Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Talc is a hydrous magnesium rich layered silicate that is widely disseminated in the Earth from the seafloor to over 100 km depth, in ultra-high pressure metamorphism of oceanic crust. In this paper we determine the single crystal elastic constants at pressures from 0 to 12 GPa of talc triclinic (C 1¯) and monoclinic (C2/c) polytypes using ab initio methods. We find that talc has an extraordinarily high elastic anisotropy at zero pressure that reduces with increasing pressure. The exceptional anisotropy is complemented by a negative Poisson's ratio for many directions in crystal space. Calculations show that talc is not only one of very few common minerals to exhibit auxetic behaviour, but the magnitude of this effect may be the largest reported so far for a mineral. The compression (Vp) and shear (Vs) wave velocity anisotropy is 80% and 85% for the triclinic polytype. At pressures where talc is known be stable in the Earth (up to 5 GPa) the Vp and Vs anisotropy is reduced to about 40% for both velocities, which is still a very high value. Vp is slow parallel to the c-axis and fast perpendicular to it. This remains unchanged with increasing pressure and is observed in both polytypes. The shear wave splitting (difference between fast and slow S-wave velocities) at low pressure has high values in the plane normal to the c-axis, with a maximum near the alow asterisk-axis in the triclinic and the b-axis in the monoclinic polytype. The c-axis is the direction of minimum splitting. The pattern of shear wave splitting does not change significantly with pressure. The volume fraction of talc varies between 11 and 41% for hydrated mantle rocks, but the lack of data on the crystallographic preferred orientation (CPO) precludes a detailed analysis of the impact of talc on seismic anisotropy in subduction zones. However, it is highly likely that CPO can easily develop in zones of deformation due to the platy habit of talc crystals. For random aggregates of talc, the isotropic Vp, Vs and Vp/Vs ratio have significantly lower values than those of antigorite and may explain low-velocity regions in the mantle wedge. Vp/Vs ratios are more complex in anisotropic media because there are fast and slow S-waves, resulting in Vp/Vs1 and Vp/Vs2 ratios for every propagation direction, making interpretation difficult in deformed polycrystalline talc with a CPO. Talc on the subduction plate boundary can strongly influence guided wave velocity as CPO would develop in this region of intense shearing. The very low coefficient of friction (< 0.1) of talc above 100 °C could also explain silent earthquakes at shallow depths (ca 30 km) along the subduction plate boundaries, frequently responsible for tsunami.
ISSN: 0012-821X

hal-00411481
https://hal.archives-ouvertes.fr/hal-00411481
DOI : 10.1016/j.epsl.2008.07.047

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg+Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr+Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites,as their formation requires hot, hydrous, shallow mantle (> 1250 degrees C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880-1030 degrees C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions. (c) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00412594
https://hal.archives-ouvertes.fr/hal-00412594
DOI : 10.1016/j.epsl.2008.06.014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Seismic tomography and high surface heat flow models indicate an anomalously thin and hot lithospheric mantle beneath the Canadian Cordillera. To understand the thermal and deformational history of the lithospheric mantle under these conditions, we performed a study of the microstructures, crystal preferred orientations (CPO), and seismic anisotropy of mantle xenoliths collected in the Summit Lake volcano in the southern Canadian Cordillera. The xenoliths vary in composition between spinel lherzolites, cpx-rich lherzolites, and wehrlites. All samples are coarse-grained and show clear evidence for annealing, but deformation microstructures are well-preserved in the spinel lherzolites. Olivine CPO is characterized by a strong point-maximum of [100] and by a girdle distribution of [010[ and [001] axes normal to it; CPO strength is variable, spinel-lherzolites have the strongest CPO. Some samples show a weak orthopyroxene CPO consistent with the olivine one. However most pyroxenes have random CPO, interstitial shapes and undeformed appearance, which suggest a secondary origin by reactive melt percolation under static conditions, leading to crystallization of cpx + sp +/- opx at the expenses of olivine, followed by crystallization of cpx + sp +/- ol at the expenses of opx to produce the wehrlites. The studied xenoliths have typical upper mantle seismic properties patterns with fastest propagation of P-waves and polarization of S-waves parallel to the olivine [100] maxima. Maximum P- and S-waves anisotropy for lherzolites are 8 and 5% respectively. Dispersion of the olivine CPO due to neocrystallization of olivine results in weaker anisotropies (6.5 and 3.5%) in wehrlites. Seismic velocities calculated using a geotherm consistent with the equilibrium temperatures of the Summit Lake xenoliths are 3-4% lower than those expected in a normal, 100 km-thick lithosphere, consistent with seismic tomography models for the region. This high geothermal gradient may be partially explained by considering the anisotropy of thermal conductivity of olivine and subhorizontal foliations in the lithospheric mantle, which result in a decrease by up to 15% of the thermal conductivity in the vertical direction.
ISSN: 0040-1951

