Contemporaneous assembly of Western Gondwana and final Rodinia break-up: Implications for the supercontinent cycle

Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800–700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-São Francisco, West African and Amazonian cratons collided at ca. 630–600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610–600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran–Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.     

Influence of soil non-linearity on the dynamic response of high-speed railway tracks

The main objectives of this paper are the evaluation of the relevance of the non-linear behaviour of the soil on the track response and the validation of a methodology, which includes these effects through an equivalent linear analysis. The proposed numerical model is based on 2.5D finite/infinite elements method, coupled with an iterative procedure in order to obtain an agreement between the strain levels and the dynamic properties of the materials. In order to validate the model, the case study of Ledsgard was simulated, and the experimental and numerical results of displacements of the track were compared, considering several circulation speeds for the X2000 train. From the results, it is possible to recognize that the stiffness degradation, function of the strain level, plays a relevant role for the case of high-speed railway lines on soft ground. Moreover, the simulations developed with the proposed methodology provided similar results to those observed, independently of the train speed, contrary to what was obtained when the elastic linear model was used.     

Ongoing research on seismic safety assessment

Research on seismic safety assessment has been the centre of great interest among the scientific community in recent years. Although the devastating impact of earthquakes on current society should be incentive enough to increase research, the development of more realistic mechanical behaviour models and the continuous enhancement of computation capabilities are paramount factors contributing a great deal to the increase of such interest. In this context, three research areas can be identified as currently leading to important developments: code related research, especially in Europe where new design codes are in the implementation process; risk analysis, namely concerning the definition of methodologies for safety assessment that involve the evaluation of the failure probability and could be included in future generations of codes; and experimental characterization of constitutive laws which provides support for the development and calibration of accurate and realistic numerical models for seismic analysis and for the adequate characterization of limit state capacities. The paper will present some of the current scientific research trends on these three seismic safety assessment related topics. Studies addressing the seismic safety assessment of structures, either probabilistically or based on code provisions, and that consider realistic nonlinear mechanical behaviour models will be focussed. Reference will also be made to experimental research on the seismic behaviour of structural elements, emphasizing its crucial role to support the development of numerical models simulating the effects of different seismic strengthening techniques. Finally, given the development of studies leading to new trends and perspectives for performance based earthquake engineering, a possible scenario for seismic design in the future is presented, emphasizing the key issues for its implementation.     

K-feldspar IRSL dating of a Pleistocene river terrace staircase sequence of the Lower Tejo River (Portugal,western Iberia)

We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected D_e values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation, giving a minimum corrected D_e of about 1000 Gy, and thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (～Marine Isotope Stage [MIS]11); T4 is ca. 340–150 ka (～MIS9-6); T5 is 136–75 ka (～MIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that blankets T6 and some of the older terraces is 30–≥12 ka. Collectively, the luminescence ages seem to indicate that regional river downcutting events may be coincident with periods of low sea level (associated, respectively, with the MIS10, MIS6, MIS4 and MIS2).     

Late Quaternary paleoseismic evidence on the Munébrega half-graben fault (Iberian Range,Spain)

The Munébrega Plio-Quaternary half-graben is a NW-SE trending neotectonic depression located in the central sector of the intraplate Iberian Range (NE Spain). The master fault of the half-graben offsets an Upper Pleistocene pediment deposit, forming an upslope-facing scarp. A trench dug across the fault scarp exposed a 25-m wide deformation zone consisting of graben and horst fault blocks with fissures in the upper part of the scarp, and a monoclinal flexure affected by normal and reverse faults in the lower part of the scarp. We infer a minimum of three faulting events over the past 72 ka, yielding an average (maximum) recurrence interval of 24 ka. The oldest event (72–41 ka) produced an antislope scarp on the relict pediment surface, confining deposition to the downthrown block. Cross-cutting faults affecting sedimentary units deposited in the sediment trap produced by the first event provide evidence for at least two younger events (33–19? ka). The measured cumulative vertical displacement (7.4 m) yield a minimum vertical slip rate of 0.10 ± 0.01 mm/year (2σ error) for the past 72 ka. If the paleoearthquakes ruptured the whole mappable length of the fault (ca. 20 km), they probably had moment magnitudes ca. 6.9 (Stirling et al. Bull Seismol Soc Am, 2002). Such earthquakes would have been more than a magnitude unit larger than the largest ones recorded historically in the Iberian Range. These results suggest that the official seismic hazard assessments, based solely on the historic and instrumental record, may underestimate the seismic hazard in the area.     

