Recent and active tectonics in the western part of the Betic Cordillera/Tectonica reciente y activa en la parte occidental de la Cordillera Betica.
The Betic Cordillera (Fig. 1a), located in the southern part of Spain, is a region of high seismic activity within the Iberian Peninsula. Most earthquakes are of low-tomoderate magnitude (M<5.5) and usually occur in the shallow brittle crust, with the exception of two localized areas of intermediate (40-120km) and deep (>620 km) seismicity (Buforn et al., 1991). In the central and eastern sectors of the cordillera, seismicity and the associated seismic hazard have been widely studied and respectively related to NW-SE to WNW-ESE high-angle normal faults (Sanz de Galdeano et al., 1995; Alfaro et al., 2007) and NNE-SSW to NE-SW left-lateral segmented faults (Martinez Diaz, 2002; Masana et al., 2004), respectively. However, the western sector has been traditionally less known due to the scarcity and quality of the outcrops, the complex geological structure, and the lower frequency and magnitude of the seismic events. Seismic hazard at the western Betic Cordillera could be denoted as low to moderate.
The study of the potential seismic sources and their kinematics is of great interest in an area where historical consequences of seismic activity have been recorded. Repeated archeoseismic damaging events with intensity [greater than or equal to] IX MSK were reported in the Roman city of Baelo Claudia around AD 40-60 and AD 260-290 (Goy et al., 1994; Silva et al. , 2009). Historical earthquakes with maximum intensities of IX have been reported in the Guadalquivir Basin in 1504, with major effects in the city of Carmona (Gentil and de Justo, 1983; Martinez Solares and Mezcua, 2002), and in 1680 NW of Malaga (Munoz and Udias, 1988). Most widespread historical damage, including a tsunami, was caused in the Cadiz Bay by the 1755 Lisbon earthquake, with an estimated magnitude of 8.5-8.7 and EMS intensities between IV and VIII in the studied area (Martinez Solares and Lopez Arroyo, 2004) but whose epicenter was located at the Cape of San Vicente, outside the study area.
Despite the aforementioned limitations, integration of brittle microtectonic and seismicity data in several key sectors of the western Betic Cordillera provide valuable information to clarify their heterogeneous behaviour. This study compiles and integrates new results, together with available recent tectonic data, to improve the characterization of recent paleostresses and present-day stresses in the northern branch of the Gibraltar Arc and better determine the main tectonic features of active sectors.
2. Geological setting
The Gibraltar Arc (Fig. 1a), formed by the junction between the Betic and Rif Cordilleras, constitutes the westernmost end of the Mediterranean Alpine Chain at the boundary between the Atlantic Ocean and the Mediterranean Sea. The present day relief of the Betic Cordillera has been formed by the interaction of Eurasia-Africa convergence together with the westward emplacement of the Internal Zones (Dewey et al., 1989; Rosenbaum et al., 2002), mainly composed of HP-LT metamorphic rocks (Gomez-Pugnaire and Fernandez-Soler, 1987; Goffe et al. , 1989). This emplacement provoked the thrust-and-fold belt structure of the sediments deposited over the Iberian passive margin, the External Zones, during the Early-Middle Miocene (Sanz de Galdeano, 1990; Sanz de Galdeano and Vera, 1992). During this stage, the frontal disorganization of the External zones (olistostromic units) and the folding and thrusting of Cretaceous to Lower Miocene rocks of the Flysch units, located in between the External and the Internal Zones, ocurred (Lujan et al., 2003). Simultaneously, Lower to Middle Miocene extension was recorded at the sedimentary sequence of the Alboran basin (Chalouan and Michard, 2004) and at the metamorphic rocks of the Internal Zones (Aldaya et al., 1984; Garcia-Duenas et al., 1992; Platt and Vissers, 1989). Since the Late Tortonian, the main active processes have been volcanic activity, relief uplift, progressive individualization of intramontane sedimentary basins, and development of the Guadalquivir foreland basin due to the flexure of the Variscan Basement below the Betic Cordillera (Maury et al., 2000; Garcia-Castellanos et al., 2002; Serrano et al., 2002; Sanz de Galdeano and Alfaro, 2004).The surface expression of recent and active tectonic structures is highly variable along the Betic Cordillera. The relief of the central and eastern sectors is mainly determined by E-W to ENE-WSW kilometer-size folds that deform up to Holocene sediments of the Alboran Sea (Galindo-Zaldivar et al., 2003; Marin-Lechado et al., 2006). These folds also interact with NW-SE to WNW-ESE high-angle normal faults (Sanz de Galdeano et al., 1995; Alfaro et al., 2007) and, in the easternmost sector, with large NNE-SSW to NE-SW left-lateral segmented faults (Gracia et al., 2008) that deform Quaternary sediments and have related low to moderate seismicity (Martinez Diaz, 2002; Masana et al., 2004; Stich et al., 2003, 2006). In the western sector, west of Malaga, recent tectonic activity is not as evident and practically no Quaternary faults could be observed at surface. However, the area is affected by recent uplift and river incision (Schoorl and Veldkamp, 2003; Sanz de Galdeano and Alfaro, 