On the structure and evolution of the Sorbas basin, S.E. Spain

Tectonophysics 773 (2019) 228230

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On the structure and evolution of the Sorbas basin, S.E. Spain 1

Lorenzo Valetti, Ernest Rutter*, Alicia McCabe , Julian Mecklenburgh

T

Rock Deformation Laboratory, Department of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK

ARTICLE INFO

ABSTRACT

Keywords: Sorbas basin Spain Structure Stratigraphy Gravity Petrophysics

Geological mapping and gravity surveying were used to constrain the shape of the Serravallian and Tortonian (U. Miocene) rocks of the Sorbas basin, SE Spain, and hence its tectonic evolution and place in the overall regional tectonic scheme. The present-day basin floor forms a westerly-deepening trough ∼2.0 km deep, infilled with mainly clastic turbidites and mass-flow deposits. Basin-fill rocks underwent broadly ENE-WSW extension during accumulation, with extensional faulting on the flank of the Sierra Cabrera to the east. Acoustic velocity measurements demonstrate a petrophysical impact of the syndepositional deformation. Post-deposition but prior to uplift and erosion, the sediments were folded into a large-scale monoclinal structure, inferred to have formed in response to gravitational sliding of sediments off the rising western flank of the Sierra Cabrera. This fold structure was deformed and rotated by 3 km of extension-related uplift of the basement block to the south during late Tortonian time, then erosion and flooding to produce a shallow-marine Messinian sequence, partially burying the older rocks. The geodynamic setting was the back-arc sector (the Betic zone) produced by Serravallian-Tortonian rollback of the Gibraltar arc subduction zone. It is bounded to the south by the Carboneras stretching transform fault, that accumulated about 40 km of left-lateral offset coevally with sediment deposition and deformation within the Sorbas basin. These events are inferred to be geodynamically linked.

1. Introduction The Betic Cordilleras of southern Spain form part of the Alpine orogenic system in southern Europe. Tectonic activity continues to the present day, dominated by the effects of continued NW-SE oriented compression of N Africa against southern Iberia (Lonergan and White, 1997; Vissers, 2012; Pedrera et al., 2012). Within the cordillera lies a wedge-shaped area (Fig. 1) bounded by (a) the Gibraltar-Rif arc in the south-east, (b) the trans-Alborán shear zone (Leblanc and Olivier, 1984; Weijermars, 1987; Vissers, 2012; Rutter et al., 2012, 2013) extending from the eastern Rif in Morocco to the vicinity of Alicante, and (c) the Internal-External zone boundary/Crevillente fault system along a line east of Cadiz to Alicante (Sanz de Galdeano and Buforn, 2005). It is well-established that the lithosphere underwent marked ENE-WSW extension during the mid and late Miocene, and continues today but to a lesser degree (Johnson et al., 1997; Faccenna et al., 2004; Gutscher et al., 2002; Augier et al., 2005; Gutscher, 2012). The Gibraltar arc is interpreted as the surface expression of an easterly-dipping subducted segment, with an accretionary complex lying in the Atlantic Ocean to the west (Royden, 1993; Gutscher et al., 2002; Martínez-Martínez et al., 2006; Díaz et al., 2010). Calc-alkaline

volcanics of the onshore Cabo de Gata complex (Fig. 1) and equivalent offshore volcanics represent mid-and upper-Miocene subduction-related igneous activity (Soriano et al., 2014; Mattei et al., 2014). The subduction zone has arrived in its present position after rollback by approximately 160 km (Royden, 1993; Faccenna et al., 2004), mainly between the Serravallian and Messinian stages of the upper Miocene epoch, resulting in lithospheric stretching in the back-arc wedge. This direction of lithospheric stretching is supported by the corresponding WSW-ENE orientation of the orientation of fast shear wave polarization in the upper mantle beneath the Betic cordilleras (Díaz et al., 2010). Contemporaneous seismicity tends to be focused within the stillstretching wedge (Rutter et al., 2013). Present day displacement rates within the wedge have recently been determined from global positioning system (GPS) measurements (Echeverria et al., 2013, 2015, Gonzalez-Castillo et al., 2015; GalindoZaldivar et al., 2015; Fig. 1). Relative to a reference Iberian massif, in the region immediately east of Almería NW-SE shortening attributable to the convergence of Iberia with Africa is locally dominant. However, across the entire wedge westwards from Alicante, where the present day velocity relative to stable Iberia is (close to) zero, westward-directed velocities increase linearly westward to become 4.5 mm/a at the

Corresponding author. E-mail address: [email protected] (E. Rutter). 1 Present address: Mine Tech Services (UK) Ltd., Bolton, UK. ⁎

https://doi.org/10.1016/j.tecto.2019.228230 Received 11 February 2019; Received in revised form 6 October 2019; Accepted 15 October 2019 0040-1951/ © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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Fig. 1. Map of S.E. Spain showing regional geodynamic scheme. The extending wedge is bounded by the Carboneras – Alhama de Murcia left-lateral fault system (the Trans-Alborán shear zone), the right lateral Crevillente fault and the Gibraltar arc, that lies 150 km to the SE. (based on Vissers, 2012). Betic movement zone (BMZ) vectors (arrows show relative direction of movement of the upper plate) from Jabaloy et al. (1993), Vissers (2012) and new data from the Sierra Cabrera (this study). Present day GPS vectors relative to Iberia from Gonzalez-Castillo et al. (2015); Galindo-Zaldivar et al. (2015) and Echeverria et al. (2013). Arrow length is proportional to velocity. Their pattern mirrors the pattern of the middle Miocene BMZ vectors. NE-SW stretching is also supported by upper mantle elongation measurements (not shown here) from shear wave splitting data (Díaz et al., 2010). Inset map (top left) outlines the wider geodynamic scheme, with the back-arc wedge that comprises the Betic zone, formed behind the Gibraltar arc retreating towards the SW. The subduction rollback is superimposed upon a strongly arcuate orogen formed between the North African and Iberian forelands.

Gibraltar arc (Fig. 1). The latter may be taken as the present day slab retreat velocity. This is consistent with a homogeneous but slow extensional strain rate everywhere in the stretching wedge of ca 2 × 10−17 s-1 (Rutter and Valetti, 2018). The stretching wedge is bounded on its south-eastern side by the trans-Alborán shear zone, a tract of linked fault segments, onshore comprising the Carboneras, Palomares and Alhama de Murcia faults (Fig. 1). Rutter et al. (2012; 2013) proposed that these constitute the onshore section of a stretching transform system, which accommodates a velocity discontinuity separating a more rapidly deforming lithospheric segment from one that is deforming less rapidly or not at all. Whilst such a fault may be seismogenic, it more obviously separates a more seismogenic region from one that is less seismogenic (Rutter et al., 2013). Rutter and Valetti (2018) proposed that other, similar representatives of this style of deformation are to be found in the Calabrian and Hellenic-Anatolian arcs. They are all of similar ages. The Trans-Alborán shear zone developed from 13 Ma onwards (Rutter et al., 2012, 2013), cutting obliquely across the then-active volcanic arc. Magmatic dykes ascending to the surface via the Carboneras fault zone (Fig. 2) bring with them deep crustal and possibly upper mantle-derived xenoliths of mafic and ultramafic rocks, and testify to the contemporaneity of fault displacements and volcanic activity. North of the Carboneras fault, no volcanic rocks of the Cabo de Gata type are preserved onshore, and any that were deposited have now been removed by erosion or transported by fault movements to the SW so that they now subcrop beneath younger Messinian and Pliocene cover rocks.

After Alpine deformation and metamorphism, the basement rocks show evidence of tectonically-assisted rapid exhumation and cooling during lower and middle Miocene time, involving the formation of extensional nappes displaying stepwise decreases in metamorphic grade upwards (Platt et al., 2005; Platt and Vissers, 1989; Jabaloy et al., 1993; Lonergan and Platt, 1995; Platt and Whitehouse, 1999). The lowermost basement unit is the Nevado-Filábride complex, dominated by mica schists, marbles, amphibolites and metagranitoids, overlain by an extensive tectonic sheet of phyllites and dolomitic carbonates, the Alpujárride complex (Jabaloy et al., 1993; Vissers, 2012). An even higher extensional nappe unit, the non-metamorphic Maláguide complex (Lonergan and Platt, 1995), is poorly represented in the south-eastern part of the stretching wedge. Stretching of the lithosphere has led to the formation of uplifted metamorphic core complexes, with intervening intermontane sedimentary basins (Augier et al., 2005; Martínez-Martínez, 2006; Platzman and Platt, 2004). Progressive uplift and exposure of basement rocks through the Miocene has been studied by means of radiometric dating (e.g. Johnson et al., 1997). Adjacent to uplifted basement blocks, faultbounded extensional basins have formed whose sedimentary fill is dominated by rocks of Serravallian through Plio-Quaternary ages (Fig. 1), for example the Granada, Vera, Alpujárras, Almería, Albox, and Fortuna basins (Weijermars, 1991; García-Dueñas et al., 1992; Galindo‐Zaldívar et al., 1999; 2003; Booth-Rea et al., 2004a, b; Martínez-Martínez et al., 2006; Vissers, 2012; Giaconia et al., 2014). Some of the extensional fault systems appear to be linked laterally by contemporaneous strike-slip faults (Martínez-Díaz and Hernández2

