Search

The complex tectonic interaction processes between the European and African plates at the Western Mediterranean have left marked imprints in the crustal architecture of this area, particularly concerning the lateral variations in crustal thicknesses. The detailed mapping of such variations is hence of large interest, as it provides a major constraint to geophysical and geodynamic modeling at different scales. Controlled-source seismic profiling and receiver functions from natural seismicity are widely considered as major tools to constrain Moho topography. We compile here the Moho depths determined from a comprehensive number of both types of seismic surveys, to end up with a new 3D Moho depth map of the Iberian Peninsula, its continental margins and North Morocco. Since the 1970s, the lithospheric structure beneath this study area has been extensively investigated using multichannel normal incidence seismic reflection and refraction/wide-angle reflection profiling. In the last few years some high-resolution surveys at sea and inland have been acquired, the latter ones involving ~ 1000 land stations. On the other hand, the TopoIberia-IberArray experiment has triggered the investigations on crustal and lithospheric structure using natural seismicity, providing a homogeneous spatial resolution never achieved before. The availability of good quality results from both methodologies in a common area provides an excellent opportunity to check the consistency of the Moho depth estimations. The integration of both datasets has resulted in a new, high resolution map of the crustal thickness variations. The final grid evidences large Moho topography variations, including crustal imbrication in the Pyrenean range, a large and relatively undisturbed Variscan Massif in the center of Iberia and a probable delamination process beneath the Gibraltar Arc. The crustal thicknesses vary from ~ 15 km in continental margins up to values exceeding 50 km beneath the Pyrenees or the Rif Cordillera. [ABSTRACT FROM AUTHOR]

The knowledge of the anisotropic properties beneath the Iberian Peninsula and Northern Morocco has been dramatically improved since late 2007 with the analysis of the data provided by the dense TopoIberia broad-band seismic network, the increasing number of permanent stations operating in Morocco, Portugal and Spain, and the contribution of smaller scale/higher resolution experiments. Results from the two first TopoIberia deployments have evidenced a spectacular rotation of the fast polarization direction (FPD) along the Gibraltar Arc, interpreted as an evidence of mantle flow deflected around the high velocity slab beneath the Alboran Sea, and a rather uniform N100°E FPD beneath the central Iberian Variscan Massif, consistent with global mantle flow models taking into account contributions of surface plate motion, density variations and net lithosphere rotation. The results from the last Iberarray deployment presented here, covering the northern part of the Iberian Peninsula, also show a rather uniform FPD orientation close to N100°E, thus confirming the previous interpretation globally relating the anisotropic parameters to the LPO of mantle minerals generated by mantle flow at asthenospheric depths. However, the degree of anisotropy varies significantly, from delay time values of around 0.5 s beneath NW Iberia to values reaching 2.0 s in its NE corner. The anisotropic parameters retrieved from single events providing high quality data also show significant differences for stations located in the Variscan units of NW Iberia, suggesting that the region includes multiple anisotropic layers or complex anisotropy systems. These results allow to complete the map of the anisotropic properties of the westernmost Mediterranean region, which can now be considered as one of best constrained regions worldwide, with more than 300 sites investigated over an area extending from the Bay of Biscay to the Sahara platform. [ABSTRACT FROM AUTHOR]

Background Squamous cell carcinoma ( SCC) in the ungual apparatus is a rare neoplasia. Although it is the most prevalent malignant tumour in this region, its diagnosis is often delayed because it is likened to benign or infectious processes. Objectives Present a case of SCC with subungual location in a toe, and carry out a review of the literature in relation to the most important aspects of subungual SCC in toes. Methods We describe the case of a white woman aged 72, with SCC located in the pulp and in the distal area of the nail bed of the third toe, associated with subungual exostosis. The literature on SCC in the subungual area of toes, in all its forms under clinical or histopathological presentation, is reviewed by means of a search involving Medline, PubMed, and Google Academic, from January 1994 to December 2011. Results Only 36 cases of subungual SCC in toes were reported in the 18 years that were reviewed. The average age of cases reviewed was 58.92, and the male-to-female ratio was 2 : 1. The most affected toe is the hallux, in 69.4% of cases. In 50% of cases, aetiology is unknown or indeterminate; in 22.3% of cases, aetiology is metastatic; and in 19.5% of cases, it is associated with human papillomavirus ( HPV). In 58.2% of cases, partial or total resection of the distal phalanx was carried out, of the entire toe, or of the osseous ray of the foot. Conclusions Chronic tissue irritation caused by microtrauma associated with subungual exostosis is the most probable aetiology of the case presented. [ABSTRACT FROM AUTHOR]

