Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We analyze the role of slab pull acting on the Pacific plate during its early Tertiary change in motion. Slab pull forces are estimated by integrating the negative buoyancy of a 700 km long slab along a revised subduction boundary model adopting the Muller et al. (2008) seafloor age reconstructions. Our results indicate that torques predicted from a simple slab pull model match the Pacific plate Euler vectors during the Tertiary fairly well. The change of the Pacific motion at similar to 50-40 Ma appears to be driven by the onset of the Izu-Bonin-Mariana system and, soon afterwards, by the Tonga-Kermadec subduction zones.
ISSN: 0094-8276

hal-00682150
https://hal.archives-ouvertes.fr/hal-00682150
https://hal.archives-ouvertes.fr/hal-00682150/document
https://hal.archives-ouvertes.fr/hal-00682150/file/faccennaGRL2012.pdf
DOI : 10.1029/2011GL050155

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 Cambridge University Press (CUP)
Résumé : International audience
This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia-Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj-Sirjan, Kermanshah and Urumieh-Dokhtar magmatic arcs), the P-T-t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW-NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115-85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene-Eocene (60-40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30-0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj-Sirjan Zone: 20-15 Ma, High Zagros: ~12-8 Ma; Simply Folded Belt: 5-0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.
ISSN: 0016-7568

insu-00616582
https://hal-insu.archives-ouvertes.fr/insu-00616582
https://hal-insu.archives-ouvertes.fr/insu-00616582/document
https://hal-insu.archives-ouvertes.fr/insu-00616582/file/692.full.pdf
DOI : 10.1017/S001675681100046X

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2, 3. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades4, 5. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.
ISSN: 0028-0836

hal-01685757
https://hal.archives-ouvertes.fr/hal-01685757
DOI : 10.1038/nature22355

Éditeur(s) : American Geophysical Union
Résumé : We use seismic reflection and rock sample data to propose that the first-order physiography of New Caledonia Trough and Norfolk Ridge formed in Eocene and Oligocene time and was associated with the onset of subduction and back-arc spreading at the Australia-Pacific plate boundary. Our tectonic model involves an initial Cretaceous rift that is strongly modified by Cenozoic subduction initiation. Hence, we are able to explain (1) complex sedimentary basins of inferred Mesozoic age; (2) a prominent unconformity and onlap surface of middle Eocene to early Miocene age at the base of flat-lying sediments beneath the axis of New Caledonia Trough; (3) gently dipping, variable thickness, and locally deformed Late Cretaceous strata along the margins of the trough; (4) platform morphology and unconformities on either side of the trough that indicate a phase of late Eocene to early Miocene uplift to near sea level, followed by rapid Oligocene and Miocene subsidence of similar to 1100-1800 m; and (5) seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of New Caledonia Trough by 1800-2200 m since Eocene time. The Cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab. This created a deep (>2 km) enclosed oceanic trough, similar to 2000 km long and 200-300 km across, in Eocene and Oligocene time as the lower crust detached, with simultaneous uplift and local land development along basin flanks. Disruption of Late Cretaceous and Paleogene strata was minimal during this Cenozoic phase and involved only subtle tilting and local reverse faulting or folding. Basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into Norfolk Basin. We suggest that delamination of the lithosphere, with possible mixing of the lower crust back into the mantle, is more widespread than previously thought and may be commonly associated with subduction initiation, such as Cenozoic events in the Mediterranean and western Pacific.
Tectonics (0278-7407) (American Geophysical Union), 2010-03 , Vol. 29 , N. TC2004 , P. 1-16

Droits : Copyright 2010 by the American Geophysical Union.
http://archimer.ifremer.fr/doc/00001/11271/9283.pdf
DOI:10.1029/2009TC002476
http://archimer.ifremer.fr/doc/00001/11271/

