Iberian Plate Research Articles

Post Late Paleozoic tectonism in the southern Catalan Coastal Ranges (NE Spain), assessed by apatite fission track analysis

We report the first apatite fission-track thermochronologic data for 17 samples from the southern Catalan Coastal Ranges of NE Spain. Thermal histories of Carboniferous metasediments, Late Hercynian intrusions and Lower-Triassic Buntsandstein sediments from three tectonic blocks, Miramar, Prades and Priorat, are derived and interpreted within the geodynamic framework and tectonic evolution of the region. The apatite fission-track ages range from 198±24 to 38±5 Ma and mean fission-track lengths are all <13.3 μm. Samples throughout the study area underwent total track annealing during the Late Hercynian magmatic episode, followed by fast cooling prior to the deposition of Lower Triassic sediments. The Lower Triassic sediments and basement rocks underwent a temperature increase during a first Mesozoic rift phase in Middle Triassic–Early Jurassic times resulting in the complete or near complete annealing of the fission-tracks. During a second Mesozoic rifting stage, in Late Jurassic to Early Cretaceous time, differential tectonic block activity is observed in the three studied tectonic blocks. Subsequently, during Late Cretaceous a long-period of thermal stability, detected in all samples, is related to the post-rift episode. The onset of fast cooling registered in the apatite fission track system during Paleogene times is related to the Pyrenean orogeny. Compressional forces associated with the ongoing southern migration of the convergence forces at the Iberian plate boundaries caused unroofing of about 2–3 km of material of the Prades and northwestern flank of the Priorat block. Extensional collapse in Late Oligocene–Miocene related to the Western Mediterranean rifting triggered the denudation of about 2 km of material from the southeastern flank of the Miramar, Prades and Priorat blocks.

Sequential limb rotation and kink-band migration recorded by growth strata, Almazán Basin, North Spain

Abstract The geometry of growth strata in the Almazan monocline demonstrates that limb rotation and kink-band migration can coexist as folding mechanisms. The Almazan Basin, showing an overall synclinal geometry, was one of the most actively subsiding areas during the Tertiary in the inner part of the Iberian plate, with more than 3500 m of preserved Tertiary non-marine sediments. They are arranged in five tectonosedimentary units, TSU (A1 to A5), bounded by unconformities at the basin borders. The southern basin border is defined by the WNW–ESE-trending Almazan monocline, that can be followed for 25 km along strike, with a maximum structural relief of about 2500 m. The maximum changes in thickness on both limbs of the monocline occur in units A2 and A3, which constrains the maximum deformation period to between the Middle/Upper Eocene (Headonian, top of A1) and Upper Oligocene/Lower Miocene (Agenian, base of A4). The detailed geometry of the Mesozoic, pre-growth, and the Tertiary growth strata of the Almazan monocline was determined from the analysis of six seismic reflection sections, at high angles or perpendicular with the fold axis. The average dip of the monocline changes from 15° in the east to more than 45° in the western sector, being replaced westwards by a north-verging, steeply dipping thrust. In some sections, growth strata in the dipping limb show rotational offlap–onlap syntectonic unconformities, with progressively decreasing dips from bottom to top, indicating that limb rotation took place during the first stages of folding. At the limits of the dipping limb (anticline and syncline), two kink bands appear in some of the sections, the one located on the syncline being more developed. Within each kink-band, beds show a constant dip of 30–40°. The axial surfaces of each kink band converge upwards in the third and fourth tecto-sedimentary units, being parallel in the older units. The fixed axial surface occurs in a down dip position with respect to the active axial surface. The geometry of growth strata indicates that the mechanism of folding followed a sequence of: (i) a first stage of large-scale limb rotation (wavelength larger than 5 km), with minor development of kink-bands in some sections, and (ii) a second stage of more localised kink-band migration, probably favoured by fault propagation in the basement.

Tertiary compressional deformation of the Iberian plate

The Tertiary deformation of the Iberian plate is here interpreted as the result of changes in the coupling between the Iberian–African plates. During the early stages of the Africa/Iberia subduction (Palaeocene), deformation was confined at the Betic plate boundary. From the Eocene, during the collision in the southern plate margin, compressional deformation delocalized and distributed throughout the Iberian plate. First, in the Pyrenees, where the main stage of thrusting occurred during the Late Eocene – Early Oligocene. Then (mainly Oligocene – Late Miocene), in the inner part of the Iberian plate, forming basement uplifts in the Iberian Chain and the Central System, in correspondence of pre‐existing (Mesozoic and Variscan) structures. Finally, during the decay of compression inside the Iberian plate, extension took place the Mediterranean margin and the Alboran Sea.