Fig. 2. Geological map showing the disposition of the main outcrop of the Sorbas basin fill of Neogene sediments in relation to the underlying basement rocks of the Sierras Alhamilla, Polopos and Cabrera, lying to the south and east, and the Sierra de Los Filábres lying to the north. To the south-east the Carboneras left-lateral transform fault outcrops, separating the EN E-W SW stretching rocks to the north-west from the volcanic rocks of the Cabo de Gata series to the south-east. The lower (Serravallian-Tortonian) rocks of the basin fill are substantially tilted and deformed and make unconformable and faulted contacts with the metamorphic basement rocks. Outliers to the southeast of the lower parts of this succession are highlighted. Unconformably-overlying Messinian and Pliocene rocks overstep onto basement rocks, forming a gently flexed antiform/synform pair. Section line L-M-N-P-Q is shown in Fig. 7, and section lines S-R, T-U and V-W are shown in Fig. 8. Arrows in the central part of the basin show sediment transport lines and senses inferred from turbidite basal lineations and clast types to be dominantly from Sierra de Los Filábres to the NE. BMZ = Betic Movement Zone, the regional detachment fault at the base of the Alpujárride low-grade metamorphic sequence.

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Enrile, 2004), although care has to be taken to distinguish true linking strike-slip faults from initially dip-slip extensional faults that may have been rotated to the vertical as a result of folding and/or uplift of the basement blocks. The Neogene basins usually display some combination of unconformable or faulted contacts with uplifted blocks of metamorphic basement. Almost everywhere there is a marked unconformity at the base of the Messinian (about 7 Ma BP), testifying to the most dramatic effects of extension-related deformation on supracrustal rocks occurring shortly before that time. The Sorbas basin is such a Neogene extensional basin (Montenat and Ott d’Estevou, 1990; Montenat et al., 1990; Haughton, 1994; Poisson et al., 1999; Haughton, 2000, 2001; Giaconia et al., 2011, 2013; 2014; Andrić et al., 2018). It lies immediately to the north of the Carboneras fault zone (Fig. 1), hence is well-placed to permit observation of the geometric effects of stretching deformation adjacent to the bounding strike–slip fault system, and of differential uplifts of the blocks of metamorphic basement rocks. This is the focus of the present study.

geometry is provided through measurements of bedding dip and indicators of sediment transport line. Our interpretation of the geometry of the rocks is constrained by geological mapping and over 800 such measurements, which revealed considerable complexity in the geometry of the sedimentary fill. These geometric complexities impact upon the construction of representative stratigraphic columns. A key issue was to determine the shape and depth of the basin floor. This was approached by means of a gravity survey plus modelling to determine the basin floor geometry. The rocks of the lower part (pre-Messinian) of the basin fill were deformed within 5 Ma of their deposition, probably at burial depths of no more than 2.5–3 km and fluid-saturated. These were porous clastic sedimentary rocks in which the deformation behaviour would have been strongly dependent on their porosities, according to the laws of critical state mechanics (Rutter and Glover, 2012; Wong and Baud, 2012). The present-day mineralogy, porosities and anisotropies of acoustic wave velocities were determined on a suite of samples to try to understand the development of the displacement field during deformation. Specific field localities are given throughout as grid references (GR) relative to the WGS 84 geoid, UTM Zone 30 N.

2. The Sorbas basin Fig. 2 shows the arrangement of the present-day Sorbas basin in relation to the adjacent basement uplifts of the Sierras Cabrera, Polopos, Alhamilla and Los Filábres, and the distribution of the main lithological groups. The presently exposed lower part of the sedimentary sequence of the Sorbas basin has been deformed as a result of the lateTortonian uplift of the Sierra de Polopos (lying between the Sierra Cabrera to the east and Sierra Alhamilla to the west). It is clear from the 1:50,000 regional geological map (Garcia Monzón et al., 1974) that the uplift was finished largely before the deposition of the unconformablyoverlying Messinian and younger rocks, which are now gently arched over the S. de Polopos and onlap unconformably against the basement rocks of the Sierra de los Filábres to the north (Fig. 2). Pliocene rocks of the region display a similar but more subdued structure, and the recent rejuvenation of the river systems of the region testifies to continuing uplift (Booth-Rea et al., 2004a). Most published work dealing with the stratigraphic successions in the Sorbas and adjacent basins (Figs. 1 and 2) has focused on the poststretching Messinian system (e.g. Van der Poel, 1992; Martin and Braga, 1994; Fortuin and Krijgsman, 2003; Braga et al., 2003; Clauzon et al., 2015), on account of its important evaporite sequence (Yesares member) recording the Messinian dessication of the Mediterranean basin, and the record of astronomical cycles in the immediately underlying Abad marls (Sierro et al., 2001). The unconformity at the base of the Messinian sequence is represented by the Azagador member, which is diachronous between (6.0 and 7.2 Ma, uppermost Tortonian and lowermost Messinian, Martin and Braga, 1994; Berggren et al., 1995). Its sedimentary facies is highly variable, represented by high energy littoral deposits south of the S. de Polopos, reefal carbonates in the north-west and high energy littoral and fluvial deposits to the north of the S. Cabrera. High-quality microfossil-based dating has been recorded from the Messinian rocks (Ott d’Estevou et al., 1990; Van der Poel, 1992; Martin and Braga, 1994; Poisson et al., 1999; Sierro et al., 2001; Fortuin and Krijgsman, 2003; Clauzon et al., 2015), but the microfossils constraining the ages of the older sediments are rather more restricted and can suffer from poor preservation.

4. Results 4.1. Gravity survey Gravity data were collected using a LaCoste Romberg model G series gravimeter. A Trimble Pro-6H differential global positioning system (DGPS) was used to record latitude, longitude and height. Post-processing using Trimble Pathfinder software allowed station heights to be obtained to within 50 cm. A total of 248 data stations were measured on four main traverses (Fig. 3), at approximately 500–1000 m spacing. A locality 5 km east of Sorbas, at the Urra Field Centre (GR 580318.9, 410519.7) was used as a base-station. Instrumental Drift (inclusive of Earth tides), Free-Air, Latitude and Bouguer Slab corrections were performed manually each day upon completion of surveying. Some initial data reduction was carried out in the field, to locate areas where additional data stations could be used to reduce interpolation uncertainties in areas of high field gradients. Data processing followed the same procedure as in the study of Do Couto et al. (2014), using the same value for the local Moho dip gradient from the regional gravity study of Torne et al. (2000), which is significant in this region due to crustal thickness variations. The resulting average total error (mGal) applicable to the residual anomaly values is: ± 0.010 mGal (StD = 0.099 mGal). Gravity inversion and geological interpretation of the basin's likely subsurface arrangement, using surface geology to help constrain interpretation, were carried out with the help of the PyGMI software (Geological Survey of South Africa; http://patrick-cole.github.io/ pygmi/index.html). Representative bulk density measurements were required to represent the main rock types. These were made for the sedimentary rocks of the basin fill by drilling 25 mm diameter cores, oven drying at 60 °C, then weighing and measuring dimensions. For the basement rock types we used data collected for a previous study (Taylor et al., 2015). Porosity of all samples was measured using helium pycnometry. All these data for the sedimentary rocks are shown in the section on petrophysical properties and in Table 1. In the buried state the pore spaces of all rock types will be saturated with water. In the case of basement rocks porosity is usually sufficiently small (< 2%) that the difference between dry and water-saturated density is insignificant for the purpose of gravity modelling. On the other hand, some of the basin-fill sediments (e.g. turbiditic sandstones) may have porosities between 10 and 20%, such that water saturation significantly affects density, and hence calculated gravity profiles. All densities measured here were therefore recalculated to include the effect of water saturation. Additionally, experimental compaction with

3. Methods employed Because the pre-Messinian part of the Neogene basin fill records the effects of the main part of the extensional deformation that led to the formation of this and other basins, together with the effects of the uplift of the basement blocks, we have focused on these rocks. Their outcrop extent is severely limited by partial burial beneath the Messinian and younger rocks. Because the greater part of the sedimentary succession is dominated by a monotonous sequence of turbiditic sandstones with interbedded limestones and marls, the main constraint on the internal 4

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Fig. 3. Contour map of the floor of the Sorbas basin from modelling of the results of the gravity survey (tabulated in supplementary data file). Contour heights are with respect to present-day sea-level and the distribution of measuring stations is shown. Deeper levels are shaded more lightly between contours. The E-W trending, westerly-deepening topographic trough corresponds to the location of the Serravallian and Tortonian (older) sedimentary basin fill. Thick lines show the northern and southern limits of the outcrop of the post-orogenic sediments, including the Messinian and younger rocks (Fig. 2), which partially bury the older sediments.

