the distance of the metamorphic layering to the rigid body. The experimental work presented here was performed with rotating inclusion s in bulk simple shear and introduces a new variable, the distance of the m arker layer from the rigid inclusion (d), expressed in terms of E, the rati o between the spacing between marker layers (D) and the greater principal d imension of the inclusion (a). Contrary to previous experimental work in th e literature, we used planar marker layers without concomitant boudinage. O ur results show new processes of development of sheath folds from planar la yers, and also that some non-cylindrical layer deflections are transient an d, 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, sheat h folds develop by drag pull processes on originally planar marker layers. Transient non-cylindrical deflections develop on originally planar marker l ayers when the distance between the marker layers is smaller than the longe st principal dimension of the rigid particle. The monoclinic symmetry of the drag sheath fold patterns associated with ro tating rigid inclusions indicate non-coaxial flow and can be used as a shea r sense criterion (quarter structure). (C) 2001 Elsevier Science Ltd. All r ights reserved.