Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc

TECTO-126617; No of Pages 16 Tectonophysics xxx (2015) xxx–xxx

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Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc L. Barcos ⁎, J.C. Balanyá, M. Díaz-Azpiroz, I. Expósito, A. Jiménez-Bonilla Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Carretera de Utrera km 1, Sevilla, 41013, Spain

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Article history: Received 30 December 2014 Received in revised form 4 May 2015 Accepted 11 May 2015 Available online xxxx Keywords: Map-view curves Gibraltar Arc Oblique convergence Triclinic transpression Strain partitioning

a b s t r a c t Complex strain patterns in the Gibraltar Arc derive from the interaction between the westward drift — and concomitant back-arc extension — of the arc hinterland (Alboran Domain) and the Europe–Africa convergence. In order to explore strain partitioning modes within the arc and the role played by large-scale oblique structures, we have studied the kinematics of the Torcal Shear Zone located at the northern branch of the Gibraltar Arc. The Torcal Shear Zone is a 70 km-long, E–W brittle-ductile shear zone that underwent overall dextral transpression during the Late Miocene to Quaternary time. Within the Torcal Shear Zone strain is highly partitioned at multiple scales into shortening, oblique, extensional and strike-slip structures. Moreover, strain partitioning is heterogeneous along-strike giving rise to four distinct structural domains. In the central sector the strain is pure-shear dominated, although narrow sectors parallel to the shear walls are simple-shear domi! nated. A single N99°E–N109°E trending horizontal velocity vector (V ) could explain the kinematics of the entire central sector of the Torcal Shear Zone. Lateral domains have different strain patterns and are comparable to splay-dominated and thrust-dominated strike-slip fault tips. The Torcal Shear Zone provokes the subvertical extrusion of the External Betics units against the Alboran Domain ! and a dextral deflection of the structural trend. Moreover, the estimated V points to the importance of the westward motion of the hinterland relative to the external wedge and fits well with the radial outward thrusting pattern identified in the arc. © 2015 Published by Elsevier B.V.

1. Introduction Map traces of orogenic belts describe a great variety of geometries that often result in a set of interconnected large-scale, curving trend lines (e.g. Marshak, 2004). Within the external wedges of orogens, studies carried out on a variety of natural cases together with analogue modelling have shown that the controls on these curving geometries are very different in nature. Variations in stress trajectories, predeformational thickness, indenter shape, tectonic transport direction patterns, and strength of detachment levels have been identified as the main controlling factors (Macedo and Marshack, 1999; Marshak, 2004). Furthermore, apparent similar geometries could be acquired by following very different strain paths (Hindle and Burkhard, 1998). Map–view curves in fold-and-thrust belts develop either convex (salient) or concave (recess) to the foreland. Study cases have shown that transition zones between them (including the end points of Macedo and Marshak, 1999) correspond to continuous or discontinuous map-view inflexion zones in which strike-slip tectonics commonly occur. Examples of these are: the strike-slip fault zones between the Richardson Mountains and the Mackenzie salient (Mazzotti and ⁎ Corresponding author. E-mail address: [email protected] (L. Barcos).

Hyndman, 2002), the SW faulted border of the Western Alpine Arc (Collombet et al., 2002), and the strike-slip fault zones at both sides of the Sulaiman salient in Pakistan (Marshak, 2004). These strike-slip deforming zones trend oblique to the dominant structural grain and could be accompanied by a wide zone of distributed deformation, as is the case of the western end of the Himalayan Arc (Mohadjer et al, 2010). From a kinematic perspective, transpressive shear zones could be expected as one of the most common cases in the lateral parts of salients given that they typically develop in oblique convergence settings. Apart from different sources of data, the study of these bands has progressively incorporated results from analogue (Casas et al., 2001; Leever et al., 2011; Schreurs and Colletta, 1998; Tikoff and Peterson, 1998) and analytical modelling (Fernández and Díaz-Azpiroz, 2009; Jones et al., 2004; Lin et al., 1998). In this regard, some works carried out on ductile transpressive shear zones have successfully compared natural strain markers with the finite strain ellipsoids produced by analytical models (Czeck and Hudleston, 2003; Fernández et al., 2013). In the westernmost Mediterranean, the hinge zone of the Gibraltar Arc is a 300-km-long salient that ends at the apex of two recessing zones (Balanyá et al., 2007; Fig. 1). Salient-recess transitions are characterized in both cases by essentially brittle strike-slip dominated shear zones: the Torcal Shear Zone (TSZ) (Barcos et al., 2011; Díaz-Azpiroz et al., 2014) and the Jebha Fault Zone (Leblanc and Olivier, 1984).

http://dx.doi.org/10.1016/j.tecto.2015.05.002 0040-1951/© 2015 Published by Elsevier B.V.

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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Fig. 1. Tectonic map of the main tectonic domains forming the Gibraltar Arc, modified from García-Dueñas et al. (1992) and Balanyá et al. (2007). Location of the Torcal Shear Zone is shown.

Kinematics of the central part of the TSZ has been recently studied by applying theoretical analytical models of general transpression (DíazAzpiroz et al., 2014), but a regionally integrated structural analysis of the complete TSZ has not been attempted yet. Moreover, theoretical models have not been applied out of the Torcal de Antequera massif within the central sector of the shear zone. As a consequence, the tectonic implications of the TSZ in terms of large-scale strain localization and its significance within the Gibraltar Arc kinematic frame remain unexplored. The present work offers a complete revision of the entire TSZ and its relationships with neighbouring areas, based on previous and new kinematic data together with the application of theoretical analytical models. We discuss: 1) the overall kinematics of the TSZ from an exhaustive inventory of coeval structures within the shear zone, including their lateral tip zones, 2) the changing strain partitioning modes along the TSZ as viewed from the application of an analytical model of oblique transpression, and 3) the kinematic role played by the TSZ in the context of a salient-recess transition within the Gibraltar Arc. 2. Tectonic setting The Western Mediterranean Sea was formed during the Neogene as a back-arc basin related to the activity of the Western Mediterranean Subduction Zone (Faccenna et al., 2004). The Calabrian and Gibraltar arcs close the system to the East and to the West respectively. The Betic and Rif chains build up the two branches of the Gibraltar Arc enclosing the Alboran back-arc basin (Comas et al., 1999). Although the internal zones of the arc (the Alboran Domain) were mostly built during the Paleogene (Azañón et al., 1997; Balanyá et al., 1997), the curved pattern of the Gibraltar Arc is a Neogene feature. The Arc formed by the westward motion of the Alboran Domain, thrusted during the

Early to Middle Miocene onto two foreland domains: the South Iberian and Maghrebian domains that gave way to the Betic and Rif fold-andthrust belts, respectively. In-between the Alboran Domain and the deformed forelands, there are tectonic slices of the so-called Flysch Complex (Luján et al., 2006), which contains very deep water sediments witnessing the existence of a Jurassic to Early Cretaceous oceanic lithosphere now subducted (Durand-Delga et al., 2000; Faccenna et al, 2004). Different lithospheric mechanisms causing arc migration and coeval back-arc extension have been proposed, such as asymmetric lithosphere delamination (García-Dueñas et al., 1992), subduction slab retreat (Faccenna et al., 2004; Royden, 1993; Thiebot and Gutscher, 2006) and hybrid subduction–delamination models (Booth-Rea et al., 2007; Duggen et al., 2005). The salient geometry of the hinge zone of the Gibraltar Arc is defined by diverse shortening structures that form the main structural grain (Balanyá et al., 2007): the Alboran Domain outer tectonic boundary, the folds and thrusts of the deformed foreland units and the Flysch Complex, and the late Miocene folds within the emerged Alboran Domain. The northern end point of the salient could be approximately placed in the apex of the adjacent recess zones close to the 4°30′W meridian (Fig. 1), i.e. along the Torcal Shear Zone (Barcos et al., 2011; Díaz-Azpiroz et al., 2014) that involves rocks belonging to the innermost South Iberian Domain. Actually, the southern TSZ wall forms the current boundary between the South Iberian units and the Alboran Domain. The South Iberian Domain is composed in the studied area by three main rock groups: a) marls with evaporites, limestones and dolostones of Triassic age, b) dolostones, oolitic and nodular limestones (Lower to Upper Jurassic), and c) white and red marly limestones (Lower

