Description:

_Journal of Structural Geology 32 (2010) 490–506_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Normal fault growth and fault-related folding in a salt-influenced rift basin: South Viking Graben, offshore Norway_ _Karla E. Kane a,*, Christopher A-L. Jackson a, Eirik Larsen b,1_ _a Department of Earth Science & Engineering, Imperial College, London SW7 2AZ, UK_ _b Statoil ASA, Forusbeen 50, N-4035 Stavanger, Norway_ _article info_ _Article history: Received 7 February 2009; Received in revised form 19 February 2010; Accepted 22 February 2010; Available online 1 March 2010_ _Keywords: Syn-rift North Sea Normal faulting Fault-propagation folding Salt tectonics_ _abstract_ _Three-dimensional seismic data were analysed to reconstruct the structural and stratigraphical development of a salt-influenced rift basin and thus gain an understanding of the relationships between normal fault growth, salt tectonics and the evolution of syn-rift depocentres. The Sleipner Basin, South Viking Graben, northern North Sea, is ca. 30 km long by 8 km wide and is bound to the east by a major extensional fault zone (Sleipner Fault Zone). Two types of fault-related fold are identified within the basin: (1) A fault-parallel monocline, interpreted as an extensional forced-fold, which formed through the upward propagation of the Sleipner Fault Zone through ductile evaporites of the Zechstein Supergroup and (2) three fault-perpendicular, salt-cored anticlines that compartmentalise the basin into four sub-basins and are related to displacement gradients along-strike of the Sleipner Fault Zone. Detailed seismic-stratigraphic analysis of pre- and syn-rift stratal units reveals a complex interplay between fault growth and salt movement which strongly controlled the evolution of syn-rift depocentres. During the early syn-rift, a series of depocentres, separated along-strike by the fault-perpendicular folds, were offset into the axis of the basin (ca. 3–4.5 km to the west of the Sleipner Fault Zone) by the fault-propagation fold. Later in the rift event, the influence of the fault-perpendicular folds depleted, resulting in a larger, interconnected depocentre that shifted into the immediate hangingwall of the fault as the surface of the fault-propagation fold was breached. The results of this study have implications for normal fault growth and sedimentary depocentre development in salt-influenced rift basins, and contribute to the general understanding of the controls on salt migration._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _1.1. Fault-related folding in extensional basins_ _Large (i.e., several tens of kilometres length) basin-bounding fault zones in extensional basins typically develop through the interaction and linkage of a series of initially isolated normal fault segments (e.g., Peacock and Sanderson, 1991; Anders and Schlische, 1994; Cartwright et al., 1995; Dawers and Anders, 1995). This fault growth process commonly results in the development of intra-basin hangingwall folds with axes that are oriented broadly perpendicular to the strike of the basin-bounding fault (e.g., Gawthorpe and Hurst, 1993; Anders and Schlische, 1994; Gawthorpe et al., 1994; Schlische, 1995); 'transverse anticlines' or 'intra-basin highs' develop adjacent to segment boundaries or areas of displacement minima, whereas 'transverse synclines' or 'intra-basin lows' are associated with displacement maxima located at the centre of the constituent fault segments (e.g., Anders and Schlische, 1994; Gawthorpe et al., 1994; Schlische, 1995). In addition, fault-parallel folds, with axes that are oriented broadly parallel to the strike of the basin-bounding fault, are often observed. These structures typically form breached or unbreached basinward-facing monoclines which develop as a result of folding ahead of the upward propagating fault tip (e.g., Withjack et al., 1990; Schlische, 1995; Gawthorpe et al., 1997; Corfield and Sharp, 2000). Previous studies have shown that the development of these fault-propagation or forced folds (sensu Withjack et al., 1990; Withjack and Callaway, 2000) can significantly influence the distribution and thickness of sediment within the evolving basin, particularly during the early phase of rifting (e.g., Withjack et al., 1990; Gupta et al., 1999; Sharp et al., 