Chronostratigraphic framework and evolution of the Fortuna basin (Eastern Betics) since the Late Miocene

ABSTRACT A Tortonian to Pliocene magnetostratigraphy of the Fortuna basin supports a new chronostratigraphic framework, which is significant for the palaeogeographical and geodynamic evolution of the Eastern Betics in SE Spain.The Neogene Fortuna basin is an elongated trough which formed over a left‐lateral strike‐slip zone in the Eastern Betics in the context of the convergence between the African and Iberian plates. Coeval with other basins in the Alicante–Cartagena area (Eastern Betics), rapid initial subsidence in the Fortuna basin started in the Tortonian as a result of WNW–ESE stretching. This led to transgression and deposition of marine sediments over extensive areas in open connection with the neighbouring basins. Since the late Tortonian, N–S to NW–SE compression led to inversion of older extensional structures. The transpressional tectonics along the NE–SW‐trending Alhama de Murcia Fault is related to the rising of a structural high which isolated the Fortuna basin from the open Mediterranean basin. The progression of basin confinement is indicated by the development of restricted marine environments and deposition of evaporites (7.8–7.6 Ma). The new basin configuration favoured rapid sediment accumulation and marine regression. The basin subsided rapidly during the Messinian, leading to the accumulation of thick continental sequences. During the Pliocene, left‐lateral shear along the Alhama de Murcia Fault caused synsedimentary folding, vertical axis block rotations and uplift of both the basin and its margins. The overall sedimentary evolution of the Fortuna basin can be regarded as a developing pull‐apart basin controlled by NE–SW strike‐slip faults. This resembles the evolution that has taken place in some areas of the Eastern Alboran basin since the late Tortonian.

The Malpica–Lamego Line: a major crustal-scale shear zone in the Variscan belt of Iberia

The Malpica–Lamego Line (MLL) is a deformation zone in the Variscan belt of NW Iberia (NW Spain and N Portugal) that runs parallel to the chain for at least 275 km, bearing I-type granodiorite plutons along most of its length. The MLL affects previous structures by which high pressure and ophiolitic rocks were exhumed and emplaced on the Iberian plate during earlier deformation phases. Correlation and reconstruction of the stratigraphy of these sheets or tectonic units at both sides of the shear zone allows a preliminary estimate of the accumulated vertical and horizontal offsets after the tectonic activity of the fault. The value of the separations, of crustal-scale proportions, reaches a maximum 15 km of vertical offset that decreases gradually to the south. The structural record found in the rocks indicates a strike-slip regime that, in general, does not fit the geometry of the offsets. We suggest that the MLL went through two different stages during the same orogenic cycle: a first dip-slip episode, a reverse faulting event, overprinted by a later strike-slip reactivation.

The Submarine Volcanic System of the Errigoiti Formation (Albian-Santonian of the Basque-Cantabrian Basin, Northern Spain): Stratigraphic Framework, Facies, and Sequences

ABSTRACT The Errigoiti Formation, located in the central domain of the Basque-Cantabrian basin, is an Albian to Santonian bathyal submarine volcanic system of alkali basaltic character. These volcanics occurred in an extensional geodynamic context generated by the drifting of the Iberian plate with respect to the European plate. Stratigraphic analyses reveal the facies distribution and depositional architecture of a longitudinal cross section of the Errigoiti volcanic system. Four facies groups were distinguished: sheet lavas (SL), derived from low-viscosity lava flows at high magma discharge rate; pillow lavas (PL), derived from intermediate-viscosity lava flows at moderate discharge rate; pillow breccias (PB), derived either from in situ fragmentation of lavas (PB1) or rock-falling from pillow-lava slopes (PB2); and stratified volcaniclastites (VC), derived from volcaniclast resedimentation through gravity flows, either by early remobilization (VC1) or post-volcanic reworking (VC2). The facies groups occur in a predictable repetitive succession, essentially from base to top, an SL-PL-PB-VC stacking pattern. This is interpreted to reflect an upward waning in the magma discharge rate. Five major volcanic sequences make up the Errigoiti Formation. Each comprises packages tens to hundreds of meters thick, deposited during short time spans of volcanic activity separated by long time intervals of volcanic inactivity. Consequently, boundaries between sequences are associated with marked hiatuses, reworked volcaniclasts, and/or turbidite-pelagic sedimentation. Analysis of lateral and vertical facies evolution enabled us to propose a depositional model consisting of three depositional environments. From proximal to distal settings, these are: amalgamated pillow volcanoes, scattered pillow volcanoes, and volcanic fringe, basically responding to a proximal to distal PL/PB, PL/VC, and SL/VC evolution of facies associations. Contemporaneous fossiliferous turbidite-pelagic sequences that surround the volcanic succession allow paleobathymetry to be estimated on the basis of biofacies. Sites with the highest rate of volcanic emission correspond to an average depth of 900 m; sites with thinner volcanic pile accumulated at a minimum depth of 1100 m. The facies sequence established in this work and the processes involved in its formation are comparable with the sequences and processes described in other submarine basic volcanic systems. Consequently, the depositional model for the Errigoiti Formation can perhaps serve as a reference model for this type of volcanic system.

