Distortion of planar structures around rigid spherical bodies

Abstract

Theoretical and experimental results show that determination of sense of rigid rotation from drag-patterns of foliation around a rigid spherical inclusion (such as a porphyroblast of garnet) is not as simple as was previously thought. An asymmetric drag-pattern can develop even when the bulk deformation is non-rotational, provided a pre-existing foliation was initially at an angle to the principal axes of strain. In simple shear, the drag-pattern of a pre-existing foliation around the rigid inclusion may belong to any one of the following four types: 1. (1) asymmetric pattern, with the same sense of drag all over the contact. 2. (2) more or less symmetrical pattern, with different senses of drag at different parts of the contact. 3. (3) asymmetrical pattern, with different senses of drag. 4. (4) pattern with inward bowing of foliation (i.e. convex toward the rigid inclusion). The geometry of the distorted foliation will depend on the amount of simple shear and the initial angle between the pre-existing foliation and the direction of simple shear. If the development of schistosity and the subsequent development of drag around a rigid porphyroblast are results of the same continuous simple-shear movement, a wellpreserved drag-pattern may be utilized to confirm that the schistosity initially developed perpendicular to the maximum finite compressive strain, provided the total deformation is not very large.