Éditeur(s) : Gauthier-Villars
Résumé : Simple thermodynamics calculations are applied to processes of a rising plume within an environment of an upward directed gradient of viscosity in the uppermost mantle. Four stages of the development of passive margins are described. Following an initial regional uplift strong lateral movements are induced at asthenospheric levels, stretching the overlying lithosphere. In the second stage plume material also enters the low viscosity lower crust, replacing continental material and forming a new, higher crust-mantle boundary by gravitational differentiation. The inital phase lag between the laterally intruding plume material and the rest of the lithosphere disappears once the continent is rifted and sea floor spreading is established (= stage 3). Finally, at stage 4, quiet subsidence, controlled by contraction, sedimentation, and possible phase changes in the lower crust, is established. The sequence described is in accordance with many observations of geology and geophysics. Rising plumes and sinking slabs are considered equivalent expressions of the heat engine earth which determines global tectonics.
Oceanologica Acta, Special issue (0399-1784) (Gauthier-Villars), 1981

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00245/35658/34167.pdf
http://archimer.ifremer.fr/doc/00245/35658/

Éditeur(s) : HAL CCSD
Résumé : This work provides new constraints on the development and on the distribution of the deformation in the upper mantle and particularly beneath transform plate boundaries. USArray experiment and the remarkable increase of the dataset in California for the past ten years allowed us to scrutinize the lateral variations of the anisotropy in the vicinity of the San Andreas Fault zone. We have confirmed and increased the detection of two layers of anisotropy beneath this plate boundary. The first layer, located in the lithosphere, is related to the deformation induced at the fault, and the other one, located in the asthenosphere, is coherent with the anisotropy observed far from it, its origin is however less clear. We show that the deformation zone associated both to the San Andreas, Calaveras and Hayward Faults, is likely 40 km wide at 70 km depth. We then performed numerical thermomechanical modeling (ADELI) of the displacement of a transform plate boundary associated with the computation of the development of crystallographic fabrics using a viscoplastic selfconsistent approach (VPSC). We analyzed the distribution of the deformation in the model ant looked after the possible interactions at depth between deformation caused at surface by the strike-slip dynamic of the fault and the shearing at the base of the lithosphere caused by the horizontal displacement of the plates. Elastic properties derived from the crystallographic fabrics modeled, show that such interactions exist and induce, beneath the fault zone, a progressive rotation of the crystallographic fabrics with depth. Seismological signature of these smooth rotations is however not relevant with the presence of two anisotropic layers as proposed beneath California. We thus consider that a decoupling zone exists between the lithosphere and the asthenosphere beneath the California to account for the sharp separation between a lithospheric and an asthenospheric deformation. We furthermore estimate that anisotropy observed far form the San Andreas Fault in California cannot be explained only by the drag of the asthenosphere by the North America lithosphere as proposed in our article. Indeed, we can only expect few tenths of second of splitting delay from the anisotropic properties derived from the numerical modeling of a plate moving in the same direction and in the same velocity than the North American lithosphere only for 10 Ma of displacement. As delays observed in California rather reach 1.5 s, anisotropy in this region thus requires the existence of an active asthenospheric flow to be explained.
Le travail réalisé pendant cette thèse a permis d'apporter de nouvelles contraintes sur le développement et la distribution de la déformation dans le manteau supérieur et plus particulièrement au niveau des grandes limites de plaques décrochantes. Grâce à l'apport de l'expérience USArray et d'une dizaine d'années d'enregistrements sismologiques supplémentaires, nous avons pu étudier, de manière précise, les variations d'anisotropie dans le voisinage de la Faille de San Andreas. Nous avons confirmé et étendu l'observation de deux couches anisotropes sous cette limite de plaque. On y observe une première couche localisée dans la lithosphère marquant la déformation induite à la limite de plaque, et une autre, asthénosphérique, cohérente avec l'anisotropie observée loin de la faille et d'origine plus discutée. Nous avons montré que la zone de déformation associée aux failles de San Andreas, Calaveras et d'Hayward a, vraisemblablement, une largeur d'au moins 40 kilomètres en base de lithosphère, sous chacune de ces failles. Nous avons ensuite procédé à la modélisation thermomécanique (ADELI) de la migration d'une limite de plaques décrochante couplée à une modélisation du développement de fabriques cristallographiques par une approche viscoplastique auto-cohérente (VPSC). Ceci nous a permis d'y observer le développement de la déformation et les conséquences des possibles interactions entre la déformation décrochante en surface et le cisaillement en base de lithosphère dû au déplacement horizontal des plaques. Les propriétés élastiques déduites des fabriques cristallographiques modélisées montrent que de telles interactions existent et provoquent, sous la limite de plaques, une rotation des orientations cristallographiques avec la profondeur. Le signal associé à ces rotations progressives n'est toutefois pas cohérent avec la présence de deux couches d'anisotropie comme proposée sous la faille de San Andreas. Nous pensons par conséquent qu'il existe, sous la Californie, une zone de découplage entre la lithosphère et l'asthénosphère, permettant d'individualiser une déformation lithosphérique d'une déformation asthénosphérique. Nous estimons, en outre, que l'anisotropie observée dans l'asthénosphère sous la Californie ne peut être expliquée seulement par le cisaillement induit par le déplacement de la lithosphère Nord Amérique. En effet, les propriétés anisotropes obtenues par modélisation à partir d'une plaque se déplaçant dans une direction et une vitesse proche de celle de la plaque Amérique du Nord montrent qu'on ne peut espérer guère plus que quelques dixièmes de seconde de délai au bout de 10 Ma de déplacement. Les déphasages mesurés en Californie étant de l'ordre de 1,5 s, il est donc nécessaire d'invoquer la présence d'écoulements mantelliques actifs sous cette région.
https://tel.archives-ouvertes.fr/tel-00767124