2000; Dawers and Underhill, 2000; Ford et al., 2007)._ _K.E. Kane et al. Journal of Structural Geology 32 (2010) 490–506_ _491_ _1.2. Salt mobility and fault-related folding_ _Models of normal fault growth and fault-related folding have largely been developed for rift basins involving extension of brittle, 'basement-type' lithologies. However, several studies have indicated that the structural style of a rift basin can be significantly influenced by the presence of ductile components such as evaporites (herein referred to as 'salt') within the pre-rift sedimentary succession (e.g., Koyi et al., 1993; Withjack and Callaway, 2000; Stewart et al., 1996, 1997; Richardson et al., 2005; Marsh et al., 2009). The viscous nature of such a substrate can restrict the upward propagation of a fault tip, resulting in a sub-salt, basement-involved fault that is decoupled from a forced-fold within the supra-salt cover strata (Withjack et al., 1990; Withjack and Callaway 2000; Stewart et al., 1996, 1997; Ford et al., 2007). Through time, the basement fault often propagates through the mechanically weak layer, leading to breaching of the earlier formed fold and the generation of a fault scarp at-surface (Jackson and Vendeville, 1994; Stewart et al., 1996, 1997; Pascoe et al., 1999). The development of normal faults and forced-folds commonly leads to salt migration which accommodates spatial variations in strain. Experimental modelling and seismic analysis have indicated that the geometry and distribution of normal faults and associated forced folds are dependent on several variables, including magnitude and rate of fault displacement; thickness contrast between the ductile layer and the brittle cover strata; physical properties of the ductile substrate (e.g., density and viscosity); and degree of differential loading in the overburden._ _(Vendeville et al., 1995; Koyi et al., 1993; Stewart et al., 1996; Richardson et al., 2005; Withjack and Callaway, 2000)._ _1.3. Study aims_ _Many studies have provided what is essentially a two-dimensional understanding of the relationship between extensional faulting and salt mobility (e.g., Vendeville and Jackson, 1992a,b; Pascoe et al., 1999; Withjack and Callaway, 2000). However, given the along-strike complexity associated with the evolution of major normal fault systems, it should be expected that a significant component of salt movement will occur out of section during rifting. Here, 3D seismic and well data are used to investigate the relationships between normal fault growth and evaporite mobility within the Sleipner Basin, a sub-basin of the South Viking Graben, offshore Norway (Fig. 1). This area is particularly complex because late Middle to Late Jurassic rifting was initiated across a preexisting structural template related to earlier, Triassic to Middle Jurassic salt movement. Critical to this study is the recognition that fault growth and salt migration results in variable rates and magnitude of subsidence that is documented by the coeval syn-rift succession. Changes in the thickness and seismic-stratigraphic architecture of the syn-rift units are used to constrain the temporal and spatial variation of surface deformation associated with the developing structures (see Gupta et al., 1998; Dawers and Underhill, 2000; Sharp et al., 2000; McLeod et al., 2000, 2002; Young et al., 2002). The results of this study indicate that the development of fault-related folds in salt-influenced rift basins is intimately_ _Fig. 1. Geological Setting of the Sleipner Basin (a) Map of South Viking Graben, northern North Sea (b) Map of Zechstein Supergroup facies distribution (redrawn from Thomas and Coward, 1996) (c) Regional cross-section showing location of Sleipner Basin study area on hangingwall dipslope of the South Viking Graben._ _492_ _K.E. Kane et al. Journal of Structural Geology 32 (2010) 490–506_ _associated with the segmentation and linkage history of the extensional faults and that the spatial and temporal distribution of syn-rift sedimentary depocentres is largely controlled by the interplay between fault displacement and salt migration. In addition, it is apparent that sediment loading within syn-rift sedimentary depocentres may influence salt migration, particularly in the early phase of rift basin evolution._ Ключевые слова: e, r, o