Neogene tectonic evolution of the Gibraltar Arc: New paleomagnetic constrains from the Betic chain

Earth and Planetary Science Letters 250 (2006) 522 – 540 www.elsevier.com/locate/epsl

Neogene tectonic evolution of the Gibraltar Arc: New paleomagnetic constrains from the Betic chain M. Mattei a , F. Cifelli a,⁎, I. Martín Rojas b , A. Crespo Blanc c , M. Comas d , C. Faccenna a , M. Porreca a a

b

Dpt. Scienze Geologiche, Università degli Studi Roma TRE, Italy Dpt. de Ciencias de la Tierra y del Medio Ambiente, Universidat de Alicante, Spain c Dpt. De Geodinamica, Universidat de Granada, Spain d CSIC — Universidat de Granada, Spain

Received 28 April 2006; received in revised form 1 August 2006; accepted 14 August 2006 Available online 27 September 2006 Editor: S. King

Abstract New paleomagnetic results from Neogene sedimentary sequences from the Betic chain (Spain) are here presented. Sedimentary basins located in different areas were selected in order to obtain paleomagnetic data from structural domains that experienced different tectonic evolution during the Neogene. Whereas no rotations have been evidenced in the Late Tortonian sediments in the Guadalquivir foreland basin, clockwise vertical axis rotations have been measured in sedimentary basins located in the central part of the Betics: the Aquitanian to Messinian sediments in the Alcalà la Real basin and the Tortonian and Messinian sediments in the Granada basin. Moreover, counterclockwise vertical axis rotations, associated to left lateral strike-slip faults have been locally measured from sedimetary basins in the eastern Betics: the Middle Miocene to Lower Pliocene sites from the Lorca and Vera basins and, locally, the Tortonian units of the Huercal-Overa basin. Our results show that, conversely from what was believed up to now, paleomagnetic rotations continued in the Betics after Late Miocene, enhancing the role of vertical axis rotations in the recent tectonic evolution of the Gibraltar Arc. © 2006 Elsevier B.V. All rights reserved. Keywords: Gibraltar Arc; Betics; Neogene; Paleomagnetic rotations

1. Introduction The tectonic processes responsible of the shape and evolution of the Gibraltar Arc are controversial and different models have been proposed to account for its arcuate shape: (1) presence of an intermediate microplate (Alboran plate) that moved westward between Africa and Iberia [1,2], (2) westward roll back of a ⁎ Corresponding author. Tel.: +39 0654888058; fax: +39 0654888201. E-mail address: [email protected] (F. Cifelli). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.08.012

subduction slab [3–7], (3) extensional collapse of the earlier collisional Betic–Rif orogen caused by convective removal of deep lithospheric roots [8,9], slab detachment [10], or delamination of lithospheric mantle [11–13]. All these tectonic models take into account the role played by huge opposite vertical axis rotations in shaping the narrow and tight Gibraltar Arc, through progressive bending of the Betics and Rif segments. In fact, a large amount of paleomagnetic data show that the arcuate shape of the Gibraltar Arc, such as the other major arcs in the Mediterranean region, is a secondary

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feature achieved through opposite vertical axis rotations along the two limbs of the arc [3,14]. Most of the paleomagnetic data incorporated in these geodynamic models come from Mesozoic rocks of the Betic and Rif chain (see [14] for a review), whereas only a limited amount of paleomagnetic results from Neogene rocks is presently available in this region (see [15] for a recent review). In this paper we present new paleomagnetic results from an extensive sampling in the Neogene sedimentary basins belonging to the Betic Cordillera in order to better constrain the time and the amount of paleomagnetic rotations in the Gibraltar Arc. The selected basins lie either over the Internal or the External zones, or even seal the Internal–External zone boundary. We discuss ours and previously published paleomagnetic data in order to provide further constrains on the geodynamic models on the origin of the Gibraltar Arc and on the Neogene tectonic evolution of the Betic chain. Our results evidence that paleomagnetic rotations in the Gibraltar Arc may be younger than generally supposed, as Late Miocene vertical axis rotations are evidenced in this paper. Such differential rotations show that strikeslip tectonics and rotations around vertical axis may have played a major role during the Late Neogene and Quaternary times, to accommodate the complex kinematics along the European–African boundary.

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2. Tectonic settings The Gibraltar Arc is located along the complex plate boundary between the European and the African plates. Together with the Rif (North Africa), the Betic Cordillera (southern Spain) forms the Gibraltar Arc and represents the westernmost segment of the Alpine– Mediterranean Belt. The Betic Cordillera is traditionally subdivided into three main structural domains [16]: the Internal Zones ([1], similar to the Alboran Domain of [17]), the Flysch Trough Units, and the Prebetic and Subbetic Zones [11] (Fig. 1). The Internal Zones or Alboran Domain is located onshore and offshore in the inner part of the arc. It consists mostly of metamorphic units, which constitute the remnants of a Paleogene orogen [18]. The Flysch Trough Units are constituted by siliciclastic sediments of Cretaceous to Early Miocene age, deposited in a deep marine or oceanic setting near the northern African margin [19]. They outcrop in the western part of the Betic chain and form actually an inactive accretionary prism, whose structural trend is mainly NNW–SSE to N–S directed [20]. The Subbetic and Prebetic Zones represent the outer margin of the chain and are formed by Mesozoic and Tertiary sediments deposited in basinal (Subbetic) and shelf (Prebetic) environments of the rifted paleomargin

Fig. 1. Schematic structural map of the Betic chain showing the main structural features. The boxed areas include the study Neogene sedimentary basins.