2004; Zazo et al., 1999, 2003) and active folding and fault activity has been reported at Sierra de Cartama and the Malaga Basin, mainly based on gravity data and drainage pattern analysis (Capote et al., 2002; Insua Arevalo et al., 2004; Insua Arevalo, 2008). The first neotectonic study performed in the area (Benkhelil, 1976) revealed the existence of Quaternary NE-SW oriented left-lateral and NW-SE right-lateral faults. In a later detailed study of the Bolonia area, SE of Vejer, Silva et al. (2006) compiled a neotectonic map and defined the NE-SW Cabo de Gracia strike-slip fault as the main active fault in this sector, with an estimated activity younger than 128 ka. Northwards, in the San Fernando Sector, Upper Pleistocene beach deposits are affected by open joints filled by calcretes dated as 19.9 ka (Gracia et al., 2008). Paleostress analysis of microtectonic structures affecting several outcrops of Upper Miocene to Quaternary sediments were also analyzed by Camacho et al. (1999), showing that most common brittle structures are NW-SE and NE-SW high-angle faults with oblique slip. At a frontal position, deformation and shallow seismicity reveal present-day activity of the mountain front in the Moron de la Frontera area (Ruiz-Constan et al., 2009). A recent compilation of Plio-Quaternary slip data, together with moment tensor solutions and borehole breakout measurements, reveal a NW-SE [SH.sub.max] trajectory in the mountain front that deflects to NNW-SSE at the Gibraltar Strait area (Pedrera et al., 2011).
[FIGURE 1 OMITTED]
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3. Seismicity distribution
Between the Cape San Vincent area and Algeria, deformation along the Eurasia-Africa plate boundary zone is accommodated over a wide area with distributed seismicity, showing a complex pattern of GPS velocities, as well as heterogeneous earthquake focal mechanisms and stress patterns (e.g. Morel and Meghraoui, 1996; Buforn et al., 2004; Stich et al., 2006; Serpelloni et al., 2007; De Vicente et al., 2008; Perouse et al., 2010). This area contrasts with more concentrated and homogeneous plate boundary deformation further east, showing mainly NWNNW directed thrusting events at the north Algerian margin, and further west, showing mainly strike-slip faulting along the oceanic portion of the plate boundary (Grimison and Chen, 1986; Buforn et al., 1988). Along the Iberia-Maghreb sector, earthquakes are of low-to-moderate magnitude (M<5.5) and occur at shallow depth, except in two localized areas of intermediate (40-120 km) and deep (>620 km) seismicity around Malaga and Granada, respectively (Buforn et al., 1991; Morales et al., 1997) as well as subcrustal earthquakes (< 60 km) in old oceanic lithosphere in the Atlantic (Stich et al., 2005; Geissler et al., 2010). Within the Betic Cordillera, faulting style changes from predominately strike-slip in the eastern part to normal faulting in the central Betics and to reverse and strike-slip faulting in the western Betics (Stich et al., 2010).Within the western Betics, seismicity is usually of lower magnitudes (rarely exceeding magnitude 4) and less frequent than in the eastern and central sectors.
Figure 1c shows the seismicity distribution from the Instituto Geografico Nacional database for magnitudes greater than 2 between 1980 and 2011 (IGN database, http://www.ign.es). The patchy pattern of the epicenter map indicates that seismic catalogues in this low seismicity area are actually dominated by a few instrumentally recorded sequences, yet we assume that basic properties of the recorded seismicity, such as focal depth and faulting style, are representative for the study area. We selected four regions where the amount of shallow instrumental seismicity is highest to show frequency histograms for the distribution of hypocenters with depth. The most active region is located near the town of Moron de la Frontera, in the northwestern front of the cordillera. Seismicity in this area is mainly related to seismic swarms occurring between the end of 2006 and the end of 2009. The four largest events show magnitudes between 4 and 5. Seismicity is concentrated in the upper crust, above 12 km depth (Figs. 1 and 2). No earthquakes occur at depths greater than 24 km. In the other selected areas, the amount of seismicity is significantly lower than in the Moron area (Fig. 2). To the east, in the Teba sector, two earthquakes of magnitude between 4 and 4.5 have been recorded, and seismicity is mostly concentrated between 8 and 12 km (Figs. 1c and 2). The most active periods in the region were 1996, 2006 and the first months of 2011. Further to the east, around Villanueva de Algaidas and Mollina, seismic activity is mainly related to a seismic crisis that took place in 1989 (Figs. 1c and 2). Faulting appears to be slightly shallower than in the former regions. Magnitude in this sector is generally lower than 3, with only eight earthquakes comprised between 3 and 3.5. To the north, we include one sector located in the Guadalquivir basin near the town of Montilla, related to activity mainly during 1985 (Figs. 1c and 2). Although the amount of seismic activity is lower than in the other areas, a 5.1 earthquake and four events between 4 and 5 have been recorded. Again, estimates for the hypocentral depth indicate faulting within the upper crust (Fig. 2).
