Internal Betics Research Articles

The paleotectonic evolution of the western Mediterranean: provenance insights from the internal Betics, southern Spain

Since the early Cenozoic, the closure of the Alpine Tethys in the Western Mediterranean has been accomplished by protracted subduction, followed by collision and orogenic collapse. The internal zones of the Betic Cordillera (southern Spain) and Rif (northern Morocco) experienced subduction metamorphism and subsequent exhumation due to the westward migration of the orogenic system. The detrital provenance of these meta-sedimentary units contains crucial insights into their pre-subduction stratigraphic arrangement, which is essential to constrain the pre-Cenozoic paleogeography and tectonic evolution of the Western Mediterranean. This study focuses on the Nevado-Filábride Complex (NFC) and the Eastern Alpujárride Complex in the Internal Betic Cordillera. New depth-profile zircon U-Pb LA-ICP-MS data from the NFC (N = 72) and Alpujárride (N = 21) and in situ apatite U-Pb data from a metabasite within the NFC allow us to establish the pre-subduction stratigraphy, sedimentary provenance, and paleotectonic configuration of the Iberian rifted margin during the Paleozoic–Early Mesozoic. Our dataset demonstrates that the NFC represents an intact Devonian to Early Jurassic stratigraphic sequence that records the evolution of the Western Mediterranean from the Variscan orogeny to rifting and opening of the Alpine Tethys. Detrital zircon U-Pb age modes of the NFC remain remarkably similar for over 200 Myr with only the differences being the progressive addition of new zircon modes related to depositional ages. Additionally, a comparison between the provenance record of the NFC and Alpujárride reveals notably similar zircon signatures of Carboniferous and Permian strata. This study reports the first data from the Triassic–Jurassic strata that record the opening of the Alpine Tethys. Additional evidence for rifting is present in the metabasites from the Veleta unit yielding in situ apatite U-Pb ages of 197 Ma, recording CAMP magmatism and Early Jurassic rifting in the Alpine Tethys. The data demonstrate a palinspastic connection between the NFC and the Alpujárride. Based on these extensive new provenance data, we propose a pre-subduction tectonic configuration in which the NFC and Alpujárride both represent adjacent attenuated continental fragments that are separated from the southern Iberian rifted margin by a narrow oceanic domain of the Alpine Tethys in the early Mesozoic.

40Ar/39Ar Age Constraints on HP/LT Metamorphism in Extensively Overprinted Units: The Example of the Alpujárride Subduction Complex (Betic Cordillera, Spain)

AbstractWidespread overprinting of early high‐pressure/low‐temperature (HP/LT) subduction stages due to subsequent collisional or late‐orogenic tectono‐metamorphic events is a common feature affecting the interpretation of geochronologic data from HP/LT orogens. The Betic‐Rif orogen is exemplary in this connection as a great majority of published radiometric ages are found to cluster around 20 Ma. This clustering is commonly interpreted as reflecting a short, yet complex, succession of tectono‐metamorphic events spanning only over a few Myr, including back‐arc extension and overthrusting of the Internal Zones on the External Zones. An alternative explanation consists in the poor preservation of a much earlier HP/LT metamorphic event, presumably Eocene, coeval with subduction and crustal thickening in the Internal Zones, and particularly the Alpujárride Complex. However, this age is vividly debated due to widespread resetting by the Early Miocene HT/LP overprint. In this study, we provide new 40Ar/39Ar evidence from white micas selected along an E‐W section of the Internal Betics, from the central to the eastern Alpujárride Complex. Our new data show (a) that exceptionally well‐preserved HP/LT parageneses in this unit retain a well‐defined Eocene age around 38 Ma, and (b) that widespread 20 Ma ages recorded all along the section correspond to a regional stage of exhumation, coeval with a major change in the kinematics of back‐arc extension. Our study provides conclusive evidence that 40Ar/39Ar dating of carefully targeted HP/LT associations can overcome the problem of extensive late‐orogenic overprinting, testifying for an Eocene HP event around 38 Ma in the Betic‐Rif orogen.