Éditeur(s) : HAL CCSD
Résumé : Today, one of the main challenges in the oil industry is the exploitation of new resources in naturally fractured reservoirs often located in structurally complex areas such as plate boundaries, mountain ranges or near salt structures. While numerical models of rock deformation based on continuum mechanics are becoming industry standard in providing efficient means for modelling natural fractures in reservoirs, the knowledge of past perturbed stress fields, which is referred as the type (normal, wrench or reverse), orientation and magnitude of the regional or local tectonic stresses that needs to be applied as boundary condition in numerical simulations, is often unknown or approximated.The essence of this thesis is to improve the recovery of the paleostress field needed to constrain numerical simulations. I first described and discuss through a comprehensive mechanically-based multi-parametric study how far we can rely on the commonly used paleostress inversion methods based on fault slip data and the Wallace and Bott assumptions. I then compared this method with a new generation technique based on geomechanics using field observations. Since slip markers on faults are hardly observed in core or image log, I used observed natural fracture data as main drivers for the inversion of the paleostress using geomechanically-based method. I demonstrated, through various outcrop and subsurface examples, how this can efficiently be done. Finally, I aimed at addressing the problem of polyphase fracture data sets with unknown mechanical type (joints, faults, stylolites ...) and expanded the mechanical stress inversion to the separation of tectonic phases.
Aujourd'hui, l'un des principaux défis dans l'industrie pétrolière est l'exploitation de nouvelles ressources dans les réservoirs naturellement fracturés souvent situés dans des zones structurellement complexes tels que les frontières de plaques, les chaînes de montagnes ou les structures de proches des dômes de sel. Alors que les modèles numériques de déformation des roches basée sur la mécanique des milieux continus fournissant des moyens efficaces pour la modélisation de fractures naturelles dans les réservoirs, sont devenus la norme dans l'industrie, les paléo-contraintes régionales ou locales appliquées comme condition aux limites dans les simulations numériques, définies par le régime (normal, décrochant ou inverse), l'orientation, et la magnitude des contraintes tectoniques, sont souvent inconnues ou approximées.L'essence de cette thèse est d'améliorer la prédiction du champ de paléo-contrainte nécessaire pour contraindre les simulations numériques. J’ai décrit dans un premier temps à travers une étude comparative multi-paramétrique complète basée sur la mécanique, dans quelle mesure nous pouvions compter sur les méthodes d'inversion des paléo-contraintes basées sur les hypothèses de Wallace et Bott et les données de glissement sur les failles. J’ai ensuite comparé ces méthodes avec une technique d’inversion de nouvelle génération basée sur la géomécanique en utilisant les observations de terrain. Puisque les marqueurs de glissement sur les failles sont rarement observés le long du forage, j’ai utilisé l’inversion géomécanique des contraintes pour les données de fractures naturelles observées. J’ai alors démontré, l’efficacité de cette technique à travers divers exemples d'affleurements et de subsurface. Enfin, j’ai proposé une solution afin de résoudre le problème des données de fractures polyphasées à type cinématique inconnu (joints, failles, stylolites ...) et étendu l’inversion de contrainte basée sur la géomécanique à la séparation des phases tectoniques.
https://hal.archives-ouvertes.fr/tel-01324110

tel-01324110
https://hal.archives-ouvertes.fr/tel-01324110
https://hal.archives-ouvertes.fr/tel-01324110/document
https://hal.archives-ouvertes.fr/tel-01324110/file/theselejri.pdf

Éditeur(s) : Elsevier
Résumé : We present results from multibeam bathymetric data acquired during 2005 and 2006, in the region of maximum slip of the 26 Dec. 2004 earthquake (Mw 9.2). These data provide high-resolution images of seafloor morphology of the entire NW Sumatra forearc from the Sunda trench to the submarine volcanic arc just north of Sumatra. A slope gradient analysis of the combined dataset accurately highlights those portions of the seafloor shaped by active tectonic, depositional and/or erosional processes. The greatest slope gradients are located in the frontal 30 km of the forearc, at the toe of the accretionary wedge. This suggests that long-term deformation rates are highest here and that probably only minor amounts of slip are accommodated by other thrust faults further landward. Obvious N-S oriented lineaments observed on the incoming oceanic plate are aligned sub-parallel to the fracture zones associated with the Wharton fossil spreading center. Active strike-slip motion is suggested by recent deformation with up to 20-30 m of vertical offset. The intersection of these N-S elongated bathymetric scarps with the accretionary wedge partly controls the geometry of thrust anticlines and the location of erosional features (e.g. slide scars, canyons) at the wedge toe. Our interpretation suggests that these N-S lineaments have a significant impact on the oceanic plate, the toe of the wedge and further landward in the wedge. Finally, the bathymetric data indicate that folding at the front of the accretionary wedge occurs primarily along landward-vergent (seaward-dipping) thrusts, all unusual style in accretionary wedges worldwide. The N-S elongated lineaments locally act as boundaries between zones with predominant seaward versus landward vergence.
Earth and Planetary Science Letters (0012-821X) (Elsevier), 2008-11 , Vol. 275 , N. 3-4 , P. 201-210

