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_Journal of Structural Geology 32 (2010) 693-702_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Palaeostress perturbations near the El Castillo de las Guardas fault (SW Iberian Massif) Encarnaci?n Garc?a-Navarro*, Carlos Fern?ndez Departamento de Geodin?mica y Paleontolog?a, Universidad de Huelva, 21071-Huelva, Spain Article info Article history: Received 23 June 2009 Received in revised form 7 April 2010 Accepted 11 April 2010 Available online 18 April 2010 Keywords: Fault Palaeostress Stress trajectories Iberian Massif Abstract Use of stress inversion methods on faults measured at 33 sites located at the northwestern part of the South Portuguese Zone (Variscan Iberian Massif), and analysis of the basic dyke attitude at this same region, has revealed a prominent perturbation of the stress trajectories around some large, crustal-scale faults, like the El Castillo de las Guardas fault. The results are compared with the predictions of theoretical models of palaeostress deviations near master faults. According to this comparison, the El Castillo de las Guardas fault, an old structure that probably reversed several times its slip sense, can be considered as a sinistral strike-slip fault during the Moscovian. These results also point out the main shortcomings that still hinder a rigorous quantitative use of the theoretical models of stress perturbations around major faults: the spatial variation in the parameters governing the brittle behaviour of the continental crust, and the possibility of oblique slip along outcrop-scale faults in regions subjected to general, non-plane strain. ? 2010 Elsevier Ltd. All rights reserved. 1. Introduction Brittle fracture of crustal rocks is a process controlled by physical parameters and relations (confining pressure, pore pressure, strain rate, temperature, differential stress, stress changes, friction and elastic coefficients, among many others) whose exact values are largely unknown for a given rock massif, and only general friction laws are available (Byerlee, 1978; Scholz, 1990; Engelder, 1993). These parameters condition the attitude of fractures, their displacement, and the triggering of seismic events (Pollard and Segall, 1987; Cowie and Scholz, 1992; King et al., 1994). Rock massifs are geometrically and mechanically heterogeneous (e.g., Bieniawski, 1989) and hence deviation of stress fields are common in fractured areas (Zoback et al., 1987; Homberg et al., 1994). Major faults are commonly composed of distinct segments that can show independent slip (Schwartz and Sibson, 1989). Apart from these complexities, the conditions under which failure occurs in rocks are characterised by the widely used Coulomb failure criterion. Rocks deformed experimentally are shown to approximately obey this criterion (e.g., Jaeger and Cook, 1979). The stress deviation near the tip of pre-existing faults can * Corresponding author. E-mail address: navarro@uhu.es (E. Garc?a-Navarro). 0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.04.004 be theoretically modelled in an elastic half space containing a fault that accomplishes the Coulomb criterion and is subjected to slip under an external stress field. As a consequence of this line of research, it has been observed that a cortege of secondary fractures and other geological structures can appear near the tips of the slipped segment (Anderson, 1951; McKinstry, 1953; Chinnery, 1966; Price, 1968; Lajtai, 1968). More recent models analyse the influence of the distinct physical parameters and the geometry of the stress trajectories around faults (e.g., Pollard and Segall, 1987; Mandl, 1988; Petit and Barquins, 1990; Homberg et al., 1997, 2004; Toda and Stein, 2002; Maerten et al., 2002). Study of second order structures (stylolites, tension gashes, dykes, joints, minor faults) at relevant field cases has allowed determining stress-field deviations near natural faults (e.g., Petit and Mattauer, 1995; Casas-S?inz and Maestro-Gonz?lez, 1996; Sim?n et al., 1999). Knowledge of the geometry of stress perturbations obtained from the study of second order structures can be used to deduce the slip sense of old, major faults. This work describes the stress field associated with the displacement of a major fault during Late Variscan times, the El Castillo de las Guardas fault. Second order faults are analysed using techniques of fault-slip inversion. Several dyke systems were emplaced coeval to these faults. Dyke and fault data are used to determine stress trajectories and the results are tested against the predictions of the theoretical models of Homberg et al. (1997) and Toda and Stein (2002). 