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of South Iberia. These rocks were detached from their hercynian basement from Early Miocene onwards, and formed a NNE–SSW to NE–SW trending fold-andthrust belt, with associated foredeep and foreland basins along its outer margin (Guadalquivir Complex and Guadalquivir Basins, respectively). Extensional tectonics during Miocene times played an important role in the development and evolution of the sedimentary basins which lie over the internal or external domains of the Betics (e.g., [13,17]), including the present-day Alboran Sea Basin [21,22]. In particular, the formation of the Alboran Sea was largely coeval with the development of the Flysch Trough, Subbetic and Prebetic fold-and-thrust belt. Finally, during the Late Miocene, the Gibraltar region experienced a drastic modification of the tectonic regime, possibly related with the halting of the roll-back processes in the Gibraltar Arc subduction system [7,23], in turn possibly related with a change of Africa–Eurasia plate convergence vector from a N-S direction to a NW-SE one (e.g., [24]). As a consequence, the whole Betics and the Alboran Sea Basin underwent a complex pattern of compressional and strike-slip tectonics which, in some cases, inverted previous extensional structures [21,22]. Volcanism accompanied and postdated Neogene extension, with calc-alkaline, potassic and basaltic volcanism scattered across the eastern sector of Alboran Sea and Betic–Rif chain. According to GPS data, Africa and Europe are presently undergoing convergence, which in the region of Gibraltar is about 4 mm/yr with a NW orientation [25]. Along the Southern Iberia and North Africa margin the African–Eurasia plate convergence is accommodated by a wide and diffuse region of deformation, mainly characterized by WSW oriented active thrust fronts and NNE oriented left lateral strike-slip faults. This plate boundary is also the source of some of the largest earthquakes in western Europe and north Africa, such as the 1755 Lisbon (estimated Mw = 8.5) or the 1980 Ms = 7.3 El Asnam earthquakes [26,27] in northern Algeria. 3. Previous paleomagnetic results Paleomagnetic data from the Gibraltar Arc come mostly from Jurassic to Late Cretaceous sedimentary units from external Betics in Spain [12,28–32] and External Rif in Morocco [33], and show mostly clockwise and counterclockwise rotations, respectively north and south of the Gibraltar Strait (see [14] for a critical review). Paleomagnetic results from internal metamorphic units also show the same opposite pattern of

vertical axis rotations along the two arms of the Arc, which are supposed to be Early Miocene in age [34]. Concerning Miocene to Pliocene sites, [30] measured large CW rotations in sedimentary rocks of Aquitanian age in eastern Betics, while no rotations were measured in one Tortonian site. Platzman et al., Calvo et al. and Platzman et al. [35–37] measured variable amount of CW vertical axis rotation in Lower Miocene mafic dikes in the Malaga and in the Alpujarride regions. In the Murcia–Cabo de Gata region (Fig. 1), [38] measured complex vertical axis rotations (mainly CCW) in Upper Miocene to Pliocene sedimentary and volcanic rocks. This complex pattern of rotations is possibly related to regional left-lateral strike-slip faults activity. More recently, magnetostratigraphic investigations have been carried out on Late Miocene to Pliocene sedimentary sections in several internal and foreland basins of both the Betics and Rif [39–45]. In that case, paleomagnetic data generally show no significant vertical axis rotations since the Late Tortonian [15]. 4. Sampling and paleomagnetic methods The location of the studied basins is reported in Fig. 1. We have selected sedimentary basins located in very different areas of the Betics, in order to obtain paleomagnetic data from structural domains that experienced different tectonic evolution during the Neogene. In particular, we sampled Miocene sediments in: (i) the Guadalquivir foreland basin, (ii) the intramontane basins of central Betics (Alcalá-la-Real, Granada, Guadix, Huercal-Overa), and (iii) the Vera, Lorca, and Mula basins in eastern Betics, which were mainly deformed by strike-slip tectonics. We sampled 61 sites, with 641 oriented samples, almost homogeneously distributed in the different sedimentary basins. Paleomagnetic analyses have been carried out at the paleomagnetic laboratories of Università di Roma TRE and ETH of Zurich using standard methods. The samples were demagnetised using both thermal and AF procedures. Reliable paleomagnetic results were gathered from 41 sites and are reported below. 5. Paleomagnetic results The nature of the magnetic carriers was investigated using different rock magnetic techniques. The stepwise acquisition of an isothermal remnant magnetization (IRM) was carried out by using a pulse magnetiser which applies magnetic fields up to 2.1–2.5 T. Different fields (0.12 T, 0.6 T and 1.7 T) were also applied along

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the three orthogonal specimen axes and the threecomponent IRM was subsequently thermally demagnetised [46]. Both low coercivity and high-coercivity magnetic carriers were identified in the analysed samples (Fig. 2a). In some samples, the multi-component IRM is characterized by high coercivity fractions, which show maximum unblocking temperature of 670 °C, indicating hematite as main magnetic carriers (Fig. 2b). In other samples, the low coercivity fraction prevails, with a maximum unblocking temperature around 280°– 370° and 580 °C, which in that case indicates iron sulphides and magnetite as the main magnetic carriers (Fig. 2c).

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Specimens were demagnetised by means of progressive stepwise thermal demagnetization, using temperature increments, or stepwise alternating field demagnetization, according to preliminary results obtained from pilot specimens from each site. Magnetic cleaning was terminated when Natural Remnant Magnetization (NRM) reached the instrument sensitivity level or when a random change of the paleomagnetic direction appeared, due to mineral transformation. This change was often accompanied by an increase in the magnetic susceptibility. In some cases samples were too weakly magnetized to allow a reliable, complete, stepwise demagnetization (NRM values about 50 × 10− 6 A/m).

Fig. 2. IRM acquisition curves showing the presence of low coercivity and high coercivity magnetic carriers (a). Thermal demagnetization curves of composite IRM are shown for hematite-bearing (b) and magnetite-bearing (c) samples. Vector component diagrams (Zijdervald diagrams, in geographic coordinates) for the progressive thermal demagnetization of representative samples (d–m). Demagnetization step values are in degrees Celsius. Open and solid symbols represent projection on the vertical and horizontal planes, respectively.

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Demagnetization data were analysed using orthogonal vector diagrams (Fig. 2d–k) and directions of the remanence components were estimated using principal component analysis [47]. Samples yielding maximum angular deviation (MAD) N 15° were rejected and were not considered for further analyses. In the other samples ChRMs or remagnetization circles were isolated, which in most of the samples show MAD b10°. The site mean paleomagnetic directions were then calculated using the great circle method of McFadden and McElhinny (1988). In 32 sites a well defined (α95 b 15.0°) ChRM was isolated, while in other 10 sites the ChRM is less defined (α95 b 25.8°) but is coherent with the other sites from the same basins, and therefore has been considered for further tectonic interpretations. The 42 reliable site mean directions before and after tectonic correction are reported in Table 1. In the following subsections we report separately paleomagnetic results obtained in the different basins. 5.1. The Guadalquivir basin The Guadalquivir basin is located along the external front of the Betic chain with a WSW–ENE orientation from the Gulf of Cadiz to the west and the Prebetic arc to the east (Fig. 1). The basin formed during middle Miocene as a foredeep basin and evolved during the Tortonian to Pliocene times as a foreland basin [48]. Sedimentary sequences filling the foreland basin are mainly composed by Late Miocene pelagic sediments, with some shallow marine episodes. The outcropping sequences become progressively younger and thicker towards the west, ranging respectively from Tortonian to Late Tortonian–Messinian in the western part. They are undeformed in the northernmost external part of the basin, where the sedimentary rocks overlie the Hercynian rocks of the Iberian foreland. To the east, the sedimentary sequences of the Guadalquivir foreland basin are overthrust by the Meso- to Cenozoic rocks forming the Prebetic fold-and-thrust belt [32,49]. In the southern part of the Guadalquivir basin and off-shore the Gulf of Cadiz the Late Miocene units are overthrust by the gravity driven nappes defined as olistrostrome unit by [50]. We sampled 4 sites in the sub-horizontal Tortonian marly clays outcropping in the eastern part of the basin, which represent the lower depositional sequence recognized in the Guadalquivir Basin [51] (Fig. 3a). All the sites gained reliable results, and the main magnetic carrier has been identified as a low coercivity fraction, probably magnetite (Fig. 2a). Both normal and reverse