Droits : 2008 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2008/publication-5166.pdf
DOI:10.1016/j.epsl.2008.04.053
http://archimer.ifremer.fr/doc/00000/5166/

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : We study the Africa-Iberia plate boundary in the vicinity of Gibraltar. Numerous models have been proposed for that region throughout the last decades, proposing mechanisms that range widely from continental delamination, convective removal, to subduction of oceanic lithosphere. To better constrain upper-mantle structure under the region, we study waveforms of P-waves that traverse the Alboran Sea region between Spain and Morocco. These show dispersive behavior, which, together with early arrival times, confirms the presence of an anomalous upper mantle structure under the Alboran Sea. The dispersion is consistent with that expected from subducted lithosphere. Waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the Alboran Sea region
ISSN: 0094-8276

hal-00407670
https://hal.archives-ouvertes.fr/hal-00407670
DOI : 10.1029/2007GL030964

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We investigate upper mantle anisotropy beneath the Pyrenean range along three N-S profiles across the mountain belt. The results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere. We present the results of two other profiles that ran in 1995-1996 and 1996-1997 in the eastern and western part of the belt, respectively and propose an interpretation of the whole results. Teleseismic shear waves (SKS, SKKS, and PKS) are used to determine splitting parameters: the fast polarization direction φ and the delay time δt. Teleseismic shear wave splitting in the eastern Pyrenees displays homogeneous φ values trending N100°E and δt values in the range 1.1 to 1.5 s. A station located in the southern Massif Central, 100 km north of the range, is characterized by different splitting parameters (φ = N70°E, δt = 0.7 s). In the western part of the belt, anisotropy parameters are similar across the whole belt (φ = N110°E and δt = 1.3 to 1.5 s). Most of the measured delay times, including those obtained in the central part of the range, are above the global average of the SKS splitting (around 1 s). At the belt scale, φ is generally poorly correlated with recent estimations of the absolute plate motion, which predicts a fast direction ranging between N50°E and N80°E. Instead, the orientation of φ (N100°E) is parallel to the trend of the Pyrenean belt but also to Hercynian preexisting structures. This parallelism supports an anisotropy primarily related to frozen or active lithospheric structures. We show that a signature related to the Pyrenean orogeny is likely for the stations located in the internal domains of the belt. By contrast, the anisotropy measured at the stations located on the external parts of the belt could reflect a pre-Pyrenean (Hercynian) deformation. We suggest that a late Hercynian strike-slip deformation is responsible for this frozen upper mantle anisotropy and that the Pyrenean tectonic fabric developped parallel to this preexisting fabric. Finally, no particularly strong splitting is related to the North Pyrenean Fault, commonly believed to represent the plate boundary between Iberia and Eurasia.
ISSN: 2169-9313

hal-01388841
http://hal.univ-reunion.fr/hal-01388841
http://hal.univ-reunion.fr/hal-01388841/document
http://hal.univ-reunion.fr/hal-01388841/file/Barruol_Aniso_Pyr_JGR1998.pdf
DOI : 10.1029/98JB02790

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Gulf of Cadiz lies astride the complex plate boundary between Africa and Eurasia west of the Betic-Rif mountain belt. We report on the results of recent bathymetric swathmapping and multi-channel seismic surveys carried out here. The seafloor is marked by contrasting morphological provinces, spanning the SW Iberian and NW Moroccan continental margins, abyssal plains and an elongate, arcuate, accretionary wedge. A wide variety of tectonic and gravitational processes appear to have shaped these structures. Active compressional deformation of the wedge is suggested by folding and thrusting of the frontal sedimentary layers as well as basal duplexing in deeper internal units. There is evidence for simultaneous gravitational spreading occurring upslope. The very shallow mean surface and basal slopes of the accretionary wedge (1° each) indicate a very weak decollement layer, geometrically similar to the Mediterranean Ridge accretionary complex. Locally steep slopes (up to 10°) indicate strongly focused, active deformation and potential gravitational instabilities. The unusual surface morphology of the upper accretionary wedge includes “raft-tectonics” type fissures and abundant sub-circular depressions. Dissolution and/or diapiric processes are proposed to be involved in the formation of these depressions.
ISSN: 0264-8172

insu-00401683
https://hal-insu.archives-ouvertes.fr/insu-00401683
DOI : 10.1016/j.marpetgeo.2007.11.008

