Geometrical relationships between unconformities and subsequent folding: the Arro fold system (southern Pyrenees)Relations entre discordances et plissement dans le système de plis et chevauchements d'Arro (bassin de Graus-Tremp,Pyrénées)

The Arro system is an oblique fold system involving the Eocene sediments of the Graus-Tremp basin. It consists of westward-verging folds, trending NNW–SSE, some of them related with thrusts, in piggyback sequence. Seismic profiles allow to infer the geometry of structures at depth: folds and thrusts are ‘decolled’ over an unconformity between turbiditic and platform sediments. Re-activation of bedding surfaces by folding in the underlying units resulted in folding and thrusting in the upper series. To cite this article: A.M. Casas et al., C. R. Geoscience 334 (2002) 765–772.     

Datation UPb sur zircons des dolérites tholéiitiques pyrénéennes (ophites) à la limite Trias–Jurassique et relations avec les tufs volcaniques dits « infra-liasiques » nord-pyrénéens

The tholeiitic doleritic magmatism (ophites) in the Aspe valley of the Pyrenees has been dated on the Triassic–Jurassic boundary (199±2 Ma) by the UPb method on zircon (SHRIMP). Emplacement of the ophites was probably synchronous with that of the earliest ‘Infraliassic’ Ségalas tuffs. The ophites are thus related to the Central Atlantic Magmatic Province (CAMP) whose emplacement linked with the fracturing of Pangea, preceded the opening of the Atlantic Ocean. To cite this article: P. Rossi et al., C. R. Geoscience 335 (2003).     

Tectono-sedimentary evolution of transverse extensional faults in a foreland basin: Response to changes in tectonic plate processes

Late Paleocene to Middle Eocene strata in the easternmost part of the Southern Pyrenees, up to 4 km thick, provide information on tectono-sedimentary evolution of faults transversal to the Pyrenean chain. To know how changes in tectonic plate processes control the structural evolution of transverse faults and the synchronous thickness and lithological distribution of sedimentary strata in a foreland basin, field observations, interpretation of 2D seismic lines tied to lithostratigraphic data of exploration wells and gravity modelling constrains were carried out. This resulted in the following two tectono-sedimentary phases in a foreland basin: first phase, dominated by transverse extensional faulting, synchronous with deposition of marine carbonates (ca. 57 to 51 Ma); and second phase, characterized by transverse contractional faulting, coeval to accumulation of marine and transitional siliciclastics (51 to 44 Ma). During the first phase, Iberia and Adria were moving to the east and west respectively. Therefore, lithospheric flexure in the easternmost part of the Iberian plate was developed due to that Sardinia was over-thrusting Iberia. Consequently, activation of E-dipping normal faults was generated giving rise to thick-deep and thin-shallow carbonate platform deposits across the hanging walls and footwalls of the transverse structures. During the second phase, a shearing interaction between Iberia and Sardinia prevailed re-activating the transverse faults as contractional structures generating thin-shelf and thick-submarine fan deposits across the hanging walls and footwalls of the transverse structures. In the transition between the first and second phases, evaporitic conditions dominated in the basin suggesting a tectonic control on basin marine restriction. The results of our study demonstrate how thickness and lithology distribution, controlled by transverse faulting in a compressional regimen, are influenced by phases related to processes affecting motions and interactions between tectonic plates and continental blocks.     

A point-sourced calciclastic submarine fan complex (Eocene Anotz Formation, western Pyrenees): facies architecture, evolution and controlling factors

A complete transect of a fossil carbonate ramp slope has been reconstructed using outcrop data from the Lower-Middle Eocene Anotz Formation in the western Pyrenees. The Anotz Formation contains four calciclastic (mostly bioclastic) members encased within hemipelagic marl/limestone alternations, each calciclastic member representing a submarine fan system. Individual fans are composed of a gullied upper slope, a leveed feeder channel, a channelized lobe area, an unconfined lobe zone and a peripheral lobe fringe that grades downcurrent into basinal deposits. Quantitative data on the dimensions and degree of lateral continuity and vertical connectivity of the Anotz calciclastic fan elements are presented. These data provide information for a better understanding of the intrinsic nature of calciclastic submarine fans and to model their reservoir potential. The long-term evolution of the Anotz carbonate slope was generally progradational, as evidenced by the four discrete episodes of calciclastic-fan development. The location of the fans was controlled by the synsedimentary tectonic activity of the Pamplona fault, which created a slope valley along which the reworked shallow-water calciclastic sediments were funnelled. In addition, episodic basinward tilting of the shallow-water carbonate ramp, linked to the development of the South Pyrenean foreland basin, switched on and off the process of calciclastic resedimentation and determined the growth or abandonment of the fan systems.     

Fluid flow record from fracture-fill calcite in the Eocene limestones from the South-Pyrenean Basin (NE Spain) and its relationship to oil shows

The presence of oil shows associated with fractures provides a significant opportunity to a) unravel the type, origin and evolution of fluids involved in fracture-fills, and b) examine how they relate to oil migration. Two stages of calcite cement (C1 and C2) were distinguished in the fractures of the Eocene Armàncies platform carbonates; C1 is characterised by fence-like crystals, exhibits dull red luminescence and contains abundant twin planes, inclusions and δ¹⁸O values that range from − 6.2‰ to − 4.8‰ VPDB. C2 consists of blocky clean crystals, is characterized by dark brown-red luminescence that alternates with yellow bands, and contains hydrocarbon fluid inclusions with homogenisation temperatures of approximately 120 °C. δ¹⁸O values range from − 9.6‰ to − 8.9‰ VPDB. The remaining porosity after C2 precipitation is filled with liquid oil that reached 115 °C. This would seem to indicate that free oil and fluid inclusions oil probably come from the same migration pulse. Oil migration timing was coeval with C2 and continued after calcite cementation was completed.     