Opposite Symmetry in the Lithospheric Structure of the Alboran and Algerian Basins and Their Margins (Western Mediterranean): Geodynamic Implications

AbstractThe geodynamic evolution of the Western Mediterranean for the past 35 My is a matter of debate. Present‐day structure and composition of the lithosphere and sublithospheric mantle may help in constraining the geodynamic evolution of the region. We use an integrated geophysical‐petrological modeling to derive and compare the present‐day thermal, density and compositional structure of the lithosphere and sublithospheric mantle along two NNW‐SSE oriented transects crossing the back‐arc Alboran and Algerian basins, from onshore Iberia to the northern Africa margin. The crust is constrained using available seismic data and geological cross‐sections, whereas seismic tomography and mantle xenoliths constrain the upper mantle structure and composition. Results show a thick crust (37 and 30 km) and a relative deep LAB (130 and 150 km) underneath the HP/LT metamorphic units of the Internal Betics and Greater Kabylies, respectively, which contrast with the 16 km thick magmatic crust of the Alboran Basin and the 10 km thick oceanic crust of the Algerian Basin. The sharp change in lithosphere thickness, from the orogenic wedge to the back‐arc basins, contrasts with the gentler lithosphere thickening toward the respective opposed margins. Our results confirm the presence of detached slabs ∼400 °C colder than upper mantle and a fertile composition than the continental lithospheric mantle beneath the External Betics and Saharan Atlas. Presence of detached quasi‐vertical sublithospheric slabs dipping toward the SSE in the Betics and toward the NNW in the Kabylies and the opposed symmetric lithospheric structure support an opposite dipping subduction and retreat of two adjacent segments of the Jurassic Ligurian‐Tethys realm.

U-Pb geochronology of detrital and igneous zircon grains from the Águilas Arc in the Internal Betics (SE Spain): Implications for Carboniferous-Permian paleogeography of Pangea

New U–Pb detrital zircon and U–Pb zircon ages of metaigneous rocks in the Águilas Arc (Betic Chain, SE Spain) allow us to determine the maximum depositional ages of the rocks. Within the Nevado-Filábride Complex, a Late Carboniferous depositional age for the Lomo de Bas schists and quartzites, and a Permian-Triassic maximum depositional age for the Tahal Fm are determined. Within the Alpujárride Complex, the maximum depositional age of the Micaschists and Quartzite Fm is Late Carboniferous and the Meta-detrital Fm was deposited in the Early Permian. Furthermore, the maximum depositional age of the Saladilla Fm in the Maláguide Complex is also Early Permian. The age distribution patterns for the Carboniferous rocks of the Nevado-Filábride and Alpujárride complexes are similar to those from the Cantabrian Zone of the Iberian Massif, suggesting deposition in Carboniferous foreland basins located eastwards of the Iberian Massif. However, age patterns in Maláguide and samples from the North-eastern Iberian Peninsula and South France show strong similarities suggesting that it can be located near those areas in the Late Carboniferous times.The samples with Early Permian maximum depositional ages from the three complexes contain more Paleozoic zircon grains relative to the older Carboniferous samples, but have similar age distribution patterns, suggesting that they were deposited in the same basin. Samples from unconformable Middle Miocene sediments have Early Permian youngest zircon populations and age distribution patterns corresponding to a mixing of detrital zircon grains from the Alpujárride and Maláguide complexes. Furthermore, there is no record of any major felsic rocks formation and/or exhumation event after the Early Permian in those two complexes.