1860 kg/m3 model cell size: x and y =1500 m; z (thickness) =100 m The model parameters proved to be an acceptable compromise between precision and noise. The gravimetric model was sliced in 14 vertical cross-sections oriented N-S and 12 oriented E-W. These cross sections where used to fit each modelled gravimetric curve to the measured one by interpreting blocks of geology in the section areas following a simple trial and error method, but using the constraints provided by the surface geology. The grid of 26 intersecting cross-sections produces a 3D geological model which was interpreted from the modelled gravimetry, and used to help constrain the geological cross sections presented later. The surface representing the shape of the basin floor (basement/cover contact) was obtained by extracting the z values of the basement/basinfill interface and replotting the surface (Fig. 3). The basin floor map is largely comparable with that produced by Do Couto et al. (2014) although there are inevitably some differences. The basin floor forms an E-W elongate trough, plunging to the west, and attaining a depth of 1.7 km below sea level. The present day land surface is about 400 m above sea level. The eastern and south-eastern parts of the basin form a near-horizontal platform at a depth of approximately 400 m below sea level (about 800 m below present day land surface). The northern limit of the Sorbas basin fill is defined by the onlap of

rising confining pressure was carried out to determine whether pressures at the bottom of the basin would be likely to provoke further permanent compaction of the more porous sediments. This is a test that is not normally carried out on porous samples encountered in basin fills for which gravity modelling is attempted. However, it was found that other than the effects of elastic compaction strains, basin pressures would be unlikely to produce enhanced permanent compaction involving grain fracturing, therefore such potential effects could be neglected. The petrophysical parameters used for modelling were the following: - -regional gravity base value of 3466.00 mgal (from European Space Agency global database) - -lithological density values (kg/m3, porosity by helium pycnometry, corrected for water saturation)

m3

“Background” (basement rocks): 2727 kg/m3 “Basin_Low” (Miocene basin sediments at > 2 km burial): 2494 kg/

“Basin_High” (pre-Messinian basin sediments < 2 km burial): 2465 kg/m3 “Messinian_Low” (Messinian sediments ex-gypsum): 2140 kg/m3 “Messinian_High” (Messinian Yesares Fm, dominantly gypsum): 5

bed dip

18 20 23 45 28 29 38 32 26 26 68 82 89 80 68 18 59

Specimen. #

R1 R3 R4 R9 R10 E1 E2 E3 E4 E5 E7 E8 E9 E10 E11 E12 E13

44 355 4 68 16 37 45 62 22 91 22 68 58 69 65 110 80

bed strike

589470 586064 584256 583971 584139 581974 581589 583886 584977 573745 574069 574854 574934 575731 577092 570793 581444

Easting

Location

4103362 4101329 4101887 4103915 4100993 4101025 4101660 4100972 4101042 4100615 4100420 4100512 4100229 4099799 4100954 4100764 4100986

Northing 0.13 0.25 0.08 0.35 0.17 0.10 0.15 0.08 0.05 0.04 0.03 0.15 0.18 0.08 0.27 0.07 0.30

Vpeak km s-1

4.25 3.39 4.21 3.96 3.14 3.54 4.74 2.75 2.15 5.44 5.12 4.43 4.54 4.54 3.09 3.31 2.40

Vmean km s-1

138 65 180 138 115 48 20 165 15 40 95 170 64 95 180 175 175

Phase angle,

48 −25 90 48 25 −42 −70 75 −75 −50 5 80 −26 5 90 85 85

Angle strike to Vmin 3.67 2.86 3.67 3.47 2.40 2.61 3.97 1.93 1.74 5.53 4.38 4.18 4.36 4.01 1.97 3.38 1.87

Vel normal bedding Vn

223 328 272 296 262 354 333 316 306 40 209 326 30 255 336 14 318

Vmin Azimuth restored 6.12 14.8 3.8 17.7 9.5 5.4 3.4 3.8 13.9 1.3 1.2 6.8 7.9 4.5 17.4 4.2 24.9

Bedding parallel Anisotropy% 4.06 3.21 4.03 3.80 2.89 3.23 4.48 2.48 2.01 5.47 4.87 4.35 4.48 4.36 2.72 3.33 2.22

Overall mean Velocity km s-1 (Vmean 3D) −13.61 −15.52 −12.71 −12.46 −23.52 −26.16 −16.28 −29.97 −19.03 1.61 −14.39 −5.55 −3.90 −11.77 −36.36 2.01 −22.25

Axial anisotropy %

8.68 8.91 7.99 9.22 10.67 11.78 5.63 16.62 20.17 2.25 6.79 5.89 5.51 3.3 20.67 25.99 18.3

He porosity %

2.480 2.490 2.520 2.420 2.410 2.400 2.554 2.311 2.206 2.569 2.540 2.558 2.554 2.644 2.070 2.009 2.216

Bulk density g cm-3

259 207 – 252 – – 241 254 198 231 – – – 268 – 222 –

Sed lineation azimuth restored

14.2 22.8 22.7 26.8 24.2 24.4 32.9 18.2 20.3 limestone limestone limestone limestone 31.4 30.7 limestone 37.9

pre-cementation porosity %

Table 1 Petrophysical properties of specimens measured. Bedding strike azimuth is given such that dip direction is 90º anticlockwise from strike direction. Acoustic P-wave directions (θ ) count + ve clockwise from strike in plane of bedding, -ve anticlockwise. Vmin and Vmean are given as km s-1 from the sinusoid fit to the data, V = Vpk sin(θ + φ) – Vmin in the plane of bedding, with strike azimuth as θ = 0, Vpk = peak amplitude, φ = phase offset angle, Vm = mean velocity in plane of bedding. Vn = velocity normal to bedding. Peak amplitude Vpk is half of peak-to-peak amplitude. Velocity measurement resolution is 0.04 km s-1, and angular resolution of wave direction is ± 5º. Anisotropy in plane of bedding (from sinusoid parameters) given as 2*100%*(Vpk − Vmin)/(Vn + 2Vmean)/3. Porosities are ± 0.1%. Transverse (axial) anisotropy is given as 100%* (Vn − Vmean3d)/ Vmean3d, where Vmean3d = (Vn + 2Vmean)/3. Vmin azimuth restored is azimuth with respect to north after rotation of bed to horizontal about its strike. Sedimentary lineation azimuth is after rotation of bedding to horizontal about its strike. Pre-cementation porosity (for siliciclastic rocks) is measured porosity less that attributable to the volume of sparry calcite cement. All specimens are right way up.

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Fig. 4. Geological map showing the strike and dip trends of the sediments forming the lower, Serravallian/Tortonian fill of the main outcrop area of the Sorbas basin. This area is contoured for the magnitude of dip, showing the occurrence of a steeply north-west dipping tract passing through Mt. Cantona. Steeply dipping beds also swing into parallelism with the uplifted basement block of the Sierras Alhamilla and Polopos to the south. With minor exceptions, beds consistently young towards the northwest over the whole outcrop area. The outcrop of the unconformity separating the older part of the basin fill from the overlying Messinian/Pliocene sequence is shown. Equal area, lower hemisphere projections of poles to bedding attitudes and their eigenvectors (triangles) for subareas in the older sequence lying west and east respectively of Mt. Cantona are presented. The rightmost stereoplot shows data for the gently-dipping rocks in the subarea indicated where there is gentle folding about a NW plunging axis. Extensional fault contacts with the S. Cabrera basement to the east are shown, and also the Gafarillos fault system that accommodates differences in the amount of vertical uplift between the Sierras Cabrera and Polopos.

rocks of the Messinian and younger sequence onto the basement rocks of the S. de los Filábres (Fig. 2), having overstepped the more steeply dipping pre-Messinian sediments lying to the south. The position of the subcrop of the base of the pre-Messinian rocks against the base of the Messinian series is tightly constrained by the distribution of units on the regional geological map (Garcia Monzón et al., 1974) to lie on an E-W trending line that also corresponds closely to the northern edge of the topographic trough in the basin floor (Fig. 2).