Seismic hazard assessment for the Itoiz dam site (Western Pyrenees,Spain)

This work summarises the seismic hazard analysis performed for the complete characterisation of strong ground-motion at the site of the Itoiz dam (Western Pyrenees, Spain). The hazard analysis includes the compilation of a composite catalogue from French and Spanish agencies, the definition of an original hybrid seismogenic source model (including zones and major faults) and the selection of ground motion prediction equations (GMPEs). Hazard results are provided as hazard curves and acceleration response spectra on rock for the 1000- and 5000-year return periods, which correspond respectively to the operating basis earthquake (OBE) and safety evaluation earthquake (SEE). The impact of truncating GMPEs at a number of standard deviations (epsilon) has been found not critical here for the return periods targeted. Subsequently, an analysis of the contribution of each source to total hazard and a hazard disaggregation analysis are performed in order to establish the earthquake-source parameters for both the OBE and SEE scenarios consistently with the seismotectonics of the region. The European Strong Motion database is then searched and a selection of records is proposed for each of the scenarios. Our results suggest that seismic hazard in the region is underestimated by the official Spanish seismic hazard map included in the current version of the code (NCSE-02), which is the reference document for the definition of seismic actions for dam projects in the whole Pyrenees.     

Estimating flash flood discharge in an ungauged mountain catchment with 2D hydraulic models and dendrogeomorphic palaeostage indicators

There is still wide uncertainty about past flash‐flood processes in mountain regions owing to the lack of systematic databases on former events. This paper presents a methodology to reconstruct peak discharge of flash floods and illustrates a case in an ungauged catchment in the Spanish Central System. The use of dendrogeomorphic evidence (i.e. scars on trees) together with the combined use of a two‐dimensional (2D) numerical hydraulic model and a terrestrial laser scan (TLS) has allowed estimation of peak discharge of a recent flash flood. The size and height distribution of scars observed in the field have been used to define three hypothetical scenarios (S_min or minimum scenario; S_med or medium scenario; and S_max or maximum scenario), thus illustrating the uncertainty involved in peak‐discharge estimation of flash floods in ungauged torrents. All scars analysed with dendrogeomorphic techniques stem from a large flash flood which took place on 17 December 1997. On the basis of the scenarios, peak discharge is estimated to 79 ± 14 m³ s^?1. The average deviation obtained between flood stage and expected scar height was ? 0·09 ± 0·53 m. From the data, it becomes obvious that the geomorphic position of trees is the main factor controlling deviation rate. In this sense, scars with minimum deviation were located on trees growing in exposed locations, especially on unruffled bedrock where the model predicts higher specific kinetic energy. The approach used in this study demonstrates the potential of tree‐ring analysis in palaeohydrology and for flood‐risk assessment in catchments with vulnerable goods and infrastructure. Copyright © 2010 John Wiley & Sons, Ltd.     

Evolution of vegetation activity on vegetated,eroded, and erosion risk areas in the central Spanish Pyrenees,using multitemporal Landsat imagery

The temporal evolution of vegetation activity on various land cover classes in the Spanish Pyrenees was analyzed. Two time series of the normalized difference vegetation index (NDVI) were used, corresponding to March (early spring) and August (the end of summer). The series were generated from Landsat TM and Landsat ETM+ images for the period 1984–2007. An increase in the NDVI in March was found for vegetated areas, and the opposite trend was found in both March and August for degraded areas (badlands and erosion risk areas). The rise in minimum temperature and the time variation of the cloud cover during the study period appears to be the most important factors explaining increased NDVI in the vegetated areas. In degraded areas, no climatic or topographic variable was associated with the negative NDVI trend, which may be related to erosion processes taking place in these regions. Copyright © 2010 John Wiley & Sons, Ltd.     

Asynchroneity of maximum glacier advances in the central Spanish Pyrenees

The deglaciation history of the Escarra and Lana Mayor glaciers (Upper Gállego valley, central Spanish Pyrenees) had been reconstructed on the basis of detailed geomorphological studies of glacier deposits, sedimentological and palynological analyses of glacial lake sediments and an accelerator mass spectrometry (AMS) ¹⁴C chronology based on minimum ages from glacial lake deposits. The maximum extent of the Pyrenean glaciers during the last glaciation was before 30 000 yr BP and pre‐dated the maximum advances of the Scandinavian Ice Sheet and some Alpine glaciers. A later advance occurred during the coldest period (around 20 000 yr BP), synchronous with the maximum global ice extent, but in the Pyrenees it was less extensive than the previous one. Later, there were minor advances followed by a stage of debris‐covered glaciers and a phase of moraine formation near cirque backwalls. The deglaciation chronology of the Upper Gállego valley provides more examples of the general asynchroneity between mountain and continental glaciers. The asynchroneity of maximum advances may be explained by different regional responses to climatic forcing and by the southern latitude of the Pyrenees. Copyright © 2002 John Wiley & Sons, Ltd.