Accretion of Grenvillian terranes to the southwestern border of the R��o de la Plata craton, western Argentina

A comprehensive review of the geological, geochronological, and isotopic features of the Mesoproterozoic Grenvillian terranes attached to the southwest of the Río de la Plata craton in Early Paleozoic times is presented in this paper. They are grouped into the northern (sierras de Umango, Maz and del Espinal and surroundings), central (Sierra de Pie de Palo, southern Precordillera and Frontal Cordillera), and southern (San Rafael and Las Matras Blocks) segments. The Mesoproterozoic basement consists mainly of arc related, intermediate to acidic and mafic?Cultramafic rocks of 1,244?C1,027?Ma, with juvenile, Laurentian affinity. Exception to it is the Maz Group, with a protracted history and reworked character. They are affected by 846?C570?Ma, extensional magmatism in the northern and central segments, which represents the Neoproterozoic breakup of the Rodinia supercontinent. Successive passive margin sedimentation is registered in Late Neoproterozoic (~640?C580?Ma) and Cambro-Ordovician (~550?C470?Ma) times. The southern segment is noted for the younger sedimentation alone, and for showing the exclusive primary unconformable relationship between the Mesoproterozoic basement and Early Ordovician cover. The effects of Early Paleozoic Famatinian orogeny, associated with the collisions of Cuyania and Chilenia terranes, are recorded as main phase (480?C450?Ma), late phase (440?C420?Ma), and Chanic phase (400?C360?Ma). Among them, the tectonothermal climax is the Ordovician main phase, to which klippe and nappe structures typical of collisional orogens are related in the northern and central segments. Preliminary data allow us to suggest a set of paired metamorphic belts, with an outboard high-P/T belt, and an inboard Barrowian P/T belt.     

GIS-based mapping for marine geohazards in seabed fluid leakage areas (Gulf of Cadiz, Spain)

This paper applies, for the first time in offshore deepwater, a method based on geographic information systems for seafloor susceptibility assessment as a first approach to marine geohazard mapping in fluid leakage areas (slope instabilities, gas escapes, seabed collapses, pockmarks, etc.). The assessment was carried out in a known seabed fluid-flow province located on the Iberian margin of the Gulf of C??diz, Spain. The method (based on statistical bivariate analysis) creates a susceptibility map that defines the likelihood of occurrence of seafloor features related to fluid flow: crater-like depressions and submarine landslides. It is based on the statistical index (Wi) method (Van Westen in Statistical landslide hazard analysis. ILWIS 2.1 for Windows application guide. ITC Publication, Enschede, pp 73?C84, 1997), in which Wi is a function of the cartographic density of seafloor features on ??factor maps??. The factors selected monitor the seafloor??s capability to store and transfer hydrocarbon gases and gravitational instability triggers: geology-lithology, gas hydrate stability zone thickness (temperature, pressure?Cwater depth and geothermal gradient), occurrence of diapirs, proximity to faults or lineaments, and slope angle of the seafloor. Results show that the occurrence of seafloor features related to fluid flow is highest where the factors ??gas source and storage?? and ??pathways of fluid escape?? converge. This means that they are particularly abundant over diapirs in contourite deposits, in the vicinity of faults, and inside theoretical gas hydrate stability fields thinned by warm undercurrents. Furthermore, the submarine landslides located on the Palaeozoic-Toarcian basement are not related to fluid leakage. This methodology provides helpful information for hazard mitigation in regional selection of potential drill sites, deep-water construction sites or pipeline routes. It is an easily applied and useful tool for taking the first step in risk assessment on a regional scale for vast areas where fluid leakage may be present, the geological model is known, and the geologically hazardous features have already been mapped.     

Rockfall detachment susceptibility map in El Hierro Island, Canary Islands, Spain

A semi-quantitative heuristic methodology is developed to map a rockfall detachment susceptibility zonation of El Hierro Island (Canary Archipelago). The rationalized procedure, which we called non-weighted bounded indicators, is based on overlapping thematic maps of conditioning factors to mass movement, which are appropriately and individually rescaled and then composed by addition to obtain a susceptibility numerical index through a GIS. As the consistency of the geomorphological analysis depends on the expert subjective criteria and the appropriate interpretation of the landscape, the use of this methodology reduces subjectivity and quantifies the degree of susceptibility. The main factors affecting the mass movement phenomena (rockfalls events), also recognized in the field and, therefore, considered in the presented GIS arrangement, are slope, profile curvature, lithology, vegetation cover and dykes density. To calculate the slope threshold or minimum angle characteristic of rockfall source areas, mixed Gaussian slope frequency decomposition is used. The curvature index reveals stepwise areas. Qualitative geomechanical characteristics are linked to a quantitative index according to a volcanic lithological-complexes classification. Both destabilization (root-wedging) and stabilization effects are considered into the vegetation cover index. The dyke density index incorporates the bearing rock capacity decrease produced in the halo around a dyke network intrusion. Slope, curvature and vegetation indexes thresholds have been fitted following field observations. A rockfall detachment susceptibility map is obtained and classified based on the histogram maxima. The rockfall inventory, based on rockfall events reported within the island, was used for the model validation. A 12?% of the whole island shows medium to very high susceptibility.     

