Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities (Alps, Norway, Mali and eastern China) through the analysis of their microstructures and lattice-preferred orientations (LPO). These samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions. Omphacite and garnet LPO were determined from electron backscatter diffraction (EBSD) technique. Garnet appears to be almost randomly oriented whereas omphacite develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the (010)-poles concentrated sub-perpendicular to the foliation. In order to analyze the deformation mechanisms that produced such omphacite LPO, we compare our observations to LPO simulated by viscoplastic self-consistent numerical models. A good fit to the measured LPO is obtained for models in which the dominant slip systems are 1/2h110i{11 ¯ 0}, [001] {110} and [001] (100). Dominant activation of these slip systems is in agreement with TEM studies of naturally deformed omphacite. Seismic properties of eclogite are calculated by combining the measured LPO and the single crystal elastic constants of omphacite and garnet. Although eclogite seismic anisotropies are very weak (less than 3% for both P-and S-wave), they are generally characterized by a maximum P-wave velocity sub-parallel to the lineation and by a minimum velocity approximately normal to foliation. The mean P-and S-wave velocities are high (respectively, 8.6 and 4.9 km/s). The S-wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation. Calculated reflection coefficients show that an eclogite/crust interface is generally a good reflector (Rc > 0.1), whereas an eclogite body embedded in the upper mantle would be hardly detectable.
ISSN: 0040-1951

hal-01389715
http://hal.univ-reunion.fr/hal-01389715
http://hal.univ-reunion.fr/hal-01389715/document
http://hal.univ-reunion.fr/hal-01389715/file/Bascou_eclo-sismo_tphy2001_hal.pdf
DOI : 10.1016/S0040-1951(01)00156-1

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This paper aims to present an overview on the influence of rheological heterogeneity and mechanical anisotropy on the deformation of continents. After briefly recapping the concept of rheological stratification of the lithosphere, we discuss two specific issues: (1) as supported by a growing body of geophysical and geological observations, crust=mantle mechanical coupling is usually efficient, especially beneath major transcurrent faults which probably crosscut the lithosphere and root within the sublithospheric mantle; and (2) in most geodynamic environments, mechanical properties of the mantle govern the tectonic behaviour of the lithosphere. Lateral rheological heterogeneity of the continental lithosphere may result from various sources, with variations in geothermal gradient being the principal one. The oldest domains of continents, the cratonic nuclei, are characterized by a relatively cold, thick, and consequently stiff lithosphere. On the other hand, rifting may also modify the thermal structure of the lithosphere. Depending on the relative stretching of the crust and upper mantle, a stiff or a weak heterogeneity may develop. Observations from rift domains suggest that rifting usually results in a larger thinning of the lithospheric mantle than of the crust, and therefore tends to generate a weak heterogeneity. Numerical models show that during continental collision, the presence of both stiff and weak rheological heterogeneities significantly influences the large-scale deformation of the continental lithosphere. They especially favour the development of lithospheric-scale strike-slip faults, which allow strain to be transferred between the heterogeneities. An heterogeneous strain partition occurs: cratons largely escape deformation, and strain tends to localize within or at the boundary of the rift basins provided compressional deformation starts before the thermal heterogeneity induced by rifting are compensated. Seismic and electrical conductivity anisotropies consistently point towards the existence of a coherent fabric in the lithospheric mantle beneath continental domains. Analysis of naturally deformed peridotites, experimental deformations and numerical simulations suggest that this fabric is developed during orogenic events and subsequently frozen in the lithospheric mantle. Because the mechanical properties of single-crystal olivine are anisotropic, i.e. dependent on the orientation of the applied forces relative to the dominant slip systems, a pervasive fabric frozen in the mantle may induce a significant mechanical anisotropy of the whole lithospheric mantle. It is suggested that this mechanical anisotropy is the source of the so-called tectonic inheritance, i.e. the systematic reactivation of ancient tectonic directions; it may especially explain preferential rift propagation and continental break-up along pre-existing orogenic belts. Thus, the deformation of continents during orogenic events results from a trade-off between tectonic forces applied at plate boundaries, plate geometry, and the intrinsic properties (rheological heterogeneity and mechanical anisotropy) of the continental plates.
ISSN: 0040-1951

hal-01389719
http://hal.univ-reunion.fr/hal-01389719
http://hal.univ-reunion.fr/hal-01389719/document
http://hal.univ-reunion.fr/hal-01389719/file/vauchez_etal_rheology_1998_hal.pdf
DOI : 10.1016/S0040-1951(98)00137-1