Sedimentary history and palaeogeography of the Cenozoic clastic wedges of the Malaguide Complex, Internal Betic Cordillera, southern Spain

The Cenozoic sedimentary cover of the Malaguide Complex (Internal Betic Cordillera, Spain), in the Almería and Málaga areas, consists of a suite of sedimentary successions from continental and shallow-marine to deep-marine environments. Structural and stratigraphic relations, and petrological and geochemical signatures reveal the sedimentary evolution of the Cenozoic Malaguide Complex (CMC) from pre-orogenic (Palocene-Eocene) to syn-orogenic (Oligocene-Early Miocene) stages. Sandstones detrital modes of the overall succession are heterogeneous testifying to a multi-source area, marked by exhumation of the Malaguide basement terranes and of the Internal Betic Zone (Alpujárride Complex) in lower position. Pre-orogenic and syn-orogenic strata consist of four main depositional sequences: the Mula Group (Paleocene), the Xiquena Group (Eocene) for the preorogenic successions; and Ciudad Granada Group (Oligocene-Aquitanian) and Viñuela Group (Burdigalian) for the synorogenic successions. Pre-orogenic strata evolve from intra-arenite to hybrid arenites to progressive increase of sandstones in abundance of detrital supply from sedimentary cover of the internal Betic units. The unroofing history of the internal Betic Units, predominantly in the Malaguide Complex, is clearly testified in strata of the synorogenic clastic units, where detrital supply is coming from the Malaguide Complex. Sedimentary lithic fragments were derived from the Mesozoic strata of the Malaguide Complex while metamorphic detritus is related to the Internal Betic Zone basement that was exhumed starting from the Oligocene. Pre-orogenic mudrocks mainly show abundance of calcite and dolomite over quartz and phyllosilicates. Syn-orogenic mudrocks, record an abrupt decrease in calcite and dolomite and an increase of phyllosilicate, quartz and feldspars mainly in the Malaga section. The geochemical signatures attest to a compositional variation of the samples from pre-to-synorogenic successions, with palaeoweathering indices showing moderate values and a weak up-section decrease. The Cenozoic Malaguide Complex played a key role in the geodynamic evolution of the Betic Cordillera, representing the key tectonic element of the western Mesomediterranean domains.

Geology, geomorphology and geohazards on a section of the Betic coastline, southern Spain

The geology of the Betic Cordillera has inspired national and international interest since the first half of the 19th century and structural and tectonic models continue to be refined. The coastal mountain ranges between Nerja and La Herradura expose rocks of the internal Betics that comprise predominantly sheared, folded and faulted schists, phyllites, quartzites and carbonates. Steep and complex terrain, adverse structure, highly disturbed rock masses, seasonally intense rainfall and coastal erosion create a challenging environment for infrastructure and urban development.

U-Pb ages of detrital zircons from the Internal Betics: A key to deciphering paleogeographic provenance and tectono-stratigraphic evolution

Zircons from the Nevado-Filábride Complex metamorphic rocks yielded 891 concordant inherited detrital ages. The 342 concordant ages from the Aulago Fm indicate a Pennsylvanian maximum depositional age, while the age density distribution indicates that this formation is closely related to the Cantabrian Zone (north of the Iberian Massif). The dominant Ediacaran and Cryogenian populations, at ca. 596 and 788 Ma respectively, relate the Nevado-Filábride Complex to northern Gondwana terranes. A discrete Mesoproterozoic zircon population at ca. 1031 Ma (Tonian-Late Estenian population) suggests that the basement of the Nevado-Filábride Complex could have been located near of the Saharan Metacraton and west of the East African Orogen at the beginning of the Paleozoic. The 549 concordant ages from the Tahal Fm yield an Early Permian age for the protoliths. The age density distribution pattern records erosion of rocks from the Variscan belt, including Late-Variscan magmatic rocks. Zircon spectra differ from those of the Alborán Domain, supporting the hypothesis that the Nevado-Filábride Complex is part of the South Iberian Domain. Furthermore, data from the Tahal Fm are similar to those of Permo-Triassic rocks from the Iberian Ranges.