Assembly of Pampia to the SW Gondwana margin: A case of strike-slip docking?

Different hypotheses have been proposed to account for the geologic evolution of the southwestern margin of Gondwana in the Early Paleozoic, involving accretion and displacement of different terranes in a protracted convergent margin. In order to constrain and understand the kinematic and paleogeographic evolution of the Pampia terrane a paleomagnetic study was carried out in different Cambrian to Devonian units of the Eastern Cordillera (Cordillera Oriental) and the Interandean Zone (Interandino) of NW Argentina. Paleomagnetic poles from the Campanario Formation (Middle to Upper Cambrian): 1.5°N 1.9°E A₉₅ = 9.2° K = 37.46 N = 8; and Santa Rosita Formation (Lower Ordovician): 8.6°N 355.3°E A₉₅ = 10.1° K = 26.78 n = 9, representative of the Pampia terrane, are interpreted to indicate a Late Cambrian significant displacement with respect to the Río de la Plata and other Gondwana cratons. A model, compatible with several geological evidences that explains this displacement in the framework of the final stages of Gondwana assembly is presented. We propose a simple dextral strike-slip kinematic model in which Pampia and Antofalla (? Arequipa?) blocks moved during Late Cambrian times from a position at the present southern border of the Kalahari craton into its final position next to the Rio de la Plata craton by the Early Ordovician.     

Sedimentary geochemical record of historical anthropogenic activities affecting Guanabara Bay (Brazil) environmental quality

A sediment core from Guanabara Bay (Brazil) was analyzed for ²¹⁰Pb dating, grain size, total organic carbon (TOC) and total nitrogen, carbon stable isotope ratio (δ¹³C), nitrogen stable isotope ratio (δ¹⁵N) and the metals Fe, Mn, Ni, Co, Cu, Pb, V and Zn, to assess the influence of land use changes on the aquatic system in a region for which large industrial and urban development is expected in the next few decades. To obtain baseline data for improving the monitoring of the expected increase in anthropogenic impacts from surrounding drainage basins, a multivariate analysis of data from different sediment layers was carried out to evaluate the dated sediment record. The geochemical data suggested three different sedimentary phases along the last 200 years. Before the 1880s, the highest clay and TOC contents were observed, where the C/N ratios and the δ¹³C values suggested a mixture of algal and terrestrial organic matter and the lowest concentrations of Co, Cu, Pb, V and Zn, for which background levels were estimated (4.6, 2.7, 14.9, 24.3 and 70.2 mg kg⁻¹, respectively). From the 1880s to the 1950s, the metal concentrations and sand particles increased, but no change in organic matter quality was observed, reflecting a period of land use change, still without significant sewage input. After the 1950s, the sedimentation rate increased from 0.42 to 0.77 cm year⁻¹ and increasing urban sewage input was evidenced by lower C/N ratios, higher δ¹⁵N, decrease of Fe and Mn concentrations and increased fluxes of metals and TOC, which showed a good relationship with population growth data.     

Influence of the properties of tropical soils in the toxicity and bioavailability of heavy metals in sewage sludge-amended lands

The toxicity and potential bioavailability of heavy metals were studied in sewage sludge-amended ferralsols and chernosols, using bioassays with earthworms (Eisenia andrei). Acute toxicity tests and avoidance assays were performed according to standard protocols. Potentially bioavailable concentrations of heavy metals were estimated by quantifying their concentrations in tissues of survival earthworms. The results revealed that soil properties played an important role in the toxicity and bioavailability of heavy metals. In this respect, higher levels of toxicity were detected for the ferralsol. The abundance of 2:1 clay minerals, high fertility and more basic values of pH seem to be very important in the reduction of toxicity levels for earthworms. Organisms exposed to the chernosols were able to uptake higher amount of metals. In that case, higher contents of nutrients in chernosols may have influenced such bioavailability processes. Avoidance responses were the most sensible indicator of toxicity. In the near future, such data can subsidize the establishment of toxic reference concentrations able to reflect the characteristics of important tropical pedological occurrences, supporting the definition of sustainable indicators for using sewage sludge in the tropical agriculture.     