A rootless suture and the loss of the roots of a mountain chain: The Variscan belt of NW Iberia

Ophiolites of different Paleozoic ages occur in North-West (NW) Iberia in a rootless suture representing the remnants of the Rheic Ocean. Associated allochthonous terranes in the hanging- and foot-walls of the suture derive from the former margins, whereas the relative autochthon corresponds to the Paleozoic passive margin of northern Gondwana. The Paleozoic tectonic evolution of this part of the circum-Atlantic region is deduced from the stratigraphical, petrological, structural and metamorphic evolution of the different units and their ages. The tectonic reconstruction covers from Cambro-Ordovician continental rifting and the opening of the Rheic Ocean to its Middle to Upper Devonian closure. Then, the Variscan Laurussia–Gondwana convergence and collision is briefly described, from its onset to the late stages of collapse associated with the demise of the orogenic roots.     

Changes in the Diffuse CO2 Emission and Relation to Seismic Activity in and around El Hierro, Canary Islands

Significant changes in the diffuse emission of carbon dioxide were recorded in a geochemical station located at El Hierro, Canary Islands, before the occurrence of several seismic events during 2004. Two precursory CO₂ efflux increases started thirteen and nine days before two seismic events of magnitude 2.3 and 1.7, which took place near El Hierro Island, Canary Islands, on March 23 and April 15, reaching a maximun value of 51.1 and 46.2 g m⁻² d⁻¹, respectively, five and eight days before the two seismic events. Other similar increases started thirteen and five days before the occurrence of two seismic events of magnitude 1.3 and 1.5 which took place on October 15 and 21 respectively, reaching the maximum values four and one day before the earthquakes. These changes were not related to variations in atmospheric or soil parameters. The Material Failure Forecast Method (FFM), which analyzes the rate of precursory phenomena, was successfully applied to forecast the first seismic event that took place in El Hierro Island in 2004.     

Stratigraphy of the tectonically imbricated lithological succession of the Neves Corvo mine area,Iberian Pyrite Belt,Portugal

Biostratigraphic research, based on palynomorphs and ammonoids, of the tectonically imbricated lithological succession of the Neves Corvo mine, in the Portuguese part of the Iberian Pyrite Belt, has yielded ages for all formerly recognised lithostratigraphic units. These can be assembled in three main lithological sequences: (1) detrital sandy/shale substrate (Phyllite-Quartzite Formation) of late Famennian age; (2) Volcano-Sedimentary Complex, divided into a lower and an upper suite, in which one basic, three dolerite sills and four felsic volcanic units and a mineralised package of massive sulphides are identified with ages which range from the late Famennian to the late Visean; (3) flysch succession (Mértola Formation) composed of shale and greywacke dated as late Visean to early Serpukhovian. Precise biostratigraphic dating of the sedimentrary host rocks of massive sulphide mineralisation constrains the age of the latter to the late Strunian (354.8–354.0 Ma). Three stratigraphic hiatuses, corresponding to early/middle Strunian, Tournaisian and early Visean respectively and a south-westward progressive unconformity were also recognised. Sequences 1 and 2 are related to extensional episodes while sequence 3 marks the beginning of compressive tectonic inversion which gave rise to south-westward flysch progradation in close relation to a foreland basin development. Our results lead to the reinterpretation of the tectonic structure of the Neves Corvo mine, with implications for the interpretation of the regional basin dynamics and metal exploration.Editorial handling: F. Tornos     

Granitic dimensional stones in Uruguay: evaluation and assessment of potential resources

In Uruguay commercial granite varieties comprise mafic rocks, granitoids, and syenitoids. There is a long tradition in Uruguay, as well as worldwide, of using dimensional stones in architecture and art, specially granitic ones. Some of the present applications of these dimensional stones are as façade cladding, countertops, and outdoor and indoor floor slabs. The color spectrum of the Uruguayan granitic dimensional stones varies from black to light gray, covering a wide variety of red and pink and minor greenish-gray. The décor of these granitic dimensional stones is mainly determined by their fabric, fundamentally the grain size and the color distribution between the different minerals that compose the rocks. In the present research the most important commercial granites were sampled to analyze their petrography and petrophysical properties. A detailed structural analysis has been performed in several deposits, as well as the application of the software 3D Block Expert for modeling the possible raw block size distribution. Other factors controlling the mining viability of the deposits were also studied (e.g., homogeneity/heterogeneity of color and décor) and the possible reserves were calculated.