Abstract: The regional mantle flow beneath the westernmost Mediterranean basin and its transition to the Atlantic domain is addressed by inspecting the anisotropic properties of the mantle. More than 100 new sites, from the Variscan core of Iberia to the northern rim of the Western African Craton, are now investigated using the data provided by different temporary and permanent broad-band seismic arrays. Our main objective is to provide a larger regional framework to the results recently presented along the Gibraltar Arc in order to check the validity of the different geodynamic interpretations proposed so far. The significant variations in the retrieved anisotropic parameters suggest that different processes must be invoked to explain the origin of the observed anisotropy. Beneath the Variscan units of the Central Iberian Massif the new results show a moderate amount of anisotropy with fast polarization directions (FPD) oriented close to E–W. Those results can only be explained in terms of global mantle flow if models accounting for contributions from surface plate motion, net lithosphere rotation and density variations are taken into consideration. One of the major results presented is the significant number of good quality data without evidence of anisotropy (“nulls”) observed beneath permanent stations in southern Portugal. Those “nulls” can be explained by the presence of a predominantly vertical mantle flow associated to large variations in the lithospheric thickness. Beneath the Gibraltar Arc the FPD show a spectacular rotation, evidenced by the results presented by Díaz et al. (2010) and Miller et al. (2013). Those results are reviewed here taking also into consideration the geodynamic modeling presented recently by Alpert et al. (2013) and other geophysical and geodetic results. Further South, the analysis of new broad-band stations installed in the Moroccan Meseta and the High Atlas show a small degree on anisotropy and a large number of “null” events, pointing again to the presence of vertical flow in the mantle. The results favor an asthenospheric origin related to present-day mantle flow for the anisotropy observed from the Variscan core of Iberia to the northern rim of the West African Craton. This flow is deflected around the high velocity slab beneath the Gibraltar Arc and seems affected locally by vertical flow associated to edge-driven convective cells. The presence of significant backazimuthal variations in the anisotropic parameters retrieved from single events suggests that a second order contribution from an anisotropic layer within the lithosphere may also exist. [Copyright &y& Elsevier]

Abstract: In the last years, the deep crustal structure of the North-Western part of the Iberian Peninsula has been explored using teleseismic receiver function (RF) analysis of P to S conversions at main crustal interfaces. This area has been previously investigated by seismic reflection and refraction experiments and therefore provides an excellent opportunity to compare the results of both approaches. The region shows the imprint of two orogenic events, the Variscan and Alpine ones that exhibit different, reverse intensities from west to east. In a first stage, a N–S transect across the Cantabrian Mountains was instrumented to study the area affected by the Alpine compressional tectonics. Later on, the limit between the undisturbed Variscan units and the reworked Alpine zones was explored by N–S and E–W transects. Finally, an array was deployed in the NW edge of Iberia, over the Variscan hinterland zone. The receiver functions are calculated by inverse filtering deconvolution of the L component from the Q component, and the resulting RF are processed using a simple form of migration to obtain images in depth of the lithosphere that can be compared to the 2-D velocity–depth models from active seismic experiments. The deep crustal structure constrained by both techniques is remarkably consistent, and provides further evidence on the crustal doubling and wedging between the Iberian and European crusts throughout the northern part of the Iberian Peninsula affected by the Alpine tectonics. The undisturbed Variscan domains, characterized by a clear subhorizontal Moho and few intracrustal convertors, show up at the southern edge of the N–S transects (southward of the Alpine deformation front) and over the Variscan hinterland in the E–W transect. The crust resulting from Alpine reworking presents a complex structure, with short and frequently dipping convertors, which in some cases seem to image preserved Variscan structures. [Copyright &y& Elsevier]

Abstract: We present here an updated compilation of crustal parameters beneath the Iberian Peninsula and surrounding waters, inferred from extensive deep seismic exploration performed in the last three decades. Firstly, the more relevant experiments developed in both terrestrial and marine domains are revisited, together with the corresponding seismic velocity–depth models published that reveal the characteristics of the different tectonic domains sampled, including oceanic zones, continental margins, orogenic and rift belts. The results are summarized in three crustal transects sketched along representative directions. Secondly, we compiled the Moho depths along all the available seismic profiles to produce a geo-referred database. This database has then been interpolated using kriging algorithms to obtain a continuous Moho depth model, in the form of a regular grid file. This integrative Moho map for a key area in the Western Mediterranean documenting the interaction between European and African plates can contribute to constrain further regional studies, either at crustal, lithospheric or mantle scales. [Copyright &y& Elsevier]