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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Cretaceous and Upper Cretaceous to Paleogene, respectively). The Flysch Complex is also represented within or adjacent to the TSZ as small, fault-bounded outcrops formed by clay and sandstones of Paleogene to Aquitanian age. Alboran Domain rocks cropping out immediately south of the TSZ consist in (i) Silurian to Carboniferous shales and greywackes, as well as Permo-Triassic red clays, sandstones and conglomerates (the so-called Malaguide Complex), and (ii) Palaeozoic to Triassic peridotite, high grade gneisses, medium to low grade schists, quartzites and phyllites (the so-called Alpujarride Complex). Lower to Middle Miocene marine deposits unconformably overlie the two Alboran Domain Complexes. This sedimentary cover, that largely crops out along the southern border of the TSZ, includes Aquitanian to Burdigalian sedimentary breccias followed by an olistostromic complex of Middle Miocene age, whose olistolites mostly derived from the Flysch Complex (La Joya Complex; Suades and Crespo-Blanc, 2013). The TSZ was formally defined by Barcos et al. (2011) as a nearly E–W trending, essentially brittle shear zone that runs across the innermost South Iberian Domain between Sierra de Teba and Sierra Gorda (Fig. 2). The kinematics of structural associations within the central sector of the TSZ (the Torcal de Antequera massif), mainly E–W dextral strike-slip faults and NE–SW oriented folds and reverse faults, indicate a dextral transpressive regime (Barcos et al., 2011; Díaz-Azpiroz et al., 2014). Preliminary data taken in other massifs along the TSZ fit well within this general kinematic scheme, though significant differences in strain-partition modes and structural relationships between the TSZ and the nearby structural domains have been described (Barcos et al., 2012). Deformation within the transpressive TSZ could be considered essentially Late Miocene to Quaternary according to age criteria previously applied to the Torcal de Antequera and Valle de Abdalajís massifs (Balanyá et al., 2012; Barcos et al., 2012). They include: structural overprinting criteria, age of deformed and undeformed rock units, geomorphologic indices and earthquakes data. 3. Structural pattern and kinematics of the Torcal Shear Zone The TSZ is a high-strain zone whose distinctive transpressive kinematics contrasts with that of the domains located at both sides, where arc-perpendicular shortening structures dominate (Balanyá et al., 2007). The TSZ is defined by a number of disconnected topographic highs roughly aligned E–W (Fig. 2). These reliefs appear as escarpmentbounded topographic highs made up of essentially Jurassic and Cretaceous carbonate rocks belonging to the South Iberian Domain. These External Betics units rise up to 800 m above the surrounded terrains

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(including Alboran Domain and Flysch Complex units as well as lateorogenic Upper Miocene rocks), suggesting that this relative uplift is later than the Middle Miocene. 3.1. Structural features of TSZ central sector: the Torcal de Antequera and Valle de Abdalajís massifs The Torcal de Antequera massif is a lens-shaped range, 13 km-long and up to 4 km wide, mainly composed of Lower–Upper Jurassic limestone and dolostone, though Cretaceous to Paleogene marly limestones crop out locally. The Torcal de Antequera massif is deformed by a variety of structures with diverse geometry and kinematics (Barcos et al., 2011; Díaz-Azpiroz et al., 2014). The main structures are brittle to brittleductile reverse, strike-slip and normal shear zones, as well as open folds (Fig. 3a). They usually appear as shear zones up to 20 m wide that generate different structures depending on the rock type. Thus, decameter spaced discrete fault planes develop in Jurassic limestones, whereas centimetre to decimetre spaced S–C-like structures are found when shearing is affecting Cretaceous to Paleogene marly limestones (Fig. 4b). These structures are not homogeneously distributed but they are distributed in two different types of structural domains with contrasting kinematics and strain partitioning patterns. Accordingly, the Torcal the Antequera massif has been divided into two outer domains coinciding with both the northern and southern massif boundaries, and an inner domain (Fig. 3a,e). The northern outer domain is a brittle shear zone, 13 km-long and 400 m-wide, roughly striking ENE–WSW. This shear zone is limited to the north by an approximately N70°E discrete fault surface, dipping northwards at between 50° and 80°, though it turns to WNW–ESE strike in its northeastern end. Late-orogenic Upper Miocene rocks crop out north of the fault, suggesting that the north block is the downthrown one. However, the low pitch of slickenlines indicates a large lateral component (Fig. 3a,e). The northern outer domain is limited to the south by a roughly E–W striking fault zone that dips steeply either to the north or south. Slickenlines (mainly fibres on C planes) indicate a dominant right lateral fault displacement. Nevertheless, at its western end, this fault zone brings Jurassic limestones of the northern outer domain into contact with Cretaceous marly limestones of the inner domain, thus indicating that the outer domain constitutes the uplifted block again (Fig. 3a). The positive flower-like structure exhibited by the outer domain fits well with its interpretation as a transpressive shear zone (Fig. 4a). Additional structures have been observed within the northern outer domain. They include a hectometre-scale upright open fold whose axial trace is

Fig. 2. Structural and shaded relief map of the Torcal Shear Zone (TSZ) and location of its main topographic and structural zones. From East to West: Cabras-Camorolos segment (CCS), Torcal de Antequera massif (TAM), Valle de Abdalajís massif (VAM) and Almargen-Peñarrubia segment (APS).

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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Fig. 3. Structural features of the central sector of the TSZ. (a) Geological and structural map of the central TSZ comprising the Valle de Abdalajís and Torcal de Antequera massifs. Within the Torcal de Antequera massif the northern outer domain (NOD), the southern outer domain (SOD) and the inner domain (ID) appear differentiated. Location of cross-sections I–I′ and II–II′ and photographs presented in Fig. 4 are shown. (b) Structural data of the inner domain of the Torcal de Antequera massif; lower hemisphere, equal area projections of faults planes, related slickenlines (black circles) with arrows indicating the movement of the hanging wall, fold axes (black diamonds) and fold facing directions (asterisks). The orientation of the strain axes (white triangles) deduced from each set of structures is also shown. (c) Main structures of the Valle de Abdalajís massif. Inset shows lower hemisphere, equal area projections of faults planes and related slickenlines (black circles). The orientation of the strain axes (black squares) deduced from each set of structures (see the main text for details) is also shown. (d) Cross-section I–I′ across the main boundaries of the Valle de Abdalajís massif, showing the structural pattern. (e) Cross-section II–II′ orthogonal to the shear zone walls of the Torcal de Antequera massif, slightly modified from Díaz-Azpiroz et al. (2014).

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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parallel with the outer domain trend, and NW–SE striking faults with an apparent dextral sense of movement on the horizontal plane (Fig. 3a). The southern outer domain is constituted by a 100 m-wide shear zone that strikes approximately E–W and dips steeply northward. The northern, upthrown hanging wall is constituted by Jurassic limestones, whereas Cretaceous marly limestones crop out in the southern downthrown footwall. The pitch of slickenlines is up to 40° mainly to the west. Taken all together, a main dextral strike-slip component can be deduced for both, northern and southern outer domains (Fig. 3a). The inner domain general structure is dominated by hectometrescale SE-vergent open folds with gently plunging axes (Fig. 3a) and no axial surface cleavage associated. NE–SW striking, NW-dipping reverse, top-to-the-SE shear zones develop related to their vertical or slightly overturned limbs (Figs. 3a,b and 4b). One of these shear zones constitutes the SE boundary of the Torcal de Antequera massif. Two systems of NW–SE and NE–SW of coeval hectometric to kilometric brittle normal fault zones, as well as hectometric joints are widely distributed within the inner domain (Fig. 3a,b). The NW–SE faults dominate and are often listric and their associated slickenlines indicate a main dip-slip fault displacement (Fig. 3b). The Valle de Abdalajís massif is defined by a set of subparallel mountain ranges roughly striking ENE–WSW (Fig. 3a). These ranges are between 2.5 and 4.5 km-long and their summits reach up to 1.200 m. The different structures that develop in the Valle de Abdalajís massif are distributed within the entire massif and they include antiformal folds, extensional faults, oblique shear zones and strike-slip fault zones (Fig. 3a,d). The main topographic features of this massif are strongly controlled by a set of parallel ENE–WSW striking oblique shear zones (Fig. 3a,d). When deforming competent Jurassic carbonates, they appear as discrete steeply SSE-dipping faults. However, when Cretaceous marly limestones are involved, S-C-like structures develop, dipping 40°–60° to the SSE (Fig. 4c). Slickenlines are sub-horizontal (less than 15° both SW or NE) on discrete fault surfaces affecting competent carbonates (Fig. 3c), the kinematic criteria indicating a major right-lateral component. However, slickenlines pitches on marly C-planes vary between 30°SW to 4°NE. In this case, the obtained dip-slip component is always reverse and the strike-slip component, though variable, is mainly right-lateral (Fig. 3c). The majority of these oblique reverse shear zones defines a footwall and hangingwall ramp geometry and place Jurassic carbonates above Cretaceous to Paleogene marly limestones. By contrast, the southernmost thrust fault, which places Triassic rocks onto Cretaceous ones (Fig. 3a), shows a flat geometry for both hanging wall and footwall. Thrusting often appears associated with the NW limb of upright kilometric folds parallel to the thrust strike, with the exception of the thrust involving Triassic rocks, which is folded by one of these structures (Fig. 3a,d). Fold axes steeply plunge towards the ENE (Fig. 3a,c). The sense and amount of fold axis plunge, together with NW–SE and, to a lesser extent NE–SW, widespread normal faults, account for the abrupt topographic drop along strike of the Valle de Abdalajís massif eastern end (Fig. 4d,e). Indeed, the role of these normal fault systems is also responsible for the western face escarpment of the whole sector, bounded by Lower Messinian rocks (Martín et al., 2001) that generally lie unconformably against normal fault-related paleo-cliffs. Regardless of the system to which they belong, normal faults cut the rest of structures. They dip 50–75° with variable dip sense and slickenlines show a main dip-slip displacement with variable throw, i.e. metre-scale in the central area of the massif and decameter-scale near the boundaries (Figs. 3c and 4g,e). The southern Valle de Abdalajís massif boundary is defined by a decametric fault zone with discrete steeply dipping faults of variable dip (Fig. 3a). The main structure of the fault zone generates a 2.300 m-long ENE fault scarp that uplifts the Jurassic limestones 200 m above the Flysch Complex outcropping in the downthrown