Creative economy and employment quality in large urban areas in Spain

The impact of the economic crisis should act as an incentive to devise less vulnerable economic development models for Spanish cities. This study confirms that the impact of the creative economy, one of the most interesting initiatives in this context, remains very limited. Furthermore, microdata obtained from a sample of one million workers in Spain, published annually by the Social Security system, confirm that the sharp contrasts emerging in employment quality between sectors and regions, the processes of ‘precariousness of talent’ and labour dualization expressed through inequalities in the occupational structure are each becoming significant threats to viable economic development. For the creative economy to be an effective instrument for metropolitan regeneration, adapted to suit the specificities of particular urban development paths, an improved understanding of the inherent complexity of creative-economy employment relations is required.     

Neotectonic basin and landscape evolution in the Eastern Cordillera of NW Argentina,Humahuaca Basin (~24°S)

The intermontane Quebrada de Humahuaca Basin (Humahuaca Basin) in the Eastern Cordillera of the southern Central Andes of NW Argentina (23°–24°S) records the evolution of a formerly contiguous foreland‐basin setting to an intermontane depositional environment during the late stages of Cenozoic Andean mountain building. This basin has been and continues to be subject to shortening and surface uplift, which has resulted in the establishment of an orographic barrier for easterly sourced moisture‐bearing winds along its eastern margin, followed by leeward aridification. We present new U–Pb zircon ages and palaeocurrent reconstructions suggesting that from at least 6 Ma until 4.2 Ma, the Humahuaca Basin was an integral part of a largely contiguous depositional system that became progressively decoupled from the foreland as deformation migrated eastward. The Humahuaca Basin experienced multiple cycles of severed hydrological conditions and subsequent re‐captured drainage, fluvial connectivity with the foreland and sediment evacuation. Depositional and structural relationships among faults, regional unconformities and deformed landforms reveal a general pattern of intrabasin deformation that appears to be associated with different cycles of alluviation and basin excavation in which deformation is focused on basin‐internal structures during or subsequent to phases of large‐scale sediment removal.     

Source subtraction for the extended Very Small Array and 33-GHz source count estimates

We describe the source subtraction strategy and observations for the extended Very Small Array (VSA), a cosmic microwave background interferometer operating at 33 GHz. A total of 453 sources were monitored at 33 GHz using a dedicated source subtraction baseline. 131 sources brighter than 20 mJy were directly subtracted from the VSA visibility data. Some characteristics of the subtracted sources, such as spectra and variability, are discussed. The 33-GHz source counts are estimated from a sample selected at 15 GHz. The selection of VSA fields in order to avoid bright sources introduces a bias into the observed counts. This bias is corrected and the resulting source count is estimated to be complete in the flux-density range 20–114 mJy. The 33-GHz source counts are used to calculate a correction to the VSA power spectrum for sources below the subtraction limit.     

Tectonic evolution of an intraplate basin: the Lower Tagus Cenozoic Basin,Portugal

This article focuses on the reinterpretation of well, seismic reflection, magnetic, gravimetric, surface wave and geological surface data, together with the acquisition of seismic noise data to study the Lower Tagus Cenozoic Basin tectono‐sedimentary evolution. For the first time, the structure of the base of the basin in its distal and intermediate sectors is unravelled, which was previously only known in the areas covered by seismic reflection data (distal and small part of intermediate sectors). A complex geometry was found, with three subbasins delimited by NNE‐SSW faults and separated by WNW‐ESE to NW‐SE oriented horsts. In the area covered by seismic reflection data, four horizons were studied: top of the Upper Miocene, Lower to Middle Miocene top, the top of the Palaeogene and the base of Cenozoic. Seismic data show that the major filling of the basin occurred during Upper Miocene. The fault pattern affecting Neogene and Palaeogene units derived here points to that of a polyphasic basin. In the Palaeogene, the Vila Franca de Xira (VFX) and a NNE‐SSW trending previously unknown structure (ABC fault zone) probably acted as the major strike‐slip fault zones of the releasing bend of a pull‐apart basin, which produced a WNW‐ESE to NW‐SE fault system with transtensional kinematic. During the Neogene, as the stress regime rotated anticlockwise to the present NW‐SE to WNW‐ESE orientation, the VFX and Azambuja fault zones acted as the major transpressive fault zones and Mesozoic rocks overthrusted Miocene sediments. The reactivation of WNW‐ESE to NW‐SE fault systems with a dextral strike‐slip component generated a series of horsts and grabens and the partitioning of the basin into several subbasins. Therefore, we propose a polyphasic model for the area, with the formation of an early pull‐apart basin during the Palaeogene caused by an Iberia–Eurasia plates collision that later evolved into an incipient foreland basin along the Neogene due to a NW‐SE to WNE‐ESE oriented Iberia–Nubia convergence. This convergence is producing uplift in the area since the Quaternary except for the Tagus estuary subbasin around the VFX fault, where subsidence is observed. This may be due to the locking or the development of a larger component of strike‐slip movement of the NNE‐SSW to N‐S thrust fault system with the exception of the VFX fault, which is more favourably oriented to the maximum compressive stress.