Abstract: An evaluation of the geological hazard related to the sinking of the Prestige oil-tanker in November 2002 in the Galicia margin, NW of Iberia, was supported by the Spanish research agency. It included a specific seismic study of the local activity and of the regional seismicity in this margin, results of which are presented here. Published global catalogues show that diffuse seismicity of moderate magnitude (events up to 5.6 mb) can be associated with the Northern flank of the Galicia Margin, with a roughly E–W distribution. However, event locations in this area are poorly constrained due to the inappropriate geometry of permanent seismic networks. We collected and analyzed new data from a dense deployment of 10 OBS around the wreck site and from a network of 12 land stations at the NW corner of Iberia. During a 7 months operation period, the land network permitted location of 120 events with magnitudes ranging from 1.6 to 4.8, more than 30% of which were not reported by the Spanish permanent array, confirming the existence of a moderate seismicity in this area. Five new focal solutions have been obtained for events on-land, providing additional information about the active faults in the region. The OBS network ran for 1 month and allowed the location of three earthquakes of magnitude≤2.5 near the sinking site. Four more events have been identified, with just a rough determination of their epicenters based on polarization of the P-wave arrivals. Besides these earthquakes, the OBS data recorded numerous seismic events of different types. They include: i) groups of very short and regularly spaced pulses that can be explained as acoustic calls generated by finback whales; ii) a large number of short-duration seismic events (2–3 s long) identified only at individual OBS; iii) local intense monochromatic signals lasting for up to 4 h that can be described as harmonic tremors. These last two types of seismic events might be related to hydrothermal activity within the uppermost sediments and deserve additional studies to appropriately establish the seismic hazard of this area. [Copyright &y& Elsevier]

Abstract: A network of 10 OBS operating in continuous recording mode was deployed in June 2003 in the Galicia passive Margin, surrounding the sinking zone of the ‘Prestige’ oil tanker. The OBSs recorded very different signals, including air-gun shots, seismic events, whale vocalizations or noise associated to ships, as well as two sets of atypical seismic signals that are the object of this contribution and can be described as harmonic tremors and short-duration events. Harmonic tremors appear in most sites as intense monochromatic signals lasting for up to 4 h, with coherent polarization and a wide range of amplitudes. Even if they may be recorded at different sites within a time interval, their envelopes are not correlated, suggesting a source origin at the vicinity of each receiver. A time periodicity close to 6 h is observed, suggesting a correlation with the tidal variation. Most OBSs show also a large number of impulsive events of short-duration, between 1 and 2 s, a very regular decrease of amplitude in the signal coda and characteristic frequency spectrum with one or two narrow peaks. As for the tremors, those events are detected only in individual sites, suggesting very local source zones. We favor the interpretation of both seismic features as coming from the resonance of fluid-filled cracks induced by impulsive pressure transients, in agreement with the theoretical model of Chouet [Chouet, B. 1988. Resonance of a fluid-driven crack: radiation properties and implications for the source of long-period events and harmonic tremor. J. Geophys. Res., 93, 4375–4400.]. The observed harmonic tremors would be the result of sustained pressure fluctuations, probably related to the stress variations induced by the tidal change of the oceanic load, while the short duration events are interpreted as the impulse response to the tremor-generating system. [Copyright &y& Elsevier]

