Northeastern Iberian Peninsula Research Articles

Recent intraplate stress field in the eastern Duero Basin (N Spain)

Palaeostresses inferred from brittle mesostructures in the eastern Duero Basin show a recent stress field characterized by an extensional regime, with local strike‐slip and compressional stress states. Orientations of the maximum horizontal stress ( SHmax) show a relative scattering with two main modes: NNE to NE–SW and NW–SE. These orientations suggest the existence of two stress sources responsible for the dominant directions of the maximum horizontal stress in northeastern Iberia. Extensional structures within a broad‐scale compressional stress field can be related to both the decrease in relative stress magnitudes from active margins to intraplate regions and rifting proccesses ocurring in eastern Iberia. Stress states with NW–SE‐trending SHmax are compatible with the dominant pattern established for western Europe. NE–SW orientations of SHmax suggest the occurrence of tectonic forces coming from the Pyrenean zone. Geological and geophysical data indicate the existence of both orientations from the upper Miocene to the present‐day in NE Iberia.

Contemporary tectonic stress in northeastern Iberia. New results from borehole breakout analysis

In situ data on the orientation of tectonic stress in northern Iberia are scarce. Fault-slip and focal mechanism data show a variety of stress orientations difficult to interpret in terms of a general stress pattern. Breakout analysis performed on oriented caliper logs from 17 hydrocarbon exploration wells has provided new data on the orientation of tectonic stress in northeastern Iberia: the southern Pyrenees, the Ebro Basin and the northwestern Valencia Trough. The majority of breakout data indicate a rather consistent NE-SW to ENE-WSW directional signature of the larger horizontal principal stress orientation. This orientation differs significantly from the NW western European mean stress orientation. It is consistent with depth within the first 4.7 km of both the sedimentary fill and the sedimentary basins substrate and also within allochthonous thrust sheets in the southern Pyrenees and in the underlying autochthonous and parautochthonous foreland basin sediments.

Lateral variations of the local magnitude at Ebre station, Northeastern Iberian Peninsula

Since 1984, a seismic surveillance at the Baix Ebre Region (NE of the Iberian Peninsula) has been carried out from a seismic network consisting of two digital and one analog stations. Large discrepancies between the recorded amplitudes and the magnitude estimates given by different agencies have been observed. To explain these discrepancies, that consist of a large reduction of amplitude and a strong dependence of the azimuth, a local magnitude formula for the area has been computed, using the analog records obtained at EBR short-period station. Global results show an average station correction factor between the computed local magnitude and the reported magnitude of the order of 1, presenting large variations around the mean value with a clear dependence on the azimuth. A detailed analysis of this variation around the mean value reveals that earthquakes can be classified according to their location in different geological units, thus obtaining for each zone a distinct station correction factor that can fluctuate as much as 0.6 around the average. These large lateral variations of the magnitude are in good agreement with previous studies of coda-Q attenuation.

Nutrient content and seasonal fluctuations in the leaf component of coark-oak (Quercus suber L.) litterfall

Nutrient content and seasonality of the leaf component in cork-oak litterfal were studied over a two year period in two cork-oak forest sites differing in biomass and edaphic condition in the north-eastern Iberian peninsula. Fallen senescent leaves compared to young leaves showed higher non-mobile nutrient concentrations and lower mobile nutrient concentrations, specially P, N, K, and Mg. At both sites, seasonal fluctuations affected both leaf production and leaf mineral content. The maximum leaf fall period correspond to the start of the vegetative growth and to the lowest N and P concentration in the falling leaves. The opposite was true for the winter, when litterfal was minimal and N and P content in falling leaves was at a maximum.

Climatic changes in western Spain

Possible interactions between climatic changes and vegetation during the present century in western Spain have been examined using a phytoclimatic model. Statistical analysis of meteorological data for the period 1901–1989 suggests a tendency towards a reduction in temperature differences with a negative tendency for maximums and variability and a positive tendency for minimum temperatures. Rainfall, number of rainy days and number of rainy sequences have shown positive tendencies since the beginning of the century. Phytoclimatic determination for the same period showed dominant mediterranean conditions until 1950. Around 1951, a point of inflection was detected, the more humid conditions thereafter becoming dominant, and more favorable to deciduous broad-leaf forests in transition towards mediterranean forests. In accordance with the results of other studies these data would indicate the changing tendencies detected could be affecting a large part of the northeastern Iberian Peninsula.

