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_Journal of Structural Geology 33 (2011) 114e131_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Shear band formation and strain localization on a regional scale: Evidence from anisotropic rocks below a major detachment (Betic Cordilleras, Spain) P. Agard a,b,*, R. Augier c, P. Moni?d a UPMC Univ Paris 06, ISTEP, F-75005, Paris, France b CNRS, UMR 7193, ISTEP, F-75005, Paris, France c Univ. d'Orl?ans, Univ. F. Rabelais-Tours, CNRS INSU UMR 6113, ISTO, F-45071, Orl?ans, France d CNRS, G?osciences Montpellier, UMR 5243, F-34095, Montpellier, France Article info Article history: Received 20 April 2010 Received in revised form 11 October 2010 Accepted 19 November 2010 Available online 2 December 2010 Keywords: Shear bands Strain gradient Progressive deformation Cleavage Betics Abstract Regional-scale deformation taking place in a strongly anisotropic, yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact (i.e., the Filabres shear zone, a major detachment in the Betic Cordilleras, Spain). The density of C30 shear bands (or extensional crenulation cleavage) reworking the earlier S2 schistosity increases exponentially towards the contact, in parallel with the decrease in the size of the shear domains. Systematic variations in angles and shape ratios are also reported. Deformation and age patterns, however, suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time. This fossilized shear strain gradient was thus produced somewhat diachronously. Such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation. ? 2010 Elsevier Ltd. All rights reserved. 1. Introduction Ductile shear zones and shear band geometries have been studied extensively over the past 40 years (e.g., Ramsay and Graham, 1970; Burg and Laurent, 1978; Berth? et al., 1979; Ramsay, 1980; Gapais et al., 1987; Dutruge et al., 1995; Carreras, 2001; Battacharyya and Hudleston, 2001; Arbaret and Burg, 2003; Michibayashi and Murakami, 2007). These studies focused on geometrical patterns (Passchier, 1980), on determining the respective contributions of simple and pure shear (Ramsay and Huber, 1983; Tikoff and Fossen, 1993), or on retrieving the history of deformation (Druguet et al., 1997; Carreras et al., 2011). So far most of them dealt with individual structures at the outcrop scale (e.g., Arbaret and Burg, 2003; Pennachioni and Mancktelow, 2007) or on their organization as networks (particularly on the Cap de Creus area; e.g., Druguet et al., 1997; Carreras, 2001; Fusseis et al., 2006). Fewer, however, attempted to trace finite deformation on a regional scale, for example as a function of the distance to a major crustal shear zone (Stipp et al., 2002; Gueydan et al., 2005; Marques et al., 2007), although such patterns contain important information. * Corresponding author. UPMC Univ Paris 06, ISTEP, F-75005, Paris, France. Fax: ?33 1 44 27 50 85. E-mail address: philippe.agard@upmc.fr (P. Agard). 0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.11.011 on the rheological evolution of strain localization. This is particularly important for the weaker, phyllosilicate-rich, schistosed lithologies, which will tend to predominantly focus deformation and thus control mid-crustal deformation (e.g., Shea and Kronenberg, 1993; Handy and Brun, 2004; Le Pourhiet et al., 2004 and references therein). Initially homogeneous, isotropic rock volumes with round or spherical initial markers, such as deformed granites or gabbros transformed to mylonites or ultramylonites (Choukroune and Gapais, 1983; Arbaret and Burg, 2003), commonly develop CeS and C0eS structures (Berth? et al., 1979; Ramsay, 1980; Hippertt, 1999) and exhibit some periodicity in the localization of shear strain (Cobbold and Ferguson, 1979; Dutruge et al., 1995; Dutruge and Burg, 1997; Gueydan et al., 2003). In contrast, phyllosilicate-rich lithologies deformed through bulk simple shear, or with the principal strain oriented at high angle to the plane of anisotropy, will tend to develop spaced shear bands termed extensional crenulation cleavage (ECC, Platt, 1979; or shear band cleavage: White et al., 1980). How these multi-scale shear fabrics progressively develop in anisotropic rocks with increasing shear strain is still unclear, with contrasting evidence of strain localization or network widening (Passchier, 1984; Platt, 1984; Fusseis et al., 2006; Handy et al., 2007; Michibayashi and Murakami, 2007; Scholz, 2007), and even caution as to whether they reflect the overall sense of shear (Behrmann, P. Agard et al. Journal of Structural Geology 33 (2011) 114e131 115 P. Agard et al. 1987; Passchier, 2001; Grasemann et al., 2003; Lexa et al., 2004). Important pending questions are: is it therefore possible to derive an empirical law relating the regional-scale shear strain gradient to deformation patterns in metapelitic shear zones (which could, in turn, be used as a way to constrain their hardening softening behaviour or strain rates; e.g., Means, 1995; Sassier et al., 2009)? How much deformation is taken up near a major contact at the hmto km-scale, and how much tectonic thinning is involved? Can we quantify the amount of ductile deformation and provide constraints for numerical models attempting to reproduce rheological behaviours as seen in rocks (e.g., Jessell et al., 2009)? The area investigated here, where ECCs were first defined (Betic Cordilleras, Spain; Platt, 1979; Platt and Vissers, 1980), exhibits a stack of rocks exhumed below a major detachment. This regional-scale detachment, with a strike length of >150 km, separates the Alpujarride from the Nevado-Filabride complexes and is well known in terms of deformation and PeT-time history (Platt and Vissers, 1980; Platt et al., 1984; Garcia-Duenas et al., 1992; Martinez-Martinez et al., 2002; Augier et al., 2005a). Importantly, the shear band network investigated here can be thought to result from a single deformation event exhuming these rocks at 350e480 x14C (D3 stage; Augier et al., 2005b), hence essentially