Abstract: The mantle anisotropic features at the northwestern part of the Iberian Peninsula have been investigated by the shear-wave splitting technique from different temporary array deployments during the last 6 years. Successive seismic transects were instrumented from east to west, to sample areas affected by the Alpine compressional tectonics (Western Pyrenees and Cantabrian Mountains), as well as hinterland parts of the Iberian Variscan belt. A remarkable consistency is found in the retrieved anisotropic parameters throughout the study area, with an average fast velocity direction close to E/W. Delay times up to 1.5s are observed in most transects, but lower values, not exceeding 1s are measured at the westernmost part. Although the averaged values are compatible among the different stations, a significant variation of the splitting parameters is observed in each station with respect to the backazimuth direction. This azimuthal dependence denotes a complex distribution at depth of the anisotropic features, and cannot be explained by considering single-layer anisotropic models, either with hexagonal or orthorhombic symmetry systems, and/or dipping axes of symmetry. Synthetic models including two distinct anisotropic layers with an orthorhombic symmetry provide a satisfactory fit. In all the retained models the two layers exhibit different thicknesses; the most prominent contribution to the observed anisotropy comes from a lower layer consistently oriented close to E/W, whereas the thinner second layer, located above, ‘modulates’ the result, and its anisotropic parameters may change along the transect to account for the observed differences in splitting delay times. The dominant E–W layer throughout could not be associated to major compressional events, as in the westernmost part the Variscan terrains exhibit N–S main lineaments. It may rather correspond to an anisotropic imprint around the lithosphere–asthenosphere transition related to the eastward displacement of the Iberian plate due to the Mesozoic extensional processes during the opening of the North Atlantic and Bay of Biscay domains. The weaker anisotropic layer could be associated either with crustal anisotropic materials, or more likely, to an additional anisotropic signature within the lithosphere led by major Variscan and Alpine orogenic processes. [Copyright &y& Elsevier]

Abstract: The present-day seismicity at the westernmost part of the Pyrenean domain reported from permanent networks is of low to moderate magnitude. However, it is poorly constrained due to the scarce station coverage of the area. We present new seismic data collected from a temporary network deployed there for 17 months that provides an enhanced image of the seismic activity and its tectonic implications. Our results delineate the westward continuity of the E–W Pyrenean band of seismicity, through the Variscan Basque Massifs along the Leiza Fault, ending up at the Hendaya Fault. This seismicity belt is distributed on a crustal scale, dipping northward to almost 30 km depth. Other relevant seismic events located in the area can be related to the central segment of the Pamplona fault, and to different E–W thrust structures. [Copyright &y& Elsevier]

The S-transform provides a framework for data-adaptive filters which take advantage of time-frequency localized spectra. These filters basically consist in a data transformation to the time-frequency domain, the data-adaptive weighting of the localized spectra, and a back transformation. We illustrate that the inverse S-transform of manipulated spectra not necessarily transforms the localized signals as expected from the imposed weighting. The time localization is not directly translated and spurious signals and noise can be generated. We discuss this problem and suggest a new inverse S-transform, which may be helpful to many applications to take more advantage of the time-frequency localization. [ABSTRACT FROM AUTHOR]

This paper introduces and discusses a new polarization filter which can aid interpretation of single or multichannel multicomponent seismograms. We define a measure of degree of polarization based on instantaneous polarization attributes of analytic traces. Computation of the data eigenstructure is not required and the measure can be used in combination with its spatial coherence to enhance polarized signals on seismic record sections. Our approach avoids suppressing signals with spatially changing characteristics such as happens in the transition to post-critical reflections or for reflections from laterally varying interfaces. The simplicity of the method permits the filter to be tailored to various data characteristics and the concept can be applied to cross-energy methods. We also show that non-linear amplification of rectilinearity and planarity weight functions in time-domain principal-component filters permits the size of data windows to be decreased, improving resolution and suppressing noise. [ABSTRACT FROM AUTHOR]

Pinnegar points in his comment to a mistake in our paper on the inverse S-transform. It seems, in fact, that the error should mainly be attributed to the discretization of the S-transform, not to our inverse S-transform. Pinnegar bases his approach on the discrete S-transform by using its representation in the frequency domain. It is shown in an accompanying paper by Simon et al. that there are differences whether one discretizes the S-transform using the time-domain or the frequency-domain expression. As a consequence, both inverse S-transforms suffer from different small artifacts. There are no changes in the physics, understanding, and conclusions of our publication. [ABSTRACT FROM AUTHOR]