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Physical modeling of oblique subduction is performed to study the mechanism of strain partitioning. The model is two-layer and includes the elasto-plastic lithosphere (the overriding and subducting plates) and the low-viscosity liquid asthenosphere. The subduction is driven by a push force from a piston and a pull force when the density contrast Ap between the subducting plate and the asthenosphere is positive. We vary both Ap and the interplate friction (frictional stresses). Slip partitioning is obtained only in the models with high interplate friction and only when the overriding plate contains a weak zone. This zone in the models corresponds either to locally thinned litho-sphere or to cut (fault). The horizontal, trench-normal component of the interplate friction force [Ffh[ can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force [Fph [ in the subduction zone. Ffh is always negative (compression), while Fph can be either negative (compressional subduction regime) or positive (extensional regime). High friction, which promotes partitioning, can completely cancel the extensional (suction) force Fph. Back arc tension and strike-slip faulting appear thus as conflicting processes, although they can coexist in the same subduction zone, depending on the relative values of relevant forces. It appears that high friction can exist only in compressional subduction zones where partitioning should develop more easily. This conclusion is supported by the comparison of two oblique subduction zones, having similar geometry: the compressional southern Kurile zone (strong partitioning) and extensional southern Ryukyu zone (no lithospheric-scale partitioning). Other factors controlling the strain partitioning are the length of the oblique subductlon zone, the boundary conditions at the transverse limits of the forearc sliver, and of course, the obliquity of subduction.
ISSN: 2169-9275

hal-01261396
https://hal.archives-ouvertes.fr/hal-01261396
https://hal.archives-ouvertes.fr/hal-01261396/document
https://hal.archives-ouvertes.fr/hal-01261396/file/chemendaJGR2000.pdf
DOI : 10.1029/1999JB900332

Éditeur(s) : HAL CCSD Elsevier
Résumé : The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.
ISSN: 0012-821X

hal-01553243
https://hal.archives-ouvertes.fr/hal-01553243
DOI : 10.1016/j.epsl.2016.12.004

Éditeur(s) : HAL CCSD Wiley
Résumé :  
Aims: Astrocaryum sciophilum (Miq.) Pulle (Arecaceae) is an understorey palm, endemic to north-eastern South America with a patchy distribution. We tested the hypothesis that the spatial distribution of this palm species is not in equilibrium but is slowly colonizing the forest understorey. Location: Inventories and seed dispersal studies were conducted in the undisturbed tropical forest close to the Nouragues research station, French Guiana. Additional data were collected in the entire territory of French Guiana. Methods: We studied the demography of A. sciophilum on a 20-ha plot located at the edge of its distribution. The age of the palms was estimated by postulating an exponentially decreasing abundance by age class. Direct seed dispersal experiments were also conducted, to estimate dispersal parameters. The seeds of A. sciophilum were dispersed only by rodents. This information was used to parameterize a forest growth simulator, to study the spatial spread of this species. Results: Within the sampling plot, the density of A. sciophilum dropped sharply from about 500 individuals per hectare to zero. The maturation age was estimated to be 170 ± 70 years, and over 55 years with 95% confidence. Seed-dispersal experiments yielded an average seed dispersal distance of 11 m and a maximum estimated dispersal distance of 125 m across a generational span of 55 years to maturity. Therefore, the maximal estimated colonization speed is 2.3 m/y. Conclusions: Empirical results and numerical simulations suggest that the boundary of the A. sciophilum population is a colonization front, and that the range of this species is slowly expanding. The implications of this result in respect of palaeoenvironmental changes in this region are discussed.
ISSN: 1466-822X

hal-01032154
https://hal.archives-ouvertes.fr/hal-01032154
DOI : 10.1046/j.1466-822X.2003.00020.x