4. Earthquake focal mechanisms
Twenty-one shallow earthquake focal mechanism solutions are available for the study area (Fig. 1b and Table 1). Eighteen solutions were obtained from the time domain moment tensor inversion of full waveform recordings at regional distances, and are included in the moment tensor catalogue of the Instituto Andaluz de Geofisica (Stich et al., 2003, 2006 and 2010). Most moment tensor solutions are for earthquakes since 2006, a period characterized by more frequent than average M>3.5 earthquakes in this sector, as well as denser seismic broadband instrumentation than previous years. We also included in this analysis two earthquake focal mechanisms between Montilla and Cordoba calculated through the first motion of P waves (850526 and 960416; Buforn et al., 1995; Mezcua and Rueda, 1997) and one solution for the same area based on full waveform inversion of digitized historic seismograms of a magnitude 5.1 earthquake in 1930 (300705; Batllo et al., 2010).
The overall picture emerging from the shallow earthquake focal mechanisms, shown in Figure 1, is a predominance of strike-slip and reverse faulting, as well as a preferred NW-SE orientation of focal mechanism P-axes. However, there is a relevant level of heterogeneity, like the presence of strike-slip mechanisms with opposite kinematics. Plotting faulting style into Frohlich diagrams (Fig. 1b; Frohlich and Apperson, 1992; Celerier, 2012) confirms that most solutions correspond to reverse and strike slip faulting. Only one mechanism is of pure extension. The geographical distribution of available focal mechanisms is very heterogeneous, following the pattern of recent seismicity. Fourteen of them are concentrated at the mountain front, near Moron de la Frontera, and the faulting style ranges from pure reverse to pure strike-slip. Reverse faults show NE-SW nodal planes and hypocenter depths of 4-12 km. Depths of strike-slip solutions at the mountain front, with mainly ~NW-SE and ~SW-NE striking nodal planes, vary between 4 and 16 km. To the east, in the Teba sector, two similar earthquake focal mechanism solutions were determined (060311 and 060326 events). Both of them are strike-slip faults with NW-SE and NE-SW nodal planes and depths of 14 and 22 km. To the north, four earthquake focal mechanisms are located along the Guadalquivir Basin, between Montilla and Cordoba. The shallowest (850526) is a NNE-SSW reverse fault located at a depth of 5 km. Between 8 and 12 km there are two similar strike-slip faults with NW-SE and N E-SW nodal planes (960416 and 030124). The deepest solution (300705) corresponds to the moment magnitude Mw 5.1 earthquake that affected the town of Montilla in 1930 with an estimated epicentral intensity of VIII MSK (Karnik, 1969; Batllo et al., 2010) and is the largest event recorded instrumentally in the western Betics. The solution is a NW-SE normal fault located at 16 km depth, though depth resolution is low for this event. Finally, an oblique normal fault, with ~N-S and ~E-W nodal planes, is located onshore at the Gibraltar strait (880708).
[FIGURE 3 OMITTED]
The average stress tensor calculated from the focal mechanisms indicates a maximum NW-SE principal major stress, parallel to plate convergence, with a northwestward inclination of maximum stress, compatible with the activity of low-angle thrust faults (Ruiz-Constan et al., 2009; Pedrera et al., 2011). In addition, a high axial ratio value (as defined by Galindo-Zaldivar and GonzalezLodeiro, 1988; R=([[sigma].sub.2] - [[sigma].sub.3])/([ [sigma].sub.1] - [[sigma].sub.3])) may favour stress permutations between [[sigma].sub.2] and [[sigma].sub.1] and allow local NE-SW compression (indeed, observed for event 020915 in Table 1). However, the observed heterogeneity of faulting indicates that the average stress tensor may not accurately represent this complex study area, for which reason this work aims to provide a more specific characterization of individual areas.
5. Recent brittle deformation and paleostress analysis
We have divided the western Betic Cordillera into five different sectors for the analysis of recent brittle deformation (Fig. 3), depending on their geological setting. Analysis is focused on the sectors with recent sedimentary cover, allowing us to constrain the deformation age. Accordingly, palaeostress orientations were determined for the Middle-Late Miocene to the Quaternary through analysis of brittle meso- and micro-structures such as faults and joints, taking into account that in many cases more recent sediments do not crop out or are poorly deformed and make it impossible to differentiate phases. Stress inversion was accomplished using the Galindo-Zaldivar and Gonzalez-Lodeiro (1988) Search Grid method. It is based on a systematic search on a grid pattern that allows for the determination of overprinted stress tensors, axis orientations and axial ratios (Fig. 3). All the new results are described in Table 2, together with previous paleostress inversions performed in the area (Camacho et al., 1999; Gracia et al., 2008; Ruiz-Constan et al., 2009; Pedrera et al., 2011).
