Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present a new set of brittle microtectonic measurements carried out in the Pliocene and Quaternary rocks outcropping in several key sectors of the western Betic and Rif orogen, the so-called Gibraltar orogenic arc. This data set, along with available earthquake focal mechanisms and borehole breakouts, allowed us to compile the Pliocene and Quaternary stress map of this area. This map provides new constraints for tectonic models and the present-day tectonic activity of the proposed active eastward subduction of oceanic lithosphere beneath the Gibraltar Arc and roll-back. The horizontal maximum compressive stress (S-Hmax) is NW-SE in the Betic Orogen and N-S/NNW-SSE in the southern Rif Cordillera. There is a significant consistency between S-Hmax and the displacement field deduced from GPS measurements with respect to the African plate: both appear to reflect the NW-SE convergence between the African and the European plates that is perturbed in the Rif. We propose that part of the eastern Rif behaves as a quasi-rigid block welded to the stable African plate. This block is bounded by important faults that localized most of the deformation disturbing the stress and surface displacement field. Pliocene to Quaternary N-S to NW-SE Africa-Europe plate convergence seem to be associated to the reorganization of the remnant Early Miocene subduction system in a continental-continental collision framework. Three-dimensional reconstruction of available seismic tomography plotted against the intermediate seismicity shows that only part of the old subduction system, whose orientation ranges from N20 degrees E to N100 degrees E, remains active: the portion ranging from N30 degrees E to N40 degrees E, orthogonal to the regional convergence. (C) 2011 Elsevier Ltd. All rights reserved.
ISSN: 0264-3707

hal-00671033
https://hal.archives-ouvertes.fr/hal-00671033
DOI : 10.1016/j.jog.2010.12.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.
ISSN: 0040-1951

insu-01682810
https://hal-insu.archives-ouvertes.fr/insu-01682810
DOI : 10.1016/j.tecto.2017.11.028

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We discuss mantle structure and dynamics under the Gibraltar arc. For that region, a large variety of geodynamic models has been proposed, and there was, up to recently, no consensus on which of these models is the best one to describe the geodynamic situation of the region. The key to distinguishing the different models probably lies in the upper mantle, and we propose two specially adapted seismological techniques that help to distinguish each of the two models that are more frequently invoked. These are, on one hand, the subduction-rollback (SR) model, and on the other, continental delamination or convective removal (CDCR). The first technique that we use is based on observations of dispersed P-wavetrains that are expected for certain ray directions. Observations made on the Gibraltar arc suggest the presence of subducted oceanic lithosphere under the Alboran Sea. The second technique is based on shear-wave splitting of SKS phases, and it allows us to look for the radially-oriented flow that must be present in the mantle beneath the Alboran Sea, for a CDCR mechanism. There are however no indications of any radial flow up to this point. Both approaches therefore suggest that subduction-rollback is the best model to explain the complex geodynamic situation under the region.
ISSN: 0040-1951

hal-01388041
http://hal.univ-reunion.fr/hal-01388041
http://hal.univ-reunion.fr/hal-01388041/document
http://hal.univ-reunion.fr/hal-01388041/file/Bokelmann_Gibraltar_Tphy2011_hal.pdf
DOI : 10.1016/j.tecto.2010.08.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Seismic anisotropy of the south Iberian upper mantle is investigated using shear-wave splitting of SKS phases. We analyzed teleseismic events recorded by sixteen permanent broadband stations installed on the southern Iberian Peninsula and in northern Africa, and we determined fast polarization directions phi, and delay times delta t between fast and slow components. The area of investigation extends across two important geological structures in the Variscan Iberian Peninsula: the Variscan Iberian Massif in its center, and the Gibraltar arc in the Southeast, that represents the most westerly Alpine belt in the western Mediterranean. Shear-wave splitting measurements from stations in the Betic domain show homogeneous ENE-WSW fast directions nearly parallel to the trend of the mountain belt, and smooth spatial variations. Stations in the North, toward the southern part of the Variscan Iberian Massif show homogeneous fast directions however trending NS to NE-SW, different from those recorded in the Betic. These observations may reflect a post-Hercynian (Variscan) deformation of the Ossa-Morena zone, related to the main stages in the tectonic evolution of this part, namely transpressional stage, transtensional stage and shortening episode, or a deformation related to the posterior Alpine orogeny. Along the Gibraltar arc, we observe a smoothly varying phi trend changing from ENE-WSW in the Eastern Betics to NS in the area of Gibraltar and Ceuta, following more or less the general trend of the mountain belt around the Alboran Sea, and the coastline. Since a similar rotation is also visible in results from Pn anisotropy, this suggests that the anisotropy is vertically coherent starting from just below the Moho. Comparing the anisotropy pattern expected from various geodynamic models with the observed SKS splitting suggests that the anisotropy is best explained by a model of slab rollback, rather than by delamination models.
ISSN: 0012-821X