In the last 10–15 years, the number of high quality seismic stations monitoring the Euro-Mediterranean region has increased significantly, allowing a corresponding improvement in structural constraints. We present here new images of the seismic velocity and anisotropy variations in the uppermost mantle beneath this complex area, compiled from inversion of Pn and Sn phases sampling the whole region. The method of Hearn has been applied to the traveltime arrivals of the International Seismological Center catalogue for the time period 1990–2010. A total of 579 753 Pn arrivals coming from 12 377 events recorded at 1 408 stations with epicentral distances between 220 km and 1 400 km have been retained after applying standard quality criteria (maximum depth, minimum number of recordings, maximum residual values …). Our results show significant features well correlated with surface geology and evidence the heterogeneous character of the Euro-Mediterranean lithosphere. The station terms reflect the existence of marked variations in crustal thickness, consistent with available Moho depths inferred from active seismic experiments. The highest Pn velocities are observed along a continuous band from the Po Basin to the northern Ionian Sea. Other high velocity zones include the Ligurian Basin, the Valencia Trough, the southern Alboran Sea and central part of the Algerian margin. Most significant low-velocity values are associated to orogenic belts (Betics, Pyrenees, Alps, Apennines and Calabrian Arc, Dinarides-Hellenides), and low-velocity zones are also identified beneath Sardinia and the Balearic Islands. The introduction of an anisotropic term enhances significantly the lateral continuity of the anomalies, in particular in the most active tectonic areas. Pn anisotropy shows consistent orientations subparallel to major orogenic structures, such as Betics, Apennines, Calabrian Arc and Alps. The Sn tomographic image has lower resolution but confirms independently most of the features evidenced in the Pn tomography. [ABSTRACT FROM PUBLISHER]

Stacks of ambient noise cross-correlations are more and more routinely used to extract empirical Green's functions between station pairs. The success of the cross-correlations is due to waves which are recorded by both stations and that constructively sum at lag times which equal their propagation time between the station pair. Stacking cross-correlograms corresponding to different time spans improves the azimuthal noise coverage and further enhances the signals. Here we show how the instantaneous phase coherence can be used for a more efficient signal extraction from ambient noise cross-correlations. The instantaneous phase coherence is obtained by analytic signal processing and can be employed through the phase cross-correlation and/or through the time-frequency domain phase-weighted stack. The phase cross-correlation is more sensitive to waveform similarity but less sensitive to strong amplitude features than the conventional cross-correlation. The time-frequency domain phase-weighted stack cleans the ambient noise cross-correlograms by attenuating incoherent noise and permits an improved signal identification. We show that both approaches are powerful tools in the recovery of signals from ambient noise data and show examples where they improve the extraction of P and Rayleigh waves by considering local and global scale applications. [ABSTRACT FROM AUTHOR]

Abstract: On September 18, 2004, a 4.6 mbLg earthquake was widely felt in the region around Pamplona, at the western Pyrenees. Preliminary locations reported an epicenter less than 20 km ESE of Pamplona and close to the Itoiz reservoir, which started impounding in January 2004. The area apparently lacks of significant seismic activity in recent times. After the main shock, which was preceded by series of foreshocks reaching magnitudes of 3.3 mbLg, a dense temporal network of 13 seismic stations was deployed there to monitor the aftershocks series and to constrain the hypocentral pattern. Aftershock determinations obtained with a double-difference algorithm define a narrow epicentral zone of less than 10 km2, ESE–WNW oriented. The events are mainly concentrated between 3 and 9 km depth. Focal solutions were computed for the main event and 12 aftershocks including the highest secondary one of 3.8 mbLg. They show mainly normal faulting with some strike-slip component and one of the nodal planes oriented NW–SE and dipping to the NE. Cross-correlation techniques applied to detect and associate events with similar waveforms, provided up to 33 families relating the 67% of the 326 relocated aftershocks. Families show event clusters grouped by periods and migrating from NW to SE. Interestingly, the narrow epicentral zone inferred here is located less than 4 km away from the 111-m high Itoiz dam. These hypocentral results, and the correlation observed between fluctuations of the reservoir water level and the seismic activity, favour the explanation of this foreshock–aftershock series as a rapid response case of reservoir-triggered seismicity, burst by the first impoundment of the Itoiz reservoir. The region is folded and affected by shallow dipping thrusts, and the Itoiz reservoir is located on the hangingwall of a low angle southward verging thrust, which might be a case sensible to water level fluctuations. However, continued seismic monitoring in the coming years is mandatory in this area to infer more reliable seismotectonic and hazard assessments. [Copyright &y& Elsevier]

Seismic data recorded from a temporary network deployed at the western edge of the Pyrenees is used to study the aftershocks series following a magnitude 4.1 earthquake that took place on 2002 February 21, to the NW of Pamplona city. Aftershock determinations showed events distributed between 1 and 4 km depth in a small active area of about 4 km2, E–W oriented delineating the southern sector of the Aralar thrust unit. This seismogenic feature is supported by focal solutions showing a consistent E–W nodal plane with normal faulting following the main strike-slip rupture. The Aralar structure with its shallow activity may be interpreted as a conjugate system of the NE–SW deep-seated Pamplona active fault nearby. Cross-correlation techniques and relative location of event clusters further constrained the epicentral domain to 2 km long and 1 km wide. Statistical relations and parameters established indicate a rather low b-value of 0.8 for the Gutenberg–Richter distribution, denoting a region of concentrated seismicity, and a P-parameter of 0.9 for the Omori's law corresponding to a low decay of the aftershock activity in this area. More than 100 aftershocks were accurately located in this high-resolution experiment, whereas only 13 of them could be catalogued by the permanent agencies in the same period, due to a much sparser distribution. The results enhance the importance of using dense temporary networks to infer relevant seismotectonic and hazard constraints. [ABSTRACT FROM AUTHOR]

