La estructura sísmica de la corteza de la Zona de Ossa Morena y su interpretación geológica

El experimento de sísmica de reflexión profunda IBERSEIS ha proporcionado una imagen de la corteza del Orógeno Varisco en el sudoeste de Iberia. Este artículo se centra en la descripción de la corteza de la Zona de Ossa Morena (OMZ), que está claramente dividida en una corteza superior, con reflectividad de buzamiento al NE, y una corteza inferior de pobre reflectividad. Las estructuras geológicas cartografiadas en superficie se correlacionan bien con la reflectividad de la corteza superior, y en la imagen sísmica se ven enraizar en la corteza media. Ésta está constituida por un cuerpo muy reflectivo, interpretado como una gran intrusión de rocas básicas. La imagen de las suturas que limitan la OMZ muestra el carácter fuertemente transpresivo de la colisión orogénica varisca registrada en el sudoeste de Iberia. La Moho actual es plana y, en consecuencia, no se observa la raíz del orógeno

Geophysical model of the lithosphere across the Variscan Belt of SW-Iberia: Multidisciplinary assessment

[EN]A multidisciplinary geophysical study along a large seismic transect in the SW-Iberian Peninsula has been carried out. This study integrates the crustal structure, geometry and composition obtained from normal incidence and wide-angle seismic reflection data with other observables (geoid, gravity and topography). The internal architecture of the lithosphere across the Variscan Orogen of SW-Iberia is constrained by the 300 km long high resolution deep normal seismic reflection IBERSEIS Transect. The most prominent feature imaged by the seismic survey is the Iberseis Reflective Body (IRB), a 140 km long high amplitude reflective body located in the middle crust of the northern half of the transect. The seismic velocity (Vp) distribution within the crust and the upper mantle is constrained by two wide-angle seismic transects acquired in the same area. The velocity models show a complex crust, with a specially complex middle crust, which features higher velocities than the average continental crust. Also the wide-angle data revealed that the IRB is characterized by high velocities. This feature was then interpreted as sill-like structure built up by a series of mafic intrusions. Therefore, a key issue is to study if this relatively mafic crust is consistent with other geophysical observables. Based on the velocity models, two lithospheric density models have been derived along the IBERSEIS wide-angle transects. The geoid, gravity and topography response of these models have been calculated using a finite elements code that solves, simultaneously, the geopotential, lithostatic, and heat flow equations. The resulting values are then compared with the measured observables and the crustal and lithospheric mantle geometry and density is modified until the best fit is obtained. The initial density models calculated from the seismic data adjust quite well to the real potential field data. However, minor modifications have been required in order to properly fit the observables. The final density models are consistent with the existence of relatively high density bodies in the mid-crust providing further support to the seismic interpretation. In addition, they place new constraints on the location of the lithosphere–asthenosphere boundary and on the tectonic evolution of SW-Iberia

Rust exfoliation on carbon steels in chloride-rich atmospheres

© 2015 by De Gruyter 2015. The exposure of carbon steel in marine atmospheres can lead in certain circumstances to the formation of thick rust layers (containing a number of compact laminas) that are easily detached (exfoliated) from the steel substrate, leaving it unprotected and considerably accelerating the corrosion process. This deterioration phenomenon is of particular concern in steel infrastructures located close to the sea (civil constructions, bridges, etc.), whose service lifetime can be extraordinarily limited. High times of wetness of the metallic surface and high chloride ion deposition rates play a decisive role in the formation of this type of rust. Research has been carried out for 1 year in eight pure marine atmospheres with annual average chloride deposition rates of 70-1906 mg Cl-/m2 day. The studied carbon steels consisted of one mild steel, one conventional weathering steel (Corten A), and one high nickel (~3 wt.%) advanced weathering steel (AWS). The paper describes the environmental conditions that lead to the formation of these thick multilaminar rust layers and presents a characterisation study of this singular type of rust using a variety of analysis techniques: scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffraction, Mössbauer spectroscopy, and transmission electron microscopy/electron diffraction. The Ni-AWS shows greater resistance to the occurrence of rust exfoliation.Peer Reviewe

Variscan Metamorphism

Various segments of Variscan crust are currently exposed in Iberia in response to successive tectonic events during the Variscan orogeny itself and subsequent extensional and compressive events during the Alpine cycle, all accompanied by surface erosion, and collectively contributing to their exhumation. We review the main characteristics and geodynamic contexts of the metamorphic complexes developed in Iberia during the Variscan cycle, which include: (i) LP-HT complexes associated to the Cambrian-Early Ordovician rift stage; (ii) HP-LT complexes associated to subduction; and (iii) syn-to-post-collisional, MP and LP/HT complexes from the hinterland to the foreland fold-and thrust belts. All the above contexts are illustrated with case studies. Finally, a review of Variscan metamorphism in the Pyrenees and Catalan Coastal Ranges, located far away from the Rheic suture is also presented