Betic Orogen Research Articles

Decoupling anthropogenic, climatic, and tectonic factors in the hydrology of playa-lakes in a semi-arid, tectonically active region (betic orogen, southern Spain)

Here, we explore the dynamic interplay of climate, tectonic forces and human impacts, that have shaped the current hydrology of the 27 main endorheic continental playa-lakes in southern Spain, most of them protected as Natural Reserves. This research offers a thorough examination of the hydro-geomorphic, geochemical, and tectonic characteristics that influenced the development of these wetlands. We introduce and apply several hydro-morphological indices that effectively characterize the hydrological functioning of these wetlands. Importantly, the examination of the geological setting and analysis of recent tectonics has been found to be effective in reconstructing the evolution of some of the major playa-lakes, including Fuente de Piedra playa-lake and El Gosque playa-lake. Lastly, the analysis of satellite information using the online tool water occurrence change intensity (WOCI) of Surface Water Explorer in the studied playa-lakes was found to be inconsistent in the long-term analysis. Enhanced attention is required to refine the WOCI tool, thereby ensuring a more precise depiction of wetland dynamics in the Mediterranean. Fine-tuning will better represent complex behaviors, thus improving its utility for monitoring and managing wetlands in the region.

Preservation of the Iberian Tethys paleomargin beneath the eastern Betic mountain range

We obtain P-wave receiver functions from teleseismic earthquake recordings at a dense seismic broadband transect, deployed along 170 km across the Betic mountain range in southeastern Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the ∼50 km wide necking domain. Crustal thinning across the necking domain affects mainly the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching ∼45 km depth, and terminates abruptly at a major slab tear. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic orogen. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust. According to our profile, the present-day eastern Betics appear to have a much more significant contribution from metamorphic Iberian crust than previously thought.

Radiogenic (Sr-Nd-Hf-Os) isotope data for neogene volcanic rocks from the rif–betic orogen and lanzarote (Canary Islands) in the western Mediterranean

In this communication we present new radiogenic Sr-Nd-Hf-Os isotope data for Neogene volcanic rocks from the Rif-Betic Orogen and Lanzarote Island in the western Mediterranean along with some previously published data. Major and trace element data for the Rif samples are provided as a data supplement. Geological context and petrographic details for equivalent rocks have been published previously but the new data, especially the Hf and Os isotope data should be of use for future appraisals of the distribution of sub-crustal mantle domains in the region. Upon inspection, it would appear that alkali basalts, shoshonites and lamproites sample very different sub-continental lithospheric mantle sources in Spain and Morocco whereas there are close similarities between Lanzarote basalts and mantle xenoliths from the Atlas Mountains in Morocco.

Geodynamic Modeling of Edge-Delamination Driven by Subduction-Transform Edge Propagator Faults: The Westernmost Mediterranean Margin (Central Betic Orogen) Case Study

Lithospheric tearing at slab edges is common in scenarios where retreating slabs face continental margins. Such tearing is often accommodated via subvertical STEP (SubductionTransform Edge Propagator) faults that cut across the entire lithosphere and can result in sharp lateral thermal and rheological variations across the juxtaposed lithospheres. This setting favors the occurrence of continental delamination, i.e., the detachment between the crust and the lithospheric mantle. In order to evaluate this hypothesis, we have chosen a wellstudied natural example recently imaged with unprecedented seismic resolution: the STEP fault under the central Betic orogen, at the northern edge of the Gibraltar Arc subduction system (westernmost Mediterranean Sea). The Gibraltar Arc subduction is the result of the fast westward roll-back of the Alboran slab and it is in its last evolutionary stage, where the oceanic lithosphere has been fully consumed and the continental lithosphere attached to it collides with the overriding plate. In this study we investigate by means of thermo-mechanical modeling the conditions for, and consequences of, delamination post-dating slab tearing in the central Betics. We consider a setup based on a STEP fault separating the orogenic Betic lithosphere and the adjacent thinned lithosphere of the overriding Alboran domain. Our model analysis indicates that delamination is very sensitive to the initial thermal and rheological conditions, transitioning from a stable to a very unstable and rapidly evolving regime. We find two clearly differentiated regimes according to the time at which the process becomes unstable: fast and slow delamination. Although the final state reached in both the fast and slow regimes is similar, the dynamic surface topography evolution is dramatically different. We suggest that given a weak enough Iberian lower crust the delaminating lithospheric mantle peels off the crust and adopts a geometry consistent with the imaged southward dipping Iberian lithosphere in the central Betics. The lack of spatial correspondence between the highest topography and the thickest crust, as well as the observed pattern of uplift/ subsidence are properly reproduced by a model where relatively fast delamination (Reference Model) develops after slab tearing.