polarities have been observed which show almost antipodal directions. The mean paleomagnetic direction, when all the sites are reported to the normal polarity is D = 4.8°; I = 54.8°; K = 66.2; α95 = 11.4 (Fig. 3b), suggesting that the Guadalquivir basin underwent no rotation since Tortonian, in agreement with published results from the western part of the same basin [15]. 5.2. The Granada basin The Neogene intramontane Granada basin (Fig. 1) is located in the central part of the Betics, and seals the boundary between the External and the Internal domains (e.g., [52]), as it overlies on the Subbetic fold-and-thrust wedge and the metamorphic rocks of the Alboran Domain, north and south of the basin, respectively (Fig. 4). The sedimentary infilling of the basin took place during several sedimentary cycles, evolving from marine to continental environments [53]. The marine sedimentary sequence started in the Early Tortonian with calcarenites and conglomerates. Late Tortonian fan delta deposits and open marine marls, which progressively evolved to more restricted and temperate environments, overlie the lowermost sequence through an angular unconformity. A transition from marine to continental conditions began during the Late Tortonian as alluvial fans to shallow marine environments developed, associated with important evaporitic episodes in the deepest part of the basin. The continental sedimentation started at the end of the Tortonian– Messinian boundary, with deposits of fresh-water stromatolithic limestones passing upward to lutites with gypsum and, occasionally, with lignites and micritic limestones. Finally, the Pliocene to Lower Pleistocene sequence is characterized by alluvial fans, floodplains, and lacustrine basins, mostly developed in the central and northern part of the basin. The present day structural architecture of the Granada basin is mostly controlled by NW–SE oriented normal faults. In particular, the basin is bounded to the east and southeast by an array of such faults, which have been shown to be active [54], and that separate the Neogene sediments from the Internal Betics metamorphic domain. E–W oriented normal fault systems characterize the southern border of the basin. In the Granada Basin, 10 sites were sampled in continental marls and clays (Fig. 4a). The age of the sampled sites ranges from Late Tortonian to Late Messinian, according to [55] and [56]. Either high and low coercivity magnetic minerals have been identified in the Granada basin, which are mostly magnetite and hematite (Fig. 2a). Both normal and reverse polarities have been

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Table 1 Sites

Age

N

S0

Db

Ib

k

α95

Da

Ia

α95

k

Guadalquivir Basin BE48 Upp. Tort. BE49 Upp. Tort. BE50 Upp. Tort. BE51 Upp. Tort.

8(14) 5(9) 9(10) 7(14)

Sub-hor. Sub-hor. Sub-hor. 275, 5

178.4 18.8 20.1 1.9 7.2

−48.0 60.3 52.9 55.5 54.8

25.3 67.0 24.0 15.0 70.0

11.4 10.7 10.1 16.3 11.1

178.4 18.8 18.0 354.7 4.8

−48.0 60.3 53.1 54.9 54.8

25.3 67.0 24.0 15.0 66.2

11.4 10.7 10.1 16.3 11.4

Granada Basin BE20 Upp. Mess. BE21 Upp. Mess. BE23 Upp. Tort. BE24 Messinian BE32 Messinian BE34 Upp. Mess. BE52 Messinian BE53 Upp. Mess.

9(9) 9(9) 7(9) 9(9) 7(9) 7(7) 9(11) 6(13)

341, 31 230, 20 66, 20 0, 0 50, 20 37, 13 33, 14 306, 34

61.2 208.6 47.4 16.1 5.7 37.0 39.8 226.4 35.5

21.5 − 0.1 49.0 37.5 31.1 33.2 20.0 −19.5 27.5

43.1 10.1 58.3 12.6 43.0 32.2 52.6 7.85 14.4

7.90 14.40 8.00 15.10 9.30 10.80 8.0 25.8 15.1

51.6 208.0 52.0 16.1 11.7 37.8 39.4 213.2 34.9

13.3 −18.6 29.8 37.5 15.2 20.3 6.1 −21.8 20.6

43.1 10.1 58.3 12.6 43.0 32.2 52.6 7.85 26.4

7.9 14.4 8.0 15.1 9.3 10.8 8.0 25.8 11.0

Guadix Basin BE54 Upp. Tort. BE55 Upp. Tort. BE56 Upp. Tort. BE57 Upp. Tort. BE58 Upp. Tort. BE59 Upp. Tort. BE60 Upp. Tort.

13(14) 8(9) 7(11) 9(10) 4(10) 8(11) 10(10)

358, 34 78, 22 273, 15 186, 13⁎ 173, 5⁎ 226, 15⁎

330.8 16.1 0.4 357.4 334.9 0.4 347.4 354.7

50.9 46.1 39.5 45.3 51.4 54.0 32.3 46.6

20.8 39.2 28.8 42.1 15.2 10.5 16.3 43.6

9.3 9.0 11.4 8.1 24.4 17.9 14.3 9.2

340.2 31.1 348.3 354.5 348.8 340.1 7.0 356.7

19.2 33.1 37.2 58.1 57.8 66.6 42.2 45.8

20.8 39.2 28.8 42.1 15.2 10.5 16.3 15.3

9.3 9.0 11.4 8.1 24.4 17.9 14.3 15.9

Alcalà La Real Basin IMR20 Upp. Burdig.–Low. Langh. IMR22 Upp. Burdig.–Low. Langh. IMR23 Aquitanian IMR24 Upp. Burdig.–Low. Langh. IMR26 Low. Tort

12(12) 4(11) 12(14) 5(15) 10(12)

174, 27 Sub-hor. 83, 23 336, 18 0, 10

18.5 37.6 6.4 70.0 31.9 29.5

33.4 37.8 40.5 59.9 62.6 48.6

13.7 57.4 40.2 5940.0 17.5 16.2

14.5 13.0 8.8 1.0 11.9 19.6

32.5 37.6 22.4 40.8 24.3 31.1

56.7 37.8 31.9 56.6 54.2 47.7

13.8 57.4 40.2 5940 17.5 39.4

14.4 13.0 8.8 1.0 11.9 12.3

Vera Basin BE05 Upp. Messinian BE06 Lower Pliocene BE09 Lower Pliocene

3(9) 9(9) 10(12)

Sub-hor. Sub-hor. Sub-hor.