Deep reflection seismic images of the crustal thinning in the eastern Pyrenees and western Gulf of Lion

The results of the MCS LISA seismic survey in the Mediterranean Sea and the wide angle data recorded by the onland stations during this experiment are merged and compared with the ray tracing modeling and the deep structure known from previous refraction studies. The results are very good, arriving at a new deep seismic image of the crustal structure. The LISA seismic profile 26, shown is this publication, crosses in strike line a deep basin close to the eastern Pyrenees. The shallow part of the seismic profile is poorly imaged but, thanks to the very good ELF seismic profiles crossing the LISA survey, the acoustic basement and a decollement level can be drawn. The deep part of the LISA profile data has a much better quality than the shallow part and reveals a high velocity (7.1 km/s) lower crust, overlying a shallow Moho interpreted at 8 s TWTT. The wide angle reflections enable the comparison between the ray tracing measured by distance and the seismic image measured by time thereby making the relationship between the continent and the continental shelf clearly delineated for the first time. The thinning of the thickened crust of the eastern Pyrenees does not occur at the shore but inland at the western boundary of the Roussillon basin. However, the central part of the Gulf of Lion continental shelf forms a deep basin that overlies a thinned crust. The age of the thinning is discussed in the general context of the convergence and divergence of the European and Iberian plates including the Sardinia-Corsica block. The geometry of the transfer faults is studied and shown as not steep and to have normal and strike-slip components.

Sismotectonique des Pyrenees; principales contraintes sismologiques

Abstract This paper presents a critical review of the main seismological data which constrain the Pyrenean tectonics, mostly the spatial distribution of the hypocenters and the fault plane solutions. A new seismicity map is computed for the period 1989-99, and a compilation of the published focal solutions is presented and discussed. The Pyrenean range results from the convergence of the Iberian and Eurasian plates for about 65 Ma. This convergence followed a period of extension (-115 to -80 Ma), which is related to the opening of the Bay of Biscay. The boundary between the two plates, the North Pyrenean Fault (NPF), is recognized as a major tectonic feature running E-W to the north of the range. The eastern part of the range has been later affected by the Mediterranean tectonics (about 30 Ma ago), which generated some faults with NE-SW and NW-SE orientations in Catalonia. The Pyrenean structure is rather well known thanks to systematic studies carried out during the last two decades: crustal exploration from refraction and reflection seismic experiments, gravimetric studies, and tomographic imaging of the lithosphere using either seismological methods or magnetotelluric methods. The most important results are the detection of a 10-15 km Moho jump at the NPF with a thicker crust at the Iberian side, the evidence for a subduction of the Iberian lower crust beneath the North Pyrenean Zone along the eastern and central parts of the range, and the imaging, in the central and western Pyrenees, of two large blocks of lower crust material uplifted through the upper crust. Since 1989, several seismic networks set up by various institutions in the different parts of the range have allowed to produce a rather accurate seismicity map. Since 1997, the seismic network on the French side has been significantly upgraded, and the geographic distribution of the stations has been made more even. This network consists now in fifteen three-component, short period stations with telephonic transmissions, and five one-component stations with Meteosat satellite transmissions, which ensure an access to the data in case of damaging earthquake. Four stations of the French Atomic Energy Commission (CEA) are running in addition on the French side. 22 stations are set up on the Spanish side, with a higher density in Catalonia. With all these stations together, about 450 earthquakes with magnitudes &amp;gt; or =1.0 can be located each year in the Pyrenees, with a mean accuracy of + or -1.5 km for the horizontal coordinates, + or -3 km for depth, i.e. a factor of two better than with the previous networks. The new seismicity map reveals some new features which were not apparent in the previous map. In particular, the NPF is not a single linear feature, but it includes a multiplicity of segments which are not necessarily E-W oriented. The Adour Fault (also called "Bigorre Fault" in some previous articles), a NW-SE oriented fault in central Pyrenees, appears as a complex zone which possibly includes several segments parallel to each others. The seismicity beneath the Maladeta, which is the largest unit of the Paleozoic Axial Zone, appears more linearly distributed than in the previous map. Most of the events are located inside the upper crust (depth less than 12-15 km). Deeper events are mostly located in the western part of the range.

