Mesozoic spreading kinematics: consequences for Cenozoic Central and Western Mediterranean subduction

The highly complex tectonics of Central and Western Mediterranean subduction are well documented, but there is significant debate about the responsible dynamics. The motions of the main plates, Africa including Adria, Europe and Iberia, imposed initial and boundary conditions on the evolution of subduction that are often not considered. To quantitatively evaluate these conditions, we make a set of reconstructions from Mesozoic opening through Cenozoic closing of the Alpine Tethys, using main-plate kinematic data from several authors. Geologic and tectonic information are only added to constrain the location of the break-up boundary and a single plate-margin rearrangement at the end of the opening phase. Otherwise, the plates remain undeformed. This rigid-plate approach illustrates the context in which surface deformation and subduction occurred and provides estimates (with uncertainties) of the amount of material that should be accounted for in orogens or documented seismically in the mantle. Full tectonic reconstructions should satisfy such constraints. Opening led to alternating domains of predominantly oceanic lithosphere formed by normal spreading and domains dominated by transform motion, floored mainly by extended continental lithosphere. The transform domain structures provide logical decoupling zones to allow Penninic, Ligurian and Pyrenean basins to start subducting independently. The complex buoyancy in the transform domain linking Ligurian and Penninic basins, and obliquity between directions of opening and closing may account for a number of the oceanic basins and continental slivers often invoked to explain Alpine geology. The significant proportion of continental lithosphere in the Alboran would have favoured delamination of mantle lithosphere over subduction. The almost completely subducted Penninic slab obstructed subduction of the Ligurian domain in the direction of Africa-Europe convergence, possibly forcing the rollback of the Appeninic/Calabrian trenc

Thermally dominated deep mantle LLSVPs: A review

The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth’s thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional com- ponent. The principle evidence that LLSVPs may represent thermo-chemical ‘piles’ comes from seismic constraints, including: (i) their long-wavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocity anomalies (and elevated ratios between shear- and compressional-wave velocity anomalies); (v) anti-correlated shear-wave and density anomalies; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an isochemical, well-mixed mantle. In addition, it has been proposed that hotspots and the reconstructed eruption sites of large ig- neous provinces correlate in location with LLSVP margins. In this paper, we review recent results which indicate that the majority of these constraints do not require thermo-chemical piles: they are equally well (or poorly) explained by thermal heterogeneity alone. Our analyses and conclusions are largely based on comparisons between imaged seismic structure and synthetic seismic structures from a set of thermal and thermo-chemical mantle convection models, which are constrained by ∼ 300 Myr of plate motion histories. Modelled physical structure (temperature, pressure and composition) is converted into seismic velocities via a thermodynamic approach that accounts for elastic, anelastic and phase con- tributions and, subsequently, a tomographic resolution filter is applied to account for the damping and geographic bias inherent to seismic imaging. Our results indicate that, in terms of large-scale seismic structure and dynamics, these two provinces are predominantly thermal features and, accordingly, that chemical heterogeneity is largely a passive component of lowermost mantle dynamics

Multivariate statistical appraisal of regional susceptibility to induced seismicity: application to the Permian Basin, SW United States

Induced earthquake sequences are typically interpreted through causal triggering mechanisms. However, studies of causality rarely consider large regions and why some regions experiencing similar anthropogenic activities remain largely aseismic. Therefore, it can be difficult to forecast seismic hazard at a regional scale. In contrast, multivariate statistical methods allow us to find the combinations of factors that correlate best with seismicity, which can help form the basis of hypotheses that can be subsequently tested with physical models. Whilst strong correlations do not necessarily equate to causality, such a statistical approach is particularly important for large regions with newly emergent seismicity comprising multiple distinct clusters and multi-faceted industrial operations. Recent induced seismicity in the Permian Basin provides an excellent test-bed for multivariate statistical analyses because the main causal industrial and geological factors driving earthquakes in the region remain highly debated. Here, we use logistic regression to retrospectively predict the spatial variation of seismicity across the western Permian Basin. We reproduce the broad distribution of seismicity using a combination of both industrial and geological factors. Our model shows that the proximity to neotectonic faults west of the Delaware Basin is the most important factor that contributes to induced seismicity. The second-most important factor is salt-water disposal at shallow depths, with hydraulic fracturing playing a less dominant role. The higher tectonic stressing, together with a poor correlation between seismicity and large-volume deep salt-water disposal wells indicates a very different mechanism of induced seismicity compared to that in Oklahoma

Thorotrast Related Hepatic Malignancies. Two Case Reports

Os A.A apresentam dois casos clínicos de tumores hepáticos associados ao torotraste, dois colangiocarcinomas, observados numa enfermaria de Medicina Interna de um hospital de Lisboa. É feito um comentário sobre o efeito nefasto das radiações ionizantes em geral sobre o organismo humano, particularizando os efeitos do torotraste. Comenta-se, ainda, o prolongado tempo de latência que existiu nestes dois casos, bem como o facto de, provavelmente, estarmos perante os últimos casos de tumores induzidos pelo torotraste