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block (Figs. 3a,d and 4f). Slickenlines have low pitch and shear-sense indicators record a dominant dextral movement (Fig. 3c). 3.2. Structural features of TSZ tip areas: the Almargen-Peñarrubia and Cabras-Camorolos segments West and east of the central TSZ, this high-strain zone can be followed along several ranges that exhibit topographic and lithological similarities to the Torcal de Antequera and Valle de Abdalajís massifs, but also present significant differences in both orientation and spatial distribution of structures (Fig. 2). The western TSZ tip area is constituted by the Almargen-Peñarrubia segment. It consists of an ESE–WNW striking zone of deformation that connects the TSZ to the NE–SW striking arc-perpendicular shortening structures of the Gibraltar Arc (Balanyá et al., 2007, 2012; Fig. 2). The eastern part of this segment is mainly defined by ESE–WNW striking kilometric folds that involve, in addition to Mesozoic sequences, Messinian rocks (Martín et al., 2001; Fig. 5a). The main antiform, cored by Jurassic rocks, builds the Sierra de Peñarrubia. Its NE limb is cut by a fault zone oriented subparallel to the fold axis, which generates a mountain front (Fig. 5a). A similar structure is recognized cutting the NE end of the Sierra de Teba. The discrete fault surfaces dip between 50°SW and 80°SW, with the related slickenlines indicating reverse-dextral kinematics (Fig. 5a,b). Within the Sierra de Peñarrubia, the Jurassic rocks are cut by roughly NE–SW and, to a lesser extent, NNW–SSE dextral strike-slip faults (Fig. 5a,b). The shortening direction related to these dextral faults, including the reverse-dextral faults located at the NE boundary of the Sierra de Peñarrubia, is NW–SE. Two normal-fault systems, roughly trending SSE and ENE respectively, have been observed displacing the northeastern mountain front, being the latter also responsible for its abrupt SE boundary (Fig. 5a,b). The NW–SE folds and dextral faults that define the structural trend of the Sierra de Peñarrubia extend towards the SW, where they interfere with NE–SW striking folds related to arc-perpendicular shortening structures of the Gibraltar Arc (e.g. Sierra de las Utreras antiform) (Fig. 5a,b). The main structure of the Almargen sector is a fault zone that has an overall N120°E strike and dips between 70°SW and 90°. The associated slickenlines pitch between 0° and 30° and kinematic indicators reveal a main dextral component (Fig. 5b). As in the Sierra de Peñarrubia, the SE–NW striking structures of the Almargen sector mark the NE tectonic limit of the NE–SW striking arc-perpendicular shortening structures (and related topographic traits, e.g. Sierra de Cañete antiform) of the Gibraltar Arc (Fig. 2). The SW–NE elongated Sierra de Teba is located in the overlap between the Almargen fault zone and the Sierra de Peñarrubia. This topographic alignment is controlled by shortening structures of the same orientation, such as the kilometric Sierra de Teba antiform and the reverse faults that develop in the SE antiform limb, as well as in the limit between the Teba and Peñarrubia sierras (Figs. 4h and 5a). These faults dip more than 50°, either to the SE or NW and the latter involve Messinian rocks. Additionally, NW–SE normal faults have been observed indicating extension perpendicular to the shortening direction (Fig. 5b). The eastern TSZ tip segment (the Cabras-Camorolos segment) is defined by a set of elongated topographic highs that can be separated into two different sectors regarding the structural trend: the NW–SE striking Sierra de las Cabras, and the roughly NE–SW trending Sierra de los Camorolos (Figs. 2 and 5c). The NW–SE striking Sierra de las Cabras sector is a brittle-ductile shear zone oblique to the central sector of the TSZ. Both the NE and SW escarpments that limits the Sierra de Las Cabras are determined by approximately N120°E/80°SW fault zones, which uplift the Jurassic limestones above Cretaceous and Miocene rocks that crop out to the NE and SW (Fig. 5c). The gently pitching slickenlines (b 20°) obtained from discrete fault surfaces and S–C like structures indicate a main dextral movement (Fig. 5d). Within the Sierra de las Cabras,

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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Fig. 4. Photographs of the main structural features of the Torcal Shear Zone. Location is shown in Fig. 3. Torcal de Antequera massif: (a) positive flower-like structure uplifting the northern outer domain with respect to its northern and southern walls; (b) reverse shear zone developed at the overturned limb of a fold in the inner domain; inset shows brittle-ductile S-C-like structures associated with this reverse shear zone affecting marly limestones. Valle de Abdalajís massif: (c) ENE–WSW striking oblique shear zones with steeply SSE dipping; inset shows brittle-ductile S–C structures associated with these oblique shear zones; the slickenlines pitches on marly limestones C-planes present mainly reverse-dextral component; (d) openantiform truncated by a NW–SE normal fault; (e) NW–SE-striking normal faults; (f) WSW–ENE striking dextral strike-slip faults at the southern limit; inset shows slicken fibres on a fault plane in limestones; (g) normal fault affecting Upper Miocene rocks at El Chorro. (h) Reverse faults developed in the SE antiform limb of the Sierra de Teba.

hectometric folds develop subparallel to the dextral-fault strikes. Finally, dip-slip steeply dipping normal faults, striking either SE–NW or NE–SW, define the NE mountain front (Fig. 5c). The overall structure of the Sierra de los Camorolos is mainly dominated by ENE–WSW striking reverse faults and thrusts. With the exception of the SSE-dipping structure that limits the range to the NW, fault surfaces dip to the NNW (Fig. 5c,e). These faults

involve Jurassic, Cretaceous, and Paleogene to Middle Miocene rocks. The southernmost fault block is folded by a kilometric synform cored by Cretaceous marly limestones. Slickenlines indicate dominant dip-slip (pitch 65°–85°) movement, though the SE-dipping northernmost fault shows gently plunging striae at places. Taking into account the average attitude of slickenlines and the geometry of thrust cut-off and branch lines, these structures can be interpreted

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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Fig. 5. Geological and structural map of TSZ tip areas. (a) Geological and structural map of the eastern half of the Almargen-Peñarrubia segment located at the western TSZ tip. Location of the (h)-photograph presented in Fig. 4 is shown. (b) Lower hemisphere, equal area projections of faults planes and related slickenlines (black circles) of the Almargen-Peñarrubia segment. The orientation of the strain axes (black squares) deduced from each set of structures is shown also. (c) Geological and structural map of the Cabras-Camorolos segment located at the eastern TSZ tip. Location of the cross-section I–I′ and II–II′ are shown. (d) Equal area lower hemisphere stereoplots showing the main faults of the Cabras-Camorolos segment and associated striae. (e) Cross-section across the Sierra de Camorolos. (f) Cross-section across the Sierra del Gibalto thrust system.

as frontal structures of a thrust system (Fig. 5c,d). SE-oriented normal faults develop oblique to the shortening structures. Within the NE part of the Sierra de Camorolos, the roughly ENE– WSW oriented reverse faults change gradually to NE–SW and appear cut by a nearly ESE–WNW vertical fault zone. Most of the kinematic indicators associated to this fault zone indicate dextral strike-slip displacement, compatible with the cartographic offset (Fig. 5d). The

reverse faults of the Sierra de los Camorolos can be followed to the Sierra del Gibalto, characterized by a thrust imbricated system in which branch and cut-off lines trend consistently N30°E (Fig. 5c). Therefore, an approximately N300°E transport direction is assumed (see cross section of Fig. 5f). Both ranges are separated by a nearly WNW–ESE sinistral fault zone (Fig. 5c). In the SW Sierra de los Camorolos mountain front, ENE–WSW reverse faults are interrupted by a set of SE–NW

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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striking faults, most of them dipping between 50° and 85°NE with dextral-reverse slip, which act as a transfer zone between the Sierra de Camorolos and the Sierra de las Cabras (Fig. 5c,d).

