Evaluating Duero's basin (Spain) response to the NAO phases: spatial and seasonal variability

In this study, we analysed the influence of the North Atlantic Oscillation (NAO) on the hydrological response of the Duero River (central Spain) and its tributaries. We identified the positive and negative phases of the winter NAO for the period 1961–2006 and related precipitation and river discharge anomalies. Significant differences in precipitation and river discharge were found between the positive and negative NAO phases with negative anomalies (dry conditions) during positive NAO periods, and positive anomalies (wet conditions) during negative NAO periods. Marked differences were also found in the precipitation and river discharge response times to the NAO episodes, with an immediate response for precipitation but a lag and more sustained response for river discharges. Differing patterns were also identified in the response of river discharge to the NAO throughout the Duero basin. The physical characteristics of watersheds (including area, altitude and permeability explained most of the differences in the timing and magnitude of anomalies in the river discharge in response to the NAO. The findings highlight the great variability in the hydrological response of rivers to the NAO episodes. Copyright © 2010 John Wiley & Sons, Ltd.     

Biostratigraphic characterization by means of ammonoids of the lower Aptian Oceanic Anoxic Event (OAE 1a) in the eastern Iberian Chain (Maestrat Basin, eastern Spain)

It is now generally accepted that the Oceanic Anoxic Event 1a [OAE 1a] correlates with the lower part of the Leupoldina cabri planktonic foraminiferal Zone. Its calibration against the standard ammonite scale, however, seems to be more problematic. This is due, in part, to the fact that ammonites are scarce and/or of little diagnostic value from a biochronological viewpoint in the lower Aptian pelagic successions where the black shale horizons are better developed.We have been able to characterize OAE 1a geochemically in the relatively shallow water deposits of the eastern Iberian Chain (Maestrat Basin, eastern Spain), where ammonite faunas are rich. The interval corresponding to this event is dominated by the genera Roloboceras and Megatyloceras, accompanied by Deshayesites forbesi and Deshayesites gr. euglyphus/spathi. This assemblage is characteristic of the middle/upper part of the Deshayesites weissi Zone. The first occurrence of the species Deshayesites deshayesi (d'Orbigny), which marks the base of the overlying zone, takes place in our sections some metres above the OAE 1a interval.In the historical stratotype region of Cassis-La Bédoule (southern Provence Basin, southeastern France), the OAE 1a interval is also characterized by the presence of Roloboceras and Megatyloceras. Nevertheless, it has usually been correlated with the D. deshayesi Zone. In our opinion, this discrepancy is due to divergences in the taxonomic assignments of the deshayesitids present in these beds. In fact, the specimens attributed by French authors [Ropolo, P., Conte, G., Gonnet, R., Masse, J.P., Moullade, M., 2000. Les faunes d'Ammonites du Barrémien supérieur/Aptien inférieur (Bédoulien) dans la région stratotypique de Cassis-La Bédoule (SE France): état des connaissances et propositions pour une zonation par Ammonites du Bédoulien-type. Géologie Méditerranéenne 25, 167–175; Ropolo, P., Moullade, M., Gonnet, R., Conte, G., Tronchetti, G., 2006. The Deshayesitidae Stoyanov, 1949 (Ammonoidea) of the Aptian historical stratotype region at Cassis-La Bédoule (SE France), Carnets de Géologie / Notebooks on Geology Memoir 2006/01, 1–46.] to D. deshayesi and D. dechyi can be reinterpreted as belonging to D. forbesi.Following this reinterpretation, the Roloboceras beds (equivalent of OAE 1a) of Cassis-La Bédoule would also correspond to the D. weissi Zone. This age is additionally corroborated by data from southern England [Casey, R., 1961a. The stratigraphical palaeontology of the Lower Greensand. Palaeontology 3, 487–621; Casey, R., 1961b. A Monograph of the Ammonoidea of the Lower Greensand, part III. Palaeontographical Society, London, pp. 119–216], and by our recent observations in Le Teil (Ardèche Platform, southeastern France), where the Roloboceras faunas are also associated with Deshayesites consobrinus and Deshayesites gr. euglyphus, taxa that are characteristic of the D. weissi Zone.     

Ta and Sn concentration by muscovite fractionation and degassing in a lens-like granite body: The case study of the Penouta rare-metal albite granite (NW Spain)