hal-00618332
https://hal.archives-ouvertes.fr/hal-00618332
DOI : 10.1016/j.tecto.2011.04.014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
To quantify the relationships between anisotropy. S-wave splitting and tectonics, we determined the seismic properties of lower-crustal and upper-mantle rocks outcropping in the lvrea Zone (Northem Italy). We obtained P-and S-wave seismic velocities by laboratory direct velocity measurements and/or by calculations based on the modal compositions of the rocks, the lattice preferred orientations (LPOs), and the single crystal stiffness coefficients. Measured P-and S-wave velocities (6.0-7 .5 km s-1 and 3.6-4.2 km s-1) are typical of the lower crust. The P-wave anisotropy is in the range 0-l 0%. Shear-wave birefringence is in the range 0.0-0.6 km s-1 , with typical values between 0.0 and 0.2 km s-1 • In many cases, the birefringence is clearly related to fabric elements (foliation, lineation). Mafic rocks such as anorthosite or pyroxene-bearing gabbros exhibit low P-wave anisotropies (< 5%) and low shear-wave birefringences (less than 0.1 km s-1). In contrast, the seismic properties of felsic rocks such as biotite-bearing gneisses and mafic rocks such as amphibolites display high V P anisotropy (10%) and strong birefringence (0.3 km s-1). Biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear-wave splitting. In these rocks, maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane. To have an overview of the seismic properties of this lower-crustal section at a broader scale, we calculated from our data the anisotropie seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually. For instance, the average lower-crustal sample displays an anisotropy of 5.5% for P waves and a birefringence around 0.14 km s I for S waves propagating parallel to the foliation. We observe little splitting for waves propagating at high angle to the foliation.
ISSN: 0031-9201

hal-01389712
http://hal.univ-reunion.fr/hal-01389712
http://hal.univ-reunion.fr/hal-01389712/document
http://hal.univ-reunion.fr/hal-01389712/file/Barruol_Kern-PEPI96_hal.pdf
DOI : 10.1016/0031-9201(95)03124-3

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Melt generation in the continental crust is thought to significantly influence seismic bulk velocities and anisotropy, although existing laboratory data provide limited constraints on such seismic attributes. In this study we measured ultrasonic compressional wave speeds (Vp) and anisotropy during sample compaction, mica-breakdown and melt-generation in synthetic, foliated quartz–muscovite aggregates. Measurements were performed at peak conditions of 300 MPa hydrostatic confining pressure and 750 °C, over a six hour time period, for three separate samples where wave propagation directions were aligned at 0°, 45° and 90° with respect to the foliation plane. The experiments are initially marked by sample compaction and rapid reduction in porosity (from 25% to <2 vol%), with a corresponding increase in Vp. During initial stages of the experiment Vp ranges from 4.65 to 5.45 km/s, depending on the direction of the P wave propagation, resulting in up to ∼16% anisotropy, which originates mainly from the preferred orientation of muscovite and possible incipient melt lenses oriented parallel to foliation. As the experiment progresses the bulk Vp decreases gradually and Vp anisotropy drops from ∼16% to ∼2% after five hours into the experiment. The drop in anisotropy is caused by muscovite-breakdown and increasing amount of partial melting of the aggregate. The bulk velocities are considerably lower than predicted for a mixture of muscovite and β-quartz (stable at 300 MPa and 750 °C). Thermodynamic modeling indicates that mica breakdown and production of melt with ∼2 wt% dissolved water in the aggregate can explain the low Vp, even though β-quartz is stable; the modeled Vp is ∼4.75 km/s at 300 MPa and 750 °C. These ultrasonic attributes signify movement of the melt to fill vacant pores and coating grain boundaries, effectively connecting melt films and reducing seismic anisotropy. The results may apply to regions where crustal melting is on-going, in particular regions of elevated temperature that are only weakly deforming, with coupled anomalously low bulk velocities and seismic anisotropy.
ISSN: 0012-821X

hal-01243569
https://hal.archives-ouvertes.fr/hal-01243569
DOI : 10.1016/j.epsl.2015.05.039

