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_Journal of Structural Geology 32 (2010) 523-536_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Aspects and origins of fractured dip-domain boundaries in folded carbonate rocks_ _L. Bazalgette a,*, J.-P. Petit b, M. Amrhar c, H. OuanaГЇmi d_ _a Shell International Exploration and Production, Carbonate Research Team, Kessler Park 1, 2288 GS Risjwijk, The Netherlands_ _b GГ©osciences Montpellier, UMR 5243, CC 060, UniversitГ© Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex 5, France_ _c DГ©partement de GГ©ologie, FacultГ© des Sciences de Marrakech, B.P. 3502, Amerchich, Marrakech 40000, Morocco_ _d DГ©partement de GГ©ologie, Ecole Normale SupГ©rieure, B.P. S2400, Marrakech 40000, Morocco_ _article info_ _Article history: Received 2 November 2009; Received in revised form 3 March 2010; Accepted 6 March 2010; Available online 16 March 2010_ _Keywords: Fractures Folds Carbonate rocks Sedimentary rocks Fractured reservoirs Faults Curvature Curvature accommodation_ _abstract_ _We present comparative п¬Ѓeld studies in folded areas (Southern France, Moroccan Western Atlas and Abruzzo, Italy) giving new insights into fracture distribution within folded rocks of the shallow brittle crust. We show that the curvature in folds formed in brittle mechanical units is usually accommodated by multiple “dip-domain boundaries” (appearing as curvature discontinuities at fold scale) corresponding to relatively narrow and dense fracture zones, striking parallel or slightly oblique to the fold axis. They separate “dip-domains” where curvature is absent or moderate. It is shown that the dipdomain boundaries (which are obvious in the case of kink folds or box-fold anticlines) are currently present as multiple subtle hinges even when the curvature appears continuous at п¬Ѓrst sight. The nature of dip-domain boundaries is studied: they often cut through the whole thickness of the mechanical units. Their internal structure varies, and a non-exhaustive typology is proposed. For each type, an interpretative kinematic scenario shows how the dip-domain boundaries could initiate and develop. We suggest two kinds of origins: (1) they could correspond to the reactivation of inherited, along-strike fracture zones (opening-mode fracture concentrations such as big joints, fracture corridors, inherited faults, etc.); (2) they could be created as mechanical instabilities during the fold formation (syn-folding origin), in particular through small reverse faults. In both cases, early zones of weakness localize the dip-domain boundaries, and control the increase in curvature in association with increasing fracture density within the boundary. Because they represent well-deп¬Ѓned vertically and axially persistent sub-seismic fracture zones generally limited to the thickness of the folded unit, dip-domain boundaries could enhance the axial permeability of folded and fractured reservoirs._ _Г“ 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Folds, which are very common expressions of rock deformation in the Earth’s crust, have been extensively studied both as geometrical objects (Ramsay, 1967; Suppe, 1985) and as elements of the mechanical and structural evolution of orogens (Price and Cosgrove, 1990). In folds developed in competent lithologies in the upper part of the Earth’s crust, some of the folding strain is often accommodated by fractures. The timing and distribution of these fractures can have a significant influence on the п¬Ѓnal fold geometry, in particular, whether the fold develops a continuous or discontinuous curvature. In tri-axial buckling experiments with multi-layered parafп¬Ѓn, Bazalgette (2004) and Bazalgette and Petit (2007) produced folds with discontinuous curvature. During these experiments, the spontaneous formation of localized fracture zones caused the folds to divide into a series of planar dip-domains (Suppe, 1983), each separated by a series of “articulations” (Bazalgette, 2004) or “dip-domain boundaries” (Bazalgette and Petit, 2007). The dip-domain boundaries were essentially composed of opening mode fractures, the formation of which ultimately controlled the evolution of the fold geometry and led to the discontinuous fold curvature. Interlayer friction, layer thickness and confining pressure were shown to have a strong influence on the geometry of the experimental folds, and although a wide range of structural geometries was created, all of the folds exhibited discontinuous curvature._ _If natural folds develop in a manner