Crust and mantle lithospheric structure of the Iberian Peninsula deduced from potential field modeling and thermal analysis

We investigate the lithospheric structure of the Iberian Peninsula and lateral crustal density variations using a three-step approach. First the crustal and mantle lithosphere thicknesses are calculated from joint geoid and elevation modeling combined with thermal analysis further constrained by seismic data. We then compute the 3D gravity effect of the resulting lithospheric structure to separate the measured Bouguer anomaly into its regional and local components. Finally we invert the residual gravity anomalies to highlight lateral average crustal density variations and discuss them in terms of crustal structures. Our results show that for the majority of the study area the crustal thickness does correlate with the regional topography pattern. The highest topography – above 1500m – shows thicknesses above 44km with local values up to 48km. Crustal thicknesses in the range of 36–40km are obtained in the uplifted Alpine areas while a thinner crust is observed in sedimentary basins and in the Iberian Massif (30 to 35km) with the exception of SW Iberia region where the crust thins from 30 to 28km. Thick lithosphere – above 140km – is found along the Pyrenees, the Cantabrian Mountains, the Iberian Chain and in the Betics while the thinnest lithosphere is found in SW Iberia (90km). 3D inversion of residual anomalies show that for the majority of the area the average density of the crust is in the range of 2810±10kgm−3. The denser crust is found in the NW and SW regions of the Iberian Massif (+30kgm−3 on average) and locally in the Pyrenees (above +70kgm−3), NW of the Iberian Chain (+15kgm−3 on average) and in the southern Internal Betics (+70kgm–3). The least dense crust is found in the central and western Betic Chain (−30kgm−3 on average) and in sedimentary basin depocenters.

Does subduction polarity control metallogeny? The Mediterranean case

AbstractThe distribution of ore‐deposit types in different sectors of the circum‐Mediterranean realm that have been affected by subduction processes since the Cretaceous varies in space and time. Sectors involved in W‐directed subduction (Sardinia, Apennines–Maghrebides, Internal Betics, Tyrrhenian, Western–Eastern Carpathians) are dominated by relatively low‐sulphidation epithermal (±VMS) deposits. Orogens formed by NE‐directed subduction (Dinarides–Hellenides–Pontides–Anatolides–Taurides; DHPAT) were initially dominated by pluton‐related porphyry–skarn–high‐sulphidation epithermal associations. These distinct metallogenic styles can be related to the systematic tectono‐magmatic asymmetry of E–NE‐ and W‐directed subduction systems and are analogous to the relationship observed in circum‐Pacific belts. Exceptions to this simple pattern occurred in the DHPAT in the Cenozoic, when deposit associations typical of both E‐directed and W‐directed systems were formed. Such exceptions are interpreted to reflect superimposition of contrasting subduction trends and inheritance from earlier metallogenic stages (Apuseni) or the interference of subduction processes with subduction‐unrelated extension (Hellenides, West Anatolia).

Natural and anthropogenic controls on soil erosion in the Internal Betic Cordillera (southeast Spain)

Soil erosion in southeast Spain is a complex process due to strong interactions between biophysical and human components. Significant progress has been achieved in the understanding of soil hydrological behavior, despite the fact that most investigations were focused on the experimental plot scale. Although experimental plots allow exploring the effect of multiple biophysical and anthropogenic factors, they provide limited insights in the combined effect of all factors acting together at the landscape scale. In this study, area-specific sediment yields (SSY) have been estimated based on the volume of sediment trapped behind 36 check dams in the southeast of Spain. Low SSY-values were reported (mean = 1.40 t ha −1 year −1: median = 0.61 t ha −1 year −1). SSY variability could be explained for 67% by catchment characteristics such as drainage area, soil characteristics, land cover, average catchment slope, and annual rainfall. The low SSY values are probably caused by the agricultural abandonment that occurred over the past decades and allowed the recovery of natural vegetation. Furthermore, our results suggest that the soils have eroded in the past to such an extent that nowadays not much sediment is detached by overland flow due to residual enrichment of clay and stones. Also, sediment is to a large extent trapped locally in the catchment, as indicated by the negative relationship between SSY and catchment area.