4. Kinematic constraints on the central sector of the TSZ

The main kinematic parameters controlling deformation geometry in this model are: the extrusion obliquity υ (the angle between the extrusion direction due to the pure shearing component and the dip direction of the shear zone), the transpression obliquity ϕ (the angle between the simple shearing direction and the strike of the shear zone), the sectional kinematic vorticity number Wk (Lin et al., 1998; Truesdell, 1954), which is a function of the simple shearing to pure shearing ratio (γ = ε ), and the total amount of finite strain measured as shortening orthogonal to the main boundaries of the deformation zone (S; Jones et al., 2004; Fig. 6). The reader is referred to Fernández and Díaz-Azpiroz (2009) and references therein for a detailed description of the model. Triclinic transpression flow due to oblique simple ƒ! shearing (ϕ ≠ 0°) is produced by an inclined far-field vector F d acting on a vertical shear zone or by the displacement generated on an inclined shear zone by a horizontal far-field vector (Jones et al., 2004), which can ! then be assimilated to tectonic plates velocity vectors ( V ). In the latter case, the angle of oblique convergence α (the angle between the velocity vector and the strike of the shear zone; Fitch, 1972) can be estimated from ϕ, Wk and the dip of the shear zone (Díaz-Azpiroz et al., 2014; Jiang and Williams, 1998; Schulmann et al., 2003). The theoretical position of ! the velocity vector V is subsequently derived from α and the strike of the shear zone. According to the main structural features presented here and by Díaz-Azpiroz et al. (2014), the central sector of the TSZ meets the conditions needed to be compared with the transpression model described above (e.g., Jones et al., 2004): (1) the central sector of the TSZ is approximately tabular-shaped and its limits free to slip, (2) the structures used to estimate finite strain can be considered coeval (see Section 2) and are likely representative of the bulk strain, and (3) deformation can be assumed to be roughly isochoric. Moreover, heterogeneous deformation is minimized by analysing domains and/or structures where deformation can be regarded as homogeneous. The outer and inner domains of the Torcal de Antequera massif have been analysed individually (Díaz-Azpiroz et al., 2014). In contrast, there are not kinematically coherent domains in the Valle de Abdalajís massif, but groups of structures distributed within the massif showing compatible kinematics. Thus, two groups were defined: (1) shortening structures (namely, folds and reverse shear zones affecting marly limestones) extended along strike by coeval normal faults (folds + reverse shear zones + normal faults) and (2) dextral strike-slip shear zones, including the main fault zone defining the southern boundary and the fault zones affecting limestones at the northern part of the massif (see Section 3.1). The triclinic transpression model has not been used in other segments of the TSZ because they fail to meet some of the required conditions. Actually, the eastern and western segments cannot be approximated as tabular-shaped rock volumes and their structures hardly represent the bulk strain of the domain where they appear. The orientation of the three principal finite strain axes was approximated by the orientation of folds and by fault-slip data. According to active folding models in monoclinic transpression, fold hinges track the maximum horizontal finite strain axis during progressive deformation (Tikoff and Peterson, 1998; Titus et al, 2007; Treagus and Treagus, 1981; Wilcox et al., 1973), which corresponds to the X-axis in simpleshear-dominated transpression and to the Y-axis in pure-sheardominated transpression (Fossen and Tikoff, 1993). The structural pattern suggests that transpression in the Valle de Abdalajís massif is pure-shear-dominated and thus the Y-axis is considered subparallel with fold hinges. Accordingly, the facing direction is assumed to reveal the orientation of the X-axis (Jones et al., 2004). The orientation of the incremental strain tensor obtained via fault-slip data (see Marrett and Allmendinger, 1990 for theoretical details) can be correlated with finite strain as long as the total displacement produces shortening values under 60% of the initial length (Cladouhos and Allmendinger, 1993; Gapais et al., 2000), which is the case in the TSZ, as it will be shown below. Analyses are based on the Moment Tensor Summation for faults with known net slip (faults at the inner domain of the Torcal de 

4.1. Methods We have constrained the TSZ kinematics by using the triclinic transpression model of Fernández and Díaz-Azpiroz (2009) and the finite deformation data available. Kinematic and far-field parameters for the Torcal de Antequera and Valle de Abdalajís massifs have been obtained. The information presented in this section comprises previous results from the Torcal de Antequera massif (Díaz-Azpiroz et al., 2014), new ones from the Valle de Abdalajís massif and the first integrated evaluation of these parameters along the entire central sector of the TSZ. The model of triclinic transpression with inclined extrusion (Fernández and Díaz-Azpiroz, 2009) follows the kinematic approach used by Fossen and Tikoff (1993) or Lin et al. (1998), among others. It considers a tabular shear zone where the reference frame is fixed, with X1 parallel with the strike of the shear zone boundary, X2 normal to this boundary and X3 vertical (Fig. 6). Since the modelled shear zone is vertical, the results are rigidly rotated towards the actual orientation of the natural shear zone, thus aligning X3 to shear-zone dip direction. Flow within the modelled shear zone is considered homogeneous, steady and isochoric, being produced by the displacement of one of the undeformed bounding blocks with respect to the other ƒ! according to the far-field vector F d . The velocity gradient tensor for triclinic transpression with oblique extrusion results from the sum of noncoaxial simple shearing (γ) and coaxial pure shearing (ε). The evolution of the finite deformation geometry obtained from the integration with time of the velocity gradient tensor is given as the principal quadratic extensions (λ1 ≥ λ2 ≥ λ3) and their orientations with respect to the external reference frame. 



Fig. 6. Block diagram with the kinematic model of triclinic transpression with oblique extrusion (Fernández and Díaz-Azpiroz, 2009). The reference frame (X1, X2, X3) is defined ƒ! with respect to the shear zone orientation. F d is the convergence vector between one ! zone-bounding block and the other and V is its horizontal projection, which makes an angle α of oblique convergence with the strike of the shear zone. Transpression obliquity ϕ is the angle between the simple shearing direction (γ) and the strike of the shear zone. Extrusion obliquity υ is the angle between the extrusion direction (ε1 ) and the dip of the shear zone. Shortening orthogonal with the shear zone boundaries produced during progressive deformation is proportional to S. 





Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

L. Barcos et al. / Tectonophysics xxx (2015) xxx–xxx

Antequera massif and reverse faults of the Valle de Abdalajís massif) and on the Bingham Moment Tensor for faults with unknown net slip (mainly strike-slip shear zones and normal faults at the Valle de Abdalajís massif). Whenever possible, the shape of the finite strain ellipsoid was estimated along the three main strain axes in different cross-sections (Fig. 3d,e). Extension along the X-axis was estimated (see details in Díaz-Azpiroz et al., 2014) for both massifs by using the depth of the basal decoupling level — the base of the Triassic rocks — obtained from geophysical data (Medialdea et al., 1986; Torné et al., 1992) and the vertical uplift of the Torcal de Antequera massif with respect to the Alboran Domain units to the south (Fig. 3e). The amount of finite strain is given as the contraction across the shear zone boundaries parallel to the Z-axis (e.g., Fernández and Díaz-Azpiroz, 2009; Jones et al., 2004) and was estimated by line–length balancing in cross-sections (Fig. 3d, e) and from fold interlimb angle values according to theoretical models of folding (Titus et al., 2007; Treagus, 1997; Treagus and Treagus, 1981). Extension parallel to the Y-axis was measured directly on faults in the Torcal de Antequera massif (Díaz-Azpiroz et al., 2014). In the Valle de Abdalajís massif, a minimum Y-axis value is estimated based on fault-related topographic drops together with some faultdips and slickenlines orientation data. We consider the upper limit for this value as the maximum extension along Y-axis that satisfies the calculated extension values along X and Z axes, assuming a maximum 10% of volume increase (see Díaz-Azpiroz et al., 2014). The comparison between finite strain data from the TSZ and the outcome of the transpression model was accomplished following the standard step-by-step procedure proposed by Díaz-Azpiroz et al. (2014) and summarized in Appendix A1. In a first step, angles υ and ϕ are qualitatively constrained from geological features of the analysed high-strain zone. In the next two steps, the orientation and shape of the finite strain ellipsoid of the natural case are compared with the transpression model results to further constrain possible values for υ, ϕ and Wk (Fig. 7a–c). The fourth step compares values of α calculated using values of both ϕ and Wk obtained in the previous steps (Fig. 7d), according to the relationships between kinematic and far-field parameters (Díaz-Azpiroz et al., 2014; Jiang and Williams, 1998; Schulmann et al., 2003). Only those combinations of ϕ/Wk yielding small differences in α are considered as valid results (Fig. 7d,e). 4.2. Results Finite strain data estimated in the central sector of the TSZ are summarized in Fig. 3b and c, and in Table 1. The orientation of the finite strain ellipsoid deduced for the outer domains of the Torcal de Antequera massif (Díaz-Azpiroz et al., 2014) and the dextral shear zones of the Valle de Abdalajís massif displays the X-axis ca. 40–45° counter-clockwise away from the strike of the main massif and shallowly plunging, a steeply plunging Y-axis and a NW–SE subhorizontal Zaxis. The strain ellipsoids corresponding to the shortening structures are characterized by steeply plunging X-axes and Y-axes rotated ca. 10°–30° counter-clockwise away from the strike of the TSZ boundaries and shallowly plunging. These axes are subparallel to the Z and X axes, respectively, of the finite strain ellipsoid corresponding to the associated normal faults. The extension parallel to the X-axis is e = 0.18–0.29. The shortening across the shear zone (parallel to the Z-axis) for the Torcal de Antequera massif is − 0.2 to − 0.27 (Díaz-Azpiroz et al., 2014). For the Valle de Abdalajís massif, shortening across the shear zone has been estimated between the Sierra de Huma thrust (this structure has not been taken into account because it is considered to be previous to the transpressional deformation within the massif, see cross-section I–I′ in Fig. 3) and the northern boundary (see Fig. 3d for location), and the result has been considered representative of the entire massif. A shortening of 0.24 is accommodated by folds and reverse shear zones being assumed as a minimum value. Considering all the tilting as related to