5.1. Moron de la Frontera Sector (Mountain front)
Scarce outcrops record the present-day deformation at the northwestern mountain front of the Betic Cordillera due to several factors, including slow tectonic activity, the erodability of the sedimentary filling of the Guadalquivir basin, and the chaotic structure of the Olistostromic Complex. However, in contrast to the neighbouring sectors, this area is affected by very shallow seismicity (around 8 km depth), suggesting the presence of active tectonic structures (Ruiz-Constan et al., 2009). The present-day relief of the northwestern mountain front has a roughly NE-SW trend associated with the nucleation of NE-SW open folds. The development of these folds is linked to the present-day activity of NW-SE blind thrusts, indicated by seismic data (Stich et al., 2010), that deform the upper part of the crust without reaching the surface. Nevertheless, due to uncertainties in focal depth estimation, it could not be asserted if the seismogenic active thrusts are located in the upper part of the Variscan basement or just at its boundary with the Betic tectonic units.
Several paleostress stations located on Middle-Late Miocene marls and marly limestones cropping out in the surroundings of Moron de la Frontera and Puebla de Cazalla reveal high deformation at surface. The most important structures affecting the limit between the autochthonous Guadalquivir sediments and the Olistostromic Complex are mainly subvertical NNE-SSW and NW-SE strike-slip faults (Fig. 4a; stations 13-16 and 18). For the same age, stations located to the east, near Gilena and Osuna (stations 17 and 19), reveal the prevalence of WNWESE to NW-SE normal faults with an oblique slip. A well constrained nearly horizontal NNE-SSW extension was determined for stations 13 to 15, oblate in shape in all. In addition, a secondary local paleostress with a N-S [SH.sub.min] was determined at station 13 (Fig. 3). Stations 16 and 17 reveal radial extension with a subvertical [[sigma].sub.1] and low axial ratios. Finally, the paleostress determined for stations 18 and 19 shows prolate shapes and a [[sigma].sub.1] dipping 50-60[degrees] toward the SSW.
In the westernmost part of the mountain front, near El Palmar (station 6), more recent sediments of the Late Pliocene are deformed by N-S to NE-SW normal faults with a strike-slip component and low dip toward the W and NW, respectively, that define a well constrained NWSE [SH.sub.max] (Camacho et al., 1999). To the south, in the Jerez de la Frontera area (station 5), two sets of NW-SE and NE-SW high angle normal faults also determine a NW-SE [SH.sub.max] (Camacho et al., 1999). No reverse faults were observed at these outcrops. Finally, the only station located in the autochthonous Upper Miocene sandstones of the Guadalquivir basin, near Carmona (station 20), reveals the existence of NE-SW high angle strike-slip faults that determine well defined subhorizontal N-S extension (Camacho et al., 1999).
5.2. Teba Sector (External Zones)
Brittle deformation in Upper Miocene calcarenites cropping out in the area is scarce and constituted by a main set of NW-SE tensional joints (station 23). Recent mesoscale faults have only been identified at the boundaries of the ranges composed by Jurassic limestones in the Teba and Canete la Real area. They generally show dips higher than 60[degrees], oblique slip and transtensional right-lateral and left-lateral kinematics that vary depending on the fault plane orientations (Fig. 3, station 4). However, NESW normal faults are also frequent. Wedge-like deposits of Quaternary coarse-grained sediments that thicken towards the fault plane were observed in the hanging wall of one of these faults located near Canete la Real (Fig. 4b). It is a N60[degrees]E normal fault 300 m in length dipping 65[degrees] towards the SE. Traditionally, the development of colluvial wedges has been generally considered as evidence for earthquake related faulting (McCalpin 1996).
[FIGURE 4 OMITTED]
5.3. Ronda Sector (Neogene Intramontane Basin)
The Ronda Depression, the main intramontane basin of the western Betic Cordillera, is located at the contact between the External Zones and the Flysch unit. Its sedimentary infill is mainly constituted by Late Miocene conglomerates, marls and sandstones (Serrano, 1979; Rodriguez-Fernandez, 1982). The boundaries of the basin are unconformities. Recent deformation consists predominantly of medium to large scale NNE-SSW and WNW-ESE folds (Crespo Blanc and Campos, 2001), while brittle deformation is scarce and localized (RuizConstan et al., 2008). The NE-SW and NNW-SSE minor normal faults mainly deform Tortonian calcarenites and are mainly gathered in the southern part of the basin (Ruiz-Constan et al., 2008). Faults rarely show striation or other kinematic indicators; nevertheless, the conjugated fault sets and the displacement of reference surfaces suggest a normal component.
In the Ronda-Setenil section, the Upper Miocene sediments are mainly deformed by a metric set of conjugated NE-SW normal faults with centimetric displacement, dipping 45[degrees]-80[degrees] (stations 9 and 10; Figs. 3 and 4c). There are also two orthogonal NW-SE and NE-SW sets of vertical tension joints together with a joint spectra (Hancock, 1985) that includes tension and shear joints of other orientations (stations 11 and 12). Paleostress analysis of the micro- and meso- faults reveals N-S to NW-SE compression with low axial ratios (stations 9 and 10), whereas results of joint set analysis give a parallel direction of extension (stations 11 and 12). To the north, in the Setenil-Olvera section, Camacho et al. (1999) described two sets of faults. The first one (station 8) is constituted by NESW normal faults with a strike-slip component dipping towards the NW that cuts Upper Miocene rocks. The second one (station 7), deforming Pliocene-Quaternary sediments, is composed by high-angle NW-SE normal faults dipping towards the NE that define a NW-SE [SH.sub.max].
