Salt-influenced normal fault growth and forced folding: The Stavanger Fault System, North Sea Matthew M. Lewis a, * , Christopher A.-L. Jackson a , Rob L. Gawthorpe b a Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK b Department of Earth Science, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway article info Article history: Received 3 January 2013 Received in revised form 4 July 2013 Accepted 24 July 2013 Available online 1 August 2013 Keywords: Salt Rift basin Normal fault Forced folding Kinematic coherence Geometric coherence Egersund Basin abstract Displacement ratio (D r ) is the ratio between salt thickness (T v ) and sub-salt normal fault displacement (D)(D r ¼ T v /D), and it is typically used to predict the degree of geometric and kinematic linkage between sub- and supra-salt fault populations, and the overall structural style in salt-influenced extensional settings. However, we currently lack natural examples of how D r and the underlying geological controls vary, and how these may control the three-dimensional geometry and evolution of salt-influenced normal fault systems. Furthermore, it is currently unknown if kinematic coherence in salt-influenced extensional settings can be maintained over relatively long length-scales (10 1 e10 3 m) and for rela- tively long timeframes, and how this may impact the growth and geometry of large-throw (>500 m), salt-influenced normal fault systems. In this paper we use a 3600 km 2 , high-quality 3D seismic reflection dataset and borehole data from the Stavanger Fault System (SFS), Egersund Basin, eastern North Sea Basin to investigate; (i) how pre-rift salt thickness (T v ) and sub-salt fault throw (T) control the structural style and evolution of a basin-bounding, salt-influenced normal fault system; and (ii) the role salt plays in maintaining kinematic coherence in normal fault systems. We demonstrate that; (i) pre-rift salt dis- tribution (T v ), specifically its presence in the proto-footwall (i.e., when T v > 0), is the primary control on partitioning of faulting and (forced) folding along the fault system, and the style of linkage (i.e., hard- or soft-linkage) between sub- and supra-salt fault populations; and (ii) sub- and supra-salt fault pop- ulations represent brittle elements of a single, geometrically and kinematically coherent structure, the geometry and evolution of which is related to the ductile translation of strain on a scale (up to 8 km) and duration (c. 65 Myr) that believe is significantly greater and longer than previously documented. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. 1. Introduction In extensional systems, salt layers can act as intra-stratal de- tachments and they can thus serve to decouple sub- and supra-salt deformation, typically resulting in the development of structural styles that differ significantly from those developed in fully brittle systems (Nalpas and Brun, 1993; Jackson and Vendeville, 1994; Stewart et al., 1996, 1997; Alves et al., 2002; Richardson et al., 2005; Kane et al., 2010; Marsh et al., 2010; Duffy et al., 2012; Wilson et al., 2013). For example, due to the decoupling effect of salt, supra-salt monoclines or ‘forced folds’ (sensu Stearns, 1978) and cover-restricted, gravity-driven extensional deformation, may be especially common in salt-influenced extensional systems during the initial stages of stretching (Fig. 1)(Withjack et al., 1989, 1990; Stewart et al., 1996; Pascoe et al., 1999; Richardson et al., 2005; Ford et al., 2007). The degree of coupling between sub- and supra-salt deformation is related to five key parameters; (i) evaporite thickness, (ii) overburden thickness, (iii) the cohesive strength and ductility of the overburden, (iv) total fault displace- ment, and (v) fault displacement rate or evaporite viscosity (Koyi et al., 1993; Stewart et al., 1996, 1997; Withjack and Callaway, 2000). The combined effect of these parameters is captured in the ‘displacement ratio’ (D r ), which is defined as the ratio of salt thickness (T v ) to sub-salt fault displacement (D)(D r ¼ T v /D). This ratio thus incorporates several geological controls on fault growth (i.e., T v and D), which can vary both temporally and spatially during extension (Fig. 1)(Koyi et al., 1993). When D r ¼ <1, a hard-linked or coupled system typically forms, whereas when D r ¼ >1, a soft- linked or decoupled system forms (Koyi et al., 1993; Jackson and Vendeville, 1994; Stewart et al., 1996, 1997). D r thus provides a dimensionless parameter that can help us understand the style of * Corresponding author. E-mail addresses: matthew.m.lewis07@imperial.ac.uk, mattlewis8@msn.com (M.M. Lewis). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2013.07.015 Journal of Structural Geology 54 (2013) 156e173