The Spanish‐Portuguese Central System: An Example of Intense Intraplate Deformation and Strain Partitioning

AbstractThe intraplate deformation of Iberia during the Cenozoic produced a series of ranges and deformation belts with a wide variety of structural trends. The Spanish‐Portuguese Central System is the most prominent feature crossing over the whole of central Iberia. It is a large thick‐skinned crustal pop‐up with NE‐SW to E‐W thrusts. However, the 500‐km‐long left‐lateral strike‐slip Messejana‐Plasencia fault, also NE‐SW oriented, bends these thrusts to produce NE‐SW local paleostresses close to the fault, which seems to be consistent with a common deformational arrangement. This is also supported by the similar sedimentary infilling characteristics found in the surrounding Cenozoic basins. The moment of the maximum intraplate deformation is registered at the same time in all these basins during the upper Priabonian‐lower Chattian. As there are two possible sources for the intraplate compressive stresses, the Pyrenean (N‐S shortening) orogen to the north and the Betic (NW‐SE shortening) orogen to the south, neither can simply explain both simultaneous movements (NE‐SW strike‐slip and NE‐SW thrusting). The deduced age of the main deformation indicates a Pyrenean origin. In contrast, the concept of strain partitioning between the two types of faults gives as a result an overall north trending compression. Existing data do not support crustal detachment from the Betics neither from the Pyrenees but are consistent with a crustal uplift related to lithospheric folding. The subsequent Betic‐related stress field only slightly reworked previously Pyrenean‐related structures, except for the Portuguese sector, where tectonic activity occurred mainly in the Upper Miocene.

Extensional vs contractional Cenozoic deformation in Ibiza (Balearic Promontory, Spain): Integration in the West Mediterranean back-arc setting

Based on field work and seismic reflection data, we investigate the Cenozoic tectono-sedimentary evolution offshore and onshore Ibiza allowing the proposal of a new tectonic agenda for the region and its integration in the geodynamic history of the West Mediterranean. The late Oligocene–early Miocene rifting event, which characterizes the Valencia Trough and the Algerian Basin, located north and south of the study area respectively, is also present in Ibiza and particularly well-expressed in the northern part of the island. Among these two rifted basins initiated in the frame of the European Cenozoic Rift System, the Valencia Trough failed rapidly while the Algerian Basin evolved after as a back-arc basin related to the subduction of the Alpine–Maghrebian Tethys. The subsequent middle Miocene compressional deformation was localized by the previous extensional faults, which were either inverted or passively translated depending on their initial orientation. Despite the lateral continuity between the External Betics and the Balearic Promontory, it appears from restored maps that this tectonic event cannot be directly related to the Betic orogen, but results from compressive stresses transmitted through the Algerian Basin. A still active back-arc asthenospheric rise likely explains the stiff behavior of this basin, which has remained poorly deformed up to recent time. During the late Miocene a new extensional episode reworked the southern part of the Balearic Promontory. It is suggested that this extensional deformation developed in a trans-tensional context related to the westward translation of the Alboran Domain and the coeval right-lateral strike-slip movement along the Emile Baudot Escarpment bounding the Algerian Basin to the north.