tel-00767124
https://tel.archives-ouvertes.fr/tel-00767124
https://tel.archives-ouvertes.fr/tel-00767124/document
https://tel.archives-ouvertes.fr/tel-00767124/file/theseBonnin.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The mantle above a subducting slab is the site of complex interactions between deformation, partial melting, fluid migration and magma transport. To constrain these interactions and their effects on olivine deformation, we analyze microstructures, crystal preferred orientations, and water contents of peridotite xenoliths entrained by andesites of the Avacha volcano, southern Kamchatka arc. These xenoliths are refractory spinel harzburgites that have coarse-grained microstructures with widely spaced subgrain boundaries and sinuous grain boundaries in olivine, consistent with deformation by dislocation creep under low deviatoric stress (< 13 MPa) and with a significant contribution from diffusional processes. Analysis of crystal preferred orientations (CPO) indicates dominant activation of high-temperature, low-stress {0kl}[100] slip systems in olivine and of(100)[001] in orthopyroxene. In most samples, coarse opx crystals, elongated parallel to the lineation, enclose small olivine grains in crystallographic continuity with neighbouring crystals, indicating crystallization of orthopyroxene at the expense of olivine as a result of reactive percolation of Si-rich fluids coeval with the high-temperature deformation. Secondary crystallization of interstitial orthopyroxene led locally to development of opx-rich lenses parallel to the foliation, characterized by a decrease in olivine grain size and dispersion of the olivine CPO without changing the dominant slip systems. Half of the samples also show acicular orthopyroxene aggregates with which is associated a fine-grained matrix composed of rounded strain-free olivine, orthopyroxene, spinel, and rare amphibole crystals. This matrix occurs pervasively along grain boundaries or forms millimeter-scale irregular lenses and anastomosing veinlets that crosscut the coarse crystals and their ductile deformation structures. Both acicular orthopyroxene and the fine-grained matrix are interpreted as resulting from reactive transport of H2O-rich fluids under static conditions, probably in the lithospheric mantle. Infrared analyses show that olivine contains 1-8 center dot 6 ppm by weight of water. These low water contents are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at low pressure and dehydration during exhumation of the xenoliths. Water contents in orthopyroxene are highly variable (25-506 ppm H2O), probably recording spatially heterogeneous interaction with fluids or melts and compositional disequilibrium in the studied samples. Change in the dominant percolation mechanism from porous flow to fracturing suggests cooling, consistent with the low temperatures estimated from pyroxene thermometry (< 800-900 degrees C). The Avacha xenoliths therefore record pervasive deformation of a region of the mantle under asthenospheric conditions, followed by its accretion to the base of the lithosphere, probably as a result of cooling of the mantle wedge. Percolation of Si-rich fluids or hydrous melts is recorded at all stages; this probably enhanced diffusion and lowered deviatoric stresses during ductile deformation, but did not change the dominant slip direction in olivine from [100] to [001].
ISSN: 0022-3530