335.5 155.2 154.4 335.5

34.3 −31.1 −28.6 31.3

53.9 15.6 42.5 781.7

19.9 13.6 8.0 4.4

335.5 155.2 154.4 335.5

34.3 −31.1 −28.6 31.3

53.9 15.6 42.5 781.7

19.9 13.6 8.0 4.4

Huercal-Overa Basin BE61 Tort. BE62 Tort. BE63 Tort. BE66 Messinian BE67 Tort.

10(22) 11(12) 7(8) 8(12) 6(14)

Variable 213, 38 60, 22 354, 22 36, 18 Without BE62

155.1 113.8 171.9 178.2 166.8 156.5 167.3

5.1 −27.8 −43.8 −34.2 −46.4 −18.0 −13.5

9.5 11.5 23.8 18.0 28.8 3.9 4.2

17.7 14.1 12.6 13.5 12.9 44.3 51.2

154.1 98.4 187.2 177.5 177.3 160.1 174.7

−40.3 −16.4 −32.7 −12.2 −33.3 −31.1 − 30.2

10.0 11.5 23.8 18.0 28.8 5.8 23.4

17.2 14.1 12.6 13.5 12.9 34.6 19.4

240, 38⁎⁎ 245, 36⁎⁎ 25, 63⁎⁎ 83, 50 324, 60 224, 35

314.6 209.1 308.5 315.3 223.1 1.4

34.3 −63.5 45.4 − 0.6 50.6 13.9

7.3 9.7 40.9 25.8 20.2 71.3

20.70 20.80 10.00 16.4 12.7 6.10

316.8 150.4 317.6 326.1 278.3 348.3

25.5 −36.4 46.7 27.4 29.2 37.3

7.3 9.7 40.9 25.8 20.2 71.3

20.7 20.8 10.0 16.4 12.7 6.1

Lorca–Tercia Basin IMR01 Upp. Serravallian IMR02 Upp. Serravallian IMR03 Upp. Serr.–Lower Tort. IMR04 Upp. Tort. IMR07 Lower Burdigalian BE11 Serravallian

7(7) 7(7) 5(11) 5(10) 11(11) 9(9)

108, 27

(continued on next page)

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Table 1 (continued) Sites

Age

BE12 BE13 BE14 BE15

Lower Messinian Serravallian Serravallian Lower Tort.

N 9(9) 3(10) 8(10) 8(10)

S0

Db

57, 15 0,0 161, 80 11, 16

316.5 335.0 111.7 335.7 342.9

Ib

k 40.7 27.0 5.1 55.4 61.5

27.8 97.7 23.5 82.9 2.2

α95

Da

Ia

10.0 12.5 12.20 6.50 43.1

329.7 335.0 85.80 345.0 320.0

41.7 27.0 − 38.6 41.6 37.9

α95

k 27.8 97.9 23.5 82.9 12.9

10.0 12.5 12.2 6.5 14.0

N = number of stable directions (total number of studied samples at a site). S0 = bedding attitude (azimuth of the dip and dip values) Sites marked by ⁎⁎ have been also corrected for the plunging axis. Beddings marked by ⁎ derives from Anisotropy of Magnetic Susceptibility tensor. D, I site mean declinations and inclinations calculated before (Db, Ib) and after (Da, Ia) tectonic correction. k and α95, statistical parameters after Fisher (1953). Values in bold represent the sites mean for each basin.

observed, which show antipodal directions. The reversal test is of type Rc (γo = 16.1°; γc = 5.5°) according to [57]. Also in this basin the mean paleomagnetic direction is better grouped after (D = 34.9°, I = 20.6°, K = 26.4, α95 = 11.0) than before tectonic correction

(D = 35.5°, I = 27.5°, K = 14.4, α95 = 15.1), indicating a pre-tilting age for the isolated component of magnetization. The mean paleomagnetic direction, when all the sites are reported to the normal polarity, is D = 34.9°; I = 20.6°. Such result is coherent with previous data

Fig. 3. a) Schematic map of the Guadalquivir basin, location of the sampled sites and measured declinations (arrows). b) Mean paleomagnetic declinations from the basin before and after tectonic correction.

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Fig. 4. a) Schematic map of the Granada basin, location of the sampled sites and measured declinations (arrows). b) Mean paleomagnetic directions from the basin before and after tectonic correction.

from a magnetostratigraphic study in a Late Miocene– Lower Pliocene sedimentary section, (D = 196.5; I = − 39.7; K = 128.4, α95 = 5.9) [56] suggesting a significant post Messinian CW rotation throughout the entire Granada basin (Fig. 4b). 5.3. The Guadix basin The Guadix Basin is located in the central part of the Betic Cordillera (Fig. 1). As the Granada Basin, it seals the Alboran Domain–Subbetic boundary. This intramontane basin formed in the Late Miocene after the main compressional events that formed the main tectonic structures of the Betic Cordillera [58]. The stratigraphic record in the Guadix Basin covers the Late Miocene up to the Quaternary with no hiatuses [59]. The Late Miocene sequence began with marine calcarenites and marls. It was followed by continental fluvial and lacustrine deposits [58].

In this basin 7 sites were sampled in the Late Tortonian white marine marls (second depositional sequence of [58]) that outcrops in the northern part of the basin (Fig. 5a). The main magnetic carrier has been identified as a low coercivity fraction, probably magnetite (Fig. 2a). Notwithstanding most of the samples were characterized by low values of NRM, a stable component of magnetization were identified for all the sites. All the samples show a normal polarity and the mean directions are better grouped before (D = 354.7°, I = 46.6°, K = 43.6, α95 = 9.2) than after tectonic correction (D = 356.7°, I = 45.8°, K = 15.3, α95 = 15.9) (Fig. 5b). Furthermore, the mean direction in geographic coordinates is almost parallel to the present-day GAD magnetic field, suggesting that these sites have been remagnetized in recent times. Therefore results from the Guadix basin have not been further considered for tectonic interpretations.

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Fig. 5. a) Schematic map of the Guadix basin with the location of the sampled sites. b) Mean paleomagnetic directions from the basin before and after tectonic correction. Note that the sites show all a normal polarity and a better grouping before than after tectonic correction, suggesting a recent remagnetization.