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
In order to constrain the vertical and lateral extent of deformation and the interactions between lithosphere and asthenosphere in a context of a transpressional plate boundary, we performed teleseismic shear wave splitting measurements for 65 permanent and temporary broadband stations in central California. We present evidence for the presence of two anisotropic domains: (1) one with clear E–W trending fast directions and delay times in the range 1.5 to 2.0 s and (2) the other closely associated with the San Andreas Fault system with large azimuthal variations of the splitting parameters that can be modeled by two anisotropic layers. The upper of the two layers provides fast directions close to the strike of the main Californian faults and averaged delay times of 0.7 s; the lower layers show E–W directions and delay times in the range 1.5 to 2.5 s and thus can be compared to what is observed in stations that require a single layer. We propose the E–W trending anisotropic layer to be a 150 to 200 km thick asthenospheric layer explained by the shearing associated with the absolute plate motion of the North American lithosphere. The shallower anisotropic layer ought to be related to the dynamics of the San Andreas Fault system and thus characterized by a vertical foliation with lineation parallel to the strike of the faults localized in the lithosphere. We also propose that the anisotropic layer associated with each fault of the San Andreas Fault system is about 40 km wide at the base of the lithosphere.
ISSN: 0148-0227

hal-01236161
http://hal.univ-reunion.fr/hal-01236161
http://hal.univ-reunion.fr/hal-01236161/document
http://hal.univ-reunion.fr/hal-01236161/file/Bonnin_SKS_California_JGR2010.pdf
DOI : 10.1029/2009JB006438

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The contourite depositional system (CDS) along the southwestern Iberian Margin (SIM), within the Gulf of Cadiz and offshore areas of western Portugal bear the unmistakable signal of Mediterranean Outflow Water (MOW) exiting the Strait of Gibraltar. This locality records key information concerning the effects of tectonic activity on margin sedimentation, the effects of MOW dynamics on Atlantic circulation, and how these factors may have influenced global climate. Over the last four decades, numerous studies have been conducted on the late Miocene, Pliocene and Quaternary sedimentary stacking pattern of Neogene basins along the SIM for both academic and resources exploration purposes. However, understanding of the region rests primarily on basic seismic stratigraphy calibrated with limited data from only a few exploration wells. The Integrated Ocean Drilling Program (IODP) Expedition 339 recently drilled five sites in the Gulf of Cadiz and two sites on the western Iberian margin. The integration of core and borehole data with other geophysical databases leads us to propose a new stratigraphic framework. Interpretation of IODP Exp. 339 data along with that from industry sources and onshore outcrop analysis helps refine our understanding of the SIM's sedimentary evolution.We identify significant changes in sedimentation style and dominant sedimentary processes, coupled with widespread depositional hiatuses along the SIM within the Cadiz, Sanlucar, Doñana, Algarve and Alentejo basins. Following the 4.5 Ma cessation of a previous phase of tectonic activity related to the Miocene–Pliocene boundary, tectonics continued to influence margin development, downslope sediment transport and CDS evolution. Sedimentary features indicate tectonic pulses of about 0.8–0.9 Ma duration with a pronounced overprint of ~ 2–2.5 Ma cycles. These more protracted cycles relate to the westward rollback of subducted lithosphere at the convergent Africa-Eurasia plate boundary as its previous NW–SE compressional regime shifted to a WNW–ESE direction. Two major compressional events affecting to the Neogene basins at 3.2–3 Ma and 2–2.3 Ma help constrain the three main stages of CDS evolution. The stages include: 1) the initial-drift stage (5.33–3.2 Ma) with a weak MOW, 2) a transitional-drift stage (3.2–2 Ma) and 3) a growth-drift stage (2 Ma-present time) with enhanced MOW circulation into the Atlantic and associated contourite development due to greater bottom-current velocity. Two minor Pleistocene discontinuities at 0.7–0.9 Ma and 0.3–0.6 Ma record the effects of renewed tectonic activity on basin evolution, appearing most prominently in the Doñana basin. Several discontinuities bounding major and minor units appear on seismic profiles. Quaternary records offer the clearest example of this, with major units of about 0.8–0.9 Ma and sub-units of 0.4–0.5 Ma. Sedimentation is controlled by a combination of tectonics, sediment supply, sea-level and climate. This research identifies time scales of tectonic controls on deep-marine sedimentation, specifically over periods of 2.5–> 0.4 Ma. Shorter-term climatic (orbital) mechanisms control sedimentation at time scales of ≤ 0.4 Ma. The role of bottom water circulation and associated processes in shaping the seafloor and controlling the sedimentary stacking pattern on continental margins has to be seriously reconsidered in future multidisciplinary studies. This is not only because of the common occurrence of sandy contourite deposits in deep water setting and their economic interest for hydrocarbon exploration, but principally because they archive the heartbeat of the interior Earth and therefore have important sedimentary and paleoceanographic implications.
ISSN: 0025-3227

