On stress and strain in a continuous-discontinuous shear zone undergoing simple shear and volume loss Å. Fagereng Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa article info Article history: Received 15 November 2011 Received in revised form 16 February 2012 Accepted 25 February 2012 Available online xxx Keywords: Mélanges Shear zones Simple shear Fibre stress Polyphase rheology abstract I summarise observations within a continuous-discontinuous shear zone to discuss the local stress and strain conditions experienced within a mixed rheology shear zone undergoing volume loss and defor- mation approximating simple shear. The Chrystalls Beach Complex, New Zealand, comprises phacoids formed from dismembered beds by layer-parallel extension, enclosed within a relatively incompetent matrix. Local extension is generally subparallel to the regional direction of shortening, and overall it appears that layer-parallel extension is a geometrical necessity in low angle shear zones where signifi- cant flattening occurs in response to simple shear accompanied by volume loss. Preferential stress loading of phacoids is predicted by fibre-loading theory, and the failure of phacoids by brittle fracture is thereby governed by fibre stresses transferred from the matrix. The principal stress orientations in a phacoid are likely rotated relative to the matrix, and either parallel or perpendicular to the phacoid-matrix interface. As preferential loading of phacoids decreases the stress level in the matrix, an increased volume fraction of phacoids increases the strength of the shear zone as a whole. However, only small matrix volume fractions are required for the composite to act nearly as weak as the matrix. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In intact, isotropic rocks subject to a regional triaxial stress field defined by the three principal compressive stresses s 1  s 2  s 3 , simple shear zones are inferred to initiate at 45  to s 1 and parallel to a plane containing s 2 (e.g. Ramsay, 1980). During progressive simple shear the finite strain ellipsoid (defined by the principal extensional strains X  Y  Z) rotates so that, after significant finite shear strain, X lies at a low angle to the shear zone walls and subparallel to the transport direction (e.g. Escher and Watterson, 1974; Ramsay, 1980). Therefore, it is common for low angle shear zones accommodating horizontal shortening to have a shallow plunging greatest extensional strain with a trend subparallel to the regional shortening direction (e.g. Kvale, 1953; Bridgwater et al., 1973; Barr et al., 1986). Within exhumed examples of underthrust sediments at the base of paleo-accretionary prisms, subvertical extension veins have been observed at a high angle to subhorizontal shear planes, inferred to indicate a local subhorizontal s 3 within low angle shear zones in a regional compressional tectonic regime (Byrne and Fisher, 1990; Fagereng, 2011b). In the active Barbados accretionary margin, AMS (Anisotropy of Magnetic Susceptibility) measurements on drill core, collected in a vertical hole through the prism and into the décollement, indicate a sharp change from subhorizontal short- ening within the accretionary wedge, to subvertical shortening within underthrust sediments (Housen et al., 1996). Therefore, within some subduction-related shear zones accommodating regional horizontal shortening, local horizontal extension occurs within the shear zone. In exhumed analogues this local extension is in particular observed through boudinage of relatively competent subhorizontal layers and prevalence of subvertical extension veins (Byrne and Fisher, 1990; Raimbourg et al., 2009; Fagereng, 2011b). These observations relate to rotation of the greatest principal extensional strain towards the shear plane and accompanying subvertical shortening in the case of shallow dipping shear zones (e.g. Escher and Watterson, 1974). However, subvertical extension veins indicate a locally subhorizontal s 3 , incompatible with a compressional stress regime (e.g. Anderson,1951). Thus, although finite strain orientations within shear zones, assuming bulk simple shear, are relatively well understood, the local stress field within a simple shear zone is not so well known. Here I present a simple analysis of the local strain and stress state within a shear zone deforming by progressive bulk simple shear, particularly as expe- rienced by competent inclusions, inspired by observations in a continuous-discontinuous shear zone in an exhumed accre- tionary prism. E-mail address: ake.fagereng@uct.ac.za. Contents lists available at SciVerse ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2012.02.016 Journal of Structural Geology xxx (2012) 1e10 Please cite this article in press as: Fagereng, Å., On stress and strain in a continuous-discontinuous shear zone undergoing simple shear and volume loss, Journal of Structural Geology (2012), doi:10.1016/j.jsg.2012.02.016