694 E. Garc?a-Navarro, C. Fern?ndez Journal of Structural Geology 32 (2010) 693-702 2. Geological setting 2.1. General features The South Portuguese Zone is located at the southernmost part of the Variscan Iberian Massif (Julivert et al., 1974). The northern boundary of the South Portuguese Zone (Fig. 1) has been interpreted as a Variscan tectonic suture (Bard, 1969; Crespo-Blanc and Orozco, 1991; D?az-Azpiroz et al., 2006; Ribeiro et al., 2007). The South Portuguese Zone is considered as a part of a Variscan orogenic wedge (Simancas et al., 2003) subjected to an intense progressive deformation that evolved from large-scale lateral convergence of continental blocks to an essentially transcurrent tectonics late during the Variscan orogenesis (Garc?a-Navarro and Fern?ndez, 2004; Mantero et al., 2007). The rocks of the South Portuguese Zone are Devonian and Carboniferous and include sedimentary and igneous protholiths variably affected by the Variscan deformation and metamorphism. Structurally, the South Portuguese Zone is a fold and thrust belt, vergent to the south and southwest, and generated in response to thin-skinned tectonics (Silva et al., 1990; Quesada, 1998; Simancas et al., 2003). The Castilblanco de los Arroyos batholith (de la Rosa, 1992), composed of a large variety of basic to acid igneous rocks, intruded at shallow crustal levels the rocks of the South Portuguese Zone (Fig. 1). Intrusion ages range from Late Visean to Moscovian (de la Rosa, 1992; Giese et al., 1993), although most of the batholith was emplaced during the Bashkirian and Moscovian, and its external contacts crosscut the Variscan folds and thrusts. The rocks of the Castilblanco de los Arroyos batholith were affected by a brittle deformation that has been considered as the last stage of the Variscan orogeny (Garc?a-Navarro and Fern?ndez, 2004). This brittle deformation stage lasted from the Moscovian to the Permian and it consisted of several phases of transtension and transpression, accompanied by the intrusion of distinct systems of basic dykes (Garc?a-Navarro et al., 2003). Finally, the eastern part of the South Portuguese Zone was covered by the post-orogenic Early Permian sediments of the Viar basin, which mostly postdate the aforementioned brittle deformation phases. The Neogene sediments of the Guadalquivir basin unconformably cover the southern boundary of the South Portuguese Zone (Fig. 1). 2.2. Faults and diabase dykes at the eastern part of the South Portuguese Zone The major fractures outcropping at the eastern part of the South Portuguese Zone are (Fig. 1): the Southern Iberian shear zone, the El Garrobo-Burguillos shear zone, the Cala dam fault, and the El Castillo de las Guardas fault. The Southern Iberian shear zone marks the northern limit of the South Portuguese Zone and it was generated during the main stages of the Variscan collision (Crespo-Blanc and Orozco, 1988; D?az-Azpiroz and Fern?ndez, 2005). The El Garrobo-Burguillos shear zone is N110 x14E-striking, and it is characterised by a plano-linear ductile fabric with associated kinematic criteria indicating a predominant strike-slip, sinistral displacement. Later brittle overprinting of the ductile structures shows dextral slip. The Cala dam fault is a sinistral strike-slip fault, with a minimum length of 20 km. The azimuth of this fault varies from N40 x14E (southwest) to N60 x14E (northeast). The analysis of the geological map (Fig. 1) allows estimating a horizontal displacement of around 7 km for the Cala dam fault. Most of this displacement took place early during the Late Variscan brittle deformation, because the Cala dam fault was unfavourably oriented for slip after the Moscovian (Garc?a-Navarro and Fern?ndez, 2004). The Early Permian sediments of the Viar basin cover the northeastern tip of the Cala dam fault. The El Castillo de las Guardas fault, a largely strike-slip structure, shows a minimum length of 46 km and its average azimuth is N130 x14E, parallel to the southern boundary of the Castilblanco de los Arroyos batholith. Notwithstanding, some segments of the El Castillo de las Guardas fault are E-W-oriented, and its southern tip is covered by the Neogene sediments of the Guadalquivir basin. It is not possible to determine the magnitude of displacement along the El Castillo de las Guardas fault due to the absence of suitable markers. Furthermore, displacement along the El Castillo de las Guardas fault has alternated between sinistral and dextral during the Late Carboniferous and Early Permian (Garc?a-Navarro and Fern?ndez, 2004), which complicates the determination of the amount of slip for each episode. The Southern Iberian shear zone was disrupted by an en echelon system of brittle, sinistral, NE-SW directed faults (Simancas, 1985). Fig. 1. Geological map of the northeastern part of the South Portuguese Zone, with location of the major faults and ductile shear zones (Southern Iberian shear zone, SISZ; El Castillo de las Guardas fault, CGF; El Garrobo-Burguillos shear zone, GBSZ; and the Cala dam fault, CDF). 695 The western tip of the El Garrobo-Burguillos shear zone is crosscut by the El Castillo de las Guardas fault. 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