hal-01356166
https://hal.archives-ouvertes.fr/hal-01356166
DOI : 10.1016/j.margeo.2015.09.013

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
No major earthquake occurred in North Chile since the 1877 Mw 8.6 subduction earthquake that produced a huge tsunami. However, geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming, which reveals some degree of locking on the subduction interface. This accumulation of elastic deformation is likely to be released in a future earthquake. Because of the long elapsed time since 1877 and the rapid accumulation of deformation (thought to be 6-7 cm yr−1), many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high. We present a new Global Positioning System (GPS) velocity field, acquired between 2008 and 2012, that describes in some detail the interseismic deformation between 18°S and 24°S. We invert for coupling distribution on the Nazca-South America subduction interface using elastic modelling. Our measurements require that, at these latitudes, 10 to 12 mm yr−1 (i.e. 15 per cent of the whole convergence rate) are accommodated by the clockwise rotation of an Andean block bounded to the East by the subandean fold-and-thrust belt. This reduces the accumulation rate on the subduction interface to 56 mm yr−1 in this area. Coupling variations on the subduction interface both along-strike and along-dip are described. We find that the North Chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling. This coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region. Intersegment zones (Iquique, Mejillones) correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates. The rupture of either the Paranal or the Loa segment alone could easily produce a Mw 8.0-8.3 rupture, and we propose that the Loa segment (from 22.5°S to 20.8°S) may be the one that ruptured in 1877.
ISSN: 0956-540X

hal-00941279
https://hal.archives-ouvertes.fr/hal-00941279
DOI : 10.1093/gji/ggt183

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 : Carbon emission at volcanic centers requires a constant balance between output (mostly by volcanism, either at plate boundaries or intraplate) and input (mostly at trench settings) of carbon from and to the Earth's mantle. The form of carbon that resides in the mantle is controlled by depth (pressure) and oxygen fugacity, the latter in turn depending on the depth and the concentration of iron in the mantle. In the shallow, lithospheric mantle, carbon is likely to be present in the oxidized form of CO2 (except under cratons where carbon is reduced to graphite or diamond). Below approximately 90 km, in the asthenosphere, the oxidized form of carbon is carbonate, either mineral or melt, depending on the thermal regime. At depths greater than approximately 150 km, the asthenospheric mantle is too reducing for carbon to stay in its oxidized form and only diamond is present, unless there is sufficient hydrogen to form reduced C–H fluids. Hence, the region located in the depth range of 90 to 150 km deep is where carbonatitic melts can most likely be produced and impregnate the surrounding mantle through metasomatism. The upper bound of this region is called the carbonate ledge. This limit prevents carbonate (either solid or molten) from ascending because of degassing and CO2 liberation. The lower bound is a redox front where redox melting (that is, melting caused by oxidation) may take place in an ascending portion of carbon-containing mantle. Carbonatite eruptions and presence of carbonate mineral inclusions in deep-seated diamonds provide evidence that these boundaries can be trespassed in some cases.An analysis of the experimental data that has bearing on silicate melting in the presence of carbon further shows that the carbonate ledge is a melting curve with a negative or flat Clapeyron (dP/dT) slope. In the carbonated ultrabasic (peridotite) systems, the carbonate ledge is located between ~ 2–3 GPa. The ledge divides the pressure–temperature space into a region of low-pressure silicate melt production, and a high-pressure region where carbonatites can be produced. Carbonatitic melts in equilibrium with mantle peridotite have compositions close to dolomitic (approximately equal amounts of Ca and Mg) with a general trend of becoming markedly more magnesian with increasing pressure. Calcic carbonatites may be stable at pressures < 2 GPa if clinopyroxene is absent. At mantle transition zone pressure range, there seems to be a melting temperature decrease (negative fusion slope), which may be caused by the stabilization of majoritic garnet.The carbonated basic (broadly eclogitic) system is more complex than the peridotitic one, because of the strong control of bulk silicate composition on melting temperatures, and hence, on melt composition. In carbonated eclogite systems, we propose that the effect of bulk composition upon all observed features can perhaps be related to silica super-saturation, or lack thereof. In some cases, high calcium (> 80 mol% CaCO3) melts can be produced, making the melting of carbonated eclogites an appealing scenario for the genesis of calcio-carbonatites in the Earth's mantle. Comparison with modeled pressure–temperature paths of subducted oceanic lithosphere shows that fusion of carbonated eclogite at depths shallower than 200 km should be expected for hot (Cascadian-type) subduction thermal regimes. On the other hand, in the case of cooler thermal regimes (Honshu-type, for instance), subducted carbonates may be stable to greater depths in Earth at trench settings, depending on the bulk composition of the system.Furthermore, high-pressure experiments show evidence of a continuum among carbonatitic, kimberlitic, melilititic, and basaltic liquids, for increasing melting degree of carbonated peridotite. This continuum has not been documented in the case of fusion of carbonated eclogite. It may be present, however, when certain sediments are fused, although the silicate melts are granitic to rhyodacitic instead of being kimberlitic in composition. Additional high-pressure work on phase relations in the simple binary system CaCO3–MgCO3 and specific focus on oxide solubility in the vapor phase have the potential to further clarify phase relations on complex silicate–carbonate systems at mantle conditions.
ISSN: 0009-2541