4.3. The outcrop area and ages of the pre-Messinian basin-fill rocks The main area of outcrop of the pre-Messinian basin-fill rocks is shown in Figs. 2 and 4. These comprise boulder beds and brecciaconglomerates both at the base and at the presently highest-exposed parts of the succession, plus well-bedded sandstones and siltstones, and limestones and marls particularly in the higher exposed parts of the succession (north of Lucainena de las Torres, Fig. 2, Kleverlaan, 1989). Throughout the more than 2.5 km thickness (Fig. 5; measured normal to bedding), are repetitive sedimentary structures and lithologies characteristic of siliciclastic turbidites and mass-flow deposition. Individual beds up to 1 m thickness are often graded, with cross-laminated tops giving an unequivocal upward younging sense. Fig. 4 shows that except in a few localities affected by folding, the sense of younging everywhere is consistently towards the north-west. Mesoscale cross-bedded units and imbrication of boulders indicate a general sense of transport from north-east to south-west (Figs. 2 and 6), although further west, in the region currently termed the Tabernas basin (Fig. 1), there is evidence of transport from north-west towards south-east (Haughton, 1994, 2000; 2001; Montenat and Ott d’Estevou, 1990; Ott d’Estevou et al., 1990). Individual bedded units of constant thickness and groups can often be traced laterally for several kilometres and are easily seen on Google Earth imagery. This large thickness of rocks, commonly grouped as the Chozas Formation (Ruegg, 1964), is also known as the Grey Upper Clastics of Poisson et al. (1999) or the Tortonian II of Montenat and Ott d’Estevou (1990). Fossil content indicates marine deposition in water depths up to several hundreds of metres (Ott d’Estevou et al., 1990; Poisson et al., 1999). These authors describe fossil assemblages indicative of upper Tortonian deposition with some extension into the Messinian, although with inclusion of reworked fossils of Serravallian age. Immediately underlying the rocks of the Chozas Fm, sediments comprise extensive sheets, up to > 100 m thickness, of red conglomerates and boulder beds (the Red Lower Clastics of Poisson et al., 1999,

4.2. Geological mapping and lithologic characterization Geological mapping was carried out at scales of 1:10,000 and 1:25000, focusing on the geometry of the lower (pre-Messinian) part of the basin fill. Fig. 4 highlights the main area of outcrop of the lower part of the basin fill (up to the base of the Messinian rocks). However, before uplift of the Sierras Alhamilla, Polopos and Cabrera, erosion and then deposition of the Messinian units, the rocks of this older series initially covered a much wider area than the present main outcrop, extending southwards into what is now termed the Nijar basin (Serrano and Gonzáles Donoso, 1989; Montenat et al., 1990; Ott d’Estevou et al., 1990; Serrano, 1990; Rutter et al., 2012; 2013), eastwards onto the S. Cabrera, and south-eastwards onto basement rocks lying immediately north of the Carboneras fault zone. This greater extent is indicated by rare outcrops of these rocks either in unconformable or faulted contact with basement rocks (Fig. 2). Following uplift of the basement ranges lying to the south of the S. de Los Filábres during late Tortonian time, a large fraction of the original content of this basin has been eroded away, in a time period on the order of 1 million years, prior to 7 Ma BP. The present-day geological map of the ‘Sorbas Basin’ gives a very limited view of the shape and extent of the pre-Messinian basin, and deformation of the rocks has caused severe reorientations of bedding and fault surfaces originally involved in producing the geometry of and sedimentation within the basin. This issue was recognized by Poisson et al. (1999). 7

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of rocks of the Chozas Fm, forms the northern flank of the S. Cabrera, in reverse fault contact with the Alpujárride basement rocks (Fig. 2). The lowest sedimentary rocks, lying unconformably upon Alpujárride and Maláguide basement rocks along the southern edge of the present-day Sorbas basin fill (between Cuevas del Pájaro in the east and Lucainena de las Torres in the west) comprise thin (∼100 m) and discontinuous outcrops of a well-cemented breccia-conglomerate consisting of fragments of the immediately underlying Alpujárride and Maláguide basement rocks (dolomitic carbonates, red siltstones and phyllitic schists). Clasts from the same source are also found in the younger Red Lower Clastics and the Chozas formation, but the latter units become dominated by clasts derived from the basement rocks of the S. de Los Filábres to the north. Until the deposition of the Messinian Azágador member, there is no significant clastic input to the Serravallian/Tortonian basin fill from the Nevado- Filábride basement rocks (graphitic mica schists) that characterize the cores of the S. Alhamilla, S. de Polopos and S. Cabrera. These older sediments always lie on a substrate of rocks of the Alpujárride or Maláguide units (Garcia Monzón et al., 1974). Consequently, we deduce that the basement rocks forming the floor of the basin everywhere must be Alpujárride and/or Maláguide rocks, because the underlying Nevado-Filábride basement will never have been exposed during sedimentation. These oldest sedimentary rocks of the basin fill are steeply dipping and interlayered with yellow sands and marls that have yielded ‘unambiguously Serravallian’ marine microfossil assemblages (13.8–11.6 Ma, Ott d’Estevou et al., 1990). The contact with the basement rocks is either unconformable or is faulted, with horizontal slickenlines when the bedding (and fault surfaces) are vertical (e.g. GR 587066, 4101174). The lower part of the sedimentary fill of the present-day Sorbas basin around Gafarillos (Fig. 2) is connected to the northern edge of the Carboneras Fault zone via a string of small outcrops on the south wall of the Gafarillos fault. From the largest outcrop within this tract (Cerro Riscos Negros, GR 591005, 4100098) Montenat and Ott D’Estevou (1990) reported a Serravallian age from microfossil evidence. In the Carboneras fault zone (around El Saltador (GR 595362, 4098749, Fig. 2) these rocks are called the Saltador Fm, lie in unconformable contact with underlying basement of the Alpujárride unit, are preserved from erosion in the cores of tight folds with a gentle westerly plunge, and also as fault-bounded blocks (Rutter et al., 2012, 2013). These have been determined to be of Serravallian or lowermost Tortonian age because they are cut by 10.6 Ma radiometrically-dated andesitic dykes injected via the Carboneras fault zone (Rutter et al., 2012, 2013). From microfossil evidence, Montenat et al. (1990) reported a Langhian age based only on an identification of Praeorbulina

Fig. 5. Outline composite stratigraphic succession for the Sorbas basin along section line L-M-N-P-Q on Fig. 2, to emphasise relative thicknesses and main rock types. The thickness of pre-Messinian deposits measured normal to bedding is not the same everywhere.

also described as Tortonian I by Ott d’Estevou et al., 1990). Clast content indicates derivation from the Sierra de Los Filábres. Tortonian I deposits were inferred to be of continental origin by Ott d’Estevou et al. (1990) and tentatively ascribed to a lowermost Tortonian age (10 to 1 1Ma BP). On the western flank of the S. Cabrera, they are found in extensional faulted contact with basement rocks of the Alpujárride tectonic unit. An extensive outcrop of this unit, overlain by a thin tract

Fig. 6. (a) Looking north-east. NW-dipping graded sandstones and conglomerate of the Chozas Fm cutting coarsely cross-bedded sands with foresets dipping westwards. Image is approximately 2 m wide. Location GR 583944, 4100950. (b) Optical thi N-S ection (crossed polars) of siliciclastic microbreccia of the Chozas Fm, consisting of metamorphic rock fragments cemented with sparry calcite (examples highlighted with letter ‘c’), untwinned (undeformed), infilling spaces between grains and often enclosing several grains with a single crystal. (c) Optical thin section (plane polarized light), same rock type as in (b), treated with Alizarin Red dye to highlight sparry calcite (examples indicated with ‘c’) infilling cracks and fragmented grains. The calcite cement formed after compaction and deformation of the granular matrix, when the rock was more porous.

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sp. from the singular occurrence of basin-fill sediments that lie with unconformity on graphitic mica schists of the Nevado-Filábride basement, at GR 596714, 4099486. This older age appears inconsistent with local geological evidence, and the fossil occurrences may be reworked. The easternmost occurrence of basin-fill sediment, cut by andesitic intrusive rocks, lies further NE, transported along the northern strand of the Carboneras fault, about 2 km from the sea (GR 600997, 4102781), near El Agua del Medio. South of the Carboneras fault (at GR 599853, 4100848), lying beneath the oldest part of the Cabo de Gata volcanic complex, marly sediments were found of Burdigalian age (Serrano and Gonzáles Donoso, 1989; Serrano, 1990, 1992; Scotney et al., 2000; Rutter et al., 2012) that are not associated with the Sorbas basin, but have been transported by fault movements from some 40 km to the SW.