Mantle wedge temperatures and their potential relation to volcanic arc location

The mechanisms underpinning the formation of a focused volcanic arc above subduction zones are debated. Suggestions include controls by: (i) where the subducting plate releases water, lowering the solidus in the overlying mantle wedge; (ii) the location where the mantle wedge melts to the highest degree; and (iii) a limit on melt formation and migration imposed by the cool shallow corner of the wedge. Here, we evaluate these three proposed mechanisms using a set of kinematically-driven 2D thermo-mechanical mantle-wedge models in which subduction velocity, slab dip and age, overriding-plate thickness and the depth of decoupling between the two plates are systematically varied. All mechanisms predict, on the basis of model geometry, that the arc-trench distance, D, decreases strongly with increasing dip, consistent with the negative D-dip correlations found in global subduction data. Model trends of sub-arc slab depth, H, with dip are positive if H is wedge-temperature controlled and overriding-plate thickness does not exceed the decoupling depth by more than 50 km, and negative if H is slab-temperature controlled. Observed global H-dip trends are overall positive. With increasing overriding plate thickness, the position of maximum melting shifts to smaller H and D, while the position of the trenchward limit of the melt zone, controlled by the wedge's cold corner, shifts to larger H and D, similar to the trend in the data for oceanic subduction zones. Thus, the limit imposed by the wedge corner on melting and melt migration seems to exert the first-order control on arc position

Continental margin subsidence from shallow mantle convection: Example from West Africa

Spatial and temporal evolution of the uppermost convecting mantle plays an important role in determining histories of magmatism, uplift, subsidence, erosion and deposition of sedimentary rock. Tomographic studies and mantle flow models suggest that changes in lithospheric thickness can focus convection and destabilize plates. Geologic observations that constrain the processes responsible for onset and evolution of shallow mantle convection are sparse. We integrate seismic, well, gravity, magmatic and tomographic information to determine the history of Neogene-Recent (+100 °C providing ∼103 m of support. Beneath the Mauritania basin average excess temperatures are <−100 °C drawing down the lithosphere by ∼102 to 103 m. Up- and downwelling mantle has generated a bathymetric gradient of ∼1/300 at a wavelength of ∼103 km during the last ∼23 Ma. Our results suggest that asthenospheric flow away from upwelling mantle can generate downwelling beneath continental margins

Remotely Searching for Noctiluca Miliaris in the Arabian Sea

Reversing monsoonal winds in the Arabian Sea result in two seasons with elevated biological activity, namely the annual summer Southwest Monsoon (SWM; June to September) and winter Northeast Monsoon (NEM; November to March) [Wiggert et al., 2005]. Generally speaking, the SWM and NEM create two geographically distinct blooms [Banse and English, 2000; Levy et al., 2007]. In the summer, winds from the southwest drive offshore Ekman transport and coastal upwelling along the northwestern coast of Africa, which brings nutrient-rich water to the surface from below the permanent thermocline [Bauer et al., 1991]. In the winter, cooling of the northern Arabian Sea causes surface waters to sink, which generates convective mixing that injects nutrients throughout the upper mixed layer [Madhupratap et al., 1996]. This fertilization of otherwise nutrient-deplete surface waters produces one of the most substantial seasonal extremes of phytoplankton biomass and carbon flux anywhere in the world [Smith, 2005]

Genótipos prostrados de feijão-caupi no Cerrado do Amapá.

O cerrado do Estado do Amapá apresenta totais condições para a produção de grãos, sendo o feijão-caupi uma das opções mais viáveis. Assim, selecionar cultivares para este ambiente deve ser uma das metas da pesquisa. No caso em estudo, o experimento objetivou avaliar linhagens de porte prostrado e semi-prostrado. Em Latossolo Amarelo, clima do tipo Ami, usou-se o delineamento experimental de blocos ao acaso e quatro repetições, adubação de 80 kg ha-1 (superfosfato triplo) e 50 kg ha-1 (cloreto de potássio). A semeadura manual ocorreu em fileiras espaçadas de 0,80 m, com área útil de 8 m2 e população de 100 mil plantas ha-1. A análise de variância mostrou diferença significativa pelo teste de Tukey (p< 0,05). O melhor desempenho produtivo de 1.216,7 kg ha-1 foi da linhagem MNC02-676F-1, que apresenta grão da subclasse mulato, seguida pela cultivar BRS-Pajeú com 1.119,0 kg ha-1, com grão da subclasse sempre verde. A linhagem BRS Juruá alcançou menor período de floração, enquanto o maior comprimento de vagem foi do genótipo MNC02-677F-2. Todas as linhagens mostraram tolerância à incidência de doenças.Disponível em: http://www.conac2012.org/resumos/pdf/133c.pdf. Acesso em: 22 jul. 2013