Velocity field and tectonic strain in Southern Spain and surrounding areas derived from GPS episodic measurements

Abstract The goal of this paper is to study the velocity field and deformation parameters in Southern Spain and surrounding areas (Ibero-Maghrebian region) using GPS episodic measurements. Results are compared to those previously published as well as deformation parameters derived from seismic data. For this purpose, a geodetic GPS network of 12 stations was observed during eight field campaigns from 1998 to 2005 by the San Fernando Naval Observatory (ROA), Spain. Relative GPS velocities in the Gulf of Cadiz with respect to the stable part of Eurasia are ∼4.1 mm/yr in a NW–SE to NNW–SSE direction. In the Betics, Alboran Sea and North of Morocco, velocities are ∼4.4 mm/yr in a NW–SE direction, and they are ∼2.3 mm/yr in a N–S direction in the eastern part of the Iberian Peninsula. These results are in agreement with the anticlockwise rotation of the African plate. GPS strain tensors are determined from the velocity model, to obtain a more realistic crustal deformation model. The Gulf of Cadiz is subjected to uniform horizontal compression in a NNW–SSE direction, with a rotation to N–S in the Alboran Sea and Northern Morocco. An extensional regime in a NW–SE direction, which rotates to W–E, is present in the Internal Betics area. In the Betic, Alboran Sea and North of Morocco regions we compare seismic deformation rates from shallow earthquakes with the determined GPS deformation rates. The comparison indicates a seismic coupling of 27%, while the remaining 73% might be generated in aseismic processes. Deformations measured in the Ibero-Maghrebian region with GPS could be interpreted in terms of either elastic loading or ductile deformation.

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.

Structure of the Maláguide Complex near Vélez Rubio (Eastern Betic Cordillera, SE Spain)

The Maláguide Complex of the Betic Cordillera represents a tectonic element which mostly underwent brittle deformation during the Alpine orogeny, and that covers the high‐pressure/low‐temperature (HP/LT) complexes of the internal Betic Cordillera. The evolution of this complex can help to determine both when the lower HP/LT complexes began to exhume and the behavior of the upper plate during this process. Moreover, this complex is a key element to understanding the relationships between the late orogenic extension in the internal Betic Cordillera and the coeval compression in the External Zones of the same mountain chain. This paper focuses in the geometry and kinematics of the Alpine structures of this Maláguide Complex in the eastern Betic Cordillera, near the village of Vélez Rubio. We propose that these Alpine structures can be grouped into three main stages. First stage (Paleogene) began with a synmetamorphic slaty cleavage produced in anchizone conditions. This foliation is associated with northward vergent structures and was probably connected with the superposition of the Maláguide Complex over the Alpujárride during the middle Eocene. This first stage ended with the thinning of the Maláguide Complex (Oligocene) by an extensional detachment with a top‐to‐the‐NNW sense of movement. Second stage records the convergence of the External and Internal Zones during the Aquitanian‐Burdigalian. This convergence was a right‐lateral transpression that produced back thrusts and extensional structures that exhumed the HP/LT rocks of the Alpujárride Complex. Third stage corresponds to the evolution from the late Burdigalian to the present‐day, when the Internal and External zones were welded together.

Tectonic and metamorphic evolution of the Temsamane units, External Rif (northern Morocco): implications for the evolution of the Rif and the Betic–Rif arc

Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP–LT) metamorphism (at c . 7–9 kbar and 330–430 °C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east–west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP–LT Temsamane units: (1) an internal origin related to the subduction and the HP–LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic–Rif region.