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5.4. Alboran coast sector (Internal Zones)
Upper Tortonian calcarenites cropping out in the area of El Chorro (N of Ardales), Alora and Pizarra, have undergone recent tectonic uplift and tilting. They are a reliable marker for estimating recent uplift rates because they were deposited in coastal or very shallow marine environments during the Tortonian and at Present they are located, from north to south, between 620 and 450 m a.s.l. Upper Tortonian shallow-water calcarenites were deposited on inner platforms at depths of less than 30 m (Braga et al., 2003) approximately in the time interval from 8.5 to 7.5 Ma. In addition, Pliocene marine deposits are now situated between 180 and 114 m a.s.l. Uplift rates in this sector have been estimated in 0.16-2.76 mm/a during the Tortonian-Messinian, decreasing to 0.01-0.015 mm/a during the Pliocene, then increasing again to 0.040.1 mm/a during the Pleistocene (Schoorl and Veldkamp, 2003). However, these rocks are scarcely deformed by faults. Two sets of orthogonal joints with N-S and E-W orientations were measured in the surroundings of Ardales (station 22). This setting determines the incision of the fluvial networks of the Alboran coast, with areas of very abrupt topography.
5.5. Atlantic coast sector (westernmost orogen boundary)
Brittle microtectonic data measured on Upper Miocene to Quaternary rocks cropping out in localized sections constitute the available long-term stress indicators in this area (Camacho et al., 1999; Silva et al., 2009; Pedrera et al., 2011). Station 1 (Gracia et al., 2008) points to ENE-WSW extension based on the existence of N160[degrees]E tensional joints deforming an Upper Pleistocene calcrete near San Fernando. Pedrera et al. (2011) described brittle deformation affecting Pliocene and Quaternary shallow-marine sedimentary rocks in two sections of the Atlantic coast: the Roche Cape-Conil section and the Trafalgar Cape-Barbate section. The former section is strongly deformed by two sets of normal faults with NW-SE to WNW-ESE orientations, indistinctly dipping around 50[degrees]-60[degrees] to the NE or SW (station 2; Fig. 4d). These faults determine a subvertical maximum stress axis, and NE-SW subhorizontal extension. Some faults of the WNW-ESE set show a right-lateral component. Compressive structures are less frequent and are constituted by minor Upper Pliocene-Lower Pleistocene E-W to ENE-WSW reverse faults (Silva et al., 2009). In addition, NW-SE vertical open joints together with N130[degrees]E and N170[degrees]E conjugated shear joints deform the Upper Pliocene-Lower Pleistocene beach sediments (station 3; Pedrera et al., 2011) and indicates NE-SW extension and NW-SE compression. To the south, along the Trafalgar Cape-Barbate section, the same NW-SE to WNW-ESE sets of normal faults and joints highly deform the Tortonian and Messinian calcareous sandstones. The recent activity of these faults is well constrained because they broke up terraces that developed during the MIS 5c marine deposition (~105 ka). In the Vejer area, located between the two sections, NW-SE normal faults deform Upper Miocene sandstones and determine a well defined subhorizontal N-S extension (station 21; Camacho et al., 1999).
Despite variability, analysis of brittle deformation in the whole area reveals the activity of faults having a favorable orientation with respect to the NW-SE convergence between Eurasia and Africa. Most of the paleostresses calculated are compatible with this context, showing a prevailing NW-SE subhorizontal compression and orthogonal extension.
Although there are no prominent active faults cropping out in the western Betic Cordillera, low to moderate earthquakes occur in this area regularly. Here we summarize deformation patterns, seismicity distribution and recent and present-day stresses, with discussion of their active tectonics and geodynamic implications.
The northwestern mountain front is probably one of the sectors of the western Betics with the greatest concentration of earthquakes during recent times (seismic series of Moron de la Frontera, Teba and Villanueva de Algaidas/ Mollina). Seismicity is associated with the activity of NE-SW blind thrusts and NW-SE transfer faults (Ruiz-Constan et al., 2009). These structures, identified by means of seismic data (Stich et al., 2010), deform the upper part of the crust without reaching the surface. In addition, errors in the foci determination of the earthquake focal mechanism solutions do not allow us to confirm whether the seismogenic active thrusts deform the upper part of the Variscan basement or its contact with the Betic tectonic units. Although seismic activity is usually of low magnitude (M<5), it occurs at very shallow depths (<12 km) in an area with a widespread presence of thick plastic Triassic rocks mixed within the Olistostromic Unit that could favour seismic amplification effects.
This last unit, involving plastic Triassic rocks, extends toward the Gulf of Cadiz, where it formed the so-called Gulf of Cadiz Imbricate Wedge. Pliocene and Quaternary shallow-marine sedimentary rocks placed above this unit along the Atlantic coastal sector are deformed by small NW-SE to WNW-ESE normal faults, indistinctly dipping to the NE or SW (Pedrera et al., 2011; Camacho et al., 1999). The recent activity of these small faults is well constrained, since they broke up terraces dated as ~105 ka (Zazo et al., 1999). Compressive structures are less frequent and constituted by minor E-W to ENE-WSW reverse faults (Silva et al., 2009). In contrast to the Moron de la Frontera mountain front, seismicity is very scarce in this sector of the Gibraltar Arc.