hal-01277770
https://hal.archives-ouvertes.fr/hal-01277770
DOI : 10.1016/j.chemgeo.2015.05.018

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle–ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust–mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust–mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.
ISSN: 0012-821X

hal-01243577
https://hal.archives-ouvertes.fr/hal-01243577
DOI : 10.1016/j.epsl.2015.05.022

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
ODP (Ocean Drilling Program)/IODP (Integrated Ocean Drilling Program) Site 1256 is located on the Cocos Plate in the Eastern Equatorial Pacific Ocean, in a 15 Ma old oceanic lithosphere formed at the EPR during a period of superfast spreading (>200 mm/yr). ODP/IODP Hole 1256D reached for the first time the contact between sheeted dikes and underlying gabbros. It consequently offers a unique opportunity to study in situ, in present-day oceanic crust, the root zone of the sheeted dike complex. This root zone is a thin, 100 m thick boundary layer between the magmatic system (i.e., the axial melt lens, similar to 1100 degrees C), and the overlying high temperature hydrothermal system (<= 450 degrees C). The understanding of interactions within this boundary layer is critical to that of crustal processes along mid-ocean ridges.;This work focuses on the petrophysical characterization of the root zone of the sheeted dike complex in order to further constrain the hydrothermal circulation system in the vicinity of the axial melt lens, as recorded in non-granoblastic dikes, granoblastic dikes, and varitextured gabbros. The petrophysical properties were determined from sample measurements in the laboratory and were compared to in situ downhole geophysical probing. The porosity structure is bipolar, depending on lithology, resulting in a layered system. Non-granoblastic dikes are generally altered in the greenschist fades (similar to>250 degrees C) with relatively high and interconnected (cementation index m similar to 1.72, electrical tortuosity tau similar to 28.3) porosity (1.5%). In contrast, gabbros are retrogressively metamorphosed in the amphibolite (similar to>450 degrees C) and greenschist facies, with lower porosity (1.3%) that involves numerous fissures and cracks, resulting in a more connected medium (m similar to 1.58, tau similar to 11.8) than non-granoblastic dikes. These cracks are more abundant but also tend to close with increasing depth as indicated in downhole geophysical data. Porosity and alteration, as viewed from surface electrical conductivity, appear to be directly correlated.
ISSN: 0040-1951

hal-00544556
https://hal.archives-ouvertes.fr/hal-00544556
DOI : 10.1016/j.tecto.2010.07.013