5.4. The Alcalá-la-Real basin The Alcalá-la-Real basin is characterized by outcrops of lower Miocene to Pliocene marine sediments distributed along a NE–SW trending belt, 30 km northwest of Granada (Fig. 1). The Neogene sediments of the basin rest on top of the Mesozoic to Paleogene rocks of the fold-and-thrust wedge of the Subbetic domain, and record part of the compressional events of this part of the Betic orogen. The youngest rocks affected by the main shortening event are Aquitanian in age. Although the main shortening took place during Late Aquitanian times, deformation proceeds from Early Burdigalian to

Messinian, as the structural architecture of the basin is characterized by hecto- to kilometric-scale, widely open NE–SW oriented folds, which deform the entire Miocene sequence. In this basin, eight Aquitanian to Lower Tortonian sites were sampled. One site corresponds to the youngest rocks of the deformed Subbetic domain (IMR27, Aquitanian in age according to [60]), meanwhile sample IMR23 is the oldest rocks belonging to the Alcalá-laReal basin. Samples IMR20, 22, and 24 have been taken in Late Burdigalian to Lower Langhian calcarenites, and samples IMR21, 25 and 26 in Lower Tortonian marls (Fig. 6a). The main magnetic carrier has been identified as a low coercivity fraction, probably magnetite (Fig. 2a). Interpretable demagnetization diagrams were obtained in 6 sites, whereas 2 other sites (IMR21, IMR27) owned very weak intensity or showed an instable behaviour during demagnetization steps. Site IMR25 mean direction was almost parallel to the GAD magnetic field before tectonic correction and was suspected for recent remagnetization. This site was not considered for further tectonic considerations. The site mean paleomagnetic directions obtained from the other 5 reliable sites show a normal polarity. However the mean direction is far from the present day magnetic field in geographic coordinates and the mean basin direction is better grouped after (D = 31.1°, I = 47.7°, K = 39.4, α95 = 12.3) than before tectonic correction (D = 29.5°, I = 48.6°, K = 16.2, α95 = 19.6), indicating a pre-tilting age for the isolated component of magnetization (Fig. 6b). All the samples, from Late Burdigalian to Early Tortonian in age, show a similar clockwise rotation, and the mean paleomagnetic direction shows a 31° CW rotation. 5.5. The Huercal-Overa basin The Huercal-Overa basin, composed of Serravallian to Messinian deposits, crops out in the Eastern part of the Betics. It lies over the Alboran Domain rocks and is bounded by the Alpujarride units to the North and the Nevado–Filabride units to the South (Fig. 1). The stratigraphy of the basin can be summarised as a succession of six main units [61], which began with a coarse continental sedimentary unit, attributed to the Late Serravallian to Early Tortonian [62]. After these continental episodes, shallow and open marine turbidites and marls deposited, during the entire Late Tortonian to Early Messinian [61,63]. Seven Tortonian to Messinian sites were sampled (Fig. 7a), and reliable results were obtained from 5 sites. Either high and low coercivity magnetic minerals have

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Fig. 6. a) Schematic map of the Alcalá-la-Real basin, location of the sampled sites and measured declinations (arrows). b) Mean paleomagnetic directions from the basin before and after tectonic correction.

been identified in this basin, which are mostly magnetite and hematite, according to their blocking temperature of 580° and 680° C respectively (Fig. 2a–i). All the sites show a reverse polarity after bedding correction. The mean site directions is D = 156.5°, I = − 18.0°, K = 3.9, α95 = 44.3 before tectonic correction, whereas after tectonic correction is D = 160.1°, I = − 31.1°, K = 5.8, α95 = 34.6, which indicates a pre-tilting age for the magnetization (Fig. 7b). The basin average inclination (I = 31.1°) is significantly shallower than that expected from its palaeolatitudinal position in the Late Miocene, as for Granada basin. The mean directions sites by sites show a wide dispersion (Fig. 8). Three sites were not rotated (BE 63, 66 and 67) while the 2 other sites show different amount of CCW rotations. Meanwhile BE61 evidence a small CCW rotation, site BE62, which has been sampled along the northern margin of the basin, is strongly rotated CCW. This rotation has a local character and is related with the movement along left-lateral strike-slip faults, which characterizes this part of the Betic orogen (Fig. 1). If we discard site BE62, the mean grouping of the sites is strongly increased and the basin mean paleomagnetic direction become D = 174.7°, I = − 30.2°, K = 23.4, α95 = 19.4 (Fig. 9). Accordingly, post Tortonian CCW rotation occurred along the northern

part of the basin in connection with the activity of leftlateral strike-slip faults, whereas the rest of the basin only underwent small CCW rotations after Tortonian times. 5.6. The Vera basin The N–S oriented Vera basin is located along the eastern boundary of the Sierra de los Filabres (Fig. 7). The basin is filled with Upper Serravallian to Recent sedimentary deposits, which overlie the metamorphic units of the Internal Betics. The sedimentary sequence begins with Tortonian continental and marine units, which outcrop rarely and are strongly deformed. The Tortonian to Messinian units are made of conglomerates, turbidites, calcarenites and shallow water carbonates, which pass upward to Pliocene basinal marls and marine siliciclastic sediments. Late Pliocene to Pleistocene continental units lie on top of the sequence. The tectonic–stratigraphic evolution and the present day architecture of the basin was mainly controlled by the N20 oriented left-lateral Palomares fault system, which bounds the eastern side of the basin (Figs. 1 and 7). This fault system was active since Late Miocene up to Quaternary times, and produced approximately 15 km of left-lateral displacement [64].

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Fig. 7. a) Schematic map of the Huercal-Overa and Vera basin, location of the sampled sites and measured declinations (arrows). Mean paleomagnetic directions from the Huercal-Overa (b) and Vera (c) basins before and after tectonic correction.

Eleven sites in sub-horizontal Late Tortonian to Lower Pliocene clays and marls were sampled (Fig. 7a). Either high (Fig. 2a) and low coercivity (Fig. 2b) magnetic minerals have been identified in this basin, which are iron-sulphides, magnetite, and hematite. Eight sites showed low NRM values or were unstable during demagnetization. Three other sites have given reliable results, with antipodal normal and reverse polarities; the reversal test is of type Rb (γo = 9.8°; γc = 4.5°) according to [57]. When all the sites are reported to normal polarity, the basin mean direction is D = 335.5°, I = 31.3°, K = 781.7, α95 = 4.4. Accordingly, the Vera basin underwent a significant CCW rotation after Early Pliocene (Fig. 7c). 5.7. The Lorca basin The Lorca basin is a small depression located between ENE oriented anticlines formed by the Alboran