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The association of experimental data showing that the plastic deformation of olivine, the main constituent of the upper mantle, is highly anisotropic and the ubiquitous seismic anisotropy in the upper mantle, which indicates that olivine crystals show coherent orientations over scales of tens to hundreds of kilometers, implies that the long-term deformation in the upper mantle is anisotropic. We propose a multiscale approach, based on a combination of finite element and homogenization techniques, to model the deformation of a lithospheric plate while fully considering the mechanical anisotropy stemming from a strain-induced orientation of olivine crystals in the mantle. This multiscale model explicitly takes into account the evolution of crystal preferred orientations (CPO) of olivine and of the mechanical anisotropy during the deformation. We performed a series of numerical experiments simulating the uniaxial extension of a homogeneous (100% olivine) but anisotropic plate to test the role of the olivine CPO on the plate mechanical behavior and the link between CPO and mechanical anisotropy evolution. Even for this simple solicitation, different orientations and intensity of the initial olivine CPO result in variable plate strengths and deformation regimes. A plate with an initial CPO where the olivine [100] and [010] axes are concentrated at 45 degrees to the extension direction has high resolved shear stresses on the easy (010)[100] and (001)[100] slip systems of olivine. This results in low strength and in deformation by transtension. Plates with an initial CPO where the maximum of [100] axes is parallel or normal to the extension direction show a high initial strength. Isotropic plates have an intermediate behavior. The progressive rotation of olivine [100] axes toward the imposed stretching direction results in hardening in all models, except in those characterized by an initial concentration of olivine [100] axes normal to the imposed extension, in which softening is followed by hardening.
ISSN: 1525-2027

hal-00420063
https://hal.archives-ouvertes.fr/hal-00420063
DOI : 10.1029/2009GC002423

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
The variation of elastic- wave velocities as a function of the direction of propagation through the Earth's interior is a widely documented phenomenon called seismic anisotropy. The geometry and amount of seismic anisotropy is generally estimated by measuring shearwave splitting, which consists of determining the polarization direction of the fast shear- wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleseismic fast shear- wave component is oriented generally parallel to the strike of the trench(1), although a few exceptions have been reported (Cascadia(2) and restricted areas of South America(3,4)). The interpretation of shear- wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide(1). Here we show that the amount and the geometry of seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger- scale vertical layering consisting of dry and hydrated crust - mantle sections whose spacing is several times smaller than teleseismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear- wave splitting patterns in most subduction zones.
ISSN: 0028-0836

hal-00412016
https://hal.archives-ouvertes.fr/hal-00412016
DOI : 10.1038/nature07376

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Constraining mantle deformation beneath plate boundaries where plates interact with each other, such as beneath active or halted mountain belts, is a particularly important objective of "mantle tectonics" that may bring a depth extent to the Earth's surface observation. Such mantle deformation can be mapped at scale lengths of several tens of kilometers through the analysis of seismological data and particularly by mapping seismic anisotropy from the splitting analysis of vertically-propagating SKS waves that largely reflect the strain-induced crystal preferred orientations of the rock-forming minerals within the upper mantle. In the present study, we analyse data from approximately 50 broadband seismic stations covering the Western Alps and we provide a coherent picture of upper mantle anisotropy beneath the belt. The large-scale anisotropy pattern is characterized by fast split directions that closely follow the trend of the belt. Moreover, the maximum anisotropy magnitude is not located beneath the internal zones of the belt but instead beneath external units. All suggests that the anisotropy is likely dominated by sublithospheric mantle deformation. We propose that the observed anisotropy pattern can be explained by recent or active mantle flow around the Eurasian slab presently plunging beneath the inner parts of the Alps.
ISSN: 0012-821X

hal-00617828
https://hal.archives-ouvertes.fr/hal-00617828
https://hal.archives-ouvertes.fr/hal-00617828/document
https://hal.archives-ouvertes.fr/hal-00617828/file/Barruol_SKS_Alpes_EPSL2011_hal.pdf
DOI : 10.1016/j.epsl.2010.12.040