9

folds and faults generated during transpression a shortening of 0.36 appears as a more realistic value for shortening within the Valle de Abdalajís massif. Considering the minimum and maximum shortening values obtained from the central sector of the TSZ (0.2–0.36) and the most likely time span for transpressional deformation (up to 11 Ma), very low strain rate values, in the order of 10−16–10−15 s−1, are deduced. When applying the protocol the range of possible shortening values along the Z-axis was broadened to 0.1–0.3 for the Torcal de Antequera massif and 0.2–0.4 for the Valle de Abdalajís massif. Extension parallel to the Y-axis is 0.07 in the Torcal de Antequera massif (Díaz-Azpiroz et al., 2014) and ranges from 0.02 to 0.08 in the Valle de Abdalajís massif. Therefore, considering altogether the shortening structures and the normal faults, assumed to be coeval, the finite strain ellipsoid for this group of structures (the inner domain of the Torcal de Antequera massif and folds + reverse shear zones + normal faults of the Valle de Abdalajís massif) is dominated by the shortening structures. The NE–SW extension associated with the normal faults generates insignificant variations in the orientation of the finite strain axes deduced from shortening structures (see above). Moreover, such extension produces slight enlargement and shortening of, respectively, the Y and X axes, thus resulting in finite strain ellipsoid with general oblate shape. A summary of the kinematic and far-field transpression parameters obtained from the application of the step-by-step procedure to the data of the central sector of the TSZ is shown in Figs. 7e and 8, and Table 2 (the complete dataset for the Torcal de Antequera massif can be found in Díaz-Azpiroz et al., 2014 and that of the Valle de Abdalajís massif is presented as supplementary material to this work in Appendix A2). For the group of folds + reverse shear zones + normal faults of the Valle de Abdalajís massif, good fits between the natural data and the model outcome are attained for υ = 0° ± 5°, ϕ = 5°–10° and Wk = 0.45 and also for υ = − 5°, ϕ = 5° and Wk = 0.60. Additionally, fair fits are obtained with a wider range of values, including combinations that span from υ = − 5°, ϕ = 20° and Wk = 0.24 to υ = 5°, ϕ = 2° and Wk = 0.78. Compared with kinematic parameters estimated for the inner domain of the Torcal de Antequera massif (Díaz-Azpiroz et al., 2014, Fig. 8a and Table 2), υ angles are similar, whereas in the Valle de Abdalajís massif ϕ angles and Wk values of the Valle de Abdalajís massif are smaller. In turn, kinematic data corresponding to the dextral strike-slip shear zones of the Valle de Abdalajís massif only produce fair fits with results of the model with υ = 0°, ϕ = 2° and Wk = 0.89. These results are similar to those of the outer domains of the Torcal de Antequera massif (Fig. 8a, Table 2 and Díaz-Azpiroz et al., 2014). After the combination of the kinematic parameters obtained for both massifs of the central TSZ with dips of shear zone boundaries, the differences between the resulting α angles are not significant in most cases (Table 2). Since the main boundaries of the central sector of the TSZ are inclined, triclinic transpression can be at! tributed to horizontal velocity vectors V , which can be deduced from ! α and the strike of the massifs boundaries. The resulting range for V vector shows larger differences between sectors (N79°E–N109°E for the Valle de Abdalajís massif versus N99°E–N118°E for the Torcal de Antequera massif) than the corresponding α angles. However, the intersection between both orientation ranges (N99°E–N109°E) gives the likely position of a unique velocity vector that would account for the structure and kinematics of the central sector of the TSZ as a whole (Fig. 8b,c,d, Table 2). 5. Discussion 5.1. Dextral transpression and strain partitioning within the TSZ The northeastern branch of the hinge zone of the Gibraltar Arc, where the TSZ is defined, shows different modes of strain partitioning from the orogenic scale (Balanyá et al., 2007) to the outcrop scale. Particularly, the TSZ constitutes an upper crustal (up to 5–6 km deep to the

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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L. Barcos et al. / Tectonophysics xxx (2015) xxx–xxx

Fig. 7. Selected examples of the procedure followed to compare the kinematic data of the Valle de Abdalajís massif with the model of triclinic transpression with oblique extrusion, according to Díaz-Azpiroz et al. (2014). (a) and (b) are Flinn diagrams, representative examples, of step 2. (a) Dextral Strike-Slip shear zones compared with model results with υ = 0°/ϕ = 2°. (b) Folds + reverse shear zones + normal faults compared with model results with υ = 0°/ϕ = 10°. Both are equal area, lower hemisphere plots with the orientation of X and Y axes of the finite strain ellipsoid of the natural case (in blue, with a 95% confidence cone). Model outcomes are shown as the evolution with increasing finite strain of the theoretical loci of λ1 (solid) and λ2 (dashed) for different Wk values (in normal font for λ1 and in italic for λ2). Along each arrow, the most likely finite strain interval (see the text) is marked by a white (S = 0.2) and a black (S = 0.4) circle. Wk ranges yielding fair to good fits in each case are shown. SZB: Shear Zone Boundary; VNS: Vorticity Normal Section. (c) Step 3: Flinn diagrams showing the shape of the finite strain ellipsoid deduced for the folds + reverse shear zones + normal faults compared with the results of the model (increasing finite strain away from the origin). Grey scale is in accordance with confidence intervals (the darker area corresponds to a narrower and better constrained group of data). (d) Step 4: Comparison of α values obtained from combinations of ϕ and Wk values yielding any fit in steps 1–3. (e) Combinations of kinematic parameters that have yielded poor to good fits after applying the protocol. The first three columns correspond to the transpression parameters (υ, ϕ and Wk). The fourth to seventh columns correspond to the fit quality for steps 2 to 4. The last column shows the fit quality for the combination of steps 1–3 with step 4. In all cases, the colour code indicates the quality of fit; quality criteria: good (green), fair (yellow), poor (orange) and absent (red in diagrams and white in tables).

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

L. Barcos et al. / Tectonophysics xxx (2015) xxx–xxx

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Table 1 Summary of finite strain parameters obtained in the different domains and groups of structures of the central sector of the Torcal Shear Zone, including data from Díaz-Azpiroz et al. (2014): ODs and ID: outer domains and inner domain of the Torcal de Antequera massif (TAM); DSS and F + R + N: dextral strike-slip shear zones and folds + reverse shear zones + normal faults of the Valle de Abdalajís massif (VAM).