This illustrates the difference between the shape of the basin floor and the dip variation of the beds within the basin. Clearly, the dips within the basin have been markedly steepened relative to the initial dips at the time of sedimentation. The total sediment thickness measured normal to bedding is substantially greater than the present-day vertical depth of the basin below the present-day land surface. Along the southern margin of the present day basin the upended sediments are usually in unconformable contact with the Alpujárride or Maláguide basement rocks, although in places they have been affected by local fault movements. Thus the sediments onlap onto the basement rocks progressively towards the west. The fact that the swing in strike towards parallelism with the basement takes place close to the basin margin suggests that the sediments thin as they approach their contact with the basement. This in turn implies that during deposition the basement block formed a slowly rising sub-sea-floor sill, over which the sediments spilled, as the S. Alhamilla uplift migrated towards the west. The basin floor would also have to be uplifting, propagating westwards, resulting in the sediment bodies taking the form of prograding clinoforms (Fig. 7), rather like a modern delta. The Messinian erosion surface truncates steeply-dipping beds (Figs. 7 and 8), and there is an implied large amount of material that has been removed by erosion. The continued uplift of the S. Cabrera basement appears to have led to the erosive loss of an extensive coverage by rocks of the Chozas Fm, and the formation of the Messinian erosion surface. Throughout, clasts within the Chozas Fm are dominantly derived from the basement rocks of the S. de Los Filábres. Lithic fragments of tourmaline-bearing granitoid gneiss, garnet mica schist, banded marble, amphibolite and rare ultramafic fragments are distinctive. Although easiest to recognize in the coarse, breccia-conglomerate members, the lithic clasts matrix of even the finer grained rocks display the same mix. In thin section sand-sized grains are never seen to be microfractured; larger lithic clasts sometimes show impingement fractures, and mica grains can be buckle folded. The sedimentary transport line is sometimes revealed as erosional grooves and load casts on the base of beds. These are less common than one would like but Fig. 2 shows the orientation of such linear features measured across the east-west extent of the Chozas Fm. outcrop. Observations were made on beds of widely varying dip magnitude, therefore for comparison data were rotated about the present-day bedding-strike until the bedding became horizontal. The transport line is dominantly oriented NE-SW. Any rotations of bedding strike that may have occurred relative to present day orientations were neglected, but it seems likely that the folding of the pre-Messinian sediments to form a synformal structure as a result of end-Tortonian north-south shortening and/or basement uplift may have caused perhaps 10º or more of clockwise rotation from an original NE-SW orientation. The sense of sediment transport is inferred to be from the NE towards SW, based on the provenance of clasts from the S de Los Filábres. Giaconia et al. (2011; 2013; 2014) were first to recognize the existence of basin-bounding extensional fault systems on the western

4.4. Lithologies and structures of the rocks of the Chozas Fm The stratigraphic succession cannot be determined without first considering the structure of the rocks from sedimentary dip data and the overall thickness of the basin fill as determined from the gravity survey. Fig. 4 shows the arrangement of strike lines in the main outcrop of the pre-Messinian series and the directions of stratigraphic younging. The map is contoured to show variations in dip magnitude. In the vicinity of Mizala (GR 583357, 4100696) the beds dominantly dip gently towards the NW. Further east they undulate to form gently north- or northeast-plunging open folds. West of Mizala the dips markedly increase towards the vertical to form a 3 km-wide band centred on M. Cantona (GR 578292, 4100942), whilst continuing to dip towards the NW, so that the strike is substantially oblique to the southern boundary of the present-day basin. Approaching Lucainena de las Torres (GR 571044, 4099677) from the east, the dip angles decrease, with the strike swinging clockwise towards E-W. Along the southern margin of the present-day basin the NE-SW strike swings around towards an E-W orientation and the dips steepen to the vertical or are locally overturned. The latter change in attitude is inferred to be due to some combination of localized N-S regional shortening and/or bend folding associated with the uplift of the S. de Polopos and S. Alhamilla. Inset stereoplots on Fig. 4 illustrate the local variations in the shape of the sediment body in various places. Bearing in mind that the basin floor takes the form of an E-W trending trough with the axis passing through the latitude of Sorbas town, there is likely to be a mirror image of the above strike pattern subcropping on the base of the Messinian rocks to the north of Sorbas. The formation of the Cantona steep belt also overlies the onset of deepening of the basin floor as revealed by the gravity data, from a shelf about 700 m deep in the east to a depth of 2.5 km in the west (both relative to the present-day land surface). However, the basin floor topography is much subdued relative to the shape of the monoclinal flexure represented by the Cantona steep belt. Fig. 7 shows a cross section in two NW-SE-trending parts, drawn normal to strike of beds (section line on Fig. 2) and juxtaposed in-line to illustrate schematically the shape of the M. Cantona monoclinal fold.

Fig. 7. Composite section approximately along the length of the trough that forms the Sorbas basin with segments L-M and N-P (Fig. 2) projected onto a common NW-SE oriented line that is near normal to dominant strike orientation. No vertical exaggeration. This gives an indication of the shape of the sediment body viewed along strike and implies that a large proportion has been eroded away prior to Messinian deposition. The basin is inferred to be floored everywhere by Alpujárride basement rocks. Opposed arrows indicate senses of relative layer-parallel extension or compression from petrophysical measurements.

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Fig. 8. Approximately north-south cross sections illustrating relationships between Sorbas basin-fill rocks and underlying basement blocks (no vertical exaggeration). Locations of lines on Fig. 2. (a) Section V-W shows the deepest part of the Sorbas basin fill in the study area, subcrop of the base of the basin fill against the underside of the Messinian/Pliocene sediments, and the uplift of the S. Alhamilla basement block against the southern edge of the Serravallian/Tortonian basin fill, with concomitant bend-folding of the sediments. (b) Section R-S shows a cut through the synformally-shaped Gacia Alto/Las Sepulturas extensional basin-bounding fault that affects the older part of the basin-fill sequence. The transport line is at a high angle to the plane of section. Also shown is the dip-slip nature of the Gafarillos fault and the gentle antiform/synform pair formed by the overlying Messinian deposits. (c) Section T-U, taken where the uplift of Sierra Cabrera basement is greatest, thus Neogene rocks are only seen in the downfaulted blocks to north and south. Relatively undistorted rocks of the Sierra Cabrera suggest a near rigid-body uplift, with high-angle reverse faults on either side, producing forced bend-folding of the layered rocks overlying the Nevado-Filábride basement. Fig. 9. Examples of exposed extensional faults at the contact between the western side of the Sierra Cabrera Alpujárride basement rocks and the lower part of the Sorbas basin fill. (a) Looking north, low-angle extensional fault contact between dolomite of the Alpujárride basement (right, with dip-slip striations) against Serravallian rocks of the Lower Red Clastic series (left). Location Las Sepulturas, GR 590332, 4103683. (b) Looking east, moderately-dipping extensional fault contact between dolomitic carbonates of the Alpujárride basement complex (left) and red/yellow conglomerates and siltstones of the Lower Red Clastic series, with dip-slip slickenlines. Location: Gacia Alto, GR 588083, 4104467.

flank of the S. Cabrera, which was progressively uplifted from Tortonian times onwards, tending to create a westerly-dipping submarine paleoslope, also implied by the arrangement of bedding within the basin. The exposed extensional fault systems are highly segmented (Fig. 2), with dip-slip and oblique-slip displacements, but also the pattern is strongly curved, concave towards the basin fill, as might be enhanced in association with the folding of the pre-Messinian sediments into a large synformal structure that is half buried beneath the Messinian and Pliocene rocks. Major dip-slip faults are exposed where conglomerates of the Red Lower clastic series are downfaulted against Alpujárride basement rocks (e.g. at Gacia Alto, GR 588083, 4104467; Las Sepulturas, GR. 590332, 4103683, Fig. 10). Slickenlines on the basement carbonates and asymmetric structures in foliated cataclastic fault-rocks unambiguously show the faulting to be extensional in character. There is often evidence for multiple movements on these surfaces. Progressive uplift of the S. Cabrera is likely to have caused some steepening of the fault planes. Faulting exposed at Cerro Riscos Negros, (GR 591029. 4100140) is sub-horizontal, however. At GR 590332, 4103683 and 590332, 4103683 the exposed faulting only affects the oldest sediments of the basin fill, and the faulting is sealed by unconformably overlying younger turbiditic sands, that

overstep the faults to lie unconformably upon the basement rocks (Figs. 2, 4 and 8). The younger sediments are also likely to have had faulted contacts with the basement further east, but these have been eroded away. It seems likely that these faults merge downwards to reactivate the regional detachment surface that is the Alpujárride/Nevado-Filábride contact, representing the continued activity of those surfaces following uplift into the brittle regime. Giaconia et al. (2014) also recognized extensional faults affecting higher parts of the sedimentary fill of the basin west of Gacia Alto, and Andrić et al. (2018) described mesoscale intrabasinal extensional faults and associated localized sedimentation patterns, with an orientation and slip antithetic to the main basin-bounding faults, for example between Penas Negras (585127, 4100935) and Gafarillos (587182, 4101662) (Fig. 2). This intrabasinal faulting implies that the basin-fill rocks had continued to undergo extension during progressive sedimentation. From their detailed study of relations between sedimentation and localized faulting, Andrić et al. (2018) also demonstrated that local variations of extension direction had occurred during basin filling. The techniques employed in the present study are unable to resolve such subtleties (Fig. 9). The early basin-bounding faults are likely to have been rotated to a vertical attitude along the southern margin of the present-day basin, as a result of the N-S regional shortening displacements and the effects of 10