Evolution of depositional environments after the end of Messinian Salinity Crisis in Nijar basin (SE Betic Cordillera)

The Messinian succession is exceptionally well preserved in the intramontane sedimentary basin of Nijar in the Internal Betic Cordillera (SE Spain). Gypsum-olistostromes and large slabs of the Azagador Member are unconformably overlain by the latest Messinian sediments of the Feos Formation. These deposits, which are studied in detail in the Rambla Los Feos outcrop (basinal area), are characterized by a sedimentary cyclicity and two lithostratigraphic units can be distinguished: (1) a lower one that mainly consists of calcarenitic intervals with laminated pelites and limestone beds (2) an upper unit showing a cyclic alternation of fluvial fan deposits and lacustrine marls, that corresponds to the ‘Lago-mare episode’. The sedimentary succession records very precisely the strongly fluctuating environmental conditions during the upper Messinian, as well as the paleomorphological and paleogeographical changes that are likely related to late Miocene and Pliocene tectonic uplift of the Sierra Cabrera (Betic basement). The dominant fossil content, present throughout the upper unit in the lacustrine white marls of the Feos Formation, is represented by ostracods of the Loxocorniculina djafarovi FA, an assemblage of non-marine, aquatic communities which lived in brackish-water bodies (Lago-mare fauna). Extant ostracod faunas, similar to those documented in the Messinian fossil record and characterised by Cyprideis, Tyrrhenocythere, loxoconchids and leptocytherids, occur in hypohaline water bodies, in a salinity range of approximately 5–15‰ (mesohaline water). However, the number of species of Parathetian affinity is low, as observed in other perimediterranean areas (i.e. Apennines). The marine fauna association (foraminifera) present at levels is thought to be partly reworked, even though a marine influence at the end of the Lago-mare stage is not excluded.

Kinematics of a twisted core complex: Oblique axis rotation in an extended terrane (Betic Cordillera, southern Spain)

The Sierra de las Estancias is a metamorphic core complex that developed during the late Oligocene to early Miocene extensional collapse of the Betic Cordillera. Structural and paleomagnetic analysis shows that the northern half of the complex has been strongly modified by rotation about an inclined axis, requiring reassessment of the kinematics of the extensional phase of deformation. The extensional tectonic event produced two major detachment faults, associated mylonitic foliation and lineation, and pervasive sets of shear bands and brittle normal faults with a top‐to‐NE shear sense. An older foliation was largely transposed and reoriented during this event. The regional foliation now defines a broad antiform with a steep north dipping limb; and the trend of shear‐related lineations swings from NNE to ENE from the southern gently dipping limb into the north dipping limb. The remanent magnetization in a suite of near‐vertical early Miocene mafic dikes intruded into the north dipping limb has a declination of 243° and an inclination of −27°. Assuming that the dikes were originally vertical, a rotation of approximately 50° about an inclined SSW plunging axis preserves the near‐vertical orientation of the dikes and brings the remanence into agreement with the expected orientation for an early Miocene age. Correction for this rotation brings the foliation and lineation in the surrounding schists into orientations subparallel to those on the present southern limb of the antiform, and the dikes into a vertical E‐W orientation. The corrected orientation of the shear‐related lineation (NNE), normal faults, and dikes suggests that the original direction of extension during the late orogenic extensional event in the Internal Betic Cordillera may therefore have been around NNE/SSW, rather than E/W as currently accepted. The observed net tectonic rotation can be decomposed into a series of geologically plausible rotations involving domino‐style rotation due to slip on the late brittle normal faults, tilting about the regional fold axis related to thrust‐ramping onto the Iberian margin during Miocene collision, and a 30° clockwise rotation about a vertical axis related to oblique convergence with the Iberian margin.

Laboratory measurements of seismic P-wave velocities on rocks from the Betic chain (southern Iberian Peninsula)