from below the frictional-to-viscous transition (Handy et al., 2007). Rocks found below the contact are strongly anisotropic metapelites with shear cleavages superimposed on an earlier, pervasive, flat-lying schistosity and are chemically and structurally homogeneous at the km-scale (i.e., Paleozoic dark schists with an ubiquitous, upper greenschist facies garnetechloritoid-chloriteephengite assemblage aligned along the S2 schistosity). This area thus provides the opportunity to study the deformation of a strongly anisotropic yet homogeneous body with well-exposed XZ sections on a regional-scale, a clear deformation gradient (Platt et al., 1984) and asymmetric tectonic patterns (Augier et al., 2005a). We herein characterize the distribution of shear bands and shear geometries as a function of the distance to the Filabres shear zone through systematic measurements. The strategy adopted here is closely related to the one outlined by Choukroune et al. (1987), who stated that « useful approaches could perhaps involve statistical studies of geometric features (e.g., orientations, length, spacing and population density of structures) and of their correlations with the amount of bulk shear strain. » 2. Geological setting and deformation style 2.1. The Nevado-Filabride complex within the Betic orogen The Betic-Rif orogen results from the closure of the westernmost part of the Tethys ocean between Africa and the Iberian Peninsula (Fig. 1a). Subduction and crustal thickening leading to the formation of HP-LT metamorphic units took place from the Eocene to the Oligocene and were followed by late-orogenic extension starting after c. 35e30 Ma in the western Mediterranean (Platt et al., 1998; Jolivet and Faccenna, 2000). Lithospheric-scale causes of late orogenic extension are still debated, with contrasting models based on slab retreat (Royden, 1993; Lonergan and White, 1997; Jolivet et al., 2008), delamination of subcontinental lithosphere (Martinez-Martinez and Azan?n, 1997; Calvert et al., 2000) or convective removal of thickened crust (Platt and Vissers, 1989; Vissers et al., 1995). The internal zones of the Betic Cordilleras correspond to the initial stacking of the Nevado-Filabride, Alpujarride and Malaguide complexes (from bottom to top; Fig. 1b), currently separated from each other by major crustal-scale extensional shear zones: the Filabres Shear Zone (hereafter FSZ: Garcia-Duenas et al., 1992; BMZ in Platt and Vissers, 1989) and the Malaguide-Alpujarride Contact (MAC, Vissers et al., 1995), respectively. The age of the peak of pressure for the Nevado-Filabride (NF) complex is not precisely known but recent age constraints place it at around 30 Ma and possibly older (Augier et al., 2005b). Exhumation of the NF complex took place through a roughly EeW regional penetrative ductile extension from w30 to 18e14 Ma (Garcia-Duenas et al., 1992; Martinez-Mart?nez et al., 2002; Augier et al., 2005b). 2.2. Deformation history of the Nevado-Filabride complex The Nevado-Filabride complex is composed of three main tectonic units which are, from bottom to top (Fig. 1c; Martinez-Martinez et al., 2002 and references therein), the Ragua, Calar Alto and B?dar-Macael units, with respective structural thicknesses of 4000, 4500 and 600 m (Garcia-Duenas et al., 1988; the latter two being also referred to as the Mulhacen complex). These three units present a roughly similar lithostratigraphic succession (as for the Alpujarride units) with a thick and monotonous sequence of presumably Palaeozoic dark, graphitic micaschists and microturbidites topped by light coloured Permo-Triassic micaschists and quartzite. Ключевые слова: stage, corresponds, discussion, lc, detachment, median length, martinez martinez, nevado, france, evolution, agard journal, complex, increasing, filabres ridge, eld, geological, band, shear zone, median minimum, iv, representative area, exhumation rate, function, progressive, chosen, journal, calvert, brittle deformation, text, quartz, plot, rate, le, zone iev, time, press, volume loss, fsz detachment, function distance, schistosity, hurford, study, iii, major detachment, cobbold, temperature range, correction performed, band density, relationship, filabres, scale, mehl, handy, anisotropic rock, result, journal structural, outcrop, royden, raw data, simple shear, geology, betic, strain, sbd, spacing, brittle, stipp, tectonics, white, representative, tectonics doi, martinez, sassier, gapais, pattern, anisotropic, elsevier, alsop, average, table, trend, spain, rock pile, peak, carreras, passchier, herwegh, fsz, structural, progressive localization, filabres shear, method, geophysics, measured, ratio, tectonic, network, jessell, alpujarride unit, vissers, shear, analysis, decrease, rheological weakening, permo-triassic sample, age, aspect ratio, deformation rate, nevado-filabride, average length, shear band, increasing strain, hudleston, journal structural geology, surface occupancy, agard, area, respective contribution, exhumation, variation, structural geology, filabres detachment, distance, choukroune, sample, rheological evolution, contact, deformation pattern, fault, rock, note, recorded close, cleavage, internal consistency, tectonophysics, ?nite, length, marques, ma, extensional, correspond, platt vissers, distance detachment, systematically measured, shear domain, nevado-filabride unit, ii, domain, gradient, vissers platt, geophysical, deformation, shear strain, homogeneous, letters, jolivet, internal zone, unit, structure, burg, eld photograph, nappe stack, image analysis, density, correction, graham, nevado-filabride complex, major, fusseis, data, shear bandwidth, strain rate, localization, ?eld, applied, alpujarride, shape ratio, increase, major contact, meter scale, zone, surface, previous structure, platt, measurement, average angular, site, doi, age pattern, sbd increase, stunitz, ramsay, nf, wa, angle, augier, strain localization, bdar-macael unit, evidence, increasing distance, width, stack, scenario, ductile, regression