hal-00456053
https://hal.archives-ouvertes.fr/hal-00456053
DOI : 10.1093/petrology/egp085

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The Ronda peridotite massif in Southern Spain shows a well-defined 'recrystallization' front that separates a large-scale partial melting domain formed at the expense of the continental lithospheric mantle from a 'preserved' lithospheric domain. To investigate the processes allowing a transient lithosphere-asthenosphere boundary to propagate in the lithospheric mantle, we performed a joint petrostructural and geochemical study of an similar to 2.5 km(2) zone extending from the melting front to the mylonites that mark the western limit of the massif. This study emphasizes a feedback between heat transfer, melt percolation, and deformation in the lithospheric mantle. Petrographical observations and geochemical data show that heterogeneous reactive percolation of melts produced in the underlying partial melting domain led to refertilization of lithospheric peridotites up to 1.5 km ahead from the melting front, producing metre-scale layering of fertile and refractory mantle rocks. Within 800 m from the front, pre-existing garnet pyroxenite layers were partially molten and the resulting melts probably contributed to the refertilization process. Detailed structural mapping and analysis of the microstructures and crystal preferred orientations highlight the relations between reactive melt transport and deformation, and the control of the temperature gradient on both processes. Parallelism between the recrystallization front, compositional boundaries, and deformation structures, as well as variations in the deformation intensity of pyroxenes and spinels, suggest syn- to late-tectonic melt transport controlled by both the deformation and the thermal gradient. Variations in the strength of olivine crystal preferred orientations as a function of the modal and chemical composition of the spinel tectonites point to a higher contribution of diffusion to deformation in the most fertile rocks, corroborating the hypothesis that deformation occurred in presence of melt. Finally, the systematic dispersion of olivine [100] and orthopyroxene [001] axes in the foliation plane suggests a dominantly transpressive deformation regime.
ISSN: 0022-3530

hal-00420904
https://hal.archives-ouvertes.fr/hal-00420904
DOI : 10.1093/petrology/egp032