Remagnetization of Lower Cretaceous limestones from the southern Pyrenees and relation to the Iberian plate geodynamic evolution

Paleomagnetic study (34 sites) of thick Cretaceous ∼5000 m marine carbonate strata from the Organyà Basin (OB) in the southern Pyrenees reveals a uniformly normal polarity characteristic remanence with varying declination (D) (Berriasian‐Barremian, D = 295°; Aptian‐Albian, D = 348°, upper Cretaceous, D = 354°) that resides in magnetite and predates Late Cretaceous/Tertiary folding. The lack of reverse polarity magnetizations in the Berriasian‐Barremian series, together with their distinct hysteresis properties that are typical of remagnetized carbonates elsewhere, are taken as evidence for a secondary overprint in the lower part of the succession. A hypothesized age of remagnetization near the Barremian/Aptian boundary fits with a major tectosedimentary event in the basin that changed deposition from platform to basinal conditions and increased subsidence during the Aptian. The angular difference (∼53°) between declination from the remagnetized strata and the mean for Aptian‐Albian strata indicates that a counterclockwise rotation occurred between the two acquisition times. Part of this rotation is interpreted to be local and due to slip variation along strike of a normal fault that bounded the basin and tilted the pre Aptian series. This Cretaceous extensive phase is framed in the rift evolution of the northern margin of the Iberian plate. The inferred remagnetization age within the OB falls within the range of previously inferred widespread Cretaceous remagnetization events affecting the Iberian plate. In the OB, remagnetization is speculated to have taken place through chemical (burial diagenetic) origin rather than simple thermoviscous resetting.

Intraplate deformation and basin formation during the Tertiary within the northern Iberian plate: Origin and evolution of the Almazán Basin

The Tertiary Almazán Basin was a topographic high during the Mesozoic rifting at the northern part of the Iberian plate. It was bordered by two Mesozoic basins: (1) the Castilian Basin (with a maximum thickness of more than 2000 m) to the south and (2) the Cameros Basin (with a maximum thickness of 9000 m) to the north. To the east the Almazán Basin is bordered by Paleozoic rocks of the NW‐SE trending Aragonian Range, which underwent little subsidence during the Mesozoic. The two Mesozoic basins show an E‐W direction. During the Tertiary NNE‐SSW compression (mainly late Eocene‐early Miocene), both Mesozoic basins were inverted and uplifted, and the Almazán Basin, with an overall synclinal geometry, was filled with syn‐tectonic terrestrial sediments, reaching a maximum thickness of 3500 m. Deformation during the Tertiary was strongly controlled by preexisting structures, both Mesozoic extensional faults and reactivated Variscan folds. The depocenter of the Tertiary basin migrated southward during the Paleogene. This migration is consistent with a piggyback transport of the whole basin northward, due to the movement on the underlying, crustal‐scale north verging Cameros thrust. The geometry and sedimentary evolution of the Almazán thick‐skinned piggyback basin was controlled by the uplift associated with the main crustal‐scale thrust, which was the main source area, and was separated from the foreland basin since the early stages of filling (late Eocene). This makes the main difference with thin‐skinned piggyback basins, controlled by the foreland thrust belt and separated from the foreland basin only at their late stages.

Seismic signature of Variscan and Alpine tectonics in NW Iberia: Crustal structure of the Cantabrian Mountains and Duero basin

The seismic structure of the crust of the NW Iberian Peninsula, along the Cantabrian Mountains and their southern foreland, the Duero basin, is investigated from the first data set available from refraction and wide‐angle reflection profiles. The velocity‐depth distributions obtained along E‐W and N‐S transects evidence lateral variations in crustal structure that can be associated to Variscan and Alpine tectonic episodes. The western part of the range, toward the Variscan hinterland zones, as well as the Duero basin below the sediments, show a crustal structure similar to that of the Variscan belt elsewere in Europe. The total crustal thickness is 30–32 km, and three main layers (upper, middle, and lower crust with average velocities 6.0, 6.25, and 6.8 km/s) are resolved. Some Paleozoic structures such as the basal detachment of the Cantabrian Zone are expressed in the seismic models. The crust appears to vary eastward in the external zones. The most outstanding feature is a crustal root beneath the highest Cantabrian summits where the Moho is found at 47 km depth. A prominent Alpine reworking of the crust in the eastern part of the Cantabrian Zone is thus revealed, and the N‐S seismic transects delineate a geometry similar to the one found across the central Pyrenees in the Etude Continentale et Océanique par Reflexion et Refraction Sismique (ECORS) profile, revealing the importance of the Alpine deformations in the northern part of the Iberian plate.

