Modelling of segment structures: Boudins, bone-boudins, mullions and related single- and multiphase deformation features Xavier Maeder * , Cees W. Passchier, Daniel Koehn Institut fu¨r Geowissenschaften, Johannes Gutenberg University, 55099 Mainz, Germany article info Article history: Received 10 July 2008 Received in revised form 19 May 2009 Accepted 25 May 2009 Available online 6 June 2009 Keywords: Finite element modelling Segment structure Boudin Tension gash Flanking fold General shear deformation abstract Finite element modelling has been used to simulate the development of segment structures, deformed layer segments separated by veins, such as boudins, mullions, and bone-boudins. A parameter sensitivity analysis is used to compare the influence of the nature of the flow, the relative viscosities of veins in necks and the host rock, and the initial geometry of the layer segments. Parameter fields have been determined for the relative viscosity of veins and layers, and the kinematic vorticity number of flow. Reworked segment structures can have several shapes such as bone-, bulging, shortened bone-boudins and their asymmetric equivalents such as domino- and shearband-boudin geometry. The model for asymmetric reworked segment structures is applied to such features from the Lower Ugab Meta- turbidites in NW Namibia. The model suggests that these structures form where the neck veins are stronger than the boudinaged layer, with a significant simple shear component of the bulk flow. The quartz filled necks in the Lower Ugab are therefore stronger than the quartz-rich wall rock in greenschist facies where the progressive deformation occurred. Bone-boudins are usually interpreted to form in transpressional flow, but simulations of the rotation of tension gashes show that they can also form in simple shear or slightly transtensional shear flow. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Boudinage structures are very common in deformed layered rocks in all tectonic contexts and have been an important marker of deformation since the first recognition of pinch-and-swell struc- tures (Ramsay, 1881; Harker, 1889). They are usually defined by the disruption of layers, lenses or foliation planes in response to bulk extension along the enveloping surface (Twiss and Moores, 2007; Ramsay and Huber, 1983). The first use of the term ‘‘boudin’’, however, was to describe cylindrical bodies of deformed sandstone separated by regularly spaced quartz veins, features which have since locally been renamed as double-sided mullions (Lohest et al., 1908; Urai et al., 2001; Kenis et al., 2004, 2005; Sintubin, 2008), while similar structures were elsewhere described as shortened boudins (Passchier, 1991; Passchier and Trouw, 2005). These structures are not thought to have formed by significant extension, but by a two-stage process where a vein network first formed due to enhanced fluid pressure in an environment of low differential stress, generating quartz veins at right angles to layering (Kenis et al., 2002); this was followed by ductile shortening along the layers where a difference in rheology between the sandstone and the veins causes the formation of cuspate structures. Boudins formed by extension parallel to layering and double-sided mullions or shortened boudins can have very similar geometry because all form in response to a periodic gradient in flow parameters along layers. Many boudin or mullion structures are asymmetric in the sense that they have a monoclinic rather than orthorhombic geometry (Goscombe et al., 2004a). Some of these can be classified as flanking structures (Coelho et al., 2005; Passchier, 2001), fault drag or ‘‘hook folds’’ (Hudleston, 1989; Grasemann et al., 1999). Although most structures have similar geometry on both sides of an affected layer, creating folds of similar axial plane but opposite facing (Fig. 1), some structures in graded bedding develop only on one side of a layer and thus have an even lower symmetry. Boudins, asymmetric boudins, pinch-and-swell structures, double-sided mullions, shortened boudins, flanking structures and drag folds can all have similar 3D geometry but different inferred mechanisms of formation. We believe that in such cases, the names of structures should reflect their geometry rather than their mechanisms of formation, which is usually hard to reconstruct in the field. For this reason, and since no consensus exists as yet on * Corresponding author. Laboratory for Mechanics of Materials and Nanostructures, Empa - Materials Science & Technology, 3602 Thun, Switzerland. Tel.: þ41 332282959. E-mail address: xavier.maeder@empa.ch (X. Maeder). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.05.013 Journal of Structural Geology 31 (2009) 817–830