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.
ISSN: 0956-540X

hal-00617815
https://hal.archives-ouvertes.fr/hal-00617815
DOI : 10.1111/j.1365-246X.2011.04921.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The formation age of the Siberian cratonic mantle is not well established as yet. Re–Os data on various mantle-derived materials have shown that it contains Archaean components, but the reported Re-depletion ages show a broad variation range (3.4 to 1 Ga) and are commonly ≤ 2 Ga for peridotite xenoliths. We report Hf and Nd isotope data for cpx and garnet separated from nine refractory spinel and garnet peridotite xenoliths from the Udachnaya kimberlite. The cpx from low-opx spinel harzburgites show extremely high εHf values, from + 607 to + 2084, which testify to long-term evolution of these rocks with high Lu/Hf ratios, consistent with their residual origin and near absence of post-melting enrichments in the Lu–Hf system. Such high εHf values are unusual for cpx from other cratonic peridotites and are higher than those reported for depleted cpx in off-cratonic peridotites. The clinopyroxenes from low-opx spinel harzburgites yield Hf model ages from 1.9 to 1.7 Ga while the cpx from high-opx spinel harzburgites yield Hf model ages from 3.0 to 1.9 Ga. When plotted together, they define a Lu/Hf isochron with an age of 1.80 Ga, which we consider as a robust estimate of the formation age (melt extraction event) because it is obtained on residual rocks that show no evidence for HREE and Hf enrichments and because the model ages for three out of four individual samples are similar to each other. The cpx have high εNd of + 94 to + 123, which are inconsistent with their low Sm/NdPM of < 1 and yield no meaningful age estimates. The consistently high, positive εNd in these cpxs can be interpreted in terms of long-term evolution of refractory peridotites with high Sm/Nd, followed by relatively recent LREE enrichments.We infer that a significant part of the lithospheric mantle in the central Siberian craton may have been formed during a major event (or a series of events) at around 1.8 Ga. Older ages reported for the central Siberian craton may refer to less common materials from cratonic or other domains formed in the Archaean that were later incorporated into the cratonic roots. The transition from the “Archaean” to “modern” tectonic regimes in Siberia and possibly elsewhere may have taken place at 1.8–1.9 Ga rather than at ~ 2.5 Ga, i.e. in the second half of the Paleoproterozoic rather than at the Archaean–Proterozoic boundary, at which time the asthenospheric mantle became generally too cold to experience high-degree melting on a large scale. The ~ 1.8 Ga formation age of the Siberian cratonic mantle is coeval with that for a major part of the ancient continental crust in the central Siberian craton. The temporal crust–mantle links may be explained either by the generation of the initial source materials for continental crust in the same melting event that formed the residual peridotites or, alternatively, by subduction and melting of pre-existing proto-lithosphere destabilized by a major mantle upwelling that formed the residual mantle.
ISSN: 0009-2541

hal-01115519
https://hal.archives-ouvertes.fr/hal-01115519
DOI : 10.1016/j.chemgeo.2014.10.018

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 Elsevier
Résumé : International audience
The platinum-group element (PGE) systematics of continental mantle peridotites show large variability, reflecting petrogenetic processing of the upper mantle during partial melting and melt/fluid percolation inside the lithosphere. By removing Pd-Cu-Ni rich sulfides, partial melting events that have stabilized the sub-continental mantle lithosphere fractionated PPGEs (Palladium-group PGE; Pt, Pd) relative to IPGEs (Iridium-group PGE; Os, Ir, Ru, Rh). Residual base-metal sulfides (BMS) survive as enclosed IPGE-enriched Monosulfide Solid Solutions (Mss), which otherwise decompose into Ru-Os-Ir-rich refractory platinum-group minerals (PGMs) once the partial melts become S-undersaturated. The small-scale heterogeneous distribution of these microphases may cause extreme nugget effects, as seen in the huge variations in absolute PGE concentrations documented in cratonic peridotites. Magmas fluxing through the lithospheric mantle may change the initial PGE budgets inherited from the melting events, resulting in the great diversity of PGE systematics seen in peridotites from the sub-continental lithosphere. For instance, melt-rock reactions at increasing melt/rock ratios operate as open-system melting processes removing residual BMS/PGMs. Highly percolated peridotites are characterized by extreme PGE depletion, coupled with PGE patterns and Os-isotope compositions that gradually evolve toward that of the percolating melt. Reactions at decreasing melt-rock ratios (usually referred to as "mantle metasomatism") precipitate PPGE-enriched BMS that yield suprachondritic Pd/Ir and occasionally affect Pt/Ir and Rh/Ir ratios as well. Moreover, volatile-rich, small volume melts fractionate Os relative to Ir and S relative to Se, thereby producing rocks with supra-chondritic Os/Ir and S/Se coupled with supra-chondritic Pd/Ir and Pt/Ir. Major magmatic inputs at the lithosphere-asthenosphere boundary may rejuvenate the PGE systematics of the depleted mantle. Integrated studies of "refertilized" peridotites with worldwide provenance provide evidence for mixing between old PGM-rich harzburgitic protoliths and newly-precipitated BMS. Long-lived PGMs carry the Os-isotope compositions of ancient melt‐depletion events into seemingly undepleted fertile lherzolites. Another diagnostic feature of major refertilization processes is the increasing modal abundance of Pt-Pd-Te-Bi or Pt-As-S microphases. Due to regional-scale refertilization processes, sizeable (> 100 km) domains of the upper lithospheric mantle are now significantly enriched in Pd, Au, Cu, Se, and other incompatible chalcophile elements that are of considerable importance in PGE-ore forming events.
ISSN: 0024-4937