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We use first first-principle methods and the Peierls-Nabarro model to evaluate the resistance to glide, characterized by the Peierls stress, of glide systems for end-member MgSiO3 Perovskite at mantle pressures. [010](100) is the easiest glide system in Mg-Perovskite at all pressures. Peierls stresses increase systematically with pressure for all systems except [001](010), indicating the importance of lattice friction at lower mantle pressures. The ratio of the maximum Peierls stress for each system relative to the [010](100) value defines their critical resolved shear stress (CRSS). These CRSS are used in a visco-plastic self-consistent homogenization model to predict the evolution of crystal preferred orientations (CPO) during deformation of polycrystalline Mg-Perovskite. In axial compression, [100] tends to align with the compression direction, in agreement with in situ observations in axial compression experiments. In simple shear, [010] concentrates near the shear direction and (100), although more dispersed, tends to align near the shear plane, consistent with the dominant activity of the easier [010](100) system. The calculated seismic anisotropy for a 100% Mg-Perovskite aggregate using the CPO in simple shear and the elastic constants Of MgSiO3 perovskite at lower mantle pressures and temperatures is weak (>3% for P-waves with and >2% for S-waves) and decreases with increasing temperature and pressure. P-waves show the fastest propagation parallel to the lineation and S-waves fast polarization is in the foliation at 38 GPa and normal to the lineation at 88 GPa. This weak anisotropy is consistent with global seismological observations of a nearly isotropic lower mantle. There are however two regions where strain-induced Mg-Perovskite CPO could contribute to anisotropy; a) low temperature regions in the uppermost lower mantle, where the predicted S-wave polarization anisotropy may attain 1.6% with a fast polarization parallel to the foliation, b) in high high-temperature domains in the D '' layer, where Mg-Perovskite may be the major stable phase, leading to polarization of fast S-waves normal to the lineation for propagation directions at high angle to the lineation and an apparent isotropy for all other propagation directions.
ISSN: 0012-821X

hal-00411460
https://hal.archives-ouvertes.fr/hal-00411460
DOI : 10.1016/j.epsl.2008.03.058

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We explore the relations between deformation, annealing, and melt percolation during rifting and the effect of these processes on seismic anisotropy by analyzing the microstructures and crystal preferred orientations (CPO) in a suite of mantle xenoliths from Mega, in the southern end of the Ethiopian rift. Previous geochemical studies on these xenoliths showed evidence for interactions with variable melt types and volumes during the rifting process. The peridotites have dominantly coarse-porphyroclastic microstructures, but coarse granular or partially recrystallized microstructures also occur. The olivine CPO, characterized by orthorhombic to fiber-[100] patterns and moderate intensities, the common occurrence of (100) tilt walls, and the predominance of <0vw> rotation axes accommodating low angle misorientations in olivine support deformation by dislocation creep with dominant activation of the [100](010) system. Annealing (static recrystallization) of variable intensity followed this deformation. Modal enrichment in pyroxenes in > 60% of the studied peridotites corroborates extensive, but spatially heterogeneous reactive melt percolation leading to refertilization of the lithospheric mantle beneath the southern Ethiopian rift. The common interstitial shapes of the pyroxenes and lack of correlation between the pyroxenes and the olivine CPOs in many samples suggest that part of the refertilization is post-kinematic. However, there is no simple relation between reactive melt percolation and annealing of the olivine deformation microstructure. Comparison with data from other xenolith localities points to changes in the metasomatic imprint in the lithospheric mantle along the East African rift system correlated with the evolution in the rift maturity. Seismic properties averaged over all samples show typical lithospheric mantle patterns with fast propagation of P- and polarization of the fast S-waves parallel to the lineation. The anisotropy is moderate (< 6% for P-waves and < 4% for S-waves), implying that aligned melt must contribute to the strong seismic anisotropy measured in the Main Ethiopian rift, north of Mega.
ISSN: 0040-1951

hal-01355918
https://hal.archives-ouvertes.fr/hal-01355918
DOI : 10.1016/j.tecto.2016.05.027