Recent and active tectonics in the western part of the Betic Cordillera

Based on new seismological and recent faulting data, we analyze five main sectors in the western part of the Betic Cordillera from an active tectonic perspective: i. The northwestern frontal sector of the cordillera, including the Morón de la Frontera area, with shallow (< 15 km depth) seismogenic active NE-SW reverse and strike-slip faults that do not reach the surface. ii. The Teba area, in the External Zones, which is deformed by sparse N-S to NW-SE normal faults with associated wedges of Quaternary sediments and active strike-slip faults at intermediate crustal levels. iii. The Ronda Basin, representing the major Late Miocene intramontane basin of the western Betic Cordillera, is mainly deformed by NNE-SSW and WNW-ESE folds and locally by NW-SE normal faults. iv. The internal part of the orogen, which is characterized by shallow (<40 km) and intermediate (40 to 120 km depth) seismicity beneath Málaga and the Alborán Sea, ENE-WSW folding, relief uplift, river incision and scarce active faults with surface expression. v. The Spanish Atlantic coast, which coincides with the western end of the Betic orogen, where Pliocene to Quaternary shallowmarine sedimentary rocks are deformed by meso-scale NW-SE oriented normal faults and few ENE-WSW reverse faults. These data support the recent tectonic activity of the western Betic Cordillera determined by the NW-SE Eurasian-African convergence with variable features from the mountain front, characterized by shallow deformations, up to the Internal Zones that mainly undergo uplift and intermediate seismicity.

Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field

We present a new set of brittle microtectonic measurements carried out in the Pliocene and Quaternary rocks outcropping in several key sectors of the western Betic and Rif orogen, the so-called Gibraltar orogenic arc. This data set, along with available earthquake focal mechanisms and borehole breakouts, allowed us to compile the Pliocene and Quaternary stress map of this area. This map provides new constraints for tectonic models and the present-day tectonic activity of the proposed active eastward subduction of oceanic lithosphere beneath the Gibraltar Arc and roll-back. The horizontal maximum compressive stress ( S Hmax) is NW-SE in the Betic Orogen and N-S/NNW-SSE in the southern Rif Cordillera. There is a significant consistency between S Hmax and the displacement field deduced from GPS measurements with respect to the African plate: both appear to reflect the NW-SE convergence between the African and the European plates that is perturbed in the Rif. We propose that part of the eastern Rif behaves as a quasi-rigid block welded to the stable African plate. This block is bounded by important faults that localized most of the deformation disturbing the stress and surface displacement field. Pliocene to Quaternary N-S to NW-SE Africa-Europe plate convergence seem to be associated to the reorganization of the remnant Early Miocene subduction system in a continental–continental collision framework. Three-dimensional reconstruction of available seismic tomography plotted against the intermediate seismicity shows that only part of the old subduction system, whose orientation ranges from N20°E to N100°E, remains active: the portion ranging from N30°E to N40°E, orthogonal to the regional convergence.

Geodynamic implications for the formation of the Betic‐Rif orogen from magnetotelluric studies

Magnetotelluric data from the central Betics mountains (Spain) have been used to determine the electrical resistivity of the crust after a three‐dimensional (3D) interpretive approach. At shallow levels (<2 km), the resulting model shows good correlation between the geoelectric structures and the geologic units. At greater depths (>3 km), the most striking and well‐resolved feature of the model is an upper‐middle crust conductive body, located at the core of the Internal Betics antiform. This approximately 14‐km‐thick body is interpreted as basic or ultrabasic rocks containing a conducting mineral phase. Its structural location above the sole thrust of the Betic orogen and beneath the Nevado‐Filábride complex confirms the presence of a major suture zone between this complex and the autochthonous Iberian plate. This suture may correspond to an ancient oceanic or transitional domain developed between Iberia and the Alboran Domain during the opening of the Tethys Ocean, partially subducted and closed during the development of the Betic orogen. The possible geodynamic scenarios for the Betics have been reconsidered, taking into account this new constraint.