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the rocks, leaving only intergranular sliding as a primary compaction mechanism. Whilst the siliciclastic rocks do contain a detrital carbonate component (usually lithic dolomite or siderite), it is inferred that most of the calcite present is in the cement phase. Maximum pre-cementation porosities have therefore been estimated by adjusting the present porosity for the calcite fraction (Table 1). Porous sandstones can deform in a pervasively ductile manner involving shear-enhanced compaction, according to the laws of critical state mechanics (Wong et al., 1997; Rutter and Glover 2012; Wong and Baud, 2012). The observed capacity for these rocks to bend, fold and stretch in the Sorbas basin suggests that pre-cementation deformability was dependent on high initial porosity, with grains being able to slide with respect to each other whilst undergoing some compaction. The geometry of the Chozas formation in the Sorbas basin, showing the effects of bend folding in response to the shape of the basin floor, and with the basin bounded to the east by extensional fault systems, might be expected to show the effects of such deformation in the form of the development of seismic anisotropy. 26 mm diameter oriented core samples core samples were drilled from 17 of the above 22 rock specimens normal to the bedding orientation. These cores were variably cohesive, but displayed no mesoscale fracturing. Many cores displayed a visible preferred orientation of no N-S pheroidal grains of mica and of lithic fragments, lying preferentially in the plane of bedding. P-wave velocities under unconfined conditions were measured along the core axis (normal to bedding, Vaxial) and also in radial directions (in the plane of the bedding) at 15º intervals (Table 1). Vaxial was always slowest, as is typical of foliated rocks, but significant anisotropy was also found in the plane of bedding. Examples are shown in Fig. 11. Anisotropy in the bedding plane is defined as (Vmax − minV)/Vmean, where Vmean is the average velocity in the plane of bedding. Axial anisotropy is defined as (Vaxial − Vmean)/ Vmean. All velocities decrease markedly with increasing porosity (Fig. 11). In the plane of bedding, samples were variably anisotropic, ranging from 1% up to 25%. Axial (or transverse) anisotropy ranges between zero and 36%, and both tend to decrease markedly with increasing mean velocity. Thus less porous, better cemented and hence stiffer rocks tend to be less anisotropic in all respects. In the bedding plane, the Vmin direction is the direction of least stiffness and is expected to be the direction of greatest bedding-parallel stretching deformation (separation of grains). These orientations, after rotation of the bedding to the horizontal about the local strike direction, are shown in Fig. 4. Except for the westernmost samples, most of the data tend to be oriented at a high angle to the bedding strike, in contrast to the sedimentary transport lineations, that tend to be oriented along strike. As with the sedimentary transport lineations, there has probably been some degree of clockwise rotation associated with the formation of the southern limb of the large-scale synformal structure of the pre-Messinian beds. A resulting original WNW-ESE orientation of the stretching direction would be consistent with the orientation of basin-bounding fault slip orientations coupled with outerarc, layer-parallel stretching during the formation of the Cantona monocline (Fig. 7). The orthogonal orientations of the three westernmost data correspond to the synformal arm of the Cantona monocline, where they may have been subjected to layer-parallel compression.

Fig. 10. Schematic block diagram to illustrate the geometry of the Gafarillos fault system, forming the south-eastern margin of the present main outcrop of the rocks of the Sorbas basin. From east to west the fault changes from one accommodating uplift of the S. Cabrera, to a high-angle reverse fault accommodating uplift of the S. de Polopos and, further west, the S. Alhamilla. Basinal sediments and their basal extensional faults are rotated to the vertical against the Gafarillos fault, locally giving the impression of strike-slip faulting.

basement uplifts that have distorted the basin fill into a syncline (Fig. 8), produced intrabasinal folds and caused dip-slip thrust faulting of basement rocks over sediments along the present-day basin margin (Giaconia et al., 2014). The resultant vertical, E-W striking faults with horizontal slickenlines have led many authors to infer a substantial component of strike-slip faulting in the evolution of these basins (e.g. at GR 587055, 4101172; 588799, 4100669 and 572948, 4099421), often ignoring the impossibility of significant strike slip faults simply dying out without an accommodation mechanism at the ends. However, immediately adjacent to these places, clay-bearing foliated fault gouges in basement rocks tend to record superimposed dip-slip movements (e.g. Fig. 2) (Rutter et al., 1986). The shape of the contact between the Alpujárride and NevadoFilábride units in the S. de Polopos and S. Cabrera is clearly displaced by faulting (e.g. the Gafarillos fault), and allows the pattern of at least the vertical components of displacements to be deduced (Fig. 8). Thus the Gafarillos fault is inferred to be a dip-slip fault, accommodating uplift (decreasing westwards to zero close to Gafarillos) of the NevadoFilábride basement of the S. Cabrera to the north against the S. de Polopos basement to the south. West of Gafarillos, the same fault becomes a reverse fault owing to the relatively increasing amount of uplift of the S. de Polopos, pushing folded and upended Serravallian rocks northwards over the more gently dipping turbidites of the Chozas Fm (the North Gafarillos fault of Giaconia et al., 2014, Figs. 2, 4 and 10). 4.5. Petrophysical properties of the rocks of the pre-Messinian basin fill 22 samples of Chozas Fm rocks from its entire outcrop area and incorporating a wide range of clast sizes were analysed by X-ray diffraction and the results quantified by Rietveld analysis (Table 1 and Fig. 11) in order to obtain modal proportions of constituent minerals. The proportions of the siliciclastic components are quite tightly clustered, as would be expected for a common source. Bearing in mind the poor particle size sorting of these rocks, their porosities (now typically lying between 7 and 26 %) are inferred to have been reduced from their values at deposition. From thin section study this has been inferred to be due mainly to the formation of a sparry calcite cement, which was always seen to be clear and undeformed (no mechanical twinning), infilling cracks within fractured grains and re-entrants between grains (Fig. 6b and c). There was little or no evidence of porosity reduction by pressure solution, nor was there significant compaction involving grainscale fracturing, nor evidence of shear or compaction bands forming in

5. Discussion 5.1. The shape and evolution of the Sorbas basin through deformation of the Tortonia N-S erravallian sediments Figs. 7 and 8 show the distinctive shape of the basin fill in cross section. The oldest basin-boundary faults on the flank of the S. Cabrera displace Serravallian continental clastic sediments downwards to the west against the basement, originally potentially near sea level. Younger marine turbiditic sands of the L. Tortonian, that 11

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Fig. 11. Composition and physical properties of Tortonian rocks of the Sorbas basin. (a) Mineralogical composition of siliciclastic samples (Table 1) after removal of the calcite fraction that occurs dominantly as sparry calcite cement. (b) Examples of P-wave velocity variation in the plane of bedding in two samples (Table 1). Positive angles are clockwise from strike when viewed from above. (c) Reduction of mean P-velocity with increasing porosity (reducing stiffness). (d) Slower velocity (higher porosity) results in more velocity anisotropy, both within bedding plane and comparing bedding-normal velocity (always slowest overall) with mean velocity in bedding plane (transverse isotropy).

0.44 ± 0.02 (corresponding to a friction angle ψ of 24°), but ranging down to below 0.3 (ψ = 16°), compared to the average for a wide range of sandstones of 0.73 (Rutter and Hackston, 2017). The relative frictional weakness of the gouges is attributed to formation of oriented clay and mica surfaces, on which fine slickenlines and polishing were always observed to form (c.f. Schleicher et al., 2006; Jiménez-Millán et al., 2015). The graphite content of these gouges is always too low (< 1 wt %) to influence frictional behaviour and the pre-Messinian rocks within the basin do not contain significant amounts of evaporite material. Smectitic clay content in the basinal sediments is less than 1 wt% and therefore unlikely to affect mechanical behaviour (Morrow et al., 2017). The present–day average slope angle into the trough of the basin is 9º, but locally ranging up to 25º where low-angle faults are inferred to cut across the rocks of the Alpujárride basement. Skempton et al. (1989) measured the friction angle in sheared claybearing siliceous mudstone from the base of the Mam Tor landslide to be 14º. This permits creeping failure along the base of the landslide when the slope, with an average dip of 12º, becomes almost fully saturated with water following prolonged winter rainfall (Rutter and Green, 2013). The basal shear zone was determined to behave as a very non-linear viscous fluid, allowing creep motions when the basal shear stress was about 4% lower than that required for catastrophic failure. These observations suggest that the geometry of the base of Sorbas basin would be likely to have admitted gravitational sliding by timedependent creep motion in the phyllosilicate-bearing gouges (Fig. 12a). An additional and important factor in the formation of the large monoclinal structure in the Tortonian sediments is the mechanical state and likely behaviour of the freshly-deposited, porous sandstones in the basin. In the case of the siliciclastic rocks, estimated porosities at the time of deposition and prior to cementation (Table 1) range between 14 and 37%, averaging at 25.1% ± 7% (1 S.D.), and average grain radius is 0.32 mm. The effective hydrostatic pressure P* (burial mean pressure minus pore fluid pressure) necessary for the onset of pore collapse by grain crushing is, to a useful approximation, independent of rock type (Zhang et al., 1990; Rutter and Glover, 2012), depending only on the product of porosity φ with grain radius r (mm). Zhang et al. (1990) proposed the relationship P* = (φ r)-1.5. Several studies have made use of this relationship to estimate rock behaviour in sedimentary basins