TAM

ODs

ID

VAM

DSS

F+R+N

Axis

Trend

Plunge

X Y Z X (F) X (R) Y (F) Y (R) Z (F + R) X Y Z X (F) X (R) Y (F) Y (R) Z (F) Z (R)

227 30 134 323 299 48 33 131 18 133 288 143 2 53 222 323 129

34 65 7 65 75 3 1 20 3 82 7 85 70 7 16 5 13

(a)

n sio es

0.3 0.4

1.40–1.69

0.07

1.07

1.15

−0.20 to −0.27

0.80–0.85

0.64–0.72

s 0.18–0.29

1.18–1.30

1.40–1.69

0.02–0.08

1.02–1.08

1.04–1.17

−0.24 to −0.36

0.64–0.76

0.41–0.58

(b)

0.5

70º

no Strike-slip

(d)

ics

60º

TAM

X3

TAM

X1

1.0 90º

80º

TAMODs: 091-103 TAM: N99-118

ton

50º

υ = 0º + 5º

ec tT 0.9

φ Oblique simple shear

VAMF+R+N: 96-116

(c)

rus 0.7

TAM-ODs 40º

VAM: (079)-96-109

υ = 0º + 5º

φ = 5º-10º

Simple shear-dominated

30º

VAM

TAM

0.8

20º

φ = 2º-10º VAMDSS: 078-079

TAM-ID

10º

1.34–1.75

VAM

0.8

1.0 0º

1.05–1.27

X3

0.6

TAM

1.34–1.47

VAM

LEGEND

Triclinic transpression

0.9

1.10–1.20

VAM

0.6

VAM

S2/S3

X1

Pure shear-dominated

0.7

S1/S2

X2

5º

VAM-F+R+N

0.5

k

1.18–1.30

Th

ic T ra W nspr

0.18–0.29

Wk

clin

λ

TSZ: Torcal shear zone TAM: Torcal de Antequera massif ODs: Outer Domains ID: Inner Domain VAM: Valle de Abdalajís massif F+R+N: Folds + Reverse shear zones + Normal faults DSS: Dextral Strike-Slip shear zones

0.2

Mo

S

Pure shear υ

0º 175º 0 0.1

VAM-DSS

e

TAM X2

TAMID: 117-129

TAM

VAM: (079)-96-109 VAM

Dip-slip

TSZ: 099-109 TAM: N99-118

(e) Conto

urs o

80

f bulk

Wk

60

95

VAM

40

89 0.

TAM

0.

0.8

0

3

20

75 0. 71 0.

degree of strike-slip partitioning on simple-shear dominated zones (%)

100

0

0.2

0.4 0.6 0.8 Wk in zone of distributed deformation

1

Fig. 8. Kinematic and far-field parameters deduced for the Valle de Abdalajís massif and compared with those obtained from the Torcal de Antequera massif (Díaz-Azpiroz et al., 2014). (a) Kinematic parameters plotted in the strain triangle proposed by Díaz-Azpiroz et al. (2014) inspired in that of Jones et al. (2004). The intensity of shading is in accordance with reliability of results. The yellow and green ellipses illustrate the likely location of the kinematic transpression parameters for bulk strain within the Valle de Abdalajís and Torcal de Antequera massifs, respectively. (b)–(d) Equal area, lower hemisphere projection displaying relevant parameters of the Valle de Abdalajís massif (b), the Torcal de Antequera massif (c) and the central sector of the TSZ as a whole (d): reference frame (X1, X2, X3) related to the orientation of the zone boundaries, ranges for angles υ and ϕ and the likely orientation of the theoretical velocity vector responsible for the deformation produced at each domain and for the central sector of the TSZ. (e) Degree of strike-slip strain partitioning accommodated by the southern boundary of the Valle de Abdalajís massif and the outer domains of the Torcal de Antequera massif (ordinate axis) estimated according to Tikoff and Teyssier (1994) from: Wk values in the respective zones of distributed deformation of both massifs (abscissa axis) and bulk Wk for the TSZ as a whole (contour lines). Minimum (short dashed lines) and maximum (long dashed lines) Wk values in the zones of distributed deformation are considered. The range of average values of degree of strike-slip partitioning is depicted by thick solid double arrows. Data from the Valle de Abdalajís and Torcal de Antequera massifs are in yellow and green, respectively.

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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Table 2 Summary of the main kinematic and far-field parameters obtained for the different domains of the central sector of the Torcal Shear Zone. Results for the Torcal de Antequera massif are from Díaz-Azpiroz et al. (2014). Same acronyms as those used in Table 1. Domain

Kinematic parameters

TAM-ODs TAM-ID TAM VAM-DSS VAM-F + R + N VAM TSZ

Far-field parameters

υ

│ϕ│

Wk

SZB

α

! V

0° 0° ± 5° 0° ± 5° 0° 0° ± 5° 0° ± 5°

2°–5° 10°–15° 5°–10° 2° 5°–10° 2°–5°

0.95 0.65–0.71 0.71–0.95 0.89 0.45–0.6 0.6–0.89

086/73°N

5°–17° 31°–43° 13°–32° 14°–15° 32°–52° (15°)–32°–45°

091–103 117–129 099–118 078–079 96–116 (079)–96–109 099–109

base of the Triassic rocks; Torné et al., 1992) deformation zone that offers a great opportunity to envisage different styles of strain partitioning at multiple scales (Table 3). Most of the Late Miocene–Quaternary deformation features of the TSZ reported here are kinematically related and clearly indicate overall dextral transpression. However, there are significant differences in the map-scale structural pattern shown by each segment as a result of along-strike strain partitioning. The Teba-Peñarrubia and Cabras-Camorolos segments present distinctive features compared with the central sector, probably related to their location at the tip sectors of the TSZ. The Teba-Peñarrubia segment forms a main WNW–ESE trending zone with parallel dextral strike-slip faults and shortening structures. WNW–ESE dextral strike-slip faults probably developed initially as synthetic splay faults linked to the western termination of the TSZ, according to their orientation and sense of

Table 3 Summary of the main structural and kinematic features of the two massifs of the central sector of the TSZ evidencing contrasting styles of strain partitioning at multiple scales. VAM SCALE→ Bulk flow in the massif: Pure-shear dominated (Wk = 0.6–0.89) nearly monoclinic (ϕ = 2–5°) Strain partitioning in the massif: One simple-shear, dextral strike-slip, discrete fault zone at southern boundary

Pure-shear dominated, nearly monoclinic transpressional, wide domain of distributed deformation Structural pattern in the zone of distributed deformation: Fault dominated: Several evenly distributed oblique shear zones occasionally associated with short limbs of very open antiforms. These structures strike at small angles (10°) with the main boundaries Orthogonal normal faults extending along strike of shortening structures Strain partitioning at (the zone of distributed deformation) shortening shear zones: Strain partitioning related to lithology (discrete fault zones at limestones + distributed shear zones at marly limestones) Flow partitioning: Oblique shear zones = dextral, simple-shear, strike-slip at fault zones + pure-shear dominated shortening at reverse dextral/sinistral shear zones

TAM Bulk flow in the massif: Pure-shear/simple shear dominated (Wk = 0.71–0.95) triclinic (ϕ = 5–10°) Strain partitioning in the massif: Two simple-shear dominated (dextral strike-slip) triclinic transpressional, narrow domains at both boundaries (outer domains) Pure-shear dominated triclinic transpressional, wide domain of distributed deformation Structural pattern in the inner domain: Fold dominated: Folded structure defined by few open folds (antiforms and synforms) with reverse shear zones at short limbs These structures strike at a moderate angle (30°) with main boundaries Orthogonal normal faults extending along strike of shortening structures Strain partitioning at (inner domain) shortening shear zones: Strain partitioning related to lithology (discrete fault zones at limestones + distributed shear zones at marly limestones) No flow partitioning: Mainly reverse kinematics of the main structure reproduced at a smaller scale in faults and shear zones

064/82°S

displacement (see Kim and Sanderson, 2006). The Sierra de Teba antiform strikes at a high angle to the shear zone and probably represents a contractional stepover developed in the relay zone between two dextral segments (Jiménez-Bonilla et al., 2013). No other structures clearly linked to the TSZ have been identified westward of this sector suggesting a progressive decrease of the displacement associated with the TSZ. By contrast, at the easternmost segment of the TSZ, the Cabras-Camorolos sectors are characterized essentially by SSE-vergent thrusts, though folds, high-angle reverse faults and dextral strike-slip faults also appear. To the east, this segment finally joints, by means of a N100°E trending sinistral fault zone, the WNW–vergent Sierra del Gibalto thrust system. The cross-section of this thrust system parallel to the assumed transport direction (Cross section II–II′ in Fig. 5f) shows a displacement larger than 3 km, which is similar to the strikeslip displacement estimated for the central sector of the TSZ. Additionally, our results indicate that the orientation of the far-field velocity vector along the TSZ was nearly WNW–ESE (see Section 4.2), thus defining a low angle with respect to the thrusting direction of the Sierra del Gibalto system. Therefore, a significant part — probably most — of the TSZ lateral displacement may have been transferred to the Sierra del Gibalto shortening structures in a similar way to the thrust tip zones described in the contractional quadrant of some strike-slip faults (e.g. Kim and Sanderson, 2006). Regarding the central sector of the TSZ, the structure and kinematics deduced for the Valle de Abdalajís and the Torcal de Antequera massifs suggest that each sector corresponds to a domain with a specific bulk strain (Table 3). Such flow partitioning can be due to variability in the orientations of the far-field vectors and/or the shear zone boundaries. Our results suggest that the main structural and kinematic features of the central TSZ are compatible with a single deformation event related to a unique velocity vector, whose orientation ranges from N99°E to N109°E. Moreover, several natural examples (e.g. the Alpine fault; Norris and Toy, 2014 and references therein; the San Andreas fault; Atwater and Stock, 1998 and references therein) and analogue models (e.g., Casas et al., 2001; Leever et al., 2011) show that the main boundaries of a transpressional zone usually develop subparallel to the farfield boundaries (major tectonic limits for natural zones and backstops for models). Therefore, differences between structures and bulk kinematics of the Torcal de Antequera and Valle de Abdalajís massifs are likely related to local perturbations in the strain field due to slight differences in the orientation of the main boundaries of the TSZ (Fig. 9a). A similar situation has been previously described at the northwestern Superior Province (Lin and Jiang, 2002). We have shown (Fig. 3d) that the southern boundaries of the central sector form the current limit between the South Iberian and Alboran domains. Therefore, it is proposed that this limit, once acquired its steep geometry, favoured strain localization and the development of the TSZ. This tectonic boundary, which has an overall N65°E–N85°E strike and present vertical and steep dips to the North and South, was likely formed at the onset of the Late Miocene transpressional event. Local variations in the original orientation of this limit would result in different domains with contrasting bulk strains (Fig. 3, 9a).