The structure and geodynamics of the southern margin of the Bay of Biscay have been investigated from a set of 11 multichannel seismic reflection profiles, recorded also at wide angle offsets in an onshore-offshore network of 24 OBS/OBH and 46 land sites. This contribution focuses on the analysis of the wide-angle reflection/refraction data along representative profiles. The results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the Iberian Peninsula. Offshore, the crust has a typical continental structure in the eastern tip of the bay, which disappears smoothly towards the NW to reach crustal thickness close to 10 km at the edge of the studied area (~ 45°N, 6°W). The analysis of the velocity-depth profiles, altogether with additional information provided by the multichannel seismic data and magnetic surveys, led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic. Typical oceanic crust has not been imaged in the investigated area. Onshore, the new results are in good agreement with previous results and document the indentation of the Bay of Biscay crust into the Iberian crust, forcing its subduction to the North. The interpreted profiles show that the extent of the southward indentation is not uniform, with an Alpine root less developed in the central and western sector of the Basque-Cantabrian Basin. N-S to NE-SW transfer structures seem to control those variations in the indentation degree. [ABSTRACT FROM AUTHOR]

Consistent constraints of an over-thickened crust beneath the Rif Cordillera (N. Morocco) are inferred from analyses of recently acquired seismic datasets including controlled source wide-angle reflections and receiver functions from teleseismic events. Offline arrivals of Moho-reflected phases recorded in RIFSIS project provide estimations of the crustal thicknesses in 3D. Additional constraints on the onshore–offshore transition are inferred from shots in a coeval experiment in the Alboran Sea recorded at land stations in northern Morocco. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. This larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large crustal root, exceeding 50 km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favors models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco. [ABSTRACT FROM AUTHOR]

Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones. [ABSTRACT FROM AUTHOR]

Abstract: This work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the Central Lesser Antilles Island Arc from Martinique to Guadeloupe islands. During the whole recording period, extending from January to the end of August 2007, more than 1300 local seismic events were detected in this area. A subset of 769 earthquakes was located precisely by using HypoEllipse. We also computed focal mechanisms using P-wave polarities of the best azimuthally constrained earthquakes. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. At depth seismicity delineates the Wadati–Benioff Zone down to 170km depth. The main seismic activity is concentrated in the lower crust and in the mantle wedge, close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed. We propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense, thick and rigid crustal block and the lighter and more flexible outer forearc. Seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate. [Copyright &y& Elsevier]

Abstract: In the last decades, active seismic profiling in the northern part of the Iberian Peninsula has evidenced that the Alpine collision between the Iberian and Eurasian plates resulted in a complex crustal structure, with the Iberian crust underthrusting the Eurasian crust and reaching depths of at least 45–50km beneath the Pyrenean chain and the Cantabrian Mountains. In the transition between these two zones the situation is particularly complex, as evidenced in previous wide-angle and passive seismic studies. This contribution focuses in getting new clues on the crustal structure of this transitional zone through receiver function (RF) analysis of teleseismic data recorded at permanent and temporary stations located in both the Spanish and French sides of the Western Pyrenees. Different techniques (H-κ stacking, pseudo-migration, synthetic 2D modeling) have been considered in the analysis. Passive seismic data from previous temporary deployments in the zone have been reworked and added to the discussion. A first order result is that passive seismic data are broadly consistent with the indentation of the Iberian and Eurasian crusts inferred from active seismic profiling, thus providing a completely independent confirmation of this feature. For the first time, an Iberian Moho underlying the Eurasian crust is documented from RF beneath the stations located at the Northern side of the Pyrenean range. Moreover, clear indications of dipping interfaces are observed at some stations. The new RF results suggest that in the crustal indentation beneath the Basque Massifs area, the Eurasian crust extends farther south with respect to the image inferred from active seismic data. This new geometry implies that the Pamplona transfer zone has played a major role in the regional geodynamic history. [Copyright &y& Elsevier]