Timing of tectonic events in the Alpujárride Complex, Betic Cordillera, southern Spain

Ar–Ar and fission-track dating from four low-grade sections in the Alpujárride Complex sheds light on the timing of the early contractional history of these high-pressure metamorphic units, and their subsequent exhumation during the early Miocene extensional collapse of the Betic orogen. White mica grains from the lowest-grade rocks yield partly reset detrital Ar–Ar ages; where deformation and recrystallization are more intense, however, they yield reproducible ages as old as c . 48 Ma, which we interpret as the approximate timing of the main contractional event and associated high-pressure metamorphism in these units. Rocks from slightly higher-grade sections give reproducible Ar–Ar ages between 30 and 40 Ma, but these younger ages probably reflect the interplay of growth and cooling in fine-grained micas heated to between 300 °C and 400 °C during prograde metamorphism. In each of the four areas studied the low-grade rocks are separated by an extensional detachment or shear zone from medium-grade rocks below. In two areas these detachments have been folded around major overturned north-vergent folds. Ar–Ar data from structurally below the detachments, and fission-track ages from all structural levels, indicate the onset of significant cooling in the early Miocene, which corresponds to the age of the extensional event. The folds must therefore be early Miocene or younger.

TRANSGRESSIVE SEQUENCES ON FORELAND MARGINS: A CASE STUDY OF THE NEOGENE CENTRAL GUADALQUIVIR BASIN, SOUTHERN SPAIN

The Guadalquivir foreland basin, located between the Iberian basement northward and the Betic orogen to the South, represents the western sector of the earlier foredeep basin of the Betic Cordillera. Along the northern foreland margin, the sedimentary fill of this basin includes a Tortonian Basal Transgressive Complex (BTC), composed of five internal sequences bounded by transgressive surfaces. Two main parts are distinguished within each sequence: the lower transgressive lag deposits, and the upper stillstand/prograding sediments. Three facies associations were distinguished within this stratigraphic succession along the central sector of this basin margin: unfossiliferous conglomerates and coarse-grained sands (A), fossiliferous conglomerates and coarse-grained sands (B), and yellow medium-coarse-grained fossiliferous sands (C). A fourth facies association (D: blue silty marlstones and shales) overlies the BTC. Deposits of alluvial sediments (facies association A) and shallow-marine/foreshore sediments (facies association C), were recurrently interrupted by transgressive pulses (facies associations B and C). Every pulse is recorded by an erosional, cemented sandy-conglomerate bar with bivalves (Ostreidae, Isognomon ), balanids, gastropods and other marine bioclasts; or their transgressive equivalents. The lateral facies changes in each individual sequence of the BTC are related to: (1) the influence on the northern foreland margin of the tectonic activity of the southern orogenic margin; (2) the palaeorelief formed by irregularities of the substrate which controls the sediment dispersal; and (3) the evolution stages of the sedimentary systems.

Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P – T –time array

The Ronda peridotite in the Betic Cordillera of southern Spain is a relic of the sub-orogenic lithospheric mantle that was exhumed during earliest Miocene time from about 66 km depth. Overlying crustal rocks show an apparently coherent metamorphic zonation from high-pressure granulite-facies rocks at the contact to unmetamorphosed rocks 5 km higher in the structural sequence, indicating drastic tectonic thinning of the whole orogenic crust during exhumation. P – T paths from the peridotite and its crustal envelope indicate decompression with rising temperature to shallow depths. U–Pb ion microprobe dating of zircon, Ar/Ar dating of hornblende, Ar/Ar laserprobe dating of muscovite and biotite, and fission-track analysis of zircon and apatite reveal that cooling was extremely rapid in the interval 21.2–20.4 Ma. One-dimensional thermal modelling of the array of P – T –time paths indicates that an asthenospheric heat source at an initial depth of about 67.5 km is required to explain heating during exhumation, and that the main period of exhumation lasted 5 Ma, starting at around 25 Ma. Exhumation must therefore have directly followed removal of most, but not all, of the lithospheric mantle beneath the Betic orogen, and was coeval with a period of late orogenic extension that profoundly modified the crustal structure and created the present-day Alboran Sea in the western Mediterranean.

