Boudinage classification: end-member boudin types and modified boudin structures

Abstract

In monoclinic shear zones, there are only three ways a layer can be boudinaged, leading to three kinematic classes of boudinage. These are (1) symmetrically without slip on the inter-boudin surface (no-slip boudinage), and two classes with asymmetrical slip on the inter-boudin surface: slip being either (2) synthetic (S-slip boudinage) or (3) antithetic (A-slip boudinage) with respect to bulk shear sense. In S-slip boudinage, the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense. We have investigated the geometry of 2100 natural boudins from a wide variety of geological contexts worldwide. Five end-member boudin block geometries that are easily distinguished in the field encompass the entire range of natural boudins. These five end-member boudin block geometries are characterized and named drawn, torn, domino, gash and shearband boudins. Groups of these are shown to operate almost exclusively by only one kinematic class; drawn and torn boudins extend by no-slip, domino and gash boudins form by A-slip and shearband boudins develop by S-slip boudinage. In addition to boudin block geometry, full classification must also consider boudin train obliquity with respect to the fabric attractor and material layeredness of the boudinaged rock mass. Modified or complex boudin structures fall into two categories: sequential boudins experienced a sequence of different boudin block geometry components during progressive boudinage (i.e. continued stretch), whereas reworked boudins were modified by subsequent deformational episodes (folded, sheared and shortened types). Correct classification of boudins and recognition of their modification are the crucial first stages of interpretation of natural boudin structures, necessary to employing them as indicators of shear sense, flow regime and/or extension axes in terranes otherwise devoid of stretching lineations. © 2003 Elsevier Ltd. All rights reserved.