hal-00828626
https://hal.archives-ouvertes.fr/hal-00828626
DOI : 10.1016/j.lithos.2012.08.017

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We performed forward modelling of seismic anisotropy beneath a migrating strike-slip plate boundary to: (1) test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the San Andreas Fault and (2) constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle. The modelling involves five steps: (1) thermomechanical modelling, using a finite-element code, of the deformation field, (2) viscoplastic self-consistent modelling of the resulting olivine and pyroxene crystal preferred orientations, (3) calculation of the elasticity tensors for different domains of the finite elements (FEs) model, (4) forward modelling of seismic wave propagation through the model using ray theory, finite-frequency theory and a full-wave approach and (5) performing splitting measurements on the synthetic seismograms. SKS splitting data in central California are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the Mendocino Triple Junction. The westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the San Andreas Fault in central California. Comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two-layer models accounting for the observations in the vicinity of the San Andreas Fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary. The ability of different synthetic approaches to localize horizontally and vertically the plate boundary-related deformation differs significantly. Splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model. In contrast, splitting analysis on full-wave synthetics, which should be more representative of actual long-period SKS waves, results in smooth apparent lateral variations of anisotropy; the exact location and width of the plate boundary may only be retrieved by comparing fast-polarization profiles obtained using a multichannel analysis on waves with different periods.
ISSN: 0956-540X

hal-01236150
http://hal.univ-reunion.fr/hal-01236150
http://hal.univ-reunion.fr/hal-01236150/document
http://hal.univ-reunion.fr/hal-01236150/file/Bonnin_San_Andreas_modelling_GJI2012.pdf
DOI : 10.1111/j.1365-246X.2012.05650.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
We studied the deformation in a partially molten mantle through detailed petrostructural analysis of plagioclase lherzolites from the northwestern part of the central Lanzo massif. These peridotites display a pervasive anastomosed network of millimetre to centimetre-scale plagioclase-rich bands, which grades locally into a planar layering marked by an intercalation of plagioclase-rich and olivine-rich layers up to tens of centimetres wide. Both the anastomosed and the planar layering are subparallel to the deformation fabric, characterized by a foliation defined by the shape-preferred orientation of olivine and by crystal preferred orientation (CPO) of olivine, orthopyroxene, and clinopyroxene. This parallelism, the coexistence of microstructural evidence for melt-rock reactions and for deformation by dislocation creep, and the predominance of axial-[010] olivine CPO patterns, characterized by [010] axes clustered normal to the layering, indicate that deformation and the reactive melt percolation that formed the layering were coeval. Strong heterogeneity in mineral composition in the planar layering domains, with Fe-enrichment in olivine and spinel and highly variable Ti content in spinel in cm-scale plagioclase-rich bands, implies that the latter interacted with higher melt volumes. High Ti contents in pyroxenes and spinel in the anastomosed layering domains point to changes in melt composition, hinting to less effective melt transport. We propose therefore that the planar layering records melt segregation in layers parallel to the shear plane, whereas the anastomosed layering results from melt alignment along grain boundaries subparallel to the shear plane, without segregation. Finite strain in the different layers cannot be quantified, but comparison of the present observations with data from experiments and from other mantle outcrops displaying shear-controlled melt organization suggests that the transition from the anastomosed to the planar layering might record an increase in finite strain, that is, strain localization associated with variations in the instantaneous melt fraction.
ISSN: 0040-1951