unconformably overlie older sediments affected by early extensional faults, onlap eastwards onto Alpujárride basement rocks, therefore there has been overall subsidence, and/or sea-level rise through lower Tortonian time. Sediments would be rapidly swept south-westward down the western flank of the S. Cabrera, prograding into the basin. Although the seismic anisotropy data suggests SE-NW stretching, the overall shape of the sediment body, with the large monoclinal Cantona fold, we interpret here to have been produced by gravitationally-induced sliding of the eastern part of the sediment body into the basin, as it deepens towards the north-west. Gravity-induced sliding is common on a range of length scales, from large offshore sediment bodies such as the Niger delta (120 km, e.g. Damuth, 1994; Cobbold et al., 2009), through the Baram delta (Borneo, 50 km, Morley et al., 2008) and Sporadhes basin (Aegean sea, 50 km, Ferentinos et al., 1981) to individual creeping landslides (2 km, e.g. Mam Tor, England, Rutter and Green, 2013). It has been common to appeal to some combination of pore fluid overpressures and/or weak basal layers such as evaporite deposits to facilitate sliding down gentle slopes (e.g. Cobbold et al., 2009; Lacoste et al., 2012). However, it is hard to appeal to elevation of pore fluid pressure in the Sorbas basin to facilitate gravitational sliding. Although saturated with water, the freshly-deposited sediments will have been very porous and permeable, and therefore effectively drained. On the other hand, observed evidence of low-angle faulting detaching basal basin sediments from the metamorphic substrate, and within the phyllitic rocks of the Alpujárride complex the formation of foliated, clay-bearing fault gouges are common. Therefore as shown in Fig. 7 we propose that extensional faults affecting the basal part of the basin-fill sediments cut down into the Alpujárride substrate and eventually merge with the foliated cataclasite shear zones at the Alpujárride/Nevado-Filábride contact, that were active during the mid-Miocene detachment between these two complexes. In a separate study (PhD thesis, Valetti, 2018) the frictional behaviour of Alpujárride phyllite- and Nevado-Filábride schist-derived fault gouges was investigated over the range of normal stresses likely to have developed in the evolution of this basin. All gouge samples displayed stable frictional sliding with an average friction coefficient of 12

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Fig. 12. (a) Mohr diagram to illustrate stress state at the base of the (saturated) basin fill deposits with 3 km of overlying sediment (effective normal stress = 43 MPa, no excess pore pressure) and an induced differential stress of 25 MPa (small stress circle). Potential weak sliding planes are expected to lie between and 20º to the horizontal (shaded wedge), well outside the expected range of friction angles (16 to 24º, upper shaded wedge) in clay-bearing fault gouge. The effect of adding a horizontal tectonic extensional stress is to enlarge the circle, causing the effective stress in the gouge to move upwards towards the friction angle range, potentially favouring creep sliding. The size of the enlarged stress circle is limited to that shown by the formation of fresh intrabasinal faults. The fresh faulting failure envelope is appropriate for a porous sandstone. (b) Estimated yield curves for siliciclastic rocks of initial porosities 33%, 26% and 20% (left to right), with a burial path showing increasing stiffness with compaction. Close to each yield curve shown a bedding-parallel extensional stress is applied, causing stress path to deviate upwards, with increasing differential stress and decreasing effective mean stress.

and to interpret behaviour from mechanical tests (Fossen et al., 2011; Schultz et al., 2010; Soliva et al., 2013; Skurtveit et al., 2013). Rutter and Glover (2012) proposed P* = 0.603(φ r)-1.09, based on incorporation of more experimental data. The values of P* obtained using these two equations are 64 and 45 MPa respectively for the ‘average’ sediment of the Sorbas basin. Some compaction through elastic deformation of the aggregate and frictional sliding between grains without fracturing can take place prior to the onset of grain fracture. Assuming a maximum basin fill depth of 3.0 km and an average dry density of 2410 kg/m3 the total vertical stress (σv = σ1, assuming this to be the greatest principal stress) at the basin floor is approximately 70 MPa, or effective 40 MPa after subtracting the pore water pressure. Initial burial and mechanical compaction by intergranular readjustments leads to an increase in lateral stresses (σ2 = σ3, at a slower rate of increase than the vertical stress). A common point of reference is the assumption of no lateral strain, so that σ2 /σ1 = υ/(1 − υ), where υ is Poisson’s ratio. This loading path stress ratio, called Ko, is, for example, 0.43 for a Poisson ratio of 0.3, but the actual value depends on the degree of cementation (Jones, 1994; Karig and Morgan, 1994; Bjorlykke and Hoeg, 1997). Whilst Soliva et al. (2013) suggest that Ko = 0.5 is a reasonable average value for poorly cemented sands with no superimposed lateral tectonic stress, it could decrease with progressive burial from close to unity immediately under the sediment surface to about 0.3 after compaction and cementation. The Q/P plot is well-suited to describing the stress path and deformation history of porous sediments, where differential stress Q = (σ1 − σ3) and effective mean pressure P = (σ1 + σ2 + σ3 − 3 Pp)/3, and where Pp is pore fluid pressure. Wong et al. (1997) showed that the onset of yield in porous sandstones is to a good approximation independent of rock type when Q and P are normalized by P*. Thus

values of effective P* are respectively 39, 52 and 72 MPa. A burial path with Ko starting close to unity and decreasing to ca 0.3 is shown. The Ko slope has been transformed into Q/P coordinates using the relationship 3(1 − Ko)/(1 + 2 Ko) = 0.9 (for Ko = 0.3) at maximum slope shown. The burial stress path (no superimposed tectonic stress) approaches the successive yield curves at progressively higher effective mean and differential stresses. However, the effect whereby, as here, the overall tectonic regime imposes a horizontal supplementary extensional stress during burial is to deflect the stress path upwards (as the differential stress increases). Thus as yield is approached, the effect of the concomitant decrease in mean stress becomes apparent, and the stress path deflects towards the critical state line. Microstructurally, relative separation of the grains in the direction of stretch will occur. The amount of inelastic internal deformation of the sediment required to accommodate the formation of the monoclinal Cantona fold (outer-arc stretching) is quite small, probably less than 5% and therefore imperceptible in thi N-S ection, but some sliding must take place on some bedding surfaces to accommodate it. We suggest that the Cantona fault of Haughton (2001) and other bedding-parallel slip surfaces observed are manifestations of this. According to the critical state description of sedimentary deformation, the likely magnitudes of the induced stresses are at least qualitatively consistent with the strength of the rocks and the structures induced. All of this deformation must have been completed before the late Tortonian uplift and erosion that preceded the marine flooding and deposition of the Messinian sequence. This uplift, that appears to have led to the erosive removal of perhaps as much as 1.5 km or more of sediment, is likely to have been driven by the progressive emergence of the Sierra Cabrera, accommodated by faults on its southern and northern boundaries, the Gafarillos fault (uplift of the S. Cabrera relative to the S. de Polopos to the south) and the North Cabrera fault (Fig. 2).

(P/P* − 0.5)/).5]2 + [Q/(0.6 P*]2 = 1

5.2. Basin evolution in the Regional Dynamic Context - Relationships between the Carboneras stretching transform fault, the formation of the Sorbas basin and the regional displacement field

This is an elliptical yield surface of axial ratio 1.2, with a critical state line of the same slope (separating compactive deformation at higher mean pressures from dilatant deformation at lower mean pressures). The relationship was confirmed with additional data by Rutter and Glover (2012). Fig.12b shows such yield surfaces in Q/P coordinates for three rocks of the same (average) grain radius of 0.32 mm but a decreasing spread of porosities of 33%, 26% and 20%. Resultant

The Carboneras fault system has been shown (Rutter et al., 2012, 2013) to form a stretching transform fault, extending through the total thickness of the continental crust, between a region of mainly Serravallian-Tortonian back-arc extension to the north and less-extended 13

