Progressive development of s-type flanking folds in simple shear Ulrike Exner a, * , Neil S. Mancktelow a , Bernhard Grasemann b a Geologisches Institut, ETH-Zentrum, CH-8092 Zu ¨rich, Switzerland b Institut fu ¨r Geologische Wissenschaften, University of Vienna, A-1090 Vienna, Austria Received 11 November 2003; received in revised form 1 June 2004; accepted 1 June 2004 Available online 20 July 2004 Abstract Flanking structures are deflections of planar or linear fabric elements in a rock alongside a crosscutting element (CE), e.g. a vein or fault. This study provides new results from analogue experiments, which test and extend recent numerical models of flanking structures. A linear viscous matrix material (PDMS) was deformed in a ring shear rig that allows continuous observation to large values of shear strain. Rotational behaviour, offset and deflection of marker lines around a predefined, lubricated CE were monitored for different initial orientations of the fault with respect to the shear zone boundary, and the results were compared with numerical results and natural examples. At high initial angles to the shear zone boundary (. 1358), a structure previously described as an ‘s-type flanking fold’ develops. During progressive deformation, an initially straight marker line passing through the centre of the CE is offset in a sense synthetic with the bulk sense of shear and shows a shortening displacement across the CE. Simultaneously, this central marker line is deflected and forms symmetrical folds, which are convex in the direction of shear along the CE (i.e. normal drag). Both offset and deflection of the marker lines decrease towards the tips of the fault. Natural examples of s-type flanking folds, directly comparable with the model results, are more common than is generally appreciated. q 2004 Elsevier Ltd. All rights reserved. Keywords: Analogue modelling; Fault-related folds; Flanking structures; Simple shear 1. Introduction Several descriptive and numerical modelling studies concerning the deflection of planar passive markers around a discontinuity in a rock volume have been presented within the last few years (e.g. Grasemann and Stu ¨we, 2001; Passchier, 2001; Grasemann et al., 2003). These studies have introduced the general term flanking structures, reflecting the geometrical characteristic that they all show a symmetrical bending of marker lines around a planar central element, which is a structural or rheological discontinuity. In describing the geometry of the different structures, we follow the terminology used in previous publications (Fig. 1). The planar discontinuity in the centre of a flanking structure is called the crosscutting element (CE; Passchier, 2001). It is embedded in a homogeneous matrix, which is highlighted by passive marker lines (in 2D view) aligned parallel to the shear zone boundary (SZB). The marker line crossing the CE at its centre in the undeformed state is defined as the central marker line (CML). Generally, three types of flanking structures exist, namely a-type flanking folds, s-type flanking folds and shear bands. For a-type flanking folds there is an antithetic displacement of the central marker line along the CE, whereas s-type flanking folds and shear bands are defined by a displacement synthetic to the bulk shear sense. The offset of the CML is contractional for s-type flanking folds and extensional for shear bands, whereas a-type flanking folds can show either contractional or extensional offset. The numerical study of Grasemann et al. (2003) showed that each of these three major groups could be further subdivided into two varieties, namely normal and reverse drag structures. The term drag is used in the sense of Hamblin (1965), according to which the drag of markers along the CE can be either normal or reverse, corresponding to a deflection convex or concave in the direction of shear along the CE. The development of flanking structures is dependent on (1) the initial orientation of the discontinuity and (2) the bulk flow geometry (see figs. 5 and 6 of Grasemann et al. (2003) for details). Previous analogue studies (Hudleston, 1989; Odonne, 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2004.06.002 Journal of Structural Geology 26 (2004) 2191–2201 www.elsevier.com/locate/jsg * Corresponding author. Tel.: þ 41-1-6323702; fax: þ 41-1-6321080. E-mail address: ulrike.exner@erdw.ethz.ch (U. Exner).