First biostratigraphical constraints on the pre-Upper Ordovician sequences of the Pyrenees based on organic-walled microfossils

The first age constraint on the upper part of the pre-Upper Ordovician series of the Pyrenees is presented. Data are based on acritarchs from the Jujols Group in the La Molina area on the southern slope of the Canigó massif, near the Upper Ordovician unconformity. The new data allow us to attribute a Late Cambrian (Furongian) to Early Ordovician age to the upper part of the Jujols Group and to support the absence of well developed Middle Ordovician strata in the Pyrenees. Our results confirm the occurrence of an Upper Ordovician unconformity and highlight the “Sardic” affinity of the pre-Upper Ordovician sequences in the Pyrenees. These sequences differ markedly from the pre-Upper Ordovician series of the Iberian Massif.     

Evidence of a Cretaceous remagnetization in the Cameros Basin (North Spain): implications for basin geometry

The paleomagnetic results obtained in 23 sites from red beds of the northern border of the inverted Cameros Basin (northern Iberian Peninsula) prove a Cretaceous widespread remagnetization. Paleomagnetic and rock magnetic analyses indicate that the Natural remanent magnetization (NRM) is dominated by a stable and always normal polarity component carried by haematite. Two conglomerate test and five-fold test indicate that this component is a syn-tectonic overprint dated between Albian and Santonian times, most probably Albian, contemporary with a low-grade metamorphism. Remagnetization was acquired before compressional deformation and post-dates the main extensional stage of basin formation (Berriasian–Albian). Incremental fold tests provide best clustering solutions showing dispersed results and far from the expected direction. An alternative procedure to obtain fold test solution was applied considering asymmetric limb rotation. Mean directions obtained from these “asymmetric solutions” are consistent with the expected direction and yield a statistical value for significant grouping at the 95% confidence level in all fold tests performed. This reconstruction allows to determine the tilting of beds at the moment of acquisition of magnetization. These original dips were used to reconstruct the original geometry of the northern basin border, later modified during the Tertiary inversion stage. This extensional geometry can be characterised as a large-scale syn-sedimentary normal fault drag, with a minor roll-over anticline. The procedure developed in this paper should be tested in other inverted basins that also have undergone burial remagnetizations.     

Assessing aquifer vulnerability to pollutants by electrical resistivity tomography (ERT) at a nitrate vulnerable zone in NE Spain

There is an increasing demand for groundwater vulnerability maps which illustrate the exposure of aquifers against pollution. These maps show areas of greatest potential for groundwater contamination on the basis of local subsurface conditions. Parameters affecting vulnerability are mainly permeability and thickness of each protective layer. For unconsolidated sediments, the permeability is strongly related to the clay content, which can be deduced from indirect resistivity methods, like electrical-imaging. Such geophysical methods can be of great help in groundwater vulnerability studies because they disturb neither the structure nor the dynamics of the soil. Sensibility analysis was performed of the electrical resistivity tomography method for accurately mapping soil media. Managers and public administrators may effectively use this method for assessing the potential risk of groundwater contamination. In the studied zone, electrical resistivity exhibits a wide range of variability that can be easily correlated to soil parameters, such as clay content and hydraulic conductivity. A numerical index of protection has been assessed from the geophysical information derived from 2D electrical resistivity tomography. This work represents a preliminary approach on the natural vulnerability evaluation of shallow aquifers at the Empordà basin (NE Spain) that is highly affected by diffuse pollution by nitrates.     

Geographic characteristics of a network interdiction problem

The protection of critical infrastructure from natural and intentional events is a key component of any national security agenda. Protection schemes need to be readily identifiable and adaptable to complex changing environments. In this paper, we identify strategic geographic characteristics that impact the location of detection resources (e.g. sensors) towards the defense of regional critical infrastructure. Specifically, we seek to estimate the relationship between the results of a variation of the traditional shortest path network interdiction problem and geographical characteristics of the transportation infrastructure and the urban environment. Experiments conducted on three distinct transportation networks of different shapes and granularities (New York City—grid, Houston—radial, Boston—hybrid) underline the importance of geographic characteristics such as the proximity to resource location, attacker entry points as well as network coverage. Insights gained from this work are relevant to policy and decision makers to facilitate the development of analytical and decision-support tools capable of identifying resource allocation strategies. We discuss a heuristic-based framework that prioritizes the selection of detection resources, reflecting the importance of geographic characteristics. The findings underline the importance of geographical characteristics for the allocation of resources in a regional setting.     

Role of lateral thickness variations on the development of oblique structures at the Western end of the South Pyrenean Central Unit

The Montsec unit is one of the most important detached South-verging nappes within the South Pyrenean Central Unit (SPCU, Southern Pyrenees). A N–S cross-section of its Western sector, based on seismic reflection profiles, shows a hangingwall ramp geometry in Mesozoic strata, overlain by a syntectonic series of Lower Eocene sediments with growth geometry. The geometry of growth strata constrains the age of its movement between the Paleocene and the Middle Eocene. The geometry of the Western, oblique ramp of the South Pyrenean Central Unit is defined by a series of N–S folds, in some cases associated with underlying West-verging thrusts, as indicated by seismic reflection profiles and field data. In this paper, we propose that the geometry of the thrust wedge of Mesozoic units, progressively thinning from East to West, strongly contributed to constrain the location and geometry of the Western termination of the Montsec thrust. The hypothesis proposed is checked by a series of experimental wedges developed in a sandpack with lateral and three-dimensional thickness variations. Oblique structures form as thrusting progresses at the tip of the sand wedge.     