hal-01053509
https://hal.archives-ouvertes.fr/hal-01053509
DOI : 10.1016/j.tecto.2014.01.007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study presents microstructural evidence for deformation-controlled melt organization and for changes in olivine deformation associated with the presence of melt in an 80 m vertical section of the crust-mantle transition zone in the Oman ophiolite. This zone represents an 'end member' case for analyzing feedbacks between deformation and melt distribution in the upper mantle, since it experienced strong shear strains in presence of large melt fractions. It is characterized by a subhorizontal compositional layering at the mm to meter scale, from weakly impregnated dunites to olivine-rich gabbros, which parallels a pervasive foliation containing a strong stretching lineation. The parallelism between the compositional layering and the foliation, the diffuse limits of the layers, the alignment of elongated plagioclase-rich aggregates devoid of internal deformation structures with the elongation of olivine crystals in the dunitic layers, and the sharp compositional changes across some, but not all layer limits suggest deformation plays an essential role on the development of the layering. The variation on a mm-scale of the olivine crystal preferred orientation (CPO) symmetry as a function of the modal content: from axial-[100] symmetry in layers with <70% modal olivine to axial-[010] in more gabbroic levels (<40% olivine), which is repeated over the entire section, implies deformation in presence of variable melt fractions. Axial-[100] olivine CPO in olivine-rich layers is consistent with deformation by dislocation creep under high temperature, low pressure, dry conditions. Axial-[010] olivine CPO patterns imply additional sliding along preferentially wetted (010) grain boundaries, increase in the activity of [001] glide, or transpression localized in the melt-rich layers. Since the change in CPO symmetry is not accompanied by dispersion, instantaneous melt fractions must have remained <30-40% in all layers. The continuous variation in olivine CPO symmetry with decreasing olivine content implies therefore that the former depends on the cumulated strain in presence of melt rather than on the instantaneous melt fraction.
ISSN: 0012-821X

hal-00795508
https://hal.archives-ouvertes.fr/hal-00795508
DOI : 10.1016/j.epsl.2012.10.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study presents new geochemical data on 26 mafic lavas from the Middle Atlas and Central Morocco volcanic provinces, including Miocene nephelinites and Pliocene-Quaternary (3.9-0.6Ma) nephelinites, basanites, alkali and subalkaline basalts. Most of them represent near-primary magmas, although some alkali basalts were derived from the minor fractionation of olivine and diopside phenocrysts. These evolved samples and the subalkaline basalt display higher 207Pb/204Pb and Zr/Nb ratios and lower εNd consistent with their contamination by lower crustal granulites during an open fractionation process. The progressive enrichment in incompatible elements observed from alkali basalts to nephelinites suggests their derivation from decreasing partial melting degrees of an enriched mantle source located at the garnet-spinel transition zone. The strong negative spikes observed for K in multielement patterns indicate that this source contained a residual pargasitic amphibole. We propose that partial melting occurred at around 2GPa, i.e. near the lithosphere-asthenosphere boundary beneath the Middle Atlas (60-80km).The trace element and isotopic Sr-Nd-Pb-Hf signature of the uncontaminated lavas displays a geochemical flavour intermediate between those of high μ (HIMU), "C", and enriched mantle components. It is very similar to that of abundant metasomatic amphibole- and clinopyroxene-rich lithospheric peridotites and pyroxenites carried by Middle Atlas lavas, which likely represent an analog of the source of these lavas. It is therefore not necessary to postulate the contribution of a "fresh" asthenospheric mantle to their genesis. We propose that they resulted from the partial melting of the base of a veined lithospheric mantle metasomatised during the late Cretaceous by alkaline melts from the Central Atlantic plume, the ancestor of the Canary plume. Melting was probably triggered by the flux of a hot mantle within a regional SW-NE sub-lithospheric channel, in response to either the Alboran slab retreat or edge-driven convection around the West African craton.
ISSN: 0024-4937

insu-01056523
https://hal-insu.archives-ouvertes.fr/insu-01056523
DOI : 10.1016/j.lithos.2014.07.009

