Asymmetric boudins as shear sense indicators - an assessment from field data

Asymmetric boudins are potential but problematic shear sense indicators. They can be divided into two groups, with slip on the inter-boudin surface that is either synthetic (S-slip) or antithetic (A-slip) with respect to bulk shear sense. Since both groups have mirror-image symmetry, independent geometric criteria are needed to distinguish them if they are to be used as shear sense indicators. Investigation of asymmetric boudins in trains parallel to the main foliation from the Kaoko Belt in Namibia and elsewhere indicate that the geometry of both groups is in most cases different. Shearband boudins (formed by S-slip) have a long, curved lenticular shape and large relative displacement and synthetic drag on an inter-boudin surface that is gently inclined to the boudin exterior surface. Domino boudins (formed by A-slip) have an angular shape, an inter-boudin surface steeply inclined to the boudin exterior surface with small relative displacement and unique antithetic flanking folds instead of synthetic drag. Gash boudins are a special type of domino boudin with sigmoidal or forked inter-boudin surfaces; they form mostly as foliation boudinage. In boudin trains that were highly oblique to the attractor eigenvector (usually traced by the main foliation), all asymmetric boudin geometries formed by S-slip. Such foliation-oblique boudin trains can, nevertheless, be employed as a second class of shear sense indicator, as in all cases the boudin train as a whole must verge in the same direction as bulk shear sense. (C) 2002 Elsevier Science Ltd. All rights reserved.