Recent NW-SE to N-S (and locally NE-SW) normal faults to normal-oblique faults together with kilometric-scale open folds, generally ENE-WSW trending, are identified along the Internal Subbetics and Flysch units deforming previous Early Miocene folds-and-thrusts (e.g. Balanya et al., 2007). Only local evidence of Quaternary activity has been detected near Canete la Real, related to a N60[degrees]E normal fault dipping 65[degrees] towards the SE and measuring 300 m in length. In the eastern part of the study area, the infill of the Neogene Ronda Basin, located at the contact between the External Zones and the Flyschs, provides a good marker to constrain the timing of recent deformation. There, Upper Miocene sandstones are mainly deformed by medium to large scale NNESSW and WNW-ESE folds; meanwhile, brittle deformation is scarce, localized and generally of low intensity. NW-SE to NNW-SSE and local NE-SW normal faults deform the Upper Miocene rocks and, rarely, Pliocene-Quaternary sediments (Camacho et al., 1999; Ruiz-Constan et al., 2008). Although the NW-SE trending normal faults and the ENE-WSW folds and reverse faults are favorably oriented for reactivation under present-day stress conditions, it is very difficult to correlate earthquakes to specific faults along the Internal Subbetics and Flysch fold-and-thrust belt.
Along the Internal Zones it is likewise problematic to correlate seismicity and tectonic structures. Seismicity is concentrated within the lower crust and the lithospheric mantle, reaching 120 km in depth below the coast, probably associated with the subducted portion of the Iberian crust beneath the Betic Internal Zones (Morales et al., 1999; Buforn et al., 2004; Pedrera et al., 2011; Ruiz-Constan et al., 2011). This geodynamic scenario favoured recent tectonic uplift, as revealed by the placement of Upper Miocene calcarenites at 620-450 m a.s.l. (Schoorl and Veldkamp, 2003), the abrupt topography, and the high incision of the fluvial network. Although geomorphological data suggest active folding north of Malaga (IGME, 2012), there is no field evidence of outcropping active faults.
The scarcity and low quality of outcrops including recently deformed rocks, the complex geological structure, and the lower frequency and magnitude of the seismic events along the western Betic Cordillera come to limit our knowledge of the specific active tectonic structures in this sector. However, the integrated analysis of brittle microtectonic structures together with seismicity distribution provides evidence regarding the main tectonic feactures of active sectors. The northwerstern mountain front, in the Moron de la Frontera area, is the sector with the greatest recent earthquake activity. Although seismogenic structures do not crop out at surface, low to moderate shallow epicenters reveal the presence of active reverse and transfer faults. In addition, the presence of a thick plastic Olistostromic Unit could favour site effects. Toward the Gulf of Cadiz, Pliocene and Quaternary shallow marine sedimentary rocks placed above this unit along the Atlantic coastal sector are deformed by small NW-SE to WNW-ESE normal faults and E-W to ENE-WSW reverse faults. In contrast to the Moron de la Frontera mountain front, in this sector of the Gibraltar Arc seismicity is very scarce.
Along the Internal Subbetics and Flysch units, recent NW-SE and local NE-SW trending normal to normal-oblique faults are recognized, as well as ENE-WSW folds and reverse faults. Although these structures are favorably oriented for reactivation under the present-day stress setting, there are no direct relationships with earthquakes and only local evidences of Quaternary activity near Canete la Real. The earthquakes are progressively deeper toward the south (from 40 km to 120 km beneath the Malaga coast) and bear no direct relation with structures cropping out in the Internal Zones. Intermediate seismicity is associated with the portion of the Iberian crust subducted beneath the Betic Internal Zones. This setting favours recent uplift and river incision within the Internal Zones of the Betic Cordillera.
In summary, the Western Betics is a moderate to low seismicity region in which the complex geological structure generally impedes the outcropping of deep seismogenic faults. Moreover, small active faults recognized at surface have complex stress and kinematic relationships with the seismogenic deep faults. Both geophysical and geological data must therefore be integrated in order to characterize the active tectonics of this region.
This study was supported by the projects TOPO-IBERIA CONSOLIDER INGENIO CSD2006-00041, CGL2008-03474-E, CGL2010-2104 and CGL 2008 0367 E/ BTE of the Spanish Ministry of Science and Education, as well as by Research Group RNM-148 and the project P09-RNM-5388 of the Junta de Andalucia Regional Government.
