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_Journal of Structural Geology 32 (2010) 1721–1731_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Field evidences for the role of static friction on fracture orientation in extensional relays along strike-slip faults: Comparison with photoelasticity and 3-D numerical modeling Roger Soliva a,*, Frantz Maerten a,b, Jean-Pierre Petit a, Vincent Auzias c a Université Montpellier II, Lab. Géosciences Montpellier, UMR 5243, Place E. Bataillon, 34095 Montpellier Cedex, France b IGEOSS, Parc Euromédicine, 340 rue Louis Pasteur, 34790 Grabels, France c BERKINE SONATRACH ANADARKO, Rte de Cina, 16001 Hassi Messaoud, Algeria Article history: Received 25 May 2009 Received in revised form 14 January 2010 Accepted 18 January 2010 Available online 28 January 2010 To the memory of Maurice Mattauer, professor at the University of Montpellier II, who left us in April 2009 Keywords: Fault Friction Relay Wing cracks Damage zone Abstract Fault friction is a parameter that is difficult to assess along fault zones since its determination depends on the knowledge of any factor controlling the state of stress around faults. In brittle homogeneous rocks, a limited number of these factors, such as the shape of the fault surface, the vicinity of fault tips or the remote stress ratio, are crucial to constrain for this determination. In this paper, we propose to analyse a field example in which all these properties are met and where the nature of the slipped structure suggests differences in static friction. We compare the orientations of branching fractures at strike-slip relay zones between en echelon stylolites and en echelon joints both reactivated in shear. The field data are compared with both photoelastic and 3-D numerical models that consider the remote stress conditions and the role of the geometry of the strike-slip segments. Based on field observations, these analyses quantitatively demonstrate the significant role of fault friction on the local stress field orientation and subsequent fracture formation. This work points out that estimations of fault friction based on analyses of fracture patterns or in situ stresses must be accompanied with a thorough investigation of the 3-D fault shape, its segmentation and the remote stress state. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Static friction along faults is an extremely important parameter for the understanding of the seismic cycle, the distribution of stresses, fracture patterns and damage zones around faults. In the past decades many efforts have been made to estimate fault friction along natural faults (e.g., Hanks, 1977; Zoback and Zoback, 1980; Brace and Kohlstedt, 1980; Lachenbruch and Sass, 1980; Zoback and Healy, 1984; Mount and Suppe, 1987; Brudy et al., 1997; Zoback et al., 1987; Scholz, 2000). The measure of static friction estimated using laboratory tests on fault gouges is scale-limited, i.e. on gouge samples from a bore hole cutting crossing the fault, and therefore may not represent the frictional state of the whole surface. Other approaches, based on the analyses of heat flow (Brune et al., 1969; Lachenbruch and Sass, 1980; d’Alessio et al., 2003) or numerical modeling (e.g., Parsons, 2002; Lovely et al., 2009), allow discussion on the state of friction along the fault but are quite indirect. The analysis of in situ stresses from bore hole measurements or fracture patterns is considered as the best indicator of the frictional state along a fault (Zoback and Healy, 1984; Zoback et al., 1987; Scholz, 2000). Assuming that fault cohesion can be close to zero on an active fault (Byerlee, 1978), the static friction has been approximated by Amonton’s first law, in which the frictional coefficient (m) is expressed as a function of the shear (F) and normal (N) components of the forces applied to a frictional surface. F ≈ m * N This law states that the friction coefficient of an infinitely long fault surface is directly related to the orientation and magnitude of the stresses close to this surface (Fig. 1a). This reveals that the analysis of the stress field around a fault can be used to determine the static friction along a fault, in cases where the remote ratio of stresses applied to the sliding surface is known. Therefore, any indicators of the stress field around faults provide the opportunity to quantify the static friction. However, this analytical approach, based on Amonton’s first law, assumes that the fault plane is rectilinear and that the fault tips are infinitely far from the study area. Such a first-order approximation is quite unrealistic for natural faults having tips, being irregular, segmented or more complex in shape (Fig. 1b–d). The local orientation and magnitude of the stress field around a fault do not rely only on fault friction, which makes its determination non-unique unless we have knowledge of the other factors perturbing the local stress field. In homogeneous rocks, the first parameter that has been considered as acting on the local stress field, and more precisely on the crack angle to a fault, is the static friction coefficient (e.g., Petit and Barquins, 1988; Barquins et al., 1997; Martel, 1997; Ohlmacher and Aydin, 1997; Willemse and Pollard, 1998; Zhou, 2006; Mutlu and Pollard, 2008). However, the remote stress angle has been considered as very important (Barquins et al., 1992; Ohlmacher and Aydin, 1997) as well as the remote stress ratio (Auzias et al., 1997; Katternhorn et al., 2000; Zhou, 2006). Other factors more related to the geometry and behavior of the fault surface also seem to be very influential, such as the 3-D geometry of the faults (e.g., Segall and Pollard, 1980; King et al., 1994; Willemse, 1997; Maerten et al., 2002; Bourne and Willemse, 2001), its spatial temporal evolution (Willson et al., 2007; Lunn et al., 2008; Moir et al., 2009), and fault opening (Kattenhorn and Marshall, 2006). Therefore, any analysis of fault zones that aims to estimate the role of fault friction on the stress field or in contrast to determine the state of friction from stresses analysis must know any of these factors that can perturb the local stress field. In this paper, we analyse a field example in which these factors can be estimated. Drastic differences in fracture orientation between reactivated frictional stylolites (i.e., structures of high friction coefficient) and frictionless joints (i.e., structures of low friction coefficient) suggest that friction is a prominent property influencing the stress perturbation at the close vicinity of a fault. We chose to study fracture orientation at extensional relay zones because the stress orientation has been described as quite stable in space along a relay zone (compared to outside) due to the juxtaposition of the two extensive fault quadrants (see Fig. 1c) (e.g., Auzias et al., 1997; Ohlmacher and Aydin, 1997). We compare the field data to photoelastic and 3-D numerical models to demonstrate and quantify the significant role of static friction on the stress and fracture orientation at extensional relay zones. 1723 2. Field Data 2.1 Geological Setting The studied exposure, located close to Les Matelles (15 km North of Montpellier, France, Fig. 2), is a suitable site for the study of brittle tectonics in limestones and stress perturbations around meso-scale faults (Rispoli, 1981; Fletcher and Pollard, 1981; Petit et al., 1999). The brittle tectonic structures observed (Fig. 3a) were formed during multiphase compressive tectonics allowing the formation of joints and stylolites. These structures of similar dimension and orientation have been reactivated as slip surfaces during a late tectonic event (Petit and Mattauer, 1995). Because of their different roughness, joints and stylolites are expected to be of different frictional properties during slip. Most of them show secondary fracturing and linkage at relay zones (Fig. 3b), that can be used as indicators of the palaeostress orientation (e.g., Rispoli et al., 1981; Petit and Mattauer, 1995). It is therefore worthwhile to address with particular care on the geological setting and history of the brittle structures that will be used to constrain the role of fault friction on fracture orientation. The studied exposure has been fully described in a number of previous studies (e.g., Rispoli et al., 1981; Taha, 1986; Petit and Ключевые слова: e, r, o