Thermal history during Mesozoic extension and Tertiary uplift in the Cameros Basin, northern Spain

Detailed structural cross‐sections, analysis of extensional structures and palaeotemperatures obtained from primary fluid inclusions in quartz and calcite veins from the extensional Cameros Basin (N Spain) allow an interpretation of its thermal evolution and its geometric reconstruction to be constrained. The Cameros Basin underwent an extensional stage during the Late Jurassic to Early Cretaceous, with a maximum preserved thickness of Mesozoic deposits of about 9000 m. During the Tertiary, the basin was inverted, allowing a large part of the sedimentary sequence to be exposed. Extensional deformation in individual beds created N120E‐striking tension gashes in the synrift sequence, parallel to the master normal faults limiting the basin and dipping perpendicular to bedding. The extensional strain calculated from tension gashes varies between 4 and 12%. The number and thickness of veins increases the lower their position in the stratigraphic section. Palaeotemperatures were obtained from samples along a stratigraphic section comprising a thickness of 4000 m synrift deposits. Homogenization temperatures range from 107 to 225 °C. Palaeothermometric data and geometric reconstruction give a geothermal gradient of 27–41 °C km^?1 during the extensional stage and allow an eroded section of at least 1500 m to be inferred. Low‐grade metamorphic assemblages in lutitic rocks of the deepest part of the basin presently exposed at surface imply P–T conditions of 350–400 °C and less than 2 kbar, which implies a geothermal gradient of about 70 °C km^?1. Since the metamorphic thermal peak is dated at 100 Ma, the P–T path indicates a heating event during the late Albian, probably linked to the reaching of thermal equilibrium of the continental crust after extension. The results obtained support the hypothesis of a synclinal basin geometry, with vertical superposition of Lower Cretaceous sedimentary units rather than a model of laterally juxtaposed bodies onlapping the prerift sequence.     

Inversion of a transfer system into lateral ramps: An example from the South-Central Pyrenees (Spain)

Field work in the South-Central Pyrenees suggests that omission contacts (i.e. younger over older rocks) occur at the base of the Cadí unit (Cadí thrust), and pass laterally into thrusts. This change occurs across tear faults which are present in the hangingwall of the Cadí thrust sheet and which controlled the deposition of Upper Cretaceous sediments (Adraén formation). Detailed mapping in the contact area between the Nogueres and Cadí units has shown that the actual thrust geometry in the study area is controlled by preexisting normal and transfer faults which developed in an already compressional context. Lateral ramps or tear faults develop depending on the angle between the pre-existing extensional transfer fault and the thrust transport direction.     

Physical and Geotechnical Properties and Assessment of Sediment Stability on the Continental Slope and Basin of the Bransfield Basin (Antarctica Peninsula)

Our investigation is centred on the continental slope of the Antarctic Peninsula and adjacent basin. Type of sediments, sedimentary stratigraphy, and physical and geotechnical characterization of the sediments have been integrated. Four different types of sediments have been defined: diamictons, silty and muddy turbidites, muddy, silty and muddy matrix embedded clast contourites. There is a close correspondence between the physical properties (density, magnetic susceptibility and p-wave velocity) and the texture and/or fabric as laminations and stratification. From a quantitative point of view, only a few statistical correlations between textural and physical properties have been found. Within the geotechnical properties, only water content is most influenced by texture. This slope, with a maximum gradient observed (20°), is stable, according to the stability under gravitational loading concepts, and the maximum stable slope that would range from 22° to 29°. Nevertheless, different instability features have been observed. Volcanic activity, bottom currents, glacial loading-unloading or earthquakes can be considered as potential mechanisms to induce instability in this area.     

Full-scale dynamic testing of the Alamillo cable-stayed bridge in Sevilla (Spain)

Accelerations and displacements due to dynamic excitation by simulated traffic consisting of two trucks, were measured on the deck and tower of the Alamillo cable-stayed bridge. Also the dynamic testing program included the measurement with accelerometers of the free-damped vibration of the 26 cables achieved by means of quick-releasing force. From these measurements it was possible to obtain the dynamic parameters of the bridge (natural frequencies and damping ratios) and the real forces in the cables. In the paper, only the tests, results and conclusions related to dynamic parameters of the bridge are presented. The objective of the dynamic tests herein described was to validate the mathematical modelling and the wind-tunnel models used in the dynamic analysis of the bridge in front of traffic and wind-forces. As the agreement between dynamic parameters of the real bridge and theoretical and scaled models was very satisfactory, the correct dynamic behaviour of the bridge in response to traffic and wind (vortex shedding, flutter, etc.) can be deduced jointly with the correct alignment and expected internal forces in the permanent state in tower and deck.