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A. Ruiz-Constan * (1), A. Pedrera (2), J. Galindo-Zaldivar (3), D. Stich (4), J. Morales (4)
(1) Geosciences Montpellier, Universite Montpellier 2-CNRS, Montpellier, France email@example.com
(2) Instituto Geologico y Minero de Espana, Madrid, Spain firstname.lastname@example.org
(3) Dpto. de Geodinamica, Universidad de Granada, Instituto Andaluz de Ciencias de la Tierra, CSIC- UGR, Granada, Spain jgalindo@ugr. es
(4) Instituto Andaluz de Geofisica, Universidad de Granada, Granada, Spain email@example.com; firstname.lastname@example.org
* corresponding author
Received: 30/06/2011 / Accepted: 20/03/2012
Table 1.--Solutions for shallow earthquake focal mechanisms represented in Figure 1. The reference's article corresponds to: Batllo et al., 2010 ; Buforn et al., 1995 ; Stich et al., 2003; Mezcua and Rueda, 1997 ; Stich et al., 2006 , 2010 ). Dashes indicate data not available. Tabla 1.--Mecanismos focales de terremotos representados en la Figura 1. La referencia de los articulos se corresponde con: Batllo et al., 2010 ; Buforn et al., 1995 ; Stich et al., 2003; Mezcua and Rueda, 1997 ; Stich et al., 2006 , 2010 . Los guiones indican datos no disponibles. Date Origin time Latitude Longitude Depth (yy/mm/dd) (hh:mm:ss) [[degrees]N] [[degrees]E] [km] 300705 -- 37.56 -4.65 16 850526 18:05:10 37.80 -4.60 5 880708 23:31:11 36.21 -5.42 8 960416 22:44 37.61 -4.66 8 020915 20:54:19 37.16 -5.27 4 030124 20:35:00 37.74 -4.70 12 030725 03:37:55 36.90 -5.56 6 060311 13:43:07 36.87 -4.98 22 060326 18:15:40 36.83 -5.04 14 070102A 12:19:26 37.11 -5.39 10 070102B 15:00:47 37.16 -5.33 14 070630A 03:53:45 37.07 -5.44 8 070630B 11:29:35 37.08 -5.42 8 070918 23.20:42 37.01 -5.43 8 070914A 03:45:06 37.08 -5.47 12 070914B 03:46:49 37.08 -5.47 8 081002A 04:02:53 37.04 -5.42 4 081002B 04:05:06 37.02 -5.43 12 081002C 05:14:19 37.06 -5.43 6 081002D 09:22:14 37.06 -5.40 16 081008 16:04:47 37.06 -5.41 12 Date Seismic Magnitude Strikel Dipl (yy/mm/dd) moment [Nm] [M.sub.W] [[degrees]] [[degrees]] 300705 5.481016 5.1 123 39 850526 -- 5.1Mb 174 51 880708 0.171016 4.1 178 61 960416 -- 4.3Mb 75 76 020915 1.421015 4.1 273 68 030124 2.081015 4.2 58 86 030725 1.911014 3.5 29 40 060311 6.92 1014 3.9 243 88 060326 3.47-1014 3.7 151 70 070102A 2.981014 3.6 135 79 070102B 3.24-1014 3.6 136 71 070630A 3.881015 4.4 81 45 070630B 3.061014 3.6 103 58 070918 8.57-1014 3.9 52 34 070914A 2.391015 3.6 65 35 070914B 2.67-1015 3.6 51 54 081002A 6.89 1015 4.5 54 44 081002B 9.171014 3.9 54 39 081002C 1.191014 3.4 44 45 081002D 7.66 1013 3.2 87 66 081008 2.111014 3.5 211 69 Date Rakel Strike2 Dip2 Rake2 (yy/mm/dd) [[degrees]] [[degrees]] [[degrees]] [[degrees]] 300705 -86 298 51 -93 850526 70 25 43 114 880708 -159 78 72 -31 960416 -179 345 89 -14 020915 58 152 38 143 030124 -176 327 86 -3 030725 82 219 51 96 060311 -170 152 80 -2 060326 5 60 85 160 070102A -174 44 84 -10 070102B -152 37 64 -21 070630A 123 218 53 61 070630B 158 205 71 34 070918 103 217 57 82 070914A 107 224 56 78 070914B 97 220 36 81 081002A 103 216 47 78 081002B 92 232 51 88 081002C 109 198 48 72 081002D 166 183 77 25 081008 24 111 68 157 Date CLVD Reference (yy/mm/dd) [%] 300705 2 1 850526 -- 2 880708 19 3 960416 -- 4 020915 5 5 030124 14 5 030725 6 5 060311 1 6 060326 10 6 070102A 19 6 070102B 22 6 070630A 6 6 070630B 13 6 070918 3 6 070914A 15 6 070914B 3 6 081002A 1 6 081002B 13 6 081002C 4 6 081002D 7 6 081008 5 6 Table 2.--Middle Miocene to Quaternary kinematic data; [[sigma].sub.1]--maximum compressive stress; [[sigma].sub.2]- intermediate compressive stress; [[sigma].sub.3]-minimum compressive stress; The axial ratio values are given as R = ([[sigma].sub.2]-[[sigma].sub.3])/([[sigma].sub.1]-[[sigma].sub.3]). UP: Upper Pleistocene; PLP: Plio-Pleistocene; UPL: Upper Pliocene; UM: Upper Miocene; MUM: Middle-Upper Miocene; A/D data: assigned/total data. Table 2.