Éditeur(s) : HAL CCSD Springer-Verlag
Résumé : The seismology is the most effective method to explore the Structure of subduction zones to great depth. The distinguishing feature of the mantle in the subduction regions is the presence of hydrated phases, which transport water into the Earth's interior and release it with dramatic local consequences, triggering earthquakes and melting. The seismological detection of these hydrous phases and geo-dynamic interpretation of flow in the hydrous mantle depend on knowledge of the anisotropic elastic properties and the characteristics of the wave propagation in anisotropic media. In this paper we briefly recall the distinguishing features of anisotropic wave propagation and the observable parameters. We suggest the Vp/Vs ratio is a physically sound parameter than can be observed by seismology in anisotropic regions of the Earth, whereas the Poisson's ratio, which is often quoted, is not directly observable and does not correspond to the characteristics of wave propagation in an anisotropic or isotropic medium. We report for the first time the ratios of Vp/Vs1 and Vp/Vs2, where Vs1 is the fastest and Vs2 slowest S-wave velocities of an anisotropic media. We present the current knowledge of the anisotropic seismic properties of hydrous minerals in the upper mantle, transition zone, and lower mantle that are stable along low temperature geotherms associated with subduction, and identify which minerals are likely to influence seismological observations because they have very high volume fractions, or very high anisotropies, or both of these.;In the upper mantle antigorite and talc are exceptionally anisotropic (Vp 71% Vs 68% and Vp 65%, Vs 68%, respectively) and chlorite is also very anisotropic for S-waves (Vp 35%, Vs 76%). Comparatively less anisotropic are hornblende (Vp 27%, Vs 31%), used as proxy for tremolite in subduction zones, and clinohumite (21.8%, Vs 15.9%). Brucite at 4 GPa (Vp 26.5%, Vs 30.9%) and the dense hydrogen magnesium silicate (DHMS) phase A at 9GPa (Vp 9.3%, Vs 17.6%) are the only hydrous minerals stable in the upper mantle that have had their elastic properties measured at in situ mantle pressures. Except for the phase A, all these minerals are more anisotropic than olivine for at least one parameter (Vp, Vs, Vp/Vs1, or Vp/Vs2). In the transition zone the major phases hydrous wadsleyite and ringwoodite C have moderate to weak anisotropies (Vp 16.3%, Vs 16.5%, and Vp 1.9%, Vs 4.4%, respectively). The DHMS Superhydrous B has a moderate anisotropy (Vp 6.9%, Vs 11.6%). At greater depth the DHMS phase D Lit 24 GPa (Vp 10.8%, Vs 18.0%) is the only hydrous phase that can transport hydrogen from the transition zone into the lower mantle. The phase D is more anisotropic for S than P waves like many of the hydrous phases. From these data it is clear that hydrous phases are in general very anisotropic. However, pressure can play a strong role in reducing anisotropy. It is the case for brucite and talc, in which increasing pressure from ambient to 4 GPa reduces the anisotropy by about 50% for both P and S waves. In contrast, the anisotropy of the DHMS phase A does not change significantly with pressure.;Our picture of the seismic anisotropy of hydrated minerals remains incomplete; the elastic properties of many have not been measured even at ambient conditions (e.g., 10 angstrom phase and phase E) or not measured in their true elastic symmetry (e.g., clinochlore). The majority of hydrous minerals have not been measured at high pressure. and none related to hydrated mantle have been measured at elevated temperature. We have shown in the few cases where hydrous minerals have been measured as a function of pressure, that this variable has an important effect oil the velocity distribution and in most cases reduces the degree of anisotropy, hence we Would expect seismic anisotropy to play a key role in the determination of the shallow structure of subduction zones in the upper mantle.
Subduction zone geodynamics