Domain metamorphic units of Sierra de las Estancias, Sierra de la Tercia and Sierra Espuña (Fig. 8). To the North, it is bounded by Subbetic units (Fig. 1). The sedimentary sequence reaches locally up to 2 km of Burdigalian to Pliocene marine and continental sediments. The sedimentary sequence starts with Burdigalian to lower Serravallian marine units unconformably followed by a thick continental unit, Late Serravallian to Early-Tortonian in age, formed by red conglomerates, sandstones and silts with gypsum intercalations. These units are followed by Tortonian calcirudites and by open-marine marls and silts (Carivete marls, [65]) which locally reach up to 700 m. The calcirudites lie either upon the Lower Tortonian marine sediments, upon the Lower to Middle Miocene rocks, or directly upon the Alboran metamorphic basement, which describes a clear angular unconformity. These Tortonian sedimentary unit shows strong thickness variations, related with the position and geometry of high-angle listric growth faults

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Fig. 8. a) Schematic map of the Lorca basin, location of the sampled sites and measured declinations (arrows). b) Mean paleomagnetic directions from the basin before and after tectonic correction. Geological map from [87].

that bound the ENE and WSW margins of the Lorca basin [66]. The next sedimentary unit was deposited between the Late Tortonian and the Messinian, during the uplift and emersion of the Lorca basin. It is formed by a great variety of sedimentary facies, from shallow marine (calcarenites and calcirudites) to restricted marine, represented by organic-rich shales, cherts and dolomites, and finally evaporites (mainly gypsum, halite in the basins depocenter). This sequence is caped by lacustrine marls, together with alluvial conglomerates along the basin margins [42,65,67]. After a mostly erosional period during the Pliocene, the Quaternary is characterized by alluvial deposits whose depocenters are strongly controlled by the position and activity of the Alhama de Murcia sinistral strike-slip faults [68–70]. Fourteen sites were sampled in Serravallian to Messinian sediments (Fig. 8a). Results also include one site (IMR07) from the Mula Basin (Fig. 1). In this basin the main magnetic carrier is given by a low coercivity magnetic mineral with a blocking temperature of 550– 580 °C, identified as magnetite (Fig. 2l). In some samples also the presence of a high coercivity mineral has been observed, identified as hematite (Fig. 2a). We obtained reliable results from 10 sites. In 4 other sites the intensity of remanent magnetization was either too low or the samples showed unstable behaviour during the demagnetization steps and the results were not reliable. The site mean directions from the analysed 10 sites show a strong increase of their grade of grouping after tectonic correction (D = 320.0, I = 37.9, K = 12.9,

α95 = 14.0 after correction vs. D = 342.9, I = 61.5, K = 2.2, α95 = 43.1 before correction). After tectonic correction 8 sites have a normal polarity and 2 show a reverse polarity (reversal test is indeterminate according to [57]). When all the sites are reported to have normal polarity, the basin mean direction is D = 320.0°, I = 37.9°, to show 40° of CCW rotation of the basin after Messinian times (Fig. 8b). 6. Discussion 6.1. Analysis of paleomagnetic inclinations Tilt-corrected site mean directions from the analysed sedimentary basins show a great scatter of inclinations, which in most of the sites are significantly shallower with respect to the reference one (Table 1). As reference inclination the Geocentric Axial Dipole (GAD) field inclination (I = 56°) was used since the GAD field inclination must be very close to the expected Late Miocene inclination for the region, not showing Iberia a significant northward moving during Late Cenozoic. Large inclination errors can be observed in the HuercalOvera (generally 25° and up to 44° in difference), Vera (generally 25°) and Lorca–Tercia (generally 18° and up to 30° in difference) basins, and in particular in the Granada basin (generally 36° and up to 50° in difference). On the other hand, sites from the Guadalquivir basins do not show significant shallowing in the inclination (I = 55°).

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Inclination flattening due to depositional and/or burial compaction processes has been often observed in paleomagnetic studies from sedimentary units [71,72], and has been also observed in modern fluvial hematite-bearing deposits from the River Soan in Pakistan, which suffer about 25° inclination shallowing [73]. Tan et al. [71] showed that shallowing in inclination is particularly important in sediments containing elongate particles (such as detrital hematite), where inclination flattening of more than 30° has been observed, suggesting rock magnetism as a main factor controlling inclination shallowing. We speculate that a rock magnetic causes of shallow inclinations can be also proposed in the sedimentary basin from the Betics, where inclination flattening is particularly important in hematite-bearing sites (i.e. Granada basin), and is not observed in magnetite-bearing sites (i.e. Guadalquivir basin). Ten et al. [71] used Anisotropy of Magnetic Susceptibility (AMS) as a tool to measure compaction, due either to depositional and/or burial processes. In particular they observed that a shallowing in inclination of about 30° is observed for values of AMS foliation F b 1.053. In our samples a strong flattening is also revealed by AMS data. In particular, sites from Granada show a well defined magnetic foliation parallel to the bedding planes, with a degree of foliation (F) comprised between 1.18 and 1.014, which is compatible with the higher flattening observed in our sediments with respect to those studied by [71]. However, no clear relationship is found between inclination shallowing and the AMS foliation F. This could be due to the fact that low-field AMS in clay sediments is generally controlled by the paramagnetic clayey matrix of the rock [74], suggesting that the high variations in inclination errors can be related to compaction processes in a complex way, being influenced either from the types and relative amount of magnetic carriers, in particular hematite, and components of clay matrix. Finally, the observed conspicuous inclination error confirms the primary character of the measured magnetization in all the basins (except Guadix which does not show significant inclination flattening, due to recent remagnetization processes), since inclination shallowing can be due only to events occurring during deposition and/or diagenesis. 6.2. Analysis of paleomagnetic declinations Paleomagnetic results from Neogene intramontane basins located in different sectors of the Betic chain,

over the Internal or External Zones and in the foreland Guadalquivir Basin, evidence a complex pattern of vertical axis rotations, which took place after the Late Miocene. Menawhile no rotations have been evidenced in the Late Tortonian sediments in the Guadalquivir foreland basin, clockwise vertical axis rotations have been measured in basins situated in the central part of the Betics: the Aquitanian to Messinian sedimentary units in the Alcalá-la-Real and the Tortonian and Messinian sediments of Granada basin. By contrast, counterclockwise vertical axis rotations were measured in the Middle Miocene to Lower Pliocene sites in the Lorca and Vera basins and, locally, in the Tortonian units of the HuercalOvera basin. Such pattern of different rotations evidence that in the Betic chain, from Late Miocene onwards, different crustal blocks with a different tectonic evolution were individualized. In the following sections we briefly describe the main tectonic features of the Guadalquivir foreland basin, Central Betics and Eastern Betics in order to discuss the tectonic implications of paleomagnetic results presented in this study. 6.2.1. The Guadalquivir foreland basin The tectono-stratigraphic evolution of the Guadalquivir foreland basin has been reconstructed mainly using stratigraphic, seismic and well data [26,75]. Along its eastern boundary, the Tortonian sediments of the Guadalquivir basin are overthrust by the Mesozoic sedimentary units of the Prebetic nappes, with a NW shortening direction [14,49]. Toward the west, in the Gulf of Cadiz, Late Miocene to Pliocene units are not involved in the main compressional deformation, clearly defining the upper time boundary of the main emplacement of the allochthonous units in this region [26]. These data show that the emplacement of the allochthonous units along the outer front of the Betics mainly occurred between Late Burdigalian and Late Tortonian [14]. This compressional event has been generally interpreted as the last episode of deformation related to the westward transport of the Betic nappes, which was responsible for the Gibraltar Arc bending [14]. Offshore of the Gulf of Cadiz, however, NE–SW oriented active folds and thrusts have been detected by seismic surveys, and could be related to some of the main big historical earthquakes, which have been recorded in this region [76]. These structures have been related either to the present-day NW oriented Africa–Europe convergence [26], or have been suggested as evidences of active east dipping subduction in the front of the Gibraltar Arc [77]. In this latter hypothesis, the east dipping subduction of the oceanic