Ultrasonic laboratory measurements of seismic compressional velocities ( V p) carried out on rock samples encompasing a wide range of metamorphic grades and lithologies from the Betic chain are presented. Measurements were performed at a high confining pressure on a total of 12 representative samples of the Alboran Domain (Internal Betics). P-wave mean velocities, referred to pressure conditions of 280±10 MPa, range from 5.82 to 7.13 km s −1, the maximum values corresponding to high amphibolite facies metapelites (garnet gneiss). Under these pressure conditions, V p anisotropies range from 1.2 to 16.1%, with the lowest values being associated with granitic rocks and the highest values associated with metapelites from the upper structural units. P-wave velocities were calculated for prevailing P– T conditions at depths of 10, 20, 30 and 40 km. We discuss the significance of the analyzed rock-types in terms of metamorphic grade and derived V p at different crustal levels. P-wave velocities and densities measured on low-to-middle and middle-to-high grade metamorphic rocks are compatible with values obtained from previous seismic and gravity studies for the upper and middle crust, respectively. At lower crustal levels, the integration of our measurements with other geophysical data suggests the presence of partial melting of high-grade metamorphic rocks. The estimation of the impedance contrast from the measured physical properties suggests that the reflective character of the mid-lower crust is associated with shear/mylonitic zones and/or fine-scale lithological layering enhanced by partial melting.

A high electrical conductive zone at lower crustal depth beneath the Betic Chain (Spain)

Magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the Central Betic chain. Interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two-dimensional resistivity model along a NW–SE profile from the Guadalquivir foreland basin and across the boundary between the External and the Internal Betics. Elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected. The conductors in the upper crust coincide with the areal distribution of the Cenozoic and Mesozoic sedimentary rocks from the Guadalquivir basin and the External Betics and the conductors in the middle crust with the major Miocene extensional detachments affecting the basement rocks of the Internal Betic zone. A high conductivity zone at lower crustal levels beneath the outer Internal Betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective Moho. This conductor was interpreted as partial melting of a southeast dipping subducted Iberian lower crust. The geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the Alboran domain or to the Iberian plate. In the former case it would correspond to a major plate boundary and in the latter to an intraplate feature.

Local earthquakes seismic tomography in the Betic Cordillera (southern Spain)

A crustal tomographic image, from the surface down to 35 km depth beneath the Betic Cordillera (southern Spain), is obtained using data on local earthquakes recorded at stations from the National and Andalusian Seismic Networks. The velocity structure and the hypocentre locations are derived from the inversion of P first arrival times, using an iterative simultaneous inversion method. The reliability of the results is assessed using different control parameters. The inverted velocity field in the uppermost layers shows a significant lateral variability which reflects most of the large-scale geological features of the Betic Cordillera. Well determined local surface anomalies allow to constrain the location and geometry of the most prominent Neogene sedimentary basins. The upper crust is well resolved throughout the whole region, and is characterized by relatively high velocities in the Internal Betics and in the South Iberian Massif and lower velocities within the External Betics. A relatively well constrained event cluster displays a NNE–SSW trend, and outlines the contact zone between the Internal and the External domains. The middle and lower crustal levels show reliable results beneath the central part of the Betic Cordillera. High averaged velocities are obtained within the South Iberian and the Alboran domains, in contrast to a relatively low velocity anomaly which characterizes the boundary between them. These findings support the hypothesis of the lack of well differentiated crustal levels below the contact zone, while crustal layering is better defined beneath the Alboran and the Iberian domains.

A multidisciplinary geophysical study in the Betic chain (southern Iberia Peninsula)

The combined analysis of magnetotelluric measurements, tomographic velocity models and deep seismic reflection images confirms that the Betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties. At depth these data sets show evidence for a non-coincidence of the petrological and the seismic Moho beneath the Betics chain. The data sets reveal the geophysical properties of the Alboran domain (Internal Betics) and the Iberian Massif (External Betics). According to this, the Iberian crust features a relatively high seismic velocity, is seismically transparent in the seismic reflection images and is electrically resistive. The Alboran domain crust is characterized by a low average velocity, displays high reflectivity in the seismic reflection images and is electrically conductive. The outcrops of the metamorphic complexes (Alpujárride and Nevado Filábride), showing relatively high velocities coupled with low Vp/Vs values (1.67) derived from the Wadati slopes, suggest the existence of rocks rich in silica beneath the Alboran domain crust. An interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the Iberian upper crust. Partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels. These would account also for the reflectivity and the low Vp/Vs ratios mapped beneath the Alboran domain.