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
Peridotite xenoliths erupted by alkali basaltic volcanoes in the western Pannonian Basin can be divided into two fundamentally contrasting groups. Geochemical characteristics of the abundant protogranular, porphyroclastic and equigranular nodules suggest that these samples originate from an old consolidated and moderately depleted lithospheric mantle domain. In contrast, the geochemical features of the worldwide rare, but in the Pannonian Basin relatively abundant, poikilitic xenoliths attest to a more complex evolution. It has been argued that the origin of the peculiar texture and chemistry may be intimately linked to melt/rock reactions at successively decreasing liquid volumes in a porous melt flow system. The most likely site where such reactions can take place is the asthenosphere-lithosphere boundary. In this context, poikilitic xenoliths may provide petrological and geochemical evidence for reactions between magmatic liquids issued from the uprising asthenosphere and the solid mantle rocks of the lithosphere. These reactions are important agents of the thermal erosion of the lithosphere; thus, they could have considerably contributed to the thinning of the lithosphere in the Pannonian region. We suggest that in the Pannonian Basin, there could be a strong relation between the unusual abundance of poikilitic mantle xenoliths and the strongly eroded lithosphere.
ISSN: 0010-7999

hal-01107421
https://hal.archives-ouvertes.fr/hal-01107421
DOI : 10.1007/s00410-014-1054-y

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : International audience
The Jurassic Mirdita ophiolite crops out in the western branch of the Hellenide-Dinaride ophiolite belt in the Balkan Peninsula; it represents a remnant of the Tethyan oceanic lithosphere developed between the Apulian and Pelagonian microcontinents. Emplaced during the Cretaceous on the western margin of Pelagonia, the Mirdita ophiolite was involved in southwestward thrusting of the Hellenide-Dinaride tectonic units during the Eocene alpine tectonics. It was little affected, however, by these alpine events as marked by nearly horizontal Cretaceous limestone deposits overlying the ophiolite. A continuous section of Middle Jurassic oceanic crust, including thin gabbros, a N-S-trending sheeted dike complex, and extrusive rocks, is exposed in the central part of the ophiolite and thickens eastward within a regional, N-S-trending synform. Peridotite massifs exposed along the western and eastern edges of this synform show major structural and petrological differences. The eastern ultramafic domain has a harzburgitic mantle exhibiting a high-temperature asthenospheric foliation dipping steeply to moderately to the west. Major chromite deposits are restricted to these eastern ultramafic massifs. The western ultramafic domain has a zoned mantle, with asthenospheric harzburgite exposed at its western margin, progressively replaced eastward by plagioclase-peridotites that were highly strained at low temperatures, and that are bounded by amphibole-peridotites along with crustal rocks. The occurrence of plagioclase is ascribed to melt impregnation processes that occurred immediately before or during intense lithospheric deformation. The gabbros in the eastern massifs are thicker (1-2 km and layered, whereas they are highly thin and discontinuous in the western massifs. Mantle peridotites of the western massifs locally are in direct contact with the overlying diabase and volcanic rocks along ductile shear zones. Development of epidote-amphibolite facies metamorphism in upper-crustal rocks was produced by hydrothermal alteration in the oceanic realm, and was intense and widespread in the western domain. The Mirdita ophiolite likely originated in a short-lived, narrow ocean basin, which was closed during the Middle Jurassic by eastward overthrusting of the western domain, and westward subduction of the eastern domain. This constrictional phase was followed by dextral oblique convergence between the Pelagonian and Apulian microcontinents. This model implies a parautochtonous origin of the Mirdita ophiolite between these two microcontinents.
ISSN: 0020-6814

hal-00456063
https://hal.archives-ouvertes.fr/hal-00456063
DOI : 10.1080/00206810902823982