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lithosphere should be a continuous process since Early Miocene to the present-day, controlling the present-day geodynamics in the Betic–Rif region. Our paleomagnetic results from the analysed Tortonian marls, together with those from Messinian units of the western part of the Guadalquivir basin [15], permit to test that this foreland basin underwent no rotations since the Late Miocene. Therefore, these paleomagnetic data evidence that vertical axis rotations related to the bending of the external part of the Gibraltar Arc were either older than Late Miocene or never reached the foreland South Iberian sector and were confined to the Betic–Rif orogenic wedge. 6.2.2. Central Betics Most of the paleomagnetic data incorporated in the geodynamic models explaining the Gibraltar Arc bending have been collected in western-central Betics. In this region almost constant CW paleomagnetic rotations of about 60° have been measured in the Late Jurassic and Late Cretaceous sedimentary units of the SW–NE oriented Betics thrust belt, outcropping between Gibraltar and Granada [78]. Such huge CW rotations have been mostly measured in the allochtonous units of the Betics and have been interpreted as related either to the oblique compression which accompanied the bending of the Gibraltar Arc during the Miocene [14] or to the roll-back of a westward retreating subducting slab (i.e. [3]). In any case, paleomagnetic rotations were considered to have occurred mainly during the Early Miocene and to be already completed in the Late Miocene [14]. In central Betics we measured significant CW rotations in Granada and Alcalá-la-Real intramontane sedimentary basins. These rotations are post-Tortonian in the Alcalá-la-Real basin and post Messinian in the Granada basin. Therefore, in both basins these rotations took place after the main compressional event, which lead to the formation of the Subbetic fold-and-thrust belt in the central Betics. 6.2.3. The eastern Betics and the left-lateral strike-slip fault systems Counterclockwise paleomagnetic rotations were measured in Vera and Lorca basins and in the northwestern sites from the Huercal-Overa basin. A single site in Mula basin shows similar counterclockwise rotations. The rotated sediments are Serravallian to Early Pliocene in age, and do not show any statistical difference or trend depending of the age or the stratigraphic position of the sampled sites. In particular in the Vera basin, close to the left-lateral Palomares strikeslip fault, 25° of CCW rotations have been measured in Upper Messinian–Lowermost Pliocene sites, which

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suggests that CCW rotations occurred after that time. These CCW rotations have been observed in the different sedimentary basins located along a deformation belt dominated by Late Miocene–Quaternary leftlateral strike-slip faults, which characterized the whole South Iberian continental margin in the region of Almeria–Murcia (Fig. 1). In this region CCW rotations do not extend to sedimentary basins, which are located far away from the main strike-slip faults [15,38], and indicate the occurrence of small, fault-bounded blocks which rotate about vertical axis as a consequence of the activity of these faults. It is worth to note that some of these faults are supposed to be active [66]. Consequently, block rotation around vertical axis along left-lateral strike-slip faults could still be an active mechanism in the Almeria–Murcia region. 6.3. Tectonic implications of paleomagnetic results Contrasting models have been proposed so far to explain the formation of the arcuate structure of the Rif– Betic belt. The concomitant formation of the compressional structure in the external portion of the belt and the extensional one in the inner portion inspired the first and most popular class of models where the driven engine is internal, that is derived from the instability of the dense mantle portion of the lithosphere. This class of models includes subduction and roll-back [3,4,7] and/or delamination [11,79,80] or convective removal of the mantle lithosphere [9]. In particular, subduction and/or delamination process is supported by subduction-related calcalkaline volcanism in eastern Betics, Morocco and Alboran Sea which has been active in this region from 25 to 5 Ma ([7,23] and references therein), tomographic images, showing a narrow but long east dipping slab beneath Morocco–Gibraltar [7,76], and deep although isolated earthquakes [79]. In this view, the formation of the Gibraltar Arc is related to the westward–southwestward retreat of the trench of a small portion of the slab [3,7,23], producing CCW rotation in the Betics and CW rotation of the Rift. Geological data show that the main retreat episode vanished during the Late Miocene. The second class of models is related to external engine related to the oblique convergence along the Africa–Iberia plate boundary. The formation of large scale strike-slip structure and the present day active tectonics in this region can be best explained by this mechanism [26,27] although GPS and seismic data suggest that active deformation in such area is quite complex, and cannot simply be explained by the right lateral motion along the Iberia–Nubia plate boundary [25].

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Although geological constraints indicate that the first class of model prevail during the Early to Late Miocene while the second one from the Late Miocene onward, the interplay between the two main engine is difficult to discriminate in the narrow configuration of the Gibraltar Arc. Seismological and geodetic observations show that the present-day kinematics of the Iberia–Nubia boundary is characterized by a complex interaction between these two main processes. In fact, GPS data suggest that in central Rif an independent fault-bounded crustal block is moving southwestward, relative to Nubia, as the result of roll-back of a narrow slab of delaminated continental lithosphere, whereas the kinematics of western and central Betics can be better explained by the oblique convergence between Nubia and Iberia [81]. Paleomagnetic data presented in this paper gives important constraints on the Late Miocene to Recent kinematics and deformation. The lack of rotation in the Tortonian clays of the Guadalquivir basin indicates that the basin itself represents the northern boundary of the rotating domains of the western Betics, confining the rotations on the hanging wall of the outer front of the chain (Fig. 9). This is not surprising if the formation of the arc is related to slab roll-back, as vertical axis rotation related to the arc bending should be confined to the

overriding plate. A similar pattern of rotation has been also reconstructed in the Calabrian Arc where large opposite paleomagnetic rotations are confined in the orogenic wedge, while no rotations have been registered on the Adriatic and Hyblean foreland basins [82–84]. In the second class of hypothesis, the lack of vertical axis rotations in the Guadalquivir basin, indicates that the right-lateral shear belt, which marks the tectonic boundary between Iberia and north Africa, is placed to the south of the Guadalquivir basin, as suggested by several authors on the base of seismic and GPS data [77,85]. The interpretation of CCW paleomagnetic rotations measured in the eastern Betics (Vera, Lorca and Huerca basins), is straightforward as they are clearly related to the presence of a large-scale left-lateral strike-slip fault system, which caused the western Alboran block to move southward with respect to eastern Betics. It is worth to note that such CCW paleomagnetic rotations appear to be confined in a narrow belt close to the leftlateral strike-slip fault system and do not appear to extend to the entire eastern Betics, where magnetostratigraphic data, located away from the main left-lateral faults, do not show significant rotations (Fig. 9). More problematic is the interpretation of the CW paleomagnetic rotations measured in Alcalá-la-Real and