Abstract: An analysis of Cenozoic tectonics was carried out on the Galicia Bank Region by means of 750 km of single-channel and 155 km of multi-channel vertical seismic profiles and multibeam bathymetry. This work forms part of a general study aimed at determining the geological risk in the wreck area of the Prestige oil tanker. Several structures affecting the post-rift sedimentary units were identified: I) Faults inherited from the Lower Cretaceous propagating rift (reactivated and non-reactivated), including reactivated normal (N–S, NW–SE) and transfer (NE–SW) faults; and II) Structures resulting from compressive episodes in the Paleogene to Upper Miocene and the Upper Miocene to the Present, including a) neotectonic normal faults decoupled from the basement structure, b) folds and flexures and c) reverse faults (NE–SW to ENE–SWS) related to the Bay of Biscay subduction, that are the most striking regional structures. The characterization of structural style, sedimentary post-rift unit deformation and morphological trends were used to carry out a detailed study of the morphostructure of the Galicia Bank Region. Six morphostructural provinces were differentiated, from E to W: 1) the Galicia Interior Basin related to a main graben; 2) the Transitional Zone, which is a horst and graben territory; 3) the Galicia Bank Main Horst; 4) the Half-Graben Domain; 5) the Deep Galicia Margin, whose structure shows rotated blocks; and 6) the Northwestern Flank, characterized by reverse faults. The Transitional Zone and the Galicia Bank provinces are assumed to be the two morphostructural provinces of the Galicia Bank marginal platform. The origin of the Cenozoic deformation must have been related to reactivation processes during the Pyrenean orogeny. We propose that the fault pattern observed in the study area is related to the phase of renewed Mesozoic fault activity. Polyphase extension in the rifting stage and Cenozoic movements have produced an extensional interference pattern that is evident in the physiography. The decouplage between basement tectonics and upper sedimentary unit tectonics suggests the presence of a viscous layer. A thrust-related main antiform related to the Bay of Biscay subduction is proposed to explain the origin of the Galicia Bank morphology. [Copyright &y& Elsevier]

Abstract: Multibeam bathymetry, high (sleeve airguns) and very high resolution (parametric system-TOPAS-) seismic records were used to define the morphosedimentary features and investigate the depositional architecture of the Cantabrian continental margin. The outer shelf (down to 180–245 m water depth) displays an intensively eroded seafloor surface that truncates consolidated ancient folded and fractured deposits. Recent deposits are only locally present as lowstand shelf-margin deposits and a transparent drape with bedforms. The continental slope is affected by sedimentary processes that have combined to create the morphosedimentary features seen today. The upper (down to 2000 m water depth) and lower (down to 3700–4600 m water depth) slopes are mostly subject to different types of slope failures, such as slides, mass-transport deposits (a mix of slumping and mass-flows), and turbidity currents. The upper slope is also subject to the action of bottom currents (the Mediterranean Water — MW) that interact with the Le Danois Bank favouring the reworking of the sediment and the sculpting of a contourite system. The continental rise is a bypass region of debris flows and turbidity currents where a complex channel-lobe transition zone (CLTZ) of the Cap Ferret Fan develops. The recent architecture depositional model is complex and results from the remaining structural template and the great variability of interconnected sedimentary systems and processes. This margin can be considered as starved due to the great sediment evacuation over a relatively steep entire depositional profile. Sediment is eroded mostly from the Cantabrian and also the Pyrenees mountains (source) and transported by small stream/river mountains to the sea. It bypasses the continental shelf and when sediment arrives at the slope it is transported through a major submarine drainage system (large submarine valleys and mass-movement processes) down to the continental rise and adjacent Biscay Abyssal Plain (sink). Factors controlling this architecture are tectonism and sediment source/dispersal, which are closely interrelated, whereas sea-level changes and oceanography have played a minor role (on a long-term scale). [Copyright &y& Elsevier]