hal-00412175
https://hal.archives-ouvertes.fr/hal-00412175
https://hal.archives-ouvertes.fr/hal-00412175/document
https://hal.archives-ouvertes.fr/hal-00412175/file/Buontempo_SKS_gibraltar_EPSL2008_hal.pdf
DOI : 10.1016/j.epsl.2008.06.024

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Gibraltar arc, spans a complex portion of the Africa-Eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity. The seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold-and-thrust belt, have led to competing geodynamic models (delamination and subduction). Geophysical data acquired in the past decade provide a test for these models and support a narrow east-dipping, subduction zone. Seismic refraction studies indicate oceanic crust below the western Gulf of Cadiz. Tomography of the upper mantle reveals a steep, east-dipping high P-wave velocity body, beneath Gibraltar. The anisotropic mantle fabric from SKS splitting shows arc-parallel "fast directions", consistent with toroidal flow around a narrow, westward retreating subducting slab. The accompanying WSW advance of the Rif-Betic mountain belt has constructed a thick pile of deformed sediments, an accretionary wedge, characterized by west-vergent thrust anticlines. Bathymetric swath‐mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided. Subduction has slowed significantly since 5 Ma, but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge. Three possible origins for this deformation are discussed; gravitational spreading, overall NW-SE convergence between Africa and Iberia and finally a WSW tectonic push from slow, but ongoing roll-back subduction. In the absence of arc volcanism and shallow dipping thrust type earthquakes, evidence in favor of present-day subduction can only be indirect and remains the object of debate. Continued activity of the subduction offers a possible explanation for great (M > 8.5) earthquakes known to affect the area, like the famous 1755 Great Lisbon earthquake. Recent GPS studies show SW motion of stations in N Morocco at velocities of 3-6 mm/yr indicating the presence of an independent block, a "Rif-Betic-Alboran" microplate, situated between Iberia and Africa.
ISSN: 0040-1951

insu-00769762
https://hal-insu.archives-ouvertes.fr/insu-00769762
DOI : 10.1016/j.tecto.2012.08.038

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Mélange rocks outcrop widely in the central and western frontal sectors of the Betic Cordillera, as in many other collisional orogens where they form part of the accretionary wedges. Extrusion of Triassic plastic clays and evaporites was favored by the progressive accretion of the Betic External Zones, mixing rocks of different provenance and forming a synorogenic frontal mélange unit. MT data coupled with gravity data are a valid combined methodology to characterize the geometry of these mélange units, since the characterization of plastic rocks geometry is usually uncertain using seismic techniques. The results correlate well with known geological features (sedimentary basins, calcareous ranges, evaporitic rocks) and reveal the deep geometry. A resistive body, slightly dipping toward the SE, points to the continuity of the Iberian Massif below the Guadalquivir basin (2°) and the External Zones (6-8°). To the south, gravity models show the Iberian continental crust subducting below the Internal Zones with a roughly 20-35° slope. The main conductive bodies are related to the location of evaporitic rocks involved in the frontal mélange. They overlie the Iberian Massif and, southwards, the frontal Jurassic and Cretaceous limestone sequences of the External and Median Subbetics. In this setting, thick plastic rock units placed above the foreland could act as a lubricant facilitating continental subduction, and being progressively accreted toward a frontal mélange.
ISSN: 1525-2027

hal-00750364
https://hal.archives-ouvertes.fr/hal-00750364
DOI : 10.1029/2012GC004153

