Neogene Tectonic Evolution of the Gibraltar Arc: New Paleomagnetic Constrains from the Betic Chain ⁎ M
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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 Dpt. Scienze Geologiche, Università degli Studi Roma TRE, Italy b 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 subduction slab [3–7], (3) extensional collapse of the earlier collisional Betic–Rif orogen caused by convec- The tectonic processes responsible of the shape and tive removal of deep lithospheric roots [8,9], slab evolution of the Gibraltar Arc are controversial and detachment [10], or delamination of lithospheric mantle different models have been proposed to account for its [11–13]. All these tectonic models take into account the arcuate shape: (1) presence of an intermediate micro- role played by huge opposite vertical axis rotations in plate (Alboran plate) that moved westward between shaping the narrow and tight Gibraltar Arc, through Africa and Iberia [1,2], (2) westward roll back of a progressive bending of the Betics and Rif segments. In fact, a large amount of paleomagnetic data show that the ⁎ Corresponding author. Tel.: +39 0654888058; fax: +39 0654888201. arcuate shape of the Gibraltar Arc, such as the other E-mail address: [email protected] (F. Cifelli). major arcs in the Mediterranean region, is a secondary 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.08.012 M. Mattei et al. / Earth and Planetary Science Letters 250 (2006) 522–540 523 feature achieved through opposite vertical axis rotations 2. Tectonic settings along the two limbs of the arc [3,14]. Most of the paleomagnetic data incorporated in these geodynamic The Gibraltar Arc is located along the complex plate models come from Mesozoic rocks of the Betic and Rif boundary between the European and the African plates. chain (see [14] for a review), whereas only a limited Together with the Rif (North Africa), the Betic Cor- amount of paleomagnetic results from Neogene rocks is dillera (southern Spain) forms the Gibraltar Arc and presently available in this region (see [15] for a recent represents the westernmost segment of the Alpine– review). Mediterranean Belt. The Betic Cordillera is traditionally In this paper we present new paleomagnetic results subdivided into three main structural domains [16]: the from an extensive sampling in the Neogene sedimentary Internal Zones ([1], similar to the Alboran Domain of basins belonging to the Betic Cordillera in order to [17]), the Flysch Trough Units, and the Prebetic and better constrain the time and the amount of paleomag- Subbetic Zones [11] (Fig. 1). netic rotations in the Gibraltar Arc. The selected basins The Internal Zones or Alboran Domain is located lie either over the Internal or the External zones, or even onshore and offshore in the inner part of the arc. It seal the Internal–External zone boundary. We discuss consists mostly of metamorphic units, which constitute ours and previously published paleomagnetic data in the remnants of a Paleogene orogen [18]. The Flysch order to provide further constrains on the geodynamic Trough Units are constituted by siliciclastic sediments models on the origin of the Gibraltar Arc and on the of Cretaceous to Early Miocene age, deposited in a deep Neogene tectonic evolution of the Betic chain. Our marine or oceanic setting near the northern African results evidence that paleomagnetic rotations in the margin [19]. They outcrop in the western part of the Gibraltar Arc may be younger than generally supposed, Betic chain and form actually an inactive accretionary as Late Miocene vertical axis rotations are evidenced in prism, whose structural trend is mainly NNW–SSE to this paper. Such differential rotations show that strike- N–S directed [20]. slip tectonics and rotations around vertical axis may The Subbetic and Prebetic Zones represent the outer have played a major role during the Late Neogene and margin of the chain and are formed by Mesozoic and Quaternary times, to accommodate the complex kine- Tertiary sediments deposited in basinal (Subbetic) and matics along the European–African boundary. 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. 524 M. Mattei et al. / Earth and Planetary Science Letters 250 (2006) 522–540 of South Iberia. These rocks were detached from their vertical axis rotations along the two arms of the Arc, hercynian basement from Early Miocene onwards, and which are supposed to be Early Miocene in age [34]. formed a NNE–SSW to NE–SW trending fold-and- Concerning Miocene to Pliocene sites, [30] measured thrust belt, with associated foredeep and foreland basins large CW rotations in sedimentary rocks of Aquitanian along its outer margin (Guadalquivir Complex and age in eastern Betics, while no rotations were Guadalquivir Basins, respectively). measured in one Tortonian site. Platzman et al., Extensional tectonics during Miocene times played Calvo et al. and Platzman et al. [35–37] measured an important role in the development and evolution of variable amount of CW vertical axis rotation in Lower the sedimentary basins which lie over the internal or Miocene mafic dikes in the Malaga and in the external domains of the Betics (e.g., [13,17]), including Alpujarride regions. In the Murcia–Cabo de Gata the present-day Alboran Sea Basin [21,22]. In particular, region (Fig. 1), [38] measured complex vertical axis the formation of the Alboran Sea was largely coeval rotations (mainly CCW) in Upper Miocene to Pliocene with the development of the Flysch Trough, Subbetic sedimentary and volcanic rocks. This complex pattern and Prebetic fold-and-thrust belt. Finally, during the of rotations is possibly related to regional left-lateral Late Miocene, the Gibraltar region experienced a drastic strike-slip faults activity. More recently, magnetostrati- modification of the tectonic regime, possibly related graphic investigations have been carried out on Late with the halting of the roll-back processes in the Gib- Miocene to Pliocene sedimentary sections in several raltar Arc subduction system [7,23], in turn possibly internal and foreland basins of both the Betics and Rif related with a change of Africa–Eurasia plate conver- [39–45]. In that case, paleomagnetic data generally gence vector from a N-S direction to a NW-SE one (e.g., show no significant vertical axis rotations since the [24]). As a consequence, the whole Betics and the Late Tortonian [15]. Alboran Sea Basin underwent a complex pattern of compressional and strike-slip tectonics which, in some 4. Sampling and paleomagnetic methods cases, inverted previous extensional structures [21,22]. Volcanism accompanied and postdated Neogene exten- The location of the studied basins is reported in Fig. 1. sion, with calc-alkaline, potassic and basaltic volcanism We have selected sedimentary basins located in very scattered across the eastern sector of Alboran Sea and different areas of the Betics, in order to obtain paleo- Betic–Rif chain. magnetic data from structural domains that experienced According to GPS data, Africa and Europe are pre- different tectonic evolution during the Neogene. In sently undergoing convergence, which in the region of particular, we sampled Miocene sediments in: (i) the Gibraltar is about 4 mm/yr with a NW orientation [25]. Guadalquivir foreland basin, (ii) the intramontane basins Along the Southern Iberia and North Africa margin the of central Betics (Alcalá-la-Real, Granada, Guadix, African–Eurasia plate convergence is accommodated by Huercal-Overa), and (iii) the Vera, Lorca, and Mula a wide and diffuse region of deformation, mainly basins in eastern Betics, which were mainly deformed characterized by WSW oriented active thrust fronts by strike-slip tectonics. We sampled 61 sites, with 641 and NNE oriented left lateral strike-slip faults. This plate oriented samples, almost homogeneously distributed in boundary is also the source of some of the largest the different sedimentary basins. Paleomagnetic analyses earthquakes in western Europe and north Africa, such have been carried out at the paleomagnetic laboratories as the 1755 Lisbon (estimated Mw=8.5) or the 1980 of Università di Roma TRE and ETH of Zurich using Ms=7.3 El Asnam earthquakes [26,27] in northern standard methods.