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

L. Barcos et al. / Tectonophysics xxx (2015) xxx–xxx

13

(a) (b)

Tri

ass

ic m

arl

s an

de

vap

ori

tes

Fig. 9. Schematic block diagram illustrating the conceptual model for Late Miocene dextral transpression and strain partitioning at multiple scales in the central sector of the TSZ. ! (a) General model for the central sector of the TSZ with the main kinematic and far-field parameters obtained in this study: orientation of the main velocity vector (V ), angles of oblique convergence (α) and dip angles (δ) for the Valle de Abdalajís and Torcal de Antequera massifs, (VAM and TAM, respectively), initial width (IW) and shortening produced by transpression (S), main displacements (bold arrows) and structures within the TSZ. Note that scales are approximate, and vertical scale within the TSZ is double than outside this high-strain zone. The approximate variation of the Triassic layer thickness along the TSZ (see the main text) is also shown. (b) Block diagram with a detailed view of the oblique shear zones of the Valle de Abdalajís massif. Strain partitioning results in discrete dextral strike-slip faults in limestones and wide distributed deformation domains in marly limestones, with mainly reverse shear zones and orthogonal shortening (see the text for further discussion).

According to this hypothesis that considers a unique velocity vector, the kilometre-scale flow partitioning, responsible for the specific kinematics of each massif, was controlled by the orientation of their respective main boundaries. The average strikes of shear zone boundaries at both massifs and, hence, their respective angles of oblique convergence differ in 22°. The Valle de Abdalajís massif strikes N64°E, which corresponds to α = 35–45°, whereas the Torcal de Antequera massif strikes N86°E, yielding smaller angles of oblique convergence (α = 13–23°). This explains the larger influence of pure shear deformation in the Valle de Abdalajís massif, with smaller Wk values (0.60–0.89), as compared to the Torcal de Antequera massif (Wk = 0.71–0.95). In contrast, the boundaries of the Valle de Abdalajís massif present greater dip angles (82°) than those of the Torcal de Antequera massif (73°), which produces smaller transpression obliquities in the former (ϕ = 2–5° for the Valle de Abdalajís massif, ϕ = 5–10° for the Torcal de Antequera massif). A direct consequence of this flow partitioning is that transpression within the Valle de Abdalajís massif was likely pureshear dominated and close to a monoclinic symmetry, whereas the Torcal de Antequera massif was more probably affected by triclinic transpression close to the boundary between the pure-shear and the simple-shear dominated fields (Tables 2 and 3). Despite the steep dip angles of the shear zone boundaries, dip senses could have controlled the vergence of the main shortening structures at each massif. Southdipping boundaries led to North-vergences in the Valle de Abdalajís massif, whilst North-dipping boundaries are associated with Southvergences in the Torcal de Antequera massif (Fig. 9). At a smaller scale, the Valle de Abdalajís and Torcal de Antequera massifs present different styles of strain partitioning (Fig. 3a, Table 3). The zones with distributed deformation in both massifs can be roughly described by triclinic pure-shear dominated transpression kinematics.

However, the structural pattern accommodating such strain in each massif presents some important differences and a few similarities (Fig. 3a,d,e and Table 3). The zone of distributed deformation of the Valle de Abdalajís massif is characterized by several evenly distributed oblique shear zones that are occasionally associated with short limbs of very open antiforms. This suggests that deformation within this sector of the Valle de Abdalajís massif is controlled by faulting leading to fault-propagation-folding. In contrast, the inner domain of the Torcal de Antequera massif presents an open folded structure, with reverse shear zones developed at the short limbs and, thus, deformation is likely controlled by folding. To understand such differences it is important to emphasize that the general stratigraphic sequence of the TSZ is roughly defined by a lower weak layer (Triassic marls and evaporites) overlain by a competent layer (Upper Triassic to Paleogene carbonatic lithologies). Moreover, the total thickness of the sequence is relatively small (4.5–6.5 km) and our results suggest low strain rates for the Late Miocene transpressional event in the TSZ. Taken into account these conditions and according to analogue (Costa and Vendeville., 2002; Smit et al., 2003) and numerical models (Simpson, 2009), deformation is controlled by faulting when the weak/competent layer thickness ratio (Hw/Hc) is low (typically lower than 0.25), whereas it is controlled by folding for larger Hw/Hc ratios. Accordingly, our results suggest that the underlying weak layer of the TSZ could be thicker at the Torcal de Antequera massif (Figs. 3, 9a). This weak layer is composed of highly movable rocks. Lateral thickness variations in this layer have been documented elsewhere in the Betics and attributed to original differences in the stratigraphic sequence and/or to local displacements produced by tectonic activity previous to the Late Miocene transpression (Berástegui et al., 1998; Nieto et al., 1992). The elastic shear modulus of the upper layer might also be a controlling

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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factor (Simpson, 2009), but it is unlikely applicable in this case given that the Torcal de Antequera and Valle de Abdalajís massifs share the same competent layer stratigraphy (Fig. 3). Irrespective of the main general deformation style, at the outcrop scale shear zones are localized at contacts between limestones and marly limestones (Fig. 4b,f). In the case of the oblique shear zones associated with folds in the Valle de Abdalajís massif, differences in the structural style between these two rocks are further accompanied by flow partitioning (Table 3, Fig. 9b), which is thus controlled at this scale by lithological contrasts. As such, discrete fault zones at limestones accommodate simple shear, dextral strike-slip strain, whereas distributed zones at marly limestones mainly accommodate nearly orthogonal shortening (Fig. 3), which is also compatible with deformation produced by the antiforms associated with these shear zones. This situation resembles the discrete strain partitioning situation characteristic of dominantly brittle transpressional deformation zones (Teyssier et al., 1995), evidenced in several analogue models (e.g., Leever et al., 2011) and observed in the TSZ at a larger scale. In contrast, the kinematics of the reverse shear zones associated with folds in the Torcal de Antequera massif is overall reproduced at a smaller scale in both the limestones fault zones and the marly limestones zones of distributed deformation (Fig. 3). Therefore, there is no flow partitioning at this scale in the Torcal de Antequera massif. This difference is probably related to the kinematics of the shear zones in each domain: oblique kinematics (the zone of distributed deformation of the Valle de Abdalajís massif) favours discrete strike-slip strain partitioning and reverse kinematics (Torcal de Antequera massif inner domain) does not. The strain partitioning taking place at the shear zones associated with folds contributes to a better understanding of the kinematics of the Valle de Abdalajís and Torcal de Antequera massifs. The angle between the strike of these structures and the main boundaries of the massif is smaller at the Valle de Abdalajís massif (ca. 10°) than at the Torcal de Antequera massif (ca. 30°). This can be partly explained by smaller Wk values and larger α angles in the former. However, 10° angles should be related to very large α angles (N 60°) responding to nearly orthogonal shortening (Titus et al., 2007), a situation that departs from the 52° angle of maximum oblique convergence deduced from our results (Table 2). In the Valle de Abdalajís massif, these shear zones accommodate not only shortening, but oblique displacement, which is further decomposed into a strike-slip component parallel to the main boundaries and a shortening component at a large angle with these boundaries (see above). The resulting vector related to this oblique displacement forms an intermediate angle (b 52°) with the main boundaries of the massif. The combination of the kinematics of these structures, together with the subparallel extension produced by NW–SE-striking normal faults, accounts for the bulk triclinic pure-shear dominated transpression of this part of the Valle de Abdalajís massif. In contrast, in the Torcal de Antequera massif, triclinic transpression kinematics is related (1) to the obliquity of the folds and related shear zones (ca. 30°) which, in this case, accommodate mostly orthogonal shortening and (2), as in the Valle de Abdalajís massif, to the extension produced by NW–SEstriking normal faults. Our analysis (Fig. 8e) also suggests that (1) the degree of strike-slip partitioning is slightly larger in the Valle de Abdalajís massif (35–56%) than in the Torcal de Antequera massif (30–40%); and (2) the partitioning style is different in both domains (Table 3). In the Valle de Abdalajís massif, strike-slip partitioning is accommodated at discrete fault zones developed at its southern boundary and other evenly distributed within the entire massif. This is probably due to the presence of several oblique shear zones localized at limestones — marly limestones contacts, which favours, as previously discussed, discrete strike-slip deformation at limestones. In contrast, strike-slip partitioning in the Torcal de Antequera massif is mainly accommodated at the narrow simpleshear-dominated transpressional outer domains.