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crust to the south. The Sorbas basin lies immediately to the north of the Carboneras fault, and might therefore be expected to show the effects of stretching as EN E-W SW extension during its early development. The period of sedimentation and deformation in the Sorbas basin (Serravallian through Tortonian) was identical to the main period of displacement accumulation on the Carboneras Fault System, suggesting that they are linked geodynamically. The investigation of such effects has been a principal aim of this study. This study and those of Giaconia et al. (2011, 2013; 2014) confirm that the Serravallian and Tortonian rocks of the basin fill were deposited in a basin that was undergoing stretching towards the WSW, with low angle extensional fault contacts against the Sierra Cabrera basement, locally rotated to the vertical along the later-uplifted contact with the Sierra de Polopos-Sierra Alhamilla basement rocks (Fig. 10). Extensional faults are inferred to bottom-out within the phyllitic rocks of the Alpujárride complex (Fig. 7). Present day GPS data (GonzalezCastillo et al., 2015; Galindo-Zaldivar et al., 2015) and interpretation of shear-wave splitting observations (Díaz et al., 2010) also point to continued, albeit slower, ENE-WSW regional stretching of the crust and upper mantle in the westerly part of the Betic zone wedge. Sediments were sourced from the already-exposed basement rocks of the Sierra de Los Filábres to the north and north-east, spreading as turbidite fans down a paleoslope towards the south-west, and driven by the uplift and eventual emergence of the basement rocks of the Sierra Cabrera block. Synsedimentary deformation is recorded as relative separation of grains in the partially compacted sediments (evidenced by seismic anisotropy in the plane of bedding, Table 1) and by intrabasinal extensional faulting. Gravitational instability in the middle Tortonian led to the detachment of the sediment body from underlying basement rocks and formation of a large monoclinal structure (the Cantona fold) by sliding into the deeper, western part of the basin in the study area. The uppermost part of the Tortonian time-span was marked by generalized uplift and emergence leading to erosion and development of the marine flooding surface that marks the onset of widespread Messinian shallow marine deposition, but with the area that became the Sierras de Polopos and Sierra Alhamilla rising faster, dividing the Tortonian basin. The height difference from the present basin floor near Lucainena to the summit line of the S. Alhamilla is about 3 km, representing the likely amount of relative uplift of this basement block. The uplift of these metamorphic basement blocks is an expected consequence of extensional tectonics (cf. Sierra Nevada - Sierra de los Filábres block, Johnson et al., 1997). Lateral differences in basement uplift amounts are accommodated by dip-slip faulting (e.g. Gafarillos fault). Regionally important north-south crustal shortening throughout Neogene to Recent time has been inferred by many authors (e.g. Vissers, 2012) arising from the observation of nappe formation in the external zones of the Betic cordilleras through the tendency for the uplifted basement blocks in the Betic zone to take east-west trending antiformal shapes (e.g. S. de Los Filábres, S. Cabrera, S. Alhamilla). Faults at the boundaries between uplifted basement blocks and basin fills tend to be of high-angle reverse character (e.g. Rutter et al., 1986; Vissers, 2012), logically leading to the conclusion that these mark north-south shortening. However, a note of caution is required. Theoretical and modelling studies, and field observations (e.g. Sanford, 1959; Mandl, 1988; Prucha et al., 1965; Miller and Mitra, 2011) have shown that vertical uplift of relatively rigid basement blocks, that can arise as a result of orthogonal extensional displacements parallel to the uplifted blocks, can give rise to high-angle reverse faulting and forced bend folding of rocks forming the tops of the blocks, in the way that is seen in section across the S. Cabrera, for example (Fig. 8). Thus, whilst not suggesting that there are no effects of Neogene north-south convergence between Africa and Iberia affecting the internal Betic zone, care must be taken when interpreting local reverse faulting and bend folding in this way. Similarly, interpreting local fault displacement patterns in terms of paleostresses (paleo-displacements), inferred from

small-scale fault slickenlines, may not always lead to conclusions that have regional significance, but rather reflect local displacements applied from beneath. Argus et al. (2011) reported the present day NW-SE convergence velocity between N. Africa and Iberia to be 5.6 mm/a, but GPS vectors (Echeverria et al., 2013) east of Almería city lie typically between 1 and 2 mm/a, implying that most of the overall convergence velocity is accommodated through strains affecting the Alborán sea basin to the south. North-south shortening strain rate equals the displacement rate gradient between Cabo de Gata and S. de Los Filábres and is presently on the order of 10−15 s-1 (Echeverria et al., 2013). Since Messinian time this could give rise to (assumed homogeneous) shortening strains of ca. 10%, i.e. about 5 km shortening over the above distance, or 700 m shortening over the length of the sections in Fig. 8. Echeverria et al. (2015) have also used GPS data to estimate a present-day slip rate along the Carboneras fault, assuming the SE-directed displacement rate vectors on the north-western side, relative to a fixed Cabo de Gata block, are all dissipated through localized motion on the Carboneras fault. Their data suggest a left-lateral slip rate of about 1 mm/a at the southern part of the onshore outcrop of the fault zone, decreasing towards the north-east, but increasing south-westwards into the Alborán basin. Such a displacement rate has the capacity to accommodate 5 km of offset since the Messinian. This displacement rate gradient would be consistent with the concept of a stretching fault, along which displacement rate varies because the wallrock blocks are not rigid. Such displacement rate variations are also a necessary consequence of the westward curving pattern in the eastern part of the Betic wedge of contemporary GPS vectors, from NW- to SW-directed (Fig. 1). In the vicinity of Carboneras, however, whilst it is clear from the surface geology that most displacement was accommodated on the Carboneras fault between Serravallian and upper Tortonian time, the occurrence of Messinian extrusive volcanic rocks locally stepping across the fault (Rutter et al., 2012) with displacements of at most a few hundreds of metres implies a much smaller post-Messinian finite displacement rate. 6. Conclusions We have used geological mapping coupled with gravity surveying to infer the shape of the pre-Messinian rocks within the Sorbas basin, S.E. Spain. The basin floor takes the form of a trough plunging gently westwards, and reaching a depth of ca. 2.0 km below present ground surface. The infilling of Tortonian clastic turbidites and mass-flow deposits takes the form of a synformal structure, of which only the southern half is presently exposed, the remainder being buried beneath unconformably overlying Messinian and Pliocene rocks. The sediments were derived from the basement rocks of the S. de Los Filábres to the north and north-east, which had been exposed since Serravallian time. The synformal shape was produced in association with the lateTortonian uplift of the Sierras Alhamilla and Polopos to the south, and westward-spreading uplift of the Sierra Cabrera to the east. The uplifts led to erosion of a substantial part of the Tortonian sediments and the exposure to erosion of the crystalline Nevado-Filábride rocks of the Sierra Cabrera basement. The formation of a new marine flooding surface led to the deposition of the Messinian and Pliocene sequences. The presently-exposed older basin-fill rocks strike dominantly NESW, obliquely to the present day southern basin margin, dipping and younging systematically to the NW. Dips are gentle (20º) except for a 3 km wide tract (the M. Cantona monocline), where they approach the vertical. The bedding dips mirror the variations in the shape of the basin floor but are steeper, meaning that the turbidites must take the form of a series of clinoforms prograding to the west, driven by the westward migration of the uplift of the western flank of the S. Cabrera. The Cantona monocline is inferred to have formed in response to gravitational sliding of sediments off the rising western flank of the S. Cabrera in the later stages of Tortonian sedimentation. An investigation 14

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of the petrophysical properties of these porous sediments and fault rocks has demonstrated the feasibility of this interpretation. The overlying Messinian and Pliocene sediments display a gentle antiform/ synform pair, testifying to continued, but slower, uplift of the Sierra Cabrera and Sierrras de Polopos and Alhamilla. Differences in relative uplift of these blocks are recorded as lateral variations in the vertical displacements in the Gafarillos and North Gafarillos faults (Fig. 4). The Tortonian basin fill rocks were subjected to E-W extension during accumulation, as a result of extensional faulting on the rising western flank of the S. Cabrera, possible blind extensional faulting affecting the basin floor, and bedding-parallel extension before and during bend-folding of the Cantona monocline. The early extensional faults that are presently exposed at the base of the basin-fill sequence are tentatively inferred to merge westwards into the basal slide of the Alpujárride nappe. These faults are sealed beneath overlying Tortonian turbidite deposits that overstep eastwards onto the Alpujárride basement. Vertical, apparently strike-slip fault strands are exposed along the southern margin of the present-day basin, and are inferred to be basinbottom extensional fault surfaces that have been rotated to the vertical during late-Tortonian basement block uplift. The geodynamic setting in which these sedimentation and deformation events took place is inferred to be in the back-arc sector produced by the Serravallian-Tortonian rollback of the Gibraltar arc subduction zone, resulting in some 30% average ENE-WSW extensional strain, and accompanied by a relatively small amount of N-S shortening. The southern boundary of the sector was the Carboneras stretching transform fault, whose principal period of activity, locally accumulating ca 40 km of left-lateral offset, was identical to that of the formation and deformation of the pre-Messinian Sorbas (and adjacent) basin.

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Data accessibility Full tabulation of data used in this paper is presented in supplementary data file [dataset] SF1, and in correspondence with UK Research council requirements is deposited in the UK National Geoscience Data Centre, identified by the title of this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Part of this work was supported by UK NERC grant NE/F019475/1. Lorenzo Valetti was supported by a grant from the University of Manchester Dean’s Scholarship Fund. A. J. M. acknowledges with thanks a fieldwork grant from the University of Manchester Zochonis Travel fund. David Hodgetts carried out the post-survey DGPS corrections and we thank Rhodri Jerrett for comments on the manuscript. John Waters carried out the XRD analyses. Lindy Walsh and Paco Contreras of the Urra Field Centre, Sorbas provided local help and support. We are grateful to the editor and to reviewers Joaquina Alvarez-Marrón and Hans de Bresser for their helpful and constructive comments. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.tecto.2019.228230. 15

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