Lateral variations in the deep crustal structure at the Iberian margin of the Valencia trough imaged from seismic reflection methods

New seismic images of crustal structure have been obtained in an area of western Europe where attenuation of the continental crust by a factor of two has been reported previously. The deep lateral variations in velocity and reflectivity at the transition between the northeastern Iberian Peninsula and the Valencia trough are investigated with three seismic reflection data sets. First results of the Spanish deep vertical reflection ESCI-profile on the Iberian mainland extend the crustal image of the Ecors-Pyrenees profile to the Mediterranean coast. A highly-reflective zone about 15 km thick is imaged in the lower crust beneath the Ebro basin and the Catalan coastal ranges, where the Moho is located at 29 km depth. This deep reflective pattern appears to be disrupted by the effects of low-velocity sediments in the Neogene Reus basin, but continuity of the principal horizons is inferred from illuminations of the area using sources and receivers at wide-angle offsets. High-quality recordings up to 50 km inland of air-gun shots from the marine ESCI-profile provide a complementary image of the structure on land and show that the major crustal thinning toward the Valencia trough starts near the shoreline. The lateral variability of the crust is investigated using a second profile displaced 30 km to the northeast. Independent refraction and reflection interpretation techniques applied to this profile, which consisted of densely-spaced shots, resulted in consistent depth sections with crustal thinning beginning further inland. Thinning of 10 km takes place within 60 km at the Catalan margin. Moho-reflected waves (P MP) have been processed as far traces of a deep reflection profile. The processing included common midpoint ( cmp) sorting, stacking and migration. The inferred lateral variations in deep crustal structure near the Catalan coast are consistent with the Bouguer gravity field. They may be related to episodes of extensional block faulting that have affected the Catalan-Valencian domain since Paleogene times.

Vegetation history and human activity during the last 6000 years on the central Catalan coast (northeastern Iberian Peninsula)

In this paper, we present the results of four pollen diagrams obtained from the northeastern coast of the Iberian Peninsula. These data, together with a set of 11 radiocarbon dates, allow us to make some suggestions about human activity in this area mainly during the last 6000 years. We have established four main stages of this activity. Phase I (7000–3000 B.P.) shows some sporadic human clearance without qualitative and/or quantitative changes inside the natural woodland, except for slight oscillations in AP values. These clearances are synchronous with Neolithic and Bronze Age settlements. Phase II (3000–1500/1300 B.P.) demonstrates a different human action on the landscape along the Catalan coast. Sampling sites located in rich agricultural plains and close to urban centres show continuous woodland clearance during the Iberian period and especially during Roman times. In contrast, at boring sites far from these towns, the irregular and sporadic woodland disturbances continue, although these become more common. Phase III (1500/1300–850 B.P.) shows further woodland clearance fires along the coast. Pollen, archaeological and historical evidence allow us to relate this to the introduction of grazing in the coastal area, connected with seasonal transhumance between the littoral and mountain regions. Phase IV (850–300/150 B.P.) is characterised by the final clearance of woodland and the development of olive farming. During this phase the agrarian Mediterranean landscape was definitively formed.

Climatic Change on the Iberian Peninsula Recorded in a 30,000-Yr Pollen Record from Lake Banyoles

The Banyoles lacustrine sequence shows that the vegetational history of the northeastern Iberian Peninsula during the last 30,000 yr follows the North Atlantic pattern of climatic oscillations. The Banyoles pollen diagram, supported by two calibrated 14C dates and nine U/Th dates, shows a clear interstadial event between 30,000 and 27,000 yr B.P., a Pleniglacial period with minor oscillations that ended abruptly ca. 14,420 ± 410 yr B.P., and a late-glacial sequence that records the classical stages described in Northern Europe: the Bølling-Allerød Interstade, the Younger Dryas event at 12,000 yr B.P. (U-series age), and a short warming phase between the Younger Dryas and the last cold event (dated at 11,000 yr B.P., U-series age).