hal-00420893
https://hal.archives-ouvertes.fr/hal-00420893

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Upper mantle anisotropy beneath the African IRIS and Geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as SKS, SKKS and PKS phases. Seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station, but each station is of potential interest since it can be used to retrieve source-side seismic anisotropy at remote sites if one is able to perform station-side anisotropy correction. We performed systematic investigations of teleseismic shear wave splitting at 15 stations from the IRIS and Geoscope global seismic networks, which are located on both the oceanic and the continental parts of the African plate. Anisotropy is generally well observed at continental stations. The patterns we present generally show much more complexity than the results previously published from smaller data sets. Despite this complexity, the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting (rift structures, Archaean craton, Pan-African belt), although a deeper source of anisotropy (asthenospheric) may be present. At the oceanic stations, anisotropy measurements are much more difficult to perform because the signal is generally of poor quality. MSEY, in the Seychelles (Indian ocean), is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion, as defined by hotspot tracks.
ISSN: 0956-540X

hal-01380389
https://hal.archives-ouvertes.fr/hal-01380389
https://hal.archives-ouvertes.fr/hal-01380389/document
https://hal.archives-ouvertes.fr/hal-01380389/file/Barruol_SKS_afric_GJI2001.pdf
DOI : 10.1046/j.0956-540x.2001.01481.x

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal-preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S2 wave propagation directions and the polarization of fast split shear waves (S1) are always subparallel to olivine [100] axes of maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P wave azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and the maximum S wave polarization anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km-thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for five endmember orientations of the foliation and lineation. Comparison to seismic anisotropy data from the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies, and the low azimuthal anisotropy with with the horizontally polarized S waves (SH) faster than the vertically polarized S wave (SV) measured using surface waves are best explained by homogeneously dipping (45°) foliations and lineations in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW-SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the seismological data. The strong compositional heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3% variation in density and in up to 2.3% variation of Vp, Vs, and Vp / Vs ratio. Fe depletion by melt extraction increases Vp and Vs, but decreases the Vp / Vs ratio and density. Orthopyroxene enrichment due to metasomatism decreases the density and Vp, strongly reducing the Vp / Vs ratio. Garnet enrichment, which was also attributed to metasomatism, increases the density, and in a lesser extent Vp and the Vp / Vs ratio. Comparison of density and seismic velocity profiles calculated using the xenoliths' compositions and equilibration conditions to seismological data in the Kaapvaal highlights that (i) the thickness of the craton is underestimated in some seismic studies and reaches at least 180 km, (ii) the deep sheared peridotites represent very local modifications caused and oversampled by kimberlites, and (iii) seismological models probably underestimate the compositional heterogeneity in the Kaapvaal mantle root, which occurs at a scale much smaller than the one that may be sampled seismologically.
ISSN: 1869-9510

hal-01053516
https://hal.archives-ouvertes.fr/hal-01053516
DOI : 10.5194/se-5-45-2014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We investigated the response of the crystal structure, lattice parameters, and unit-cell volume of hydrous layered silicate phlogopite at conditions relevant to subduction zone settings. We have used first principles simulation based on density functional theory to calculate the equation of state and full elastic constant tensor. Based on the generalized gradient approximation, the full single crystal elastic constant tensor with monoclinic symmetry shows significant anisotropy with the compressional elastic constants: c11 = 181 GPa, c22 = 185 GPa, c33 = 62 GPa, the shear elastic constants c44 = 14 GPa, c55 = 20 GPa, c66 = 68 Ga, and c46 = −6 GPa; the off diagonal elastic constants c12 = 48 GPa, c13 = 12 GPa, c23 = 12 GPa, c15 = −16 GPa, c25 = −5 GPa and c35 = −1 GPa at zero pressure. The elastic anisotropy of phlogopite is larger than most of the layered hydrous phases relevant in the subduction zone conditions. The shear anisotropy, AVS for phlogopite is ∼77% at zero pressure condition and although it decreases upon compression it remains relatively high compared to other hydrous phases relevant in the subduction zone settings. We also note that the shear elastic constants for phlogopite are relatively low. Phlogopite also has a high isotropic bulk VP/VS ratio ∼2.0. However, the VP/VS ratio also exhibits significant anisotropy with values as low as 1.49. Thus, phlogopite bearing metasomatized mantle could readily explain unusual VP/VS ratio as observed from seismological studies from the mantle wedge regions of the subduction zone.
ISSN: 0031-9201

hal-01115670
https://hal.archives-ouvertes.fr/hal-01115670
DOI : 10.1016/j.pepi.2014.05.004