Éditeur(s) : HAL CCSD
Résumé : International audience
The Rif Chain (Northern Morocco) belongs, with the Betic Cordillera(Southern Spain), to the western termination of the Alpine belt. Deepestunits of the Rif Internal Zones (i.e. Lower Sebtides) outcrop in three areas, from North to South : Ceuta, Cabo Negro and Beni Bousera.The quick exhumation of these units during the Late Oligocene-EarlyMiocene is related with a crustal thinning and the Alboran Sea rifting.Despite Beni Bousera massif has been well investigated, no data areavailable in Ceuta and Cabo Negro areas. This study aims to constrainthe last steps of cooling and exhumation in the Lower Sebtides usingmedium- and low-temperature thermochronological data. Combinationof Ar-Ar on micas, fission tracks and (U-Th)/He on apatites (respectivelyAFT and AHe) from high-grade rocks allows understanding thermalhistory between 450-350°C and surface temperatures. These datasuggest a similar cooling evolution of the Ceuta and Cabo Negro units.Ages obtained with all these methods are comprised in the same interval,between 22 and 18 Ma. These data record an almost instantaneouscooling (between 50 and 200°C/Ma) associated to final exhumation afterthe Oligo-Miocene thermal event affecting these units at shallowdepths (10 km). Low-temperature data suggest that Ceuta and CaboNegro units cooled at surface temperatures before Beni Bousera ones.Since the end of Burdigalian, the Rif internal zones vertical movementsdid not exceed 1km, even during the Messinian Salinity. AFT and AHeages in the whole Sebtides are also slightly older than these obtainedin their Spanish equivalent (Alpujarrides). The Gibraltar arc is dividedinto several independent crustal blocks separated by major fault zonesperpendicular to the coast that could have induced independent verticalmovements during the entire Neogene
24 ème Réunion des sciences de la Terre 2014
Pau, France

insu-01080768
https://hal-insu.archives-ouvertes.fr/insu-01080768

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In this paper, we present the first estimate of the Holocene deformation along the southern front of Gibraltar arc (Morocco) and the first field constraints on the local 1755 CE Fes-Meknes surface rupturing earthquake which could be associated to the “Great Lisbon Earthquake” (M > 8.5) in November 1st, 1755. Using satellite imagery, aerial photographs and field investigations, we carried out a morphotectonic study along the ~ 150 km-long Southern Rif Front (SRF) to identify the most recent evidences of tectonic activity. Analyzed offset alluvial deposits confirm that (i) the last ~ 5 ka cumulative deformation leading to a slip rate of ~ 3.5 ± 1 mm/yr for this segment of the SRF is consistent with the GPS derived horizontal shortening rate of 2–4 mm/yr and (ii) a recent major earthquake ruptured a ~ 30 km-long segment along the SRF. Based on deposits dating and historical seismicity we propose that this seismic event occurred in 1755 as a local earthquake. Even though this 1755 local event cannot be considered as a strong aftershock of the main Lisbon seismic event (M > 8.5), their temporal closeness, their occurrence under the same convergent stress regime (~ NNW-SSE-oriented compression) and the fact that Fes-Meknes area was strongly shaken during the Lisbon earthquake, raises the question of the possible triggering of the Fes earthquake. Anyway, our new results suggest that most of the Nubia-Rif belt convergence is accommodated by the SRF, making it potentially the most destructive structure of the Rif.
ISSN: 0040-1951

hal-01622048
https://hal.archives-ouvertes.fr/hal-01622048
DOI : 10.1016/j.tecto.2017.05.036

Éditeur(s) : HAL CCSD University of Chicago Press
Résumé : International audience
Contrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margins. Rather than convective removal or delamination of the lithospheric root, our data strongly support Alboran geodynamic models that envisage slab rollback as the tectonic mechanism responsible for the Miocene lithospheric thinning. The Ronda Cr-rich pyroxenite dikes represent the earliest unambiguous manifestation of subduction-related magmatism in the western Mediterranean and testify to the involvement of terrigenous sediments in the primitive stages of subduction.
ISSN: 0022-1376

hal-00745853
https://hal.archives-ouvertes.fr/hal-00745853
DOI : 10.1086/663875

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
We analyze focal mechanisms of shallow-intermediate earthquakes in a NW-SE transect along the western Betic Cordillera and Alboran Sea, and deep earthquakes located in the central Betics to constrain the state of stress at the Gibraltar Arc slow convergence area. Shallow earthquakes (< 40 km) are preferably clustered at the mountain front. A general NW-SE horizontal compression is compatible with the convergence, and NW plunging compression axes are in agreement with frontal thrust activity. Toward the Alboran Sea the earthquakes are progressively deeper, reaching intermediate depths (40-120 km) near the coast and supporting the present-day activity of subduction only in this area. The Iberian continental crust concentrates most of the intermediate seismicity and is forced to partially sink into the mantle, probably through the pull of the oceanic slab. This downdip pull together with the buoyancy of the Iberian continental crust produces the slab curvature, downdip extension in the external arc of the continental slab, and downdip compression in the inner arc. T axes highly dipping to the southeast at 90-120 km depth occur at the oceanic/continental transition. Deep earthquakes (> 620 km) show very similar focal mechanisms, fitting the general slab behavior of resistance to further descent at the 660 km discontinuity. Seismicity features evidence the present-day stress distribution in a context of transition from subduction to continental collision.
ISSN: 1525-2027