Geophysical constraints on the crustal structure of the Olot Volcanic Area, northeastern Iberian Peninsula

Abstract Seismic and electric data obtained in a recent geophysical survey place new constraints on the structure at depth of the Olot region, the youngest Quaternary volcanic area of the Iberian Peninsula. The upper crust under the Olot Volcanic Zone (OVZ), Southern Pyrenees, was investigated by a dense network of seismic refraction profiles. The profiles running along the main axis of volcanism show P-velocities less than 4 km/s for up to 400 m under the OVZ. Higher velocities are observed in the Eocene sediments, 5.3 km/s in the east and 4.6 km/s in the west of the OVZ. Vertical electrical soundings clearly define the volcanic rocks. Resistivities higher than 1,000 Ω.m are typical of pyroclastics, and values above 10,000 Ω.m are found in basaltic flows less than 140 m thick. An Eocene substratum identified at the base of the soundings is characterized by resistivities of between 150 and 500 Ω.m. The seismic basement, characterized by a velocity of 6.0 km/s, was observed at 2 km depth, with a lateral continuity deepening to the southwest. Velocities higher than 6.6 km/s are suggested in the OVZ below 5 km depth. The particle motion analysis of individual seismograms has provided independent constraints from S-phases and PS-phases converted at the basement-sediment discontinuity. In the OVZ, no indications of the existence of anomalous bodies in the upper 5 km of the crust have been found. A local anomalous zone evidenced at the eastern edge of the OVZ deserves further study.

Estudio de las tierras raras en las bauxitas kársticas del Noroeste de la Península Ibérica

The bauxite deposits from Northeastern Iberian Peninsula are geochemically studied in this paper. Major, minor, trace and rare earth elements are determined by ICP spectros­ copy in fifteen samples from three different zones: Southpyrenees, Catalanides y Maestraz­ go. According to La/Y and europium anomaly values, the sedimentation environment du­ ring the genesis of bauxites had an acid and reducent character. This conclusion is agree with Petraschek's (1989) model for this type of ore deposits. The different methods here included (factor analysis, rare earth elements accumulation coefficients, etc.) reveal a great homogeneity in the characteristics of the parent rocks of the different bauxite deposits. These parent rocks had probably an intermediate geochemi­ cal character. From a paleogeographic point of view, the parents rocks were related to rocks of permic-triassic-lower jurassic age.

Craniometrical characteristics of Neomys fodiens (Pennant, 1771) (Mammalia, Insectivora) from the northeastern Iberian Peninsula

The aim of this paper was to establish the craniometrical features of Neomys fodiens (Pennant, 1771) from the northeastern Iberian Peninsula (Catalonia). The variability in 22 skull and mandible measurements in a series of forty eight specimens was studied. Fifty seven specimens from Navarre were used for comparison.

Small-scale deformations in quaternary depostits in the northeastern Iberian peninsula

Les études de dépôts quaternaires dans deux aires du Bassin tertiaire de l'Ebre (NE de l'Espagne) révèlent une déformation d'origine diapirique, dont letypereflête les différentes caractéristiques structurales et lithologiques du "socle" ! Ces déformations ont été attribuées aux processus diapiriques suivants : (a) l'intrusion de marnes du Tertiaire dans la couverture Quaternaire et (b) les mouvements halocinétiques du gypse à petite et grande échelle. Ces structures d'origine diapirique sont classifiées en quatre groupes : domal et basculements , en relation au processus (1) ; et perforant et domal-perforant , en relation au processus (2). Les effets de déformations provoquées par effondrements d'origine karstique sont aussi présentes. A quelques localités des galets striés et impressionnés ont été trouvés. Dans le cas des diapirs perforants il n'y a que des failles inverses, tandis que dans le type domal-perforant on trouve aussi des failles à composante normale. La validité de cette dernière observation est constatée par le modèle reproduit à l'échelle en laboratoire. Le type de phénomènes diapiriques observés dépend de la lithologie et de la structure du "socle" tertiaire. La plupart des structures observées se localise dans les niveaux du Quarternaire inférieur et moyen.

Palaeogene evolution of deformation in the northeastern Iberian Peninsula

AbstractDuring the Palaeogene Alpine compression three units were differentiated in the studied area (south of the Ebro Basin): the Catalan Coastal Range, dominated by NE–SW major basement faults with a sinistral movement; The Iberian Range, where the important basement faults have a NW–SE direction and a reverse movement (often with a dextral component); the Linking Zone, between these two ranges, where an E–W dominant structural direction is marked by an array of folds and thrusts (with a northward vergence) in the Mesozoic cover.From the analysis of both these major structures and the small-scale structures, it can be deduced that the compression in the studied area has evolved from a NW–SE direction (lower-middle Eocene) to a N–S direction and to a NE–SW one (uppermost Oligocene). The major structures are due to the N–S compression. Later, the stress-field progressively changed to a distensive regime.We suggest a relation between the compressive phases and the displacement direction of the Iberian Plate with reference to the European Plate.