Structure and kinematics of the South Iberian paleomargin and its relationship with the Flysch Trough units: extensional tectonics within the Gibraltar Arc fold-and-thrust belt (western Betics)

The superposition of structures observed in a selected area of the external zones of the Betic orogen, situated in the westernmost segment of the peri-Mediterranean mountain belt, reveals an alternation of compressional and extensional events during the overall convergence of the African and Eurasian plates that bound the system. An Early Miocene compressional event produced the formation of the so-called Gibraltar Arc fold-and-thrust belt, formed by the units derived from the South Iberian paleomargin and the Flysch Trough. This wedge constitutes the footwall of the Gibraltar Thrust, whose hanging wall corresponds to the Alboran Domain. The outward migration of the compressional front (W-to-NWward in the South Iberian segment) was followed by the outward migration of the extensional locus, related to the Middle Miocene rifting that led to the Alboran Basin subsidence. The tectonic inversion of the Gibraltar Thrust was accompanied by normal faulting of the innermost part of the paleomargin and the Flysch Trough units. Finally, the Alboran region underwent N–S-to-NW–SE compression from Late Miocene onwards, which caused the uplift of the Gibraltar Arc area.

The Betic orogen and the Iberian–African boundary in the Gulf of Cadiz: geological evolution (central North Atlantic)

The study of the Gulf of Cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy and tectonics. The evolution of the southern Iberian margins was more complex than in most North Atlantic margins since it entailed several phases of rifting, convergence and strike-slip motions. Three main tectonic provinces surround the internal zones of the Gibraltar Arc orogenic belt. These include in the Iberian margin of the Gulf of Cádiz the flysch units of the Campo de Gibraltar complex, the Betic External Zones, and the Neogene basins of the Guadalquivir Valley. Fault-bounded blocks of flysch and Subbetic units crops out over large areas of the southeastern Iberian shelf. The basement of the northwestern area, in contrast, is represented by the Paleozoic rocks of the Hercynian massif of Iberia. Half-graben structures determined the main structural trends of the margin during the Mesozoic, which were affected by inversion structures during the Neogene compressional stages. The Mesozoic and lower Cenozoic units are best observed in wells and seismic profiles from the northern area. These units are either obscure below a thick olistostrome deposit or are absent in most of the rest of the Gulf of Cadiz. Seven lithoseismic units from Triassic to Upper Oligocene and another seven Neogene and Quaternary units are identified based on the relationship to the depositional sequence and the emplacement of the olistostrome. The first tectonic phase was characterized by a passive margin, which was controlled by the development of half-graben extensional structures and carbonate platforms. This evolution comprises the Mesozoic and early Cenozoic. Ocean-spreading in the North Atlantic induced extensional tectonics, which deformed the Cretaceous syn-tectonic post-Aptian deposits. Increased amounts of terrigenous materials were supplied to the margin from Aptian to Albian times, controlling depositional patterns, while terrigenous siliciclastic facies replaced the Jurassic carbonate platforms. From Middle Eocene to Early Miocene the margin was influenced by the relative motions of Iberia and Africa and the development of the Alpine orogeny. The Iberian–African boundary in the Gulf of Cadiz experienced transpression and the Mesozoic basins probably underwent subduction. The emplacement of an olistostrome took place in the Gulf of Cadiz towards the Central Atlantic basin plains during the Tortonian. The end of the olistostrome emplacement during the Late Miocene coincides with accelerated tectonic subsidence, while thick progradational and aggradational depositional sequences were developed. The occurrence of closely juxtaposed regions of compression and extension during the Miocene may reflect the westwards progression of the Gibraltar Arc mountain front over a subducting thinned Tethys crust. When the motion between Iberian and Africa was N–S to NNW–SSE oriented, the migration of the arcuate mountain front into the eastern Gulf of Cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere. In contrast to the Betic–Rif belts, however, there is no geophysical evidence to postulate that continental collision took place in the Gulf of Cadiz during the Cenozoic.

