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Villasenor A.,CSIC - Institute of Earth Sciences Jaume Almera | Chevrot S.,University Paul Sabatier | Harnafi M.,Mohammed V University | Gallart J.,CSIC - Institute of Earth Sciences Jaume Almera | And 5 more authors.
Tectonophysics | Year: 2015

New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400 km, took place on a NW-SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure. © 2015 Elsevier B.V. Source


Castroviejo R.,Technical University of Madrid | Quesada C.,IGME | Soler M.,Technical University of Madrid
Mineralium Deposita | Year: 2011

The original stratigraphic relationships and structure of VMS deposits are commonly obscured by deformation. This can also affect their economic significance, as shown by several Iberian Pyrite Belt (IPB, SW Iberia) examples. The contrasting rheologic properties of the different lithologies present in an orebody (massive sulphide, feeder stockwork, alteration envelope, volcanic and sedimentary rocks) play a major role in determining its overall behaviour. Variscan thin-skinned tectonics led to stacking of the massive pyrite and stockwork bodies in duplex structures, resulting in local thickening and increased tonnage of minable mineralization. Furthermore, differential mechanical behaviour of the different sulphide minerals localised the detachments along relatively ductile sulphide-rich bands. The result was a geochemical and mineralogical reorganisation of most deposits, which now consist of barren, massive pyrite horses, bounded by base metal-rich ductile shear zones. Metal redistribution was enhanced by mobilisation of the base metal sulphides from the initially impoverished massive pyrite, through pressure-solution processes, to tensional fissures within the already ductile shear zones. In NW Iberia, VMS deposits were also strongly overprinted by the Variscan deformation during emplacement of the Cabo Ortegal and Órdenes allochthonous nappe complexes, but no stacking of the orebodies was produced. Original contacts were transposed, and the orebodies, their feeder zones and the country rock acquired pronounced laminar geometry. In lower-grade rocks (greenschist facies, Cabo Ortegal Complex), solution transfer mechanisms are common in pyrite, which remains in the brittle domain, while chalcopyrite shows ductile behaviour. In higher-grade rocks (amphibolite facies, Órdenes Complex), metamorphic recrystallisation overprints earlier deformation textures. The contrasting behaviour of the IPB and NW Iberian deposits is explained by key factors that affect their final geometry, composition and economics, such as pre-deformation structure, size and mineralogical composition of the orebody and associated lithologies, temperature, crustal level, deviatoric stress and availability of a fluid phase during deformation and the style and rate of deformation. © 2010 Springer-Verlag. Source


Oliva-Urcia B.,University of Zaragoza | Casas A.M.,University of Zaragoza | Soto R.,IGME | Villalain J.J.,University of Burgos | Kodama K.,Center for Advanced Marine Core Research
Geophysical Journal International | Year: 2011

The purpose of this paper is to explain the kinematics of the inverted extensional Organyà basin (Central Southern Pyrenees) during its extensional evolution. The study of the anisotropy of magnetic susceptibility (AMS) and the study of brittle mesostructures are the instruments used to decipher the extensional kinematics. Separation of magnetic subfabrics (analyses of AMS at low temperature and anisotropy of the anhysteretic remanence - AARM) has been also done to properly interpret the obtained magnetic fabric data. The combination of all magnetic fabric and brittle mesostructures analyses has provided new information to clarify the structural evolution of the basin, which support a new tectonic model for the Aptian-Albian basinal stage. 20 sites for the magnetic fabric analysis were sampled in Aptian-Lower Albian marls. The AMS ellipsoids obtained at room temperature show their kmin axes perpendicular to bedding and kmax axes oriented NW-SE in 70 per cent of the samples, N-S in 15 per cent and NE-SW in 15 per cent. Analyses of AMS at low temperature show a better clustering of the axes of the magnetic ellipsoid pointing out that phyllosilicates constitute the carriers of the AMS, whereas the analyses of the AARM confirm the scattering of the ferromagnetic grains. These results suggest that the obtained AMS in the Organyà basin was acquired during or shortly after deposition of the Aptian-Lower Albian marls without any subsequent overprint.The direction of the magnetic lineation obtained from the AMS at room and low temperatures, agrees with the orientation of the stress axes obtained from the analysis of brittle mesostructures (mainly faults and calcite-filled tension gashes) indicating its tectonic origin. We interpret the orientation of the magnetic ellipsoids related to the transtensional movements linked to the formation of the Organyà basin during the Aptian-Albian interval, which are consistent with the eastward displacement and rotation of Iberia with respect to Europe. Analysis of tension gashes also indicates a main NW-SE extension direction, whereas palaeostress analysis of striated faults shows a more N-S trending σ3. The transtensional model proposed from magnetic fabric and brittle mesostructures for the Organyà basin during the Aptian-Albian is consistent with the creation of a strongly subsiding area south of the Axial Zone during the Cretaceous and implies that rotation and separation of Iberia respect to Europe took place simultaneously. © 2010 The Authors Geophysical Journal International © 2010 RAS. Source


Pozo-Antonio J.S.,University of Vigo | Alejano L.R.,University of Vigo | Veiga M.,University of Vigo | Alberruche E.,IGME
Proceedings of the Institution of Civil Engineers: Engineering Sustainability | Year: 2014

There is a broad consensus in society that landscape is an important asset that needs to be protected. In recent decades, although substantial efforts have been invested in better understanding and describing landscape, the inclusion of landscape values in daily decision making in the mining and civil engineering fields is a relatively new notion. In this paper, broadening of the scope of traditional slope design is proposed (i.e. to create stable and economic slopes or retaining walls) to take account of environmental and aesthetic values. For this purpose an indirect landscape assessment method is developed, based on quantitatively rating aspects such as morphology, colour distribution and vegetation, so as to produce a score for landscape quality. The results are compared with those resulting from an online survey based on direct assessment of 30 photographed landscapes, resulting in a reasonably good correlation. This indirect assessment method is also applied to a practical case study of drystone as opposed to concrete retaining walls. Use of this method is recommended as an approach to including environmental and aesthetic values in practices like mining and civil engineering that have traditionally altered landscapes. Source


Sanchez-Garcia T.,IGME | Bellido F.,IGME | Pereira M.F.,University of Evora | Chichorro M.,New University of Lisbon | And 3 more authors.
Gondwana Research | Year: 2010

Two very different periods of magma emplacement in the crust of the Ossa-Morena zone (early and main events) in SW Iberia have been previously interpreted to record a Cambrian/Early Ordovician rifting event that is thought to have culminated in the opening of the Rheic Ocean during the Early Ordovician. New stratigraphic, petrographic, geochemical and Sm-Nd isotope data from Cambrian volcanic rocks included in six key low-grade sections in both Portugal and Spain considerably improve our understanding of these events. These data: (1) confirm the existence of two rift-related magmatic events in the Cambrian of the Ossa-Morena zone, (2) demonstrate that the early rift-related event was associated with migmatite and core-complex formation in the mid-upper crust and is represented by felsic peraluminous rocks, the parent magmas of which were derived mainly from crustal sources, and (3) show the main rift-related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks. Some of the mafic rocks show N-MORB affinity, whereas others have OIB or E-MORB affinities, suggesting different heterogeneous mantle sources (depleted and enriched, asthenospheric and lithospheric, plume-like and non-plume-like). The acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle-derived magmas. This new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge-trench collision. We interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower-crust flow in a high heat-flow setting, to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates. © 2009 International Association for Gondwana Research. Source

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