The Penouta peraluminous low-phosphorous granite is the most important low-grade, high-tonnage Sn-Ta-Nb-bearing albite granite from the Iberian Massif. A sheet or laccolith shape, instead of a stock, is inferred for the Penouta granite, maybe in relation with the low viscosity and high mobility of a fluorine-bearing melt. Subhorizontal lateral extension of the magma is also inferred via vertical and horizontal geochemical variations. The absence of compositional gaps in variation diagrams, coupled with continuous evolutionary trends of compatible and incompatible elements with height, discard a multi-pulse intrusion and point to a single magma pulse. Mineral chemistry, trace element and least-squares mass balance modelling support a differentiation process from bottom to top in the emplacement place. The absence of switch from incompatible to compatible behaviour (bell-shaped trends) in Sn, Nb and Ta variation diagrams, coupled to experimental constraints on tantalite and cassiterite saturation, suggest that Nb-Ta oxides and probably cassiterite were not fractionated mineral phases, their crystallisation being related to concentration gradients within a trapped intercumulus melt. Major and trace element modelling support that the concentration upwards of Ta and the Ta/Nb ratio could be a consequence of mineral fractionation, with a key role of muscovite (mainly primary) for the Ta/Nb ratio, as this mineral has a higher partition coefficient for Nb than Ta. Our results suggest that fluorine and peraluminosity had a limited effect in the Ta/Nb ratio variations. Hence, Ta enrichment is mainly controlled by fractional crystallisation processes. In most cases, Sn enrichment was also concomitant with Ta, indicating that crystal-melt fractionation processes also played an important role in Sn concentration. Nevertheless, the strongest Sn enrichment in the granite (e.g., central part of the granite body) does not correspond to a significant Ta enrichment. The high affinity of Sn for fluids and the high partitioning of Ta for melt could explain this decoupling. Nevertheless, the magmatic signature of cassiterites in these strongly Sn-enriched zones (central part of the granite body) rules out a hydrothermal subsolidus origin for this fluid. By analogy with models carried out in sill-like bodies it seems likely that the Sn enrichment in the central part of the granite body is related to fluid saturation/degassing occurred in the lower margin, as a consequence of cooling and crystallisation of mostly anhydrous minerals (i.e. second boiling). The vapour exsolved migrated into the hotter melt up to the central part, where it probably was reabsorbed, yielding cassiterite with a magmatic signature. Moreover, we suggest that heat loss in the upper margin of the granite body might also contribute to the formation of a second fluid-saturated zone. As a result, pegmo-aplites and greisen were developed.     

Identification and characterization of fluid escape structures (pockmarks) in the Estremadura Spur,West Iberian Margin

Located on the West Iberian margin, between Cabo Carvoeiro and Cabo da Roca, the Estremadura Spur is a trapezoidal promontory elongated in an east-west direction, extending until the Tore seamount. Recently a field with more than 70 pockmarks was discovered in the NW region of the Estremadura Spur outer shelf (Lourinhã Monocline). Pockmarks are the seabed culminations of fluid migration through the sedimentary column and their characteristic seabed morphologies correspond to cone-shaped circular or elliptical depressions. The characterization of these features and the understanding of the associated fluid escape process are the main objectives of this work. Here we characterize these structures to understand their structural and stratigraphic control based on: 1) Seismic processing and interpretation of the high resolution 2D single-channel sparker seismic dataset, 2) Bathymetric and Backscatter interpretation and 3) ROV direct observation of the seafloor.The analysis of the seismic profiles allowed the identification of six seismic units, disturbed by the migration and accumulation of fluids. The Estremadura Spur outer shelf has been affected by several episodes of fluid migration and fluid escape during the Pliocene-Quaternary that are expressed by a vast number of seabed and buried pockmarks. At present, the pockmarks are mainly inactive, as the seabed pockmarks are covered by recent sediments. It is concluded that the migration of fluids to the seabed occurred over the Pliocene-Quaternary, as indicated by the buried pockmarks at different depths below the seabed. The vertical stacking of various pockmarks suggests a cyclical fluid flow activity that can possibly be the result of the eustatic sea level variations and the subsequent changes of the hydrostatic pressure.     

Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula

Travel times of 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa–Eurasia plate convergence vector (northwest–southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east–west in its central part and north–south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established.     

Erosion surfaces and Neogene landscape evolution in the NE Duero Basin (north-central Spain)

We present a chronological model of erosion surface development in the Iberian and Cantabrian Ranges of north-central Spain. We map four erosion surfaces and interpret these to be related to Duero basin continental sediments and tectonic activity from Upper Oligocene to Plio-Pleistocene. The oldest erosion surface, SE1, formed across Upper Oligocene–Lower Miocene synorogenic deposits; while surface SE2 was contemporaneous with the Middle Miocene alluvial systems, ending with an uplift stage in the Astaracian. The two most recent erosion surfaces, SE3 and SE4, developed during extensional tectonic episodes and are associated with the deposition of Upper Páramo sedimentary units at the end of the Miocene (Upper Turolian) and alluvial fan deposits, known as rañas (Plio-Pleistocene). With the exception of SE1, which seems to be associated with a relatively wet climate, the surfaces formed during periods of marked aridity and generally warm temperatures. Through geostatistical reconstruction of the best preserved surface (SE2), applying ordinary kriging method to the topography (DEM) of the erosion surface and its correlating sedimentary plains, we identify the deformation processes which occurred on this surface after its formation.     