Éditeur(s) : HAL CCSD American Geophysical Union (AGU) AGU and the Geochemical Society
Résumé : International audience
To study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber-[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P wave velocities of the peridotites decrease weakly with depth. S wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P and S waves anisotropy is variable (2–12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]-fiber CPO patterns, if foliations are horizontal, vertically propagating S waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy-induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.
ISSN: 1525-2027

hal-01116428
https://hal.archives-ouvertes.fr/hal-01116428
DOI : 10.1002/2014GC005452

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study analyses the interactions between deformation and reactive fluid and melt percolation, and their effects on sub-arc mantle seismic properties based on microstructural observations on mantle xenoliths extracted by the Tubaf and Edison seamounts close to the Lihir Island, in the Papua New Guinea archipelago. These xenoliths sample an oceanic lithosphere, which has experienced high-temperature deformation in the presence of fluids or melts. This was followed by metasomatism under static conditions. Syn-kinematic percolation of reactive Si-rich melts or fluids in peridotites has produced pyroxene-enrichment, grain size reduction, and dispersion of olivine crystal preferred orientation (CPO). Fourier transform infrared spectroscopy analyses show that olivine has very low water contents (1-4 wt. ppm H2O), similar to spinel peridotites from other subduction zones. These low values may record both low water solubility in olivine at low pressure and dehydration during transport and exhumation. Water contents in pyroxenes are highly variable and likely result from spatially heterogeneous melt or fluid percolation. Analysis of olivine CPO indicates dominant activation of both (010)[100] and (001)[100] slip systems, which are characteristic of deformation under high temperature, low stress, low pressure and low to moderate hydrous conditions. Fast S-wave polarization and P- and Rayleigh propagation directions are thus parallel to the mantle flow direction. The pyroxene enrichment by melt-rock reactions is accompanied by dispersion of olivine CPO and induces a significant decrease of the maximum S-wave and P-wave anisotropy in the peridotites. The calculated seismic properties also show that the lowest Vp/Vs ratios (< 1.7) mapped in fore-arc mantle may only be explained by taking in consideration the CPO-induced elastic anisotropy of the peridotites.
ISSN: 0040-1951

hal-00950058
https://hal.archives-ouvertes.fr/hal-00950058
DOI : 10.1016/j.tecto.2013.09.024

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In the southeastern corner of the Tyrrhenian basin, in the central Mediterranean Sea, a tight alignment of earthquakes along a well-defined Benioff zone marks one of the narrowest active trenches worldwide, where one of the last fragment of the former Tethys ocean is consumed. Seismic tomography furnishes snapshot images of the present-day position of this slab, and seismic anisotropy allows to reconstruct the past kinematics of the subduction process. Using seismic anisotropy fast directions as a proxy for the present and past mantle flow, we look backward for the seismic traces of the slab motion through the western-central Mediterranean mantle, from the starting locus of subduction toward its present day position. The result of combining independent data sets provides a coherent pattern of anisotropy that illustrates an example of slab rollback from its initiation point to its present-day position.
ISSN: 0012-821X

hal-01389115
http://hal.univ-reunion.fr/hal-01389115
http://hal.univ-reunion.fr/hal-01389115/document
http://hal.univ-reunion.fr/hal-01389115/file/Lucente_medit_EPSL2006_hal.pdf
DOI : 10.1016/j.epsl.2005.10.041

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Upper-mantle flow beneath the South Pacific is investigated by analysing shear wave splitting parameters at eight permanent long-period and broad-band seismic stations and 10 broad-band stations deployed in French Polynesia from 2001 to 2005 in the framework of the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). Despite the small number of eventsand the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity, upper-mantle seismic anisotropy has been detected at all stations except at Tahiti where two permanent stations with 15 yr of data show an apparent isotropy. The median value of fast polarization azimuths (N67.5°W) is parallel to the present Pacific absolute platemotion direction in French Polynesia (APM: N67°W). This suggests that the observed SKS fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving Pacific Plate and preserved in the lithosphere as the plate cools. However, analysis of individualmeasurements highlights variations of splitting parameters with event backazimuth that imply an actual upper-mantle structure more complex than a single anisotropic layer with horizontal fast axis. A forward approach shows that a two-layer structure of anisotropy beneath French Polynesia better explains the splitting observations than a single anisotropic layer. Second ordervariations in the measurements may also indicate the presence of small-scale lateral heterogeneities. The influence of plumes or fracture zones within the studied area does not appear to dominate the large-scale anisotropy pattern but may explain these second-order splitting variations across the network.
ISSN: 0956-540X