The interplay of lithospheric flexure and thrust accomodation in forming stratigraphic sequences in the southern Apennines foreland basin system, Italy

Most of the regional shortening in the type area of the southern Italy orogenic belt, Calabria and southern Apennines, was accomodated by displacement on the Calabrian Terrane (Oligocene to middle Miocene), the Liguride and Sicilide Complex (lower to middle Miocene), the Alburni-Cervati-Pollino-Monti della Maddalena units (inner carbonate platform and slope; middle to upper Miocene), Lagonegro units (upper Miocene), the Monte Alpi unit (lower Pliocene) thrust systems. Inception of each thrust system generated synorogenic sediment associated with frontal thrust-tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland. The Neogene kinematic and sedimentary evolution of the southern Italy is related by shortening of the Apennines and Calabria and stretching of the Tyrrhenian Basin. The southern Italy orogenic belt consists of tectonic assemblages of Paleozoic crystalline rocks (various metamorphic rocks and plutonics), Triassic through upper Miocene platform to deep-marine carbonatic and siliceous rocks, Jurassic to Cretaceous ophiolites and related oceanic sedimentary strata, Paleogene to lower Miocene Subduction Complex assemblage, and lower Miocene through Pleistocene foreland clastic strata. Triassic through Quaternary clastic sediments are accumulated in tectonically diverse sedimentary basins, recording the key geodynamic events affected the Mediterranean region after the Late Paleozoic-Early Mesozoic breakup of Pangea. These sediments are generated following the onset of the sequential rift development and continental separation (Triassic to Early Jurassic), the Tethyan proto-oceanic rifting and open-ocean phases (Early-Middle Jurassic to Early Cretaceous), the Tethyan subduction and onset of Alpine orogenesis (Middle-Late Cretaceous to Paleogene), the Adria (or Apennines) orogenesis (early Miocene to present), and the Tyrrhenian backarc rifting (or Tyrrhenian phase; 15 Ma to present). The oldest (early-middle Triassic) siliciclastic sediments of southern Italy are on the Apulia plate (the buried base of the Apulia platform), the Monte Facito Formation of the Lagonegro Basin, and redbeds of Sardinia (Iberian plate). During Upper Triassic to Middle Jurassic, continental redbeds (rift-valley phase), and shelf-slope-basin sediments (proto-oceanic rift phase) having quartzose composition were accumulated directly on the crustal basement of the Iberia plate margin, in Calabria-Peloritani domain, but siliciclastics are absent on the southern Apulia plate. These sediments record the initial phases of the Tethyan rifting processes. Siliciclastic accumulations in southern Italy assemblages during the main alpine phases (Cretaceous to Paleogene) are scarse or absent. The Cretaceous to early Miocene alpine tectonic phases in southern Italy include remnants of the Alpine orogen (Calabrian Block) and quartzofeldspathic siliciclastic deposits of the Oligocene to early Miocene Paludi, Stilo-Capo d’Orlando and Frazzano formations, and volcanolithic, quartzolithic and quartzofeldspathic sands of the Calabro-Lucanian Flysch Unit. The siliciclastic deposits are directly related to new subduction plane of the Adriatic-Ionian oceanic lithosphere beneath the Iberia plate, generating an oceanic subduction complex (Liguride Complex of southern Italy), and a calcalkaline volcanic arc (Sardinia). From late Paleogene to the present, the siliciclastic sedimentary sequences of southern Italy filled basins that are directly related to this convergent setting, causing consumption of the oceanic lithosphere, and subsequent accretion of the Calabrian allochthonous terranes over the Adria-Africa plate, generating post-Oligocene foreland basin systems. The composition as well as stratigraphic and structural relations of clastic sediments derived from erosion of the Calabrian microplate provide constraints for paleogeographic and tectonic models of southern Italy. Clastic detritus in the following sedimentary assemblages were derived mainly from provenance terranes within the strongly deformed Calabrian Arc allochthon: (A) the Paleogene «Calabro-Lucanian Flysch Unit» accretionary wedge, deposited in a remnant ocean basin that lay east of the Calabrian Arc and west of the Adria margin; (B) Burdigalian to lower Messinian foreland basin successions, widely developed in the southern Apennines, representing progressively shifted foredeep basins and related wedgetop basins; (C) Upper Tortonian to Messinian nonmarine to shallow-marine and deep-marine successions, cropping out on the western and eastern Calabrian Arc, representing synrift clastic wedges related to backarc rifting in the peri-Tyrrhenian area (western sequences), or foreland clastic wedges in the peri-Ionian area (eastern sequences); (D) Pliocene and Quaternary of the northern Calabrian Arc, represented by foredeep and related wedge-top basins on the eastern side (Gulf of Taranto and Corigliano Basin), and a slope basin on the western side, the eastern Tyrrhenian margin (Paola Basin). These receive detritus primarily from deep erosion of northern Calabria. The modern deep-marine basins of offshore northern Calabria have many similarities to the middle to upper Miocene clastic sequences in both foreland and backarc regions of the southern Italy. The type of sedimentary provenance of the southern Italy foreland basin system, providing an example of the changing nature of the orogenic belt through time, may contribute to general understanding and application of other major orogens.