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclasespinel websterites. The 'asthenospherized' part of the Ronda peridotite contains abundant layered websterites ('group C' pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitions towards their host peridotites. Previous studies have suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modelling study of a layered outcrop of group C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization meltrock reactions. Mg-number [=Mg/(Mg + Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (similar to 0.65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (>0.1 and >1, respectively) to balance Mg-Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front. The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convex-upward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)(N) vs (Sm/Yb)(N) diagram (where N indicates chondrite normalized). Based on (Ce/Nd)(N) values, two transient, enriched domains between the light REE (LREE)-depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxeniteperidotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions. The latter are probably volatile-rich, small-volume melt fractions residual after the refertilization reactions that generated group C pyroxenites, which migrated throughout the massifincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the 'layered granular' subdomain and 'plagioclase-tectonite' domain) are on average more fertile than the residual, 'coarse granular' subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.
ISSN: 0022-3530

hal-00412200
https://hal.archives-ouvertes.fr/hal-00412200
DOI : 10.1093/petrology/egn014

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 American Geophysical Union
Résumé : International audience
We have investigated the seismic anisotropy from SKS and SKKS wave splitting along a N-S profile perpendicular to the Pyrenean mountain belt. The line crossed the North Pyrenean Fault (NPF), which probably represents the plate boundary between the European and the Iberian plates. We observed large delay times δt (1.3 to 1.5 s) and uniform fast polarization directions ϕ (N100E) at three stations located on the Iberian plate. On the other hand, δt are smaller (0.5–1.0 s) and the ϕ scattered at stations located north of the NPF. The small scale variability, the relation with surface geology and the correlation of the δt with lithospheric thickness suggest that the anisotropy could be predominantly of lithospheric origin. However, an asthenospheric contribution cannot be ruled out. In the North Pyrenean Zone, an important thermal event before the Pyrenean compression has likely erased the former mantle fabric. The observed anisotropy is then likely related to the Pyrenean orogeny. On the Iberian plate, our splitting parameters are similar to those obtained at two stations located on Hercynian structures in Central Spain. This suggests that two contributions are likely superposed to that of the Pyrenean built up: one related to the Hercynian orogeny and the other one related to the rotation of the Iberian plate.
ISSN: 0094-8276

hal-01389118
http://hal.univ-reunion.fr/hal-01389118
http://hal.univ-reunion.fr/hal-01389118/document
http://hal.univ-reunion.fr/hal-01389118/file/barruol_pyr_GRL1995.pdf
DOI : 10.1029/94GL03225

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 : International audience
Interactions between deformation and melt percolation yield important consequences for the evolution of the mantle lithosphere, controlling its composition and mechanical behavior. In the Lherz massif (Pyrenees, France), the analysis of structural relationships between harzburgites, lherzolites and pyroxenites and of the crystal-preferred orientations (CPO) of olivine and pyroxenes highlights a strong feedback between percolation of basaltic melts and deformation under near-solidus conditions at the lithosphere–asthenosphere boundary. Elongated harzburgite bodies up to tens of meters wide, which are the remnants of an old lithospheric mantle, preserve a constant foliation. This foliation is locally outlined by cm-scale flattened websteritic lenses. At the contact with the enclosing lherzolites, the harzburgite foliation is crosscut by the lherzolites foliation and by cm-wide websterite bands parallel to the contact. Strain intensity in the lherzolites increases with distance to the harzburgites. Based on these observations, we propose that reactive percolation was synchronous to the deformation and propose that variations in instantaneous melt fraction, due to pyroxenes and spinel crystallization during reactive melt transport, guided strain localization. Accordingly, the observed decrease in olivine CPO intensity and change in CPO patterns from harzburgites to distal lherzolites are interpreted as recording changes in the relative contribution of dislocation glide and diffusion processes, which is ruled by a balance between the instantaneous melt fraction and the local strain rate. We also propose that the pervasive websteritic layering in the refertilized lherzolites may result from deformation-assisted melt segregation in a system with decreasing permeability due to refertilization reactions. Finally, we discuss the possible timing and geodynamical context of the refertilization episode.
ISSN: 0012-821X

hal-00411724
https://hal.archives-ouvertes.fr/hal-00411724
DOI : 10.1016/j.epsl.2008.07.053