--Datos cinematicos del Mioceno Medio a Cuaternario; [[sigma].sub.1]--esfuerzo compresivo maximo; [[sigma].sub.2]--esfuerzo compresivo intermedio; [[sigma].sub.3]--esfuerzo compresivo minimo; La razon axial esta definida como R = ([[sigma].sub.2]-: [[sigma].sub.3])/([[sigma].sub.1]-[[sigma].sub.3]). UP: Pleistoceno Superior; PLP: Plio-Pleistoceno; UPL: Plioceno superior; UM: Mioceno superior; MUM: Mioceno Medio-Superior; A/D data datos asignados/totales. ID Location 1 San Fernando 2 Conil-Barbate 3 Conil-Barbate 4 Teba 5 Jerez de la Frontera 6 El Palmar 7 Olvera-Setenil 8 Olvera-Setenil 9 Ronda 10 Ronda 11 Ronda 12 Ronda 13 Puebla Cazalla 14 Puebla Cazalla 15 Puebla Cazalla 16 Puebla Cazalla 17 Osuna 18 Moron Ftra. 19 Gilena 20 Carmona 21 Vejer 22 Alora-Pizarra 23 Teba ID Tectonic structures 1 Joints: N160[degrees]E open joints 2 Meso-faults: NW-SE normal faults and scarce ENE-WSW reverse faults 3 N130[degrees]E and N170[degrees]E shear joints and NW-SE open joints 4 NE-SW/WNW-ESE normal and oblique slip faults 5 NW-SE and NE-SW high-angle normal faults 6 N-S to NE-SW high-angle oblique-slip faults 7 NE-SW high-angle oblique-slip faults 8 NW-SE high-angle oblique-slip faults 9 Micro-faults: conjugated NE-SW normal faults 10 Meso-faults: NNE-SSW/NNW-SSE normal faults 11 NE-SW conjugated joints 12 NW-SE to E-W tension joints 13 Meso-Faults: NE-SW to NW-SE strike slip faults 14 Meso-faults: NE-SW strike slip faults 15 Meso faults: NE-SW strike slip faults 16 Meso-faults: NE-SW/NW-SE oblique-slip faults 17 Micro-faults: NW-SE normal faults 18 Meso-faults: NE-SW/NW-SE oblique-slip faults 19 Micro-faults: WNW-ESE normal faults 20 NE-SW high-angle oblique-slip faults 21 NW-SE high-angle strike-slip faults 22 N-S/E-W tension joints 23 NW-SE tension joints ID Age/lithology Stress information [[sigma].sub.1]>[[sigma].sub.2]> [[sigma].sub.3] 1 UP calcrete ENE-WSW extension 2 PLP calcarenites 224/74 148/02 016/57 3 PLP calcarenites NW-SE compression/NE-SW extension 4 PLP NW-SE extension 5 UPL sandstones NW-SE compression 6 UPL sandstone NW-SE compression 7 UPL sandstone NW-SE compression 8 UM sandstones N-S extension 9 UM sandstones NW-SE extension 10 UM sandstones 225/77 018/12 109/06 11 UM sandstones NW-SE compression 12 UM sandstones NW-SE compression/ NE-SW extension 13 MUM marls 295/06 161/82 026/06 109/59 261/28 358/12 14 MUM marls 091/65 287/24 194/06 15 MUM marls 269/21 150/52 012/30 16 MUM marls 244/69 042/20 135/08 17 MUM marls 304/82 124/08 034/00 18 MUM marls 206/48 104/11 005/40 19 MUM marls 197/54 101/05 007/36 20 UM sandstones N-S extension 21 UM sandstones N-S extension 22 UM sandstones Radial extension 23 UM sandstones NE-SW extension ID Axial ratio A/T Data Reference 1 -- 42 Gracia et al. (2008) 2 0.03 13/20 Pedrera et al. (2011) 3 -- 86 Pedrera et al. (2011) 4 0.54 13 This study 5 0.11 14 Camacho et al. (1999) 6 0.42 11 Camacho et al. (1999) 7 0.49 6 Camacho et al. (1999) 8 0.42 10 Camacho et al. (1999) 9 -- 11 This study 10 0.28 7/7 This study 11 -- 13 This study 12 -- 9 This study 13 0.84 18/30 Ruiz-Constan et al. 0.90 20/30 (2009) 14 0.99 17/20 Ruiz-Constan et al. (2009) 15 0.68 11/22 Ruiz-Constan et al. (2009) 16 0.11 11/14 This study 17 0.17 7/10 This study 18 0.10 7/11 This study 19 0.10 15/17 This study 20 0.34 10 Camacho et al. (1999) 21 0.32 7 Camacho et al. (1999) 22 -- 20 This study 23 -- 25 This study
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|Title Annotation:||articulo en ingles|
|Author:||Ruiz-Constan, A.; Pedrera, A.; Galindo-Zaldivar, J.; Stich, D.; Morales, J.|
|Publication:||Journal of Iberian Geology|
|Date:||Jan 1, 2012|
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