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.

Interaction of tectonic and depositional processes that control the evolution of the Iberian Gulf of Cadiz margin

This study provides an integrated view of the growth patterns and factors that controlled the evolution of the Gulf of Cadiz continental margin based on studies of the tectonic, sedimentologic and oceanographic history of the area. Seven sedimentary regimes are identified, but there are more extensive descriptions of the late Cenozoic regimes because of the larger data base. The regimes of the Mesozoic passive margin include carbonate platforms, which become mixed calcareous–terrigenous deposits during the Late Cretaceous–early Tertiary. The Oligocene and Early Miocene terrigenous regimes developed, in contrast, over the active and transcurrent margins near the African–Iberian plate boundary. The top of the Gulf of Cadiz olistostrome, emplaced in the Late Miocene, is used as a key horizon to define the `post-orogenic' depositional regimes. The Late Miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the Miocene Atlantic–Mediterranean gateways. The terrigenous drift depositional regime of the Early Pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the Mediterranean outflow currents that developed after the opening of the Strait of Gibraltar. The Late Pliocene and Quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels. Deposition of shelf-margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence. During the highstand regimes of the Holocene, inner shelf prograding deltas and deep-water sediment drifts were developed under the influence of the Atlantic inflow and Mediterranean outflow currents, respectively. A modern human cultural regime began 2000 years ago with the Roman occupation of Iberia; human cultural effects on sedimentary regimes may have equalled natural factors such as climate change. Interplay of tectonic and oceanographic controls dominated the evolution of the Cadiz margin during the Cenozoic. Depositional sequences formed where the tectonic setting provided the accommodation space and the shape of the deposits has been greatly influenced by the strong unidirectional Atlantic inflow currents on the shelf and Mediterranean outflow currents on the slope. The entire cycle of the inflow and outflow deposition along the margin has been controlled first by the tectonic evolution of the Betic and Rif gateways, which become closed during the Late Miocene, and after the Messinian by the opening of the Strait of Gibraltar. Strong current development during eustatic sea level highstands of the Pliocene and Quaternary has controlled deposition because of maximum sill depths at Gibraltar for water circulation. Lowstand sea levels slowed circulation and resulted in mud drapes over the slope and regressive stratigraphic sequences over the shelf. More recently, the human industrial revolution has caused heavy metal contamination of sediment and water over the Cadiz margin. Human activity also has affected sedimentation rates because of deforestation that caused increased depositional rates near undammed rivers and decreased rates where rivers have been dammed. Future research efforts will need to focus on: (1) the effect of increased Mediterranean outflow caused by river damming plus global warming and the increased outflow as a potential trigger for new ice ages; (2) assessments of geologic hazards for planning man-made shoreline structures, developing offshore petroleum resources and maintaining undersea communications cables; and (3) confirmation of the general geologic history of the Cadiz margin.

PERIDOTITE BODIES IN THE W MEDITERRANEAN – TECTONIC SLICES IN AN ALPINE BELT FRAGMENTED BY LATE STAGE EXTENSION AND INHERENT SLAB ROLL-BACK