hal-00669889
https://hal.archives-ouvertes.fr/hal-00669889
DOI : 10.1029/2011GC003824

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In the framework of the Africa-Europe convergence, the Mediterranean system presents a complex interaction between subduction rollback and upper-plate deformation during the Tertiary. The western end of the system shows a narrow arcuate geometry across the Gibraltar arc, the Betic-Rif belt, in which the relationship between slab dynamics and surface tectonics is not well understood. The present study focuses on the Western Betics, which is characterized by two major thrusts: 1/ the Internal/External Zone Boundary limits the metamorphic domain (Alboran Domain) from the fold-and-thrust belts in the External Zone; 2/ the Ronda Peridotites Thrust allows the juxtaposition of a strongly attenuated lithosphere section with large bodies of sub-continental mantle rocks on top of upper crustal rocks. New structural data show that two major E-W strike-slip corridors played a major role in the deformation pattern of the Alboran Domain, in which E-W dextral strike-slip faults, N60 thrusts and N140 normal faults developed simultaneously during dextral strike-slip simple shear. Olistostromic sediments of Lower Miocene age were deposited and deformed in this tectonic context and hence provide an age estimate for the inferred continuous westward translation of the Alboran Domain that is accommodated by an E-W lateral (strike-slip) ramp and a N60 frontal thrust. The crustal emplacement of large bodies of sub-continental mantle may occur at the onset of this westward thrusting in the Western Alboran domain. At lithosphere-scale, we interpret the observed deformation pattern as the subduction upper-plate expression of a lateral slab tear and its westward propagation since Lower Miocene.
ISSN: 0040-1951

insu-01163695
https://hal-insu.archives-ouvertes.fr/insu-01163695
https://hal-insu.archives-ouvertes.fr/insu-01163695/document
https://hal-insu.archives-ouvertes.fr/insu-01163695/file/Frasca-2015.pdf
DOI : 10.1016/j.tecto.2015.05.017

Éditeur(s) : HAL CCSD
Résumé : International audience
In the frame of the Africa-Europe convergence, the Mediterranean tectonic system presents a complex interaction between subduction rollback and upper-plate deformation during the Tertiary. The western Mediterranean is characterized by the exhumation of the largest subcontinental mantle massif worldwide (the Ronda Peridotite) and a narrow arcuate geometryacross the Gibraltar arc within the Betic-Rif belt (the internal part being called the Alboran domain), where the relationship between slab dynamics and surface tectonics is not well understood. New structural and geochronological data are used to argue for 1/ hyperstrechting of the continental lithosphere allowing extensional mantle exhumation to shallow depths, followed by 2/ lower miocene thrusting. Two Lower Miocene E-W-trending strike-slip corridors played a major role in the deformation pattern of the Alboran Domain, in which E-W dextral strike-slip faults, N60°-trending thrusts and N140°-trending normal faults developed simultaneously during dextral strike-slip simple shear. The inferred continuous westward translation of the Alboran Domain is accommodated by a major E-W-trending lateral ramp (strike-slip) and a N60°-trending frontal thrust. At lithosphere-scale, we interpret the observed deformation pattern as the upper-plate expression of a lateral slab tear and of its westward propagation since Lower Miocene. The crustal emplacement of the Ronda Peridotites occurred at the onset of this westward motion.The Miocene tectonics of the western Alboran is therefore marked by the inversion of a continental rift, triggered by shortening of the upper continental plate and accommodated by E-W dextral strike-slip corridors. During thrusting and westward displacement of the Alboran domain with respect to Iberia, the hot upper plate, which involved the previously exhumed sub-continental mantle, underwent fast cooling.
AGU Fall Meeting 2015
San Francisco, United States