Silurian/Ordovician asymmetrical sill-like bodies from La Codosera syncline, W Spain: A case of tholeiitic partial melts emplaced in a single magma pulse and derived from a metasomatized mantle source

A Silurian/Ordovician extensional event in the southernmost sectors of the Central Iberian Zone is inferred from the Sm/Nd isochron obtained (436 ± 17 Ma) after the diabase sills from the La Codosera syncline. From the geochemical and mineralogical points of view, the diabase sills are subalkaline and range between high-Mg tholeiite diabases to tholeiite andesites. LREE enrichment, an Nb negative anomaly, the absence of a Ta trough and a high Nd isotope signature (ε_Ndt = + 6) are the most relevant geochemical features. The diabase bodies are up to 330 m in thickness and were sampled from bottom to top along several different sections, permitting the definition of an accumulation of clinopyroxene, olivine and plagioclase close to chilled margins at the bottom, and abundant pegmatoid layers at the top. Chemical profiles and mass-balance modelling suggest that the bulk rock and chilled margin compositions are not dissimilar, defining an unusual S-type vertical compositional profile for large (> 50 m thick) sills, which in turn strongly suggests a single magma pulse and a probable gravitational settling. Assuming chilled margin samples as the parental magma, as well as Cr-enriched samples as cumulate layers, a two-stage liquid line of descent has been established, the first one consisting of a clinopyroxene-plagioclase-olivine cumulate assemblage. A second stage in relation to the depletion in Ti, Fe and V is accounted for by ilmenite fractionation, along with that of clinopyroxene, plagioclase and olivine fractionation. Thermobarometric estimations reveal that the clinopyroxene (around 1100 °C and 197 MPa) was a late mineral phase, whereas the plagioclase (around 1200 °C) was pre- to syn-emplacement, in agreement with the presence only of plagioclase phenocrysts in the chilled margins and the very abundant positive Eu anomaly. The energy constraint modelling is consistent with the lack of a significant assimilation process owing to the high temperature contrast between the country rock and the magma itself. The compositional characteristics of chilled margins enable them to be ruled out as primary melts in equilibrium with mantle olivine (Fo₉₁), a certain amount of olivine fractionation being required, which might have occurred in magma conduits en route to shallow emplacement levels in the crust. Spinel lherzolitic xenoliths from the European Cenozoic alkaline magmatism appear to be unsuitable protoliths to account for the chilled margin compositions. Instead, a hybrid mantle source consisting of a small amount of OIB-mantle component (5 wt.%) and a depleted end-member mantle component seems to be a plausible protolith, resulting in a good fit with the fractionation-corrected chilled margin trends for 10% of partial melting.     

PGE distribution in massive sulfide deposits of the Iberian Pyrite Belt

We present the first platinum group elements (PGE) data on seven massive sulfide deposits in the Iberian Pyrite Belt (IPB), one of the world largest massive sulfide provinces. Some of these deposits can contain significant PGE values. The highest PGE values were identified in the Cu-rich stockwork ores of the Aguas Teñidas Este (Σ PGE 350 ppb) and the Neves Corvo (Σ PGE 203 ppb) deposits. Chondrite normalized PGE patterns and Pd/Pt and Pd/Ir ratios in the IPB massive, and stockwork ores are consistent with the leaching of the PGE from the underlying rock sequence.     

Elastic-anelastic Regional Structures for the Iberian Peninsula Obtained from a Rayleigh Wave Tomography and a Causal Uncoupled Inversion

The elastic and anelastic structure of the lithosphere and asthenosphere of the Iberian Peninsula is derived by means of tomographic techniques applied to local phase and group velocities and local attenuation coefficients of Rayleigh wave fundamental mode. The database consists of surface wavetrains recorded at the broadband stations located in the Iberian Peninsula on the occasion of the ILIHA project. Path-averaged phase and group velocities and attenuation coefficients were previously obtained by standard filtering techniques of surface wavetrains and, subsequently, local dispersion curves were computed according to the Yanovskaya-Ditmar formulation. First, a principal component analysis (PCA) and the average linkage (AL) clustering algorithm are applied to these local values in order to classify the Iberian Peninsula in several rather homogeneous domains from the viewpoint of the similarity of the corresponding local dispersion curves, without previous seismotectonic constraints. Second, averaged phase and group velocities and attenuation coefficients representing each homogeneous region are used to derive the respective elastic and anelastic models of the lithosphere and asthenosphere. This purpose is achieved by using the uncoupled causal inversion of phase and group velocities and attenuation coefficients. The main features of the homogeneous regions are discussed by taking as reference the Hercynic, Alpine and Neogene domains of the Iberian Peninsula, and two questions affecting the reliability of the elastic-anelastic models are revised. First, the coherence of the shear-velocity and Q_β⁻¹ models obtained by causal uncoupled inversion for each region is analysed. Second, the influence of the causal phase and group velocities on the shear-velocity models is evaluated by comparing elastic and anelastic models derived from causal uncoupled inversion with those deduced from non-causal inversion.