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 Elsevier
Résumé : International audience
The presence of two regional seismic networks in southeastern France provides us high-quality data to investigate upper mantle flow by measuring the splitting of teleseismic shear waves induced by seismic anisotropy. The 10 three-component and broadband stations installed in Corsica, Provence, and western Alps efficiently complete the geographic coverage of anisotropy measurements performed in southern France using temporary experiments deployed on geodynamic targets such as the Pyrenees and the Massif Central. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 1.0 and 1.5 s have been observed at most sites, but some larger delay times, above 2.0 s, have been observed at some stations, such as in northern Alps or Corsica, suggesting the presence of high strain zones in the upper mantle. The azimuths of the fast split shear waves define a simple and smooth pattern, trending homogeneously WNW–ESE in the Nice area and progressively rotating to NW–SE and to NS for stations located further North in the Alps. This pattern is in continuity with the measurements performed in the southern Massif Central and could be related to a large asthenospheric flow induced by the rotation of the Corsica–Sardinia lithospheric block and the retreat of the Apenninic slab. We show that seismic anisotropy nicely maps the route of the slab from the initial rifting phase along the Gulf of Lion (30–22 Ma) to the drifting of the Corsica–Sardinia lithospheric block accompanied by the creation of new oceanic lithosphere in the Liguro–Provençal basin (22–17 Ma). In the external and internal Alps, the pattern of the azimuth of the fast split waves follows the bend of the alpine arc. We propose that the mantle flow beneath this area could be influenced or perhaps controlled by the Alpine deep penetrative structures and that the Alpine lithospheric roots may have deflected part of the horizontal asthenospheric flow around its southernmost tip.
ISSN: 0040-1951

hal-00109313
https://hal.archives-ouvertes.fr/hal-00109313
https://hal.archives-ouvertes.fr/hal-00109313/document
https://hal.archives-ouvertes.fr/hal-00109313/file/barruol_TGRS_Tphy2004_hal.pdf
DOI : 10.1016/j.tecto.2004.08.002

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
As previously suggested by many authors, shear wave splitting rneasurernents certainly provide the best insights on the tectonic structure (or fabric) of the upper mantle. Shear wave splitting pararneters are correlated with the flow fabric developed in the deforming upper mantle. Petrophysical analysis of peridotites [e.g., Kem et al., 1996; Mainprice and Silver, 1993] shows that the largest anisotropy is recorded for shear waves propagating close to the Y structural direction (i.e., normal to the lineation in the foliation plane) and that the fast split shear wa ve is polarized in a plane parallel to the X structural axis (i.e., the lineation, marked by the olivine a axis concentration). Mapping shear wave splitting parameters over a specific tectonic domain would therefore provide an image of the mantle fabric at depth.
ISSN: 2169-9313

hal-01389720
http://hal.univ-reunion.fr/hal-01389720
http://hal.univ-reunion.fr/hal-01389720/document
http://hal.univ-reunion.fr/hal-01389720/file/Vauchez_comment_JGR_1999_hal.pdf
DOI : 10.1029/1999JB900053

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
To quantify the seismic properties of lower crustal rocks and to better constrain the origin of the lower crustal seismic reflectivity, we determined the complete 3-D seismic properties of a lower crustal section. Eight representative samples of the main lithologic and structural units outcropping in the Val Sesia (Ivrea zone) were studied in detail. The seismic velocities were calculated using the single crystal stiffness coefficients and the lattice preferred orientation (LPO) of each mineral in all samples. The 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined. Mafic and ultramafic rocks such as pyroxenite and pyroxene-bearing gabbros display complex shear wave properties. These rocks are weakly birefringent (maximum 0.1 kms-1) and it is difficult to find consistent relationships between the seismic properties and the rock structure. On the other hand, seismic properties of deformed felsic rocks are essentially controlled by mica. They display strong S-wave birefringence (0.3 km s-1) and relatively high