Integrating a wide range of multidisciplinary data, i.a., seismic tomography features, regional tectonic transport directions and uplift/cooling rates, and ages of subduction/collision, lithospheric extension and slab detachment, suggests a new model for the development of the Alpine belt in the W Mediterranean (Fig. 1; Zeck, 1996; 1997; 1999). The model implies that the backbone of the Alpine orogeny was formed by a composite SW-NE striking Betic-Ligurian subduction system, active until some time before 22 Ma (Fig. 1b). The system which consumed Mesozoic Tethyan oceanic lithosphere was W-ward dipping under the leading edge of Iberia which was drifting E-ward under the influence of the opening of the North Atlantic. Subduction activity was terminated by the formation of the Alpine orogenic belt through collision of Iberian and Betic-Ligurian continental lithospheres. A series of late stage extensional regimes, with local formation of Neogene oceanic lithosphere, in the Valencia Gulf, Provencal-Algerian basin and Tyrrhenian basin, and inherent slab roll-back subsequently have fragmented the original collisional belt (Fig. 1c). This has produced the seemingly erratic distribution of Alpine metamorphic core complexes in the western Mediterranean (Fig. 1a). Peridotite bodies are present in most of these Alpine core complexes: Betic-Rif, Calabria, Corsica and Liguria. The Ronda peridotites in the Betic-Rif belt are among the best known. These bodies outline the Arc of Gibraltar, and were shown to represent tectonic slices of mantle rocks within the major Alpujarride nappe complex. They reached their present position in three stages (Zeck, 1997). The first covers their travel from great mantle depths to crustal levels, in the order of 150-200 km (pseudomorphed diamonds). The second stage covers induction of peridotite slices in the crustal nappe pile of the Betic-Ligurian subduction zone system. Metamorphic conditions within the peridotite and its envelope at this stage are given by mineral parageneses formed at c. 11-15 kbar and 750-850oC (Tubia, 1994). Overprinting of these HP parageneses by mainly static LP parageneses (7-3 kbar and 750-850oC; Tubia, 1994) took place during the third and final stage of emplacement which started after cessation of subduction activity and slab break-off, shortly before 22 Ma as indicated by regional cooling rate studies (Zeck, 1996). Slabbreak- off caused lateral convective inflow of high-temperature and low-density asthenospheric material into the widening gap above the sinking lithospheric slab. This may have had a triggering effect on the uplift, but first of all created a HT metamorphic/anatectic regime within the overlying Betic lithosphere leading to thermal weakening in the crustal section promoting tectonic extrusion under the influence of compression between the rigid Iberian and African lithospheric plates. At higher levels, secondary extensional tectonic units, some of which contained peridotite slices, were individualized and displaced over distances of 50-200 km. Extremely high uplift/cooling rates (ca. 500oC/m.y.; Zeck, 1996) characterise this third stage of peridotite emplacement. Directions of extensional tectonic transport were variable – NW-ward for the Sa Bermeja/Alpujata peridotite body, W-ward for the body at Ceuta and SW-ward for that of Beni Bousera – and define a centrifugal pattern radiating from the central part of the Alboran Sea basin (Fig. 1c).

Seismic crustal structure in the southwest of the Iberian Peninsula and the Gulf of Cadiz

The crust under the southwestern Iberian Peninsula and the Gulf of Cadiz has been sampled by 1200 km of deep seismic refraction/wide-angle reflection profiles, together with many seismic reflection lines and bore-holes. Wide-angle seismic data were collected during the last three decades. Commercial multichannel data provide a detailed image of the uppermost crust, improving the confidence about the models of the deeper structures. P-wave velocities within the thick column of sediments in the Gulf of Cadiz range from 2.0 to 3.8 km/s, while the Algarve and the Sines areas have higher velocities of 4.3 to 4.8 km/s. The top of the Palaeozoic basement rises to the northwest, outcropping in the South Portuguese zone of the Iberian Massif, and is characterized by P-wave velocities of 5.7–5.9 km/s. High velocities of 6.4 km/s have been found at shallow depths of 7 to 10 km in the South Portuguese zone, that could be related to the mafic and ultramafic rocks in the Beja-Acebuches zone. Lower crustal velocities are in the range of 6.7–6.9 km/s. The crustal thickness shows important lateral changes from 29 km beneath the Guadalquivir Basin/Iberian Massif contact to 35 km in the southeastern part of the South Portuguese zone. From the interpretation of these seismic data, a geodynamic model of the evolution of the crust in Southwestern Iberia and the Gulf of Cadiz is proposed. The Guadalquivir Basin, and its continuation at sea, the Gulf of Cadiz, is a flexure area of the crust that could be related to the overloading due to the overthrusting of the Alboran Domain over the Iberian plate.

Subduction and obduction processes in the Swiss Alps

The significance of the Briançonnais domain in the Alpine orogen is reviewed in the light of data concerning its collision with the active Adriatic margin and the passive Helvetic margin. The Briançonnais which formerly belonged to the Iberian plate, was located on the northern margin of the Alpine Tethys (Liguro-Piémont ocean) since its opening in the early-Middle Jurassic. Together with the Iberian plate the Briançonnais terrane was separated from the European plate in the Late Jurassic–Early Cretaceous, following the northern Atlantic, Bay of Biscay, Valais ocean opening. This was accompanied by the onset of subduction along the northern margin of Adria and the closure of the Alpine Tethys. Stratigraphic and metamorphic data regarding this subduction and the geohistory of the Briançonnais allows the scenario of subduction–obduction processes during the Late Cretaceous–early Tertiary in the eastern and western Alps to be specified. HP–LT metamorphism record a long-lasting history of oceanic subduction-accretion, followed in the Middle Eocene by the incorporation of the Briançonnais as an exotic terrane into the accretionary prism. Middle to Late Eocene cooling ages of the Briançonnais basement and the presence of pelagic, anorogenic sedimentation lasting until the Middle Eocene on the Briançonnais preclude any sort of collision before that time between this domain and the active Adria margin or the Helvetic margin. This is confirmed by plate reconstructions constrained by magnetic anomalies in the Atlantic domain. Only a small percentage of the former Briançonnais domain was obducted, most of the crust and lithospheric roots were subducted. This applies also to domains formerly belonging to the southern Alpine Tethys margin (Austroalpine–inner Carpathian domain). It is proposed that there was a single Palaeogene subduction zone responsible for the Alpine orogen formation (from northern Spain to the East Carpathians), with the exception of a short-lived Late Cretaceous partial closure of the Valais ocean. Subduction in the western Tethyan domain originated during the closure of the Meliata ocean during the Jurassic incorporating the Austroalpine–Carpathian domain as terranes during the Cretaceous. The subduction zone propagated into the northern margin of Adria and then to the northern margin of the Iberian plate, where it gave birth to the Pyrenean–Provençal orogenic belt. This implies the absence of a separated Cretaceous subduction zone within the Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced the subduction to jump to the SE margin of Iberia in the Eocene, creating the Apenninic orogenic wedge and inverting the vergence of subduction from south- to north-directed.