insu-01242043
https://hal-insu.archives-ouvertes.fr/insu-01242043

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Located on the margin of the west Alboran basin, the Gibraltar Arc (Betic-Rif mountain belt) displays post-Pliocene vertical movements evidenced by uplifted marine sedimentary basins and marine terraces. Quantification of vertical movements is an important clue to understand the origin of present-day relief generation in the Betic-Rif mountain chain together with the causes of the Messinian Salinity Crisis. In this paper, we present the results of a pluridisciplinary study combining an analysis of low temperature thermochronology and Pliocene basins evolution to constrain the exhumation history and surface uplift of internals units of the Rif belt (Northern Morocco). The mean (U-Th)/He apatite ages obtained from 11 samples are comprised between 14.1 and 17.8 Ma and display a wide dispersion, which could be explained by a great variability of apatite chemistries in the analyzed samples. No correlations between altitude and age have been found along altitudinal profile suggesting a rapid exhumation during this period. Thermal modeling using our (U-Th)/He apatite ages and geochronological data previously obtained in the same area (40Ar/39Ar and K/Ar data on biotite, zircon and apatite fission track) allow us to propose a cooling history. The rocks suffered a rapid cooling at 60-100 °C/Ma between 22.5 and 19 Ma, then cooled to temperatures around 40 °C between 19 and 18 Ma. They were re-heated at around 110 °C between 18 and 15 Ma then rapidly cooled and exhumed to reach the surface temperature at around 13 Ma. The re-heating could be related to a renewal in thrusting and burying of the inner zones. Between 15 and 13 Ma the cooling resumed at a rate of 50 °C/Ma indicating an exhumation rate of 0.8 mm/y considering an average 40 °C/km geothermal gradient. This exhumation may be linked to the extension in the Alboran Sea. Otherwise biostratigraphic and sedimentological analysis of Pliocene basins of the internal Rif provided informations on the more recent events and vertical movements. Pliocene deposits of the Rifian coast represent the passive infilling of palaeo-rias between 5.33 and 3.8 Ma. The whole coastal area was uplifted at slow average rates (0.01-0.03 mm/y) in relation with a northeastward tilting of 0.2-0.3° since the Lower-Pliocene. A late Pliocene to present extensional tectonics associated to uplift has been identified all along the coastal ranges of the Internal Zone of the Rif chain. This extension was coeval with the major late Pliocene to Pleistocene extensional episode of the Alboran Sea and appears to be still active nowadays. No significant late Messinian uplift was evidenced, thus calling into question the geodynamic models relating the closure of the marine gateways and the MSC to slab roll back.
ISSN: 0264-3707

hal-01053531
https://hal.archives-ouvertes.fr/hal-01053531
DOI : 10.1016/j.jog.2014.01.006

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
A NE-SW magnetotelluric 110 km-long profile including 18 sites was acquired across the western Rif Cordillera along the Eurasian-African plate boundary, allowing to constrain its poorly known deep structure. It extends from the Internal Zones, close to the Alboran coast, crossing the External Zones and up to the Gharb foreland basin. The periods recorded range from 0.001 s to 1000 s. The combination of magnetotelluric data with available geological data provides new insight regarding the relationship between deep and shallow crustal structures of the Rif Cordillera. Analyses of structural dimensionality suggest a preferential NW-SE direction, and a 2D joint inversion was performed. A 3D inversion extending the 2D model along the strike confirmed the reliability of this approach. The magnetotelluric model shows a heterogeneous upper crust in agreement with the geological structures observed at surface. The Internal Zones correspond to resistive (metamorphic rocks) and conductive (peridotites) bodies, while the External Zones and the foreland basin are characterized by large conductive bodies of variable thickness. A crustal detachment level separating shallow geological units from a probable variscan basement was inferred. At depth, the most relevant feature consists of large resistive bodies with a shallow irregular top, located below the frontal part of the Rif. The outcrops of exotic metapelitic, granitic and gneissic blocks in the frontal part of the Cordillera suggest that these large resistive bodies may correspond to a gneissic or granitic basement surrounded by metapelitic rocks. Late contractive thrust and diapiric processes were responsible for their uplift and shallow emplacement. The Rif constitutes an active southwestward vergent orogenic wedge, oblique to the present-day NW-SE convergent Eurasian-African plate boundary.
ISSN: 1525-2027

hal-00670999
https://hal.archives-ouvertes.fr/hal-00670999
DOI : 10.1029/2011GC003783