hal-01249275
https://hal.archives-ouvertes.fr/hal-01249275
https://hal.archives-ouvertes.fr/hal-01249275/document
https://hal.archives-ouvertes.fr/hal-01249275/file/GJI2007SKS.pdf
DOI : 10.1111/j.1365-246X.2007.03475.x

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Peridotite xenoliths brought up to the surface by the volcanism of the Kerguelen Islands represent a mantle that has been affected by a high degree of partial melting followed by intense melt percolation above the Kerguelen plume. These xenoliths are therefore particularly suitable to investigate effects of melt-rock interaction on crystallographic fabrics (lattice-preferred orientation (LPO)) of peridotite minerals and on the LPO-induced seismic properties of peridotites above a mantle plume. We have studied a suite of 16 ultramafic samples representative of different degrees of partial melting and magma-rock interaction among which the protogranular harzburgites are the least metasomatised xenoliths and dunites are the ultimate stage of metasomatism. Olivine LPO is characterized by high concentration of [010] axes perpendicular to the foliation and [100] axes close to the lineation or distributed in the foliation plane in harzburgites, whereas the high concentration of [100] axes is parallel to the lineation and [010] axes is perpendicular to the assumed foliation in dunites. Olivine LPO in harzburgites is interpreted as being due to a deformation regime in axial compression or transpression. The fabric strength of olivine decreases progressively from protogranular to poikilitic harzburgites and finally to dunites, for which it remains nevertheless significant (J index 3.8). Seismic properties calculated from LPO of minerals indicate that metasomatism at higher melt/rock ratio lowers the P wave velocities. The most significant difference between harzburgites and dunites corresponds to the distribution of S wave anisotropy. Harzburgites display the maximum of anisotropy within the foliation plane and the minimum of anisotropy perpendicular to the foliation plane, whereas the lowest anisotropy is parallel to the lineation for dunites. These modifications of seismic properties as a result of metasomatic processes may induce seismic heterogeneities in the mantle above the Kerguelen plume. In addition, assuming a lithospheric mantle primarily harzburgitic and structured with a horizontal foliation, the seismic properties calculated for the Kerguelen xenoliths reconcile the rather high anisotropy evidenced by the horizontally propagating surface waves with the apparent isotropy revealed by the absence of splitting of vertically propagating teleseismic SKS waves recorded by the GEOSCOPE Kerguelen station.
ISSN: 1525-2027

hal-00366222
https://hal.archives-ouvertes.fr/hal-00366222
https://hal.archives-ouvertes.fr/hal-00366222/document
https://hal.archives-ouvertes.fr/hal-00366222/file/bascou_Kerguelen_2007GC001879_hal.pdf
DOI : 10.1029/2007GC001879

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica. However, naturally deformed micaceous rocks commonly show composite (e.g. S-C) fabrics. It is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields. An outcrop analogue for granitic mid-crustal deformed zones is used here to calibrate the seismic response against both parameters. Seismic responses are modelled using crystallographic preferred orientations for polymineralic, micaceous granitic gneisses, measured using Electron Back-Scatter Diffraction. The sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation, so-called rock and fabric recipes. The maximum P- and S-anisotropy are calculated at 16.6% and 23.9% for single-foliation gneisses but for mixed (i.e. S-C) foliation gneisses these values reduce to 5.8% and 7.5% respectively. Furthermore, mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientation of fabrics in nature. can generate substantial variations in the orientation and magnitude of seismic anisotropy (especially for shear waves) as measured for the continental crust using existing receiver function and teleseismic near-vertical incidence methods. Thus, maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth-varying deformation kinematics and tectonic decoupling.
ISSN: 0012-821X

hal-00445247
https://hal.archives-ouvertes.fr/hal-00445247
DOI : 10.1016/j.epsl.2009.09.035