similar to the experiments above, then there are significant implications for hydrocarbon exploration and production in folded and fractured reservoirs. Assuming that dip-domain boundaries are zones of intense fracturing and potential high fracture connectivity, they could behave as high permeability drains in folded and fractured reservoirs. Therefore, dip-domain boundaries would need to be explicitly included in fractured reservoir models (e.g., Rawnsley and Wei, 2001; Rawnsley et al., 2004; De Keijzer et al., 2007) because their presence might significantly impact well performance or the potential for early water breakthroughs. However, and before this can be done, we need a clear understanding of the distribution and internal organization of fractures, and in particular, fractured dip-domain boundaries in folded structures._ _A variety of conceptual models for fold-related fracture distributions have been published since the late 1960s, most based largely on the mechanical interpretation of folded and fractured outcrops (Price, 1966; Stearns, 1964; Stearns and Friedman, 1972; Price and Cosgrove, 1990). These works primarily focused on the geometry of fold-related fracture networks. More recent work has emphasized the importance of reactivated and inherited fractures (Guiton et al., 2003; Bergbauer and Pollard, 2004; Bellahsen et al., 2006), as well as the progressively evolving geometry of folds (e.g., Fischer and Wilkerson, 2000). Fold-related fracture prediction has been further enhanced by kinematic and mechanical restorations (e.g., Salvini and Storti, 2001; Maerten and Maerten, 2006), as well as curvature analysis (e.g., Lisle, 1994; Fischer and Wilkerson, 2000; Bergbauer, 2007). Despite these improvements in understanding and predicting fractures in folds, the prediction at the scale of the folded mechanical unit is still complicated by the fact that deformation processes are also strongly linked to the mechanical organization of the folded series. Attempts at correlating fracture distributions with the layer thickness in mono- and multilayered models have been proposed since the 1980s (Ladeira and Price, 1981; Bai and Pollard, 2000), but they are limited to tabular conditions. More complex approaches involving the lithological properties within folded multilayers have been also described, based on natural examples (Hanks et al., 1997; Fischer and Jackson, 1999; Hayes and Hanks, 2008). These studies have pointed out some of the influences of the multi-layering parameters on the folding style, the associated stress and strain distributions, and their impact on the development of fracture patterns._ _This paper describes the structure of natural dip-domainal folds in selected outcrops in folded carbonate formations and at different scales. In particular, we aim to describe the detailed internal structure of dip-domain boundaries, and to give insight into the way these boundaries accommodate and result in a discontinuous fold curvature during their development. We propose a simple and non-exhaustive classification of the variety of observed dip-domain_ _Fig. 1. Geological setting of the Coulazou gully outcrops: (1) Structural sketch showing the location of the Coulazou gully folded outcrops. These outcrops are situated in the “Montpellier Fold”, which is interpreted as the northernmost termination of the North-Pyrenean zone (Mattauer, 1971). (2) Structural sketch of the Coulazou gully folded and faulted area. (3) Simplified section of the Montpellier Fold area (modified from GГЁze, 1979)._ _Fig. 2. Example of dip-domainal fold with box-fold like geometry: (a) general view of the outcrop, (b) semi-interpretative sketch, (c) detail of the well-exposed northernmost dip-domain boundary._ _Fig. 3. Detailed view of the well-exposed northernmost hinge of the fold described on Fig. 5: (1) outcrop photograph, (2) semi-interpretative sketch. Zones (a), (b) and (c) are interpreted in Fig. 7._ _Fig. 4. Mechanical scenarios aiming to explain the dip-domain boundary mechanism of the fold described in Figs. 5 and 6. In zone (a), curvature is only accommodated by mode I fracture concentration and coalescence. In zones (b) and (c), curvature is initiated in beds affected by early bed-scale reverse faults. Increasing curvature is accommodated by offset on these faults and by the formation of mode I fractures in the zones of stress strain concentration._ _boundaries, and present some ideas on the parameters that most strongly influence dip-domain boundary localization._ Ключевые слова: e, r, o