Fig. 9. Paleomagnetic declinations (and relative confidence limits) from Neogene sediments in the Betic chain. White arrows come from this study and indicate cumulative paleomagnetic declinations for each study basin: a) Guadalquivir Basin (see Fig. 3); b) Granada Basin (see Fig. 4); c) Alcalà la Real Basin (see Fig. 6); d) Vera Basin (see Fig. 7); e) Huercal-Overa Basin (see Fig. 7), including basin paleomagnetic mean discarding BE62 (dashed paleomagnetic declination); f) Lorca–Tercia Basin (see Fig. 8). Black arrows come from previous studies and are indicative of mean paleomagnetic declination deduced in single magnetostratigraphic sections: 1) Chicamo section [40]; 2) and 3) Chorrico and Librilla section [39]; La Serrata section [42]; 5) Abad section [44]; Zorreras section [88]; Galera section [89]; 8) Carmona section [15]; 9) Purcal section [56]; 10) Venta de la Virgin section [90]. (Modified from [15]).

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in the Granada basins, in the central part of the Betic chain (Fig. 9). Our results are coherent with previous paleomagnetic results in the central Betics, where clockwise rotations have been already documented [14]. However, the occurrence of such rotations in the Late Miocene sedimentary units demonstrates that rotation is younger than previously supposed and occurred after the main phases of nappe emplacement. This new result implies either that bending process of the overall belt was still at work during the Late Miocene (first class of models) or that these rotations are due to right-lateral shear, related to the oblique plate convergence (second class of models) [22,25]. The first solution would imply a revaluation of the timing of the bending process of the Gibraltar Arc as a consequence of westward slab roll back. This hypothesis, recently proposed by [76], should imply the occurrence of Late Miocene to Recent opposite vertical axis rotations in the Betics (CW) and in the Rif (CCW). At the moment no paleomagnetic data from Late Miocene to Recent are available from the Rif chain, hindering the possibility to test successfully this hypothesis. However it is important to note that active west-directed roll-back of an east-dipping slab, which should cause a westward motion of Gibraltar relative to Africa, is inconsistent with the well-defined eastward motion of GPS sites observed in northwestern Morocco [85]. The second solution would enhance the role of the Nubia–Iberia oblique convergence in the recent tectonic evolution of the Betic chain and the observed CW rotations in western Betics should accommodate the right-lateral shear at the boundary between Iberia and Nubia plates. This interpretation is coherent with GPS data which suggest that the the Iberia–Nubia plate boundary is presently located in a deformation belt located between southern Iberia and northern Morocco [85]. CW rotations measured in central Betics could be a still active process which accommodate the complex pattern of Late Miocene to Recent deformation evidenced in central Betics [86]. Finally, we underline that a more straightforward constraint to the post Miocene evolution of the Gibraltar arc could be only accomplished after a detailed paleomagnetic investigations in the Rif chain, which could constrain the kinematics along the entire Gibraltar Arc, and would be able to distinguish between the two proposed classes of models. 7. Conclusions CW and CCW rotations have been measured in the different intramontane basins of the Betic chain. CCW

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rotations are confined to a deformation belt related to wrench tectonics (left lateral strike-slip faults) and continued until recent times. On the other hand, CW rotations have been measured for the first time in Late Miocene units from intramontane basins in central Betics. This makes substantially younger the age of CW rotation in the Betic chain. Our results imply a reconsideration of the timing of paleomagnetic rotations in the Betics, and enhance the role of vertical axis rotations in the Late Miocene to Recent tectonic evolution of the Gibraltar Arc. Recent paleomagnetic rotations are mostly related to the complex pattern of deformation associated to the right-lateral strike-slip component of relative motion at the Africa–Iberia plate boundary, but could also imply that the bending of the Gibraltar Arc was not completely achieved in the Late Miocene. To test this hypothesis detailed paleomagnetic investigations in the Neogene sedimentary basins from the Moroccan side of the Gibraltar Arc (Rif chain) should be carried out. Acknowledgments Financial support for this work was provided by Projects: a) 01-LECEMA22F [WESTMED] by the European Science Foundation under the EUROCORES Programme EUROMARGINS, through contract No. ERAS-CT-2003-980409 of the European Commission, DG Research, FP6 (M.M., F.C., M.P. and C.F.); b) BTE2003-05057-C02 (A.C–B) and c) REN20013868-C03-01 (M.C.) from the MEC and FEDER founding (Spain). We thank G. Booth-Rea for his help in taking samples in the eastern Betics. M.M. and C.F. are particularly grateful to Victor Garcia Duenas who introduce them to the geology of the Betics. References [1] J. Andrieux, J.M. Fontobe´, M. Durand-Delga, Sur un mode`le explicatif de l'Arc de Gibraltar, Earth Planet. Sci. Lett. 12 (1971) 191–198. [2] D. Leblanc, P. Olivier, Role of strike-slip faults in the Betic Rifian Orogeny, Tectonophysics 101 (1984) 344–355. [3] L. Lonergan, N. White, Origin of the Betic–Rif mountain belt, Tectonics 16 (3) (1997) 504–522. [4] L.H. Royden, Evolution of retreating subduction boundaries formed during continental collision, Tectonics 12 (1993) 629–638. [5] S. Duggen, K. Hoernle, P. van den Bogaard, L. Rüpke, J.P. Morgan, Deep roots of the Messinian salinity crisis, Nature 422 (2003) 602–606. [6] M.J.R. Wortel, W. Spakman, Subduction and slab detachment in the Mediterranean–Carpathian region, Science 290 (2000) 1910–1917. [7] C. Faccenna, C. Piromallo, A. Crespo-Blanc, L. Jolivet, F. Rossetti, Lateral slab deformation and the origin of the western Mediterranean arcs, Tectonics 23 (2004) TC1012, doi: 10.1029/2002TC001488.

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