Abstract: TOPO-EUROPE addresses the 4-D topographic evolution of the orogens and intra-plate regions of Europe through a multidisciplinary approach linking geology, geophysics, geodesy and geotechnology. TOPO-EUROPE integrates monitoring, imaging, reconstruction and modelling of the interplay between processes controlling continental topography and related natural hazards. Until now, research on neotectonics and related topography development of orogens and intra-plate regions has received little attention. TOPO-EUROPE initiates a number of novel studies on the quantification of rates of vertical motions, related tectonically controlled river evolution and land subsidence in carefully selected natural laboratories in Europe. From orogen through platform to continental margin, these natural laboratories include the Alps/Carpathians–Pannonian Basin System, the West and Central European Platform, the Apennines–Aegean–Anatolian region, the Iberian Peninsula, the Scandinavian Continental Margin, the East-European Platform, and the Caucasus–Levant area. TOPO-EUROPE integrates European research facilities and know-how essential to advance the understanding of the role of topography in Environmental Earth System Dynamics. The principal objective of the network is twofold. Namely, to integrate national research programs into a common European network and, furthermore, to integrate activities among TOPO-EUROPE institutes and participants. Key objectives are to provide an interdisciplinary forum to share knowledge and information in the field of the neotectonic and topographic evolution of Europe, to promote and encourage multidisciplinary research on a truly European scale, to increase mobility of scientists and to train young scientists. This paper provides an overview of the state-of-the-art of continental topography research, and of the challenges to TOPO-EUROPE researchers in the targeted natural laboratories. [Copyright &y& Elsevier]

Abstract: P and S velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection–refraction profiles in the region of active tectonics at the NE edge of the Tibetan Plateau and discussed together with respect to similar data at its Himalaya–south Tibet edge. The quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude–frequency observed between waves of different penetrations, and both in P and S modes. Attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature, ratio of in situ to solidus absolute temperatures. The latter depend on the physical conditions being of dry, wet or dehydration melting, which are found different among the regions of the northern Bayan Har and northern Qang Tang boundaries between blocks, as well as the Tethyan–Himalayas, south of the Indus–Tsangpo suture. Their in situ temperatures differ also as estimated from their different V p for a similar felsic composition. Joint measurement of several parameters, V p, V s, Q p and Q s reveals the composition, mineralogy, temperature and hydration conditions of the lower half of the thickened crust of Tibet that may be discussed in terms of evolution. The material presently in the thickened crust, even its lower part, has a felsic composition, upper to middle crustal lithology, and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic Moho. Under northern Qang Tang, the felsic material in the lower half of the crust appears as hot and dry. Its burial may have occurred earlier or may have been moderate in the postcollisional phase. This is consistent with a model of indentation of the Qang Tang crust by an originally thinner Bayan Har crust to bring part of its crust to greater depth, suggested from imaging the crustal architecture. Under northern Bayan Har, the material in the lower half of the crust appears as felsic, at low temperature and not dry conditions. This is evidence that it has been transported from a shallower depth, and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm. It supports a model of recent overriding of the middle crust of the north Kun Lun block to the north independently suggested from the image of crustal architecture. The Tethyan Himalayas case appears bracketed by these two cases in northern Tibet for V p and temperature conditions, but shows highest attenuation in the lower crust that is colder but less dry than under northern Qang Tang. [Copyright &y& Elsevier]

The dimensions, the geographical position and the complex geological history of the Iberian Peninsula makes it a unique and singular target to study its crustal and upper mantle structure and geodynamical evolution using geophysical methods. The lithospheric structure beneath Iberia has been investigated since the 1970's using deep multichannel seismic reflection and refraction/wide-angle reflection profiling. Gravimetric and magnetic data were acquired following the deployment of permanent and temporary stations, mostly during the 1990's. Beginning in the late 1990's, the progressive use of Global Navigation Satellite Systems (GNSS) instruments contributed to monitor the present-day motions. During the last decades, numerous geological and geophysical surveys have investigated the Iberian lithosphere and upper mantle in the onshore and offshore regions, the most recent ones related to the TopoIberia project. The approach developed in this contribution is twofold. Firstly, we summarize the available geophysical information over Iberia, from focusing on the upper crust to the lithosphere-asthenosphere boundary and the transition zone marking the bottom of the upper mantle. Results of GNSS data, potential fields, controlled source seismic profiles, magnetotelluric data, body and surface wave tomography, receiver functions and 2D and 3D lithospheric modeling are reviewed and compared. Secondly, we focus on the areas of greater geodynamic interest and the regions where inconsistencies within the geophysical results, or contradictions in their tectonic interpretation exist, identifying the major questions that are still under debate. Besides shedding light to the state of knowledge and pointing out present-day research challenges, this review provides a tool for the integration of the diverse geophysical datasets with the surface geology and geodynamical processes that are interpreted to have built the complex geology of the Iberian Peninsula. [ABSTRACT FROM AUTHOR]