Evidence of widespread Cretaceous remagnetisation in the Iberian Range and its relation with the rotation of Iberia

A palaeomagnetic investigation has been carried out at 13 sites of Jurassic age in the Iberian Range (northern Spain). Two components of remanent magnetisation have been found at each site. A primary high-temperature component shows an average counterclockwise rotation with respect to the north of 33±2° clockwise about a vertical axis corresponding to the absolute rotation of the Iberian plate since the Jurassic. A secondary low-temperature component shows a systematic declination difference of 16±4° with respect to the primary component. This indicates that a rotation of Iberia must have occurred between the two acquisition times. Comparison of the magnetisation directions with previous palaeomagnetic data and with sea-floor spreading data, constrains the age of the remagnetisation between 95 and 125 Ma. The remagnetisation may be associated with the extensional phases in the Iberian Basin in the Early Cretaceous (Barremian–early Albian) or Late Cretaceous (Cenomanian). A principal characteristic of the remagnetisation is its widespread character in the Iberian Range.

Crustal structure and orogenic material budget in the west central Pyrenees

Surface and subsurface data are combined to construct a crustal‐scale cross section of the western central Pyrenees (France and Spain) at the boundary between the European and Iberian plates. The position of Moho reflections in the ECORS‐Arzacq reflection profile suggests a tectonic wedge of European crust and upper mantle had indented the Iberian plate at lower crustal levels. The European wedge is overlain by an upper, Iberian wedge thus constituting a double (stacked) wedge geometry. The upper wedge was delaminated and deformed giving rise to the Pyrenean orogenic prism, manifested as a bivergent fan in upper crustal levels. The underthrust lower Iberian plate has been imaged to depths of 55–60 km, but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km. Total orogenic contraction calculated from surface structures is about 75–80 km, which was accomplished between the latest Cretaceous and the early Miocene at an averaged rate of 1.2 mm/yr. Consistent with these moderate values, exhumation of the orogen is much less than in the more shortened eastern parts of the range. Paleozoic basement and preorogenic Mesozoic rocks are little eroded, and much of the exhumation involved cannibalization of early foreland basins, which once covered the entire, poorly emergent orogen at this transect. This resulted in a continuous process of sediment recycling and, coupled with a considerable lateral arrival of material, a bulk negative erosion‐sedimentation budget.

The role of extension in the Miocene denudation of the Nevado‐Filábride Complex, Betic Cordillera (SE Spain)

The Internal Zone of the Betic Cordillera, SE Spain, consists of a nappe stack of three complexes, the deepest of which is the Nevado‐Filábride Complex. The zone is separated from the overlying Alpujárride Complex by a crustal scale shear zone that has variously been interpreted as a thrust or an extensional detachment. A suite of 74 new apatite and zircon fission track results have been obtained from the Nevado‐Filábride Complex and these have been used to define regional cooling patterns for the complex. Rapid cooling (105°C–200°C Ma−1) is spatially related to the tectonic contact with the overlying Alpujárride Complex‐Cooling to near‐surface temperatures occurred first in the east (Sierra de los Filabres) during the mid‐Serravallian (12±1 Ma) and was completed by the early Tortonian (9–8 Ma) in the west (Sierra Nevada). There is no correlation between fission track age and sample elevation. These results are consistent with tectonic unroofing of this complex, a finding that favors extension as the mechanism by which the two complexes were brought into contact. Extension spans the middle and earliest upper Miocene (12–8 Ma) in the study area and therefore lasted much longer than previously documented. A hypothesis is advanced which links oblique convergence between the Iberian plate and the Betic Internal Zones, resulting in crustal contraction at depth, with orogen parallel extension in the middle and upper crust.