5.2. Kinematic role played by the TSZ within the Gibraltar Arc In the central parts of the TSZ, the shear zone walls are steeply dipping (Torcal de Antequera massif) or nearly vertical (southern limit of the Valle de Abdalajís massif) and the structural position of the Alboran Domain corresponds to the relative downthrow block of the TSZ (cross sections I–I′ and II–II′ in Fig. 3). This greatly differs with respect to the structural arrangement of the Alboran Domain along the hinge zone of the Gibraltar Arc, in which it appears systematically thrusted upon the external zones (Balanyá et al., 2007; Expósito et al., 2012). Because the thrusting of the Alboran Domain is essentially of Early Miocene age (though continued into the Middle Miocene), the Late Miocene and later time-span of the TSZ indicates that the shear zone modified an earlier forward-thrust tectonic pile in which the Alboran Domain acted as the regional hanging wall block. Furthermore, the applied model of triclinic transpression (Fernández and Díaz-Azpiroz, 2009) in the central sector of the TSZ (Torcal de Antequera and Valle de Abdalajís massifs) assumes that shear zone walls are mechanically constrained and that slip is allowed along the boundaries. This assumption fits well with the identified strike-slip fault nature of the TSZ walls and underlines the importance of the high rheological contrasts (as the Alboran-South Iberian boundary) in the shear zone evolution and strain localization. The Gibraltar Arc was formed during the Neogene by the westward migration of the Alboran Domain (hinterland) and its subsequent collision with the South Iberian and Maghrebian paleomargins. Both the westward displacement of the Alboran Domain and the associated back-arc extension, were probably driven by migrating subduction retreat and/or subcontinental mantle delamination (Royden et al., 1993; García-Dueñas et al., 1992; Comas et al., 1999; among others), having occurring coevally to N–S to NW–SE Europe–Africa slow convergence (DeMets et al., 1994). Regarding the northern branch of the arc (Betics), in which the average trend of the Alboran Domain tectonic boundary is WSW–ENE, this combination of coeval movements must have resulted in a general dextral transpressive kinematic regime. However, the development of dextral transpressive shear zones, either parallel or oblique to the main structural grain, has been scarcely documented in the Betics (Sanz de Galdeano and López Garrido, 2012). In the Western Betics, only two cases are identified: a deformation band subparalell to the main orogenic fabric located at the NW boundary of the intermontane Ronda Basin (Jiménez-Bonilla et al., 2015), and the TSZ that forms ca. N40°E with respect to the dominant structural grain and causes their dextral deflection. Taken into account our results concerning the size, amount of deformation and specific location of the TSZ, this transpressive shear zone must be considered as the main structure that shapes the lateral northeastern end of the Gibraltar Arc hinge zone. On the whole, the TSZ and neighbouring areas could be considered as seismically active. Earthquakes concentrate in the western and eastern TSZ tip zones and in Sierra Gorda (east of the Sierra del Gibalto; Balanyá et al., 2012; Fig. 2). Available focal solutions within the TSZ and nearest zones show that present-day strain is highly partitioned and that strike-slip solutions are dominant, being related to E–W or NW–SE dextral strike-slip faults (Balanyá et al., 2012; FernándezIbáñez et al., 2007; Stich, 2003). This suggests that the present brittle deformation in the zone does not essentially differ from the kinematics outlined for the TSZ during the Late Miocene and later. Our results suggest the TSZ begins to be active in the Late Miocene and continued until the Quaternary. Transport directions along the hinge zone of the Gibraltar Arc define an outward radial thrusting pattern that has a NW–SE to WNW–ESE dominant shortening direction in the northern branch (Balanyá et al., 2007). This implies the TSZ trends oblique to the regional transport direction and explains its transpressional regime. Furthermore, the results from theoretical models applied to TSZ (Díaz-Azpiroz et al., 2014; this paper) precise the far-field velocity vector in the Torcal de Antequera and Valle de Abdalajís massifs (between N99°E and N109°E). All these results point to the importance

Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002

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N99°E to N109°E. This underlines the importance of the westward motion of the Alboran Domain relative to the external wedge and fits well with the radial outward thrusting pattern identified around the Gibraltar Arc. The identification of a variety of strain partitioning modes at multiple scales is revealed as an essential question to be taken into account in any kinematic model of this orogenic Arc.

of the westward movement of the Alboran Domain relative to the external zones. This also underlines that relationships between the N–S to NW–SE Europe–Africa convergence from the Early Miocene onwards (DeMets et al., 1994: Mazzoli and Helman, 1994) and the kinematics of the Gibraltar Arc are not simple. Our results also show, extending the question to the whole Gibraltar Arc system, that plate convergence slip vectors cannot be easily used to assume the transpressional regime of any a priori favourably oriented deformation zone. The kinematic frame accounting for this probably results in high strain partitioning at multiple scales in the Betic and Rif chains. In addition to the characteristic heterogeneity of the continental crust rheology, ultimate causes should be related to the highly complex subduction geometry of this part of the western Mediterranean, probably including subduction slab retreat following different directions and the subsequent generation of slab windows (Faccenna et al., 2004; van Hinsbergen et al., 2014).

Acknowledgements

6. Conclusions

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.tecto.2015.05.002.

1. The Torcal Shear Zone (TSZ) is a large-scale, E–W brittle-ductile shear zone that provokes the near vertical uplift of the inner external zones against the Alboran Domain. Within the TSZ the strain is highly partitioned into shortening, oblique, strike-slip and extensional structures. Accordingly, the TSZ can be divided along-strike into four main structural domains with different development of each type of structures, all of them showing evidences of overall dextral transpression during the Late Miocene to the Quaternary. 2. The western and eastern termination zones of the TSZ show different structural associations. The western tip zone results in dextral strikeslip faults linked by contractional stepovers and resembles the splaydominated large-scale strike-slip fault tips. The eastern tip zone, in contrast, merges into a WNW vergent thrust system that accommodates most of the TSZ motion. 3. Different styles of strain partitioning occur in the central sector of the TSZ at multiple scales, from pluri-kilometric to outcrop-scale, both along- and across-strike. Along-strike flow partitioning produces larger ϕ and Wk values in the Torcal de Antequera massif (triclinic pure-shear/simple-shear dominated transpression) with respect to the Valle de Abdalajís massif (nearly monoclinic pure-shear dominated transpression). In the Valle de Abdalajís massif, the southern boundary is a discrete, dextral strike-slip fault zone, whereas deformation within the massif is dominated by evenly distributed oblique-reverse shear zones. Deformation at the shear zones is partitioned into discrete dextral faults at limestones and pure-shear dominated thrusting at marly limestones. In the Torcal de Antequera massif, deformations is partitioned into two domains at the boundaries accommodating simple-shear-dominated transpression and a wider inner domain of distributed deformation affected by triclinic pure-shear-dominated transpression. This massif presents an open folded structure, with reverse shear zones developed at folds short limbs. 4. Strain partitioning in the central sector of the TSZ appears to be mainly controlled by (1) slight, along strike variations in the orientation of the Alboran Domain–South Iberian Domain boundary, (2) lateral differences in the thickness of the weak Triassic layer and (3) the rheological contrast related to the contact between limestones and marly limestones. 5. The TSZ induces the dextral deflection of the regional structural grain and represents the main structure that shapes de northern end of the Gibraltar Arc hinge zone. Due to TSZ age, it can be concluded that the present-day pattern of the westernmost Gibraltar Arc was reached in later stages of the Arc development, thus amplifying the degree of protrusion of the former orogenic curvature. 6. Structure and kinematics of the central sector of the TSZ are compatible with a single, Late Miocene to Quaternary, transpressional event related to a unique velocity vector whose orientation ranges from

This study was supported by grants TOPOIBERIA CONSOLIDERINGENIO2010-CSD-2006-0041, CGL2013-46368-P, and RNM-451. Sara Titus and an anonymous reviewer are acknowledged for their constructive comments on improving the manuscript. Appendix A. Supplementary data

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Please cite this article as: Barcos, L., et al., Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.05.002