Exhumation of high-pressure metapelites and coeval crustal extension in the Alpujarride complex (Betic Cordillera)

The current configuration of the Alpujarride complex (Betic Orogen) comprises a stack of thinned tectonic units, made up mainly of continental crust. P-T path modelling in different levels of an Alpujarride unit (the Salobreña tectonic unit) reveals a high-pressure metamorphic stage followed by subsequent decompression at nearly isothermal conditions. Nevertheless, the P-T conditions differ from one level to another: while the top of the Permo-Triassic metapelites includes carpholite-kyanite-bearing assemblages (10 kbar/425°C), intermediate levels are characterized by Mg-rich chloritoid-Zn-rich staurolite-kyanite-bearing assemblages (minimum 10.5 kbar/450°C) and the bottom of the Palaeozoic metapelite sequence presents garnet-kyanite-plagioclase-bearing assemblages (average conditions of 13 kbar/625°C). Thus, the Salobreña tectonic unit represents an entire upper continental segment containing several crustal layers which underwent high-pressure metamorphism during Alpine continental subduction. The decompression, associated with an important mineral growth stage and the development of flat-lying regional foliation, reflects the thinning of this crustal segment, which resulted in parallel disposition of the bedding and metamorphic zones. At low-pressure conditions, large-scale folds affect the regional foliation, producing stratigraphic duplication, metamorphic inversions and the general reorganization of previous Alpujarride sheets. Finally, brittle Miocene extensional tectonics, coeval with the generation of the Alboran Basin, contributed to the definitive exhumation of the Alpujarride complex.

Turbidite facies, fluid-escape structures and mechanisms of emplacement of the Oligo-Miocene Aljibe Flysch, Gibraltar Arc, Betics, southern Spain

The Aljibe Flysch, the largest tectono-sedimentary unit in the Gibraltar Arc, provides a record of Oligo-Miocene sedimentation in the Betic Orogen, southern Spain. The Aljibe Flysch can be subdivided into two sub-units, the Beneiza Flysch (Late Oligocene) and the Aljibe Arenites (Early Miocene), which have very different depositional histories and mechanisms of emplacement. The Beneiza Flysch is characterised by laterally continuous and thin-bedded lithic arenites, siltstones and mudstones which contain complete and partial Bouma sequences. These are interpreted to have been deposited in a sheet-turbidite system which may have formed part of the North African Flysch Trough. By contrast, the aljibe Arenites comprise of thick-bedded super-mature quartz arenites which are dominated by fluid-escape structures. These sandstones are similar to Facies Group B1-B2 disorganised sands of Pickering et al. (1989), which are interpreted to be the product of deposition from high-density turbidity currents. Fluid-escape structures have been classified according to the scheme of Lowe (1975) and their vertical arrangement indicates that a large volume of sediment was deposited extremely rapidly during frequent remobilisation events. Consideration of the origin of the fluid-escape structures and their vertical arrangement has identified problems in their interpretation, and identified key areas for future research. The boundary between the Beneiza Flysch and Aljibe Arenites is interpreted to be a significant stratigraphic boundary across which there is a major change in sediment source and mechanism of sediment emplacement. Several models can be put forward for the generation of this stratigraphic break, and these serve to set the agenda for future research into the emplacement of other similar flysch sequences and the origin of the Gibraltar Arc Flysch.

Geophysical and geological constraints on the evolution of the Guadalquivir foreland basin, Spain

Abstract This paper presents a compilation and reinterpretation of available geophysical and geological data recently acquired for the ENE-WSW Guadalquivir foreland basin, located on the northern margin of the Betic orogen in southern Iberia. The data include seismic reflection and refraction profiles, well logs, gravity, geoid, surface heat-flow data and field observations. The deep structure of the southern Iberian margin is characterized by large variations in crustal thickness and high heat-flow values, which result in a very low lithospheric rigidity for the whole area. Geoid and gravity data show that deformation affected the crust and the lithospheric mantle differently, producing anomalous mass distributions that could act as subsurface loads. Seismic sequence analysis of the basin infill has permitted the re-assessment of the depositional sequential arrangement of the sediments deposited from Late Langhian-Early Serravallian to Messinian. They are arranged in six sequences and do not show any E-W progradational pattern indicating that during this period the acting loads moved essentially in a NNW direction. A careful analysis of the southern border of the basin shows that the ‘so-called olistostromes’ correspond to lateral diapirs of squeezed Triassic evaporites and internally imbricated Miocene wedges. We discuss the results obtained in terms of palaeo-geographic environments, time distribution and nature of acting loads, and constraints for future basin modelling approaches.

Lithospheric boudinage in the Western Mediterranean back‐arc basin

The western Mediterranean is composed of irregular troughs formed as back‐arc basins in the hanging wall of the W‐directed Apenninic subduction which retreated eastward during Neogene and Quaternary times. The basins are progressively younger toward the east, ageing from late Oligocene–early Miocene (Valencia, Provençal, Alboran and Algerian basins), to middle Miocene–Pleistocene (Tyrrhenian Sea). The basins isolated boudins of continental lithosphere, the Sardinia–Corsica block representing the largest. The boudinage has a wavelength of 100–400 km and facilitated stretching of the continental lithosphere with formation of new oceanic crust in the Provençal, Algerian, Vavilov and Marsili basins. The boudins developed both in the earlier Alpine–Betic orogen (Alboran basin) and in its foreland (Provençal and Valencia troughs). The extension appears clearly asymmetric due to its eastward polarity, accommodated by E‐dipping master low‐angle normal faults. Moreover the thinning shows variations in boudinage wavelength and is characterized by several along‐strike transfer zones and heterogeneities. The western Mediterranean back‐arc setting is comparable with Atlantic and western Pacific back‐arc basins associated with W‐directed subduction zones that show similar large‐scale lithospheric boudinage.

Gravity-induced recumbent folds and low-angle normal faults in the Alpujarras region (Betic Cordilleras, Spain): indications of Miocene extensional tectonics in the western Mediterranean

Large north-facing recumbent folds found in the Alpujarride Domain of the Betic orogen, heretofore related with a contractional nappe-stacking episode predating the early- to middle-Miocene extension, can be reinterpreted as gravitational folds, their development being connected with the extensional event that produced the low-angle normal faults and extensional detachments in Miocene times. This conclusion is based on detailed research carried out in the eastern part of the Alpujarras region (southern Betics) and includes the establishment of the fold geometry and metamorphic frame by means of field and mineralogical techniques, and the analysis of the relationships between recumbent folds and extension-related structures such as low-angle normal faults, stretching lineations, boudinage structures, etc.

Phylogeny and biogeography of midwife toads (Alytes, Discoglossidae): a reappraisal

The allozyme data base of Arntzen & García-París (1995) on midwife toads (Alytes, Discoglossidae) is reanalysed considering each locus as a discrete character. The phylogeny thus inferred differs from the one obtained with genetic distances in the position of A. dickhilleni from the Betic region – it appears that its sister species is the widespread A. obstetricans, not the Mallorcan endemic A. muletensis. This phylogenetic hypothesis agrees with the taxonomic treatment of the genus based on morphology. A testable biogeographic hypothesis is proposed to account for the diversification of midwife toads in Iberia and the Balearics. The postulated underlying geological changes were the spread of inland saline lakes that divided Iberia (16 mY B.P.), the emergence and break-up of the Betic orogen (14 mY), and the formation of the Betic Strait (8 mY). Dispersal over sea channels or during the Messinian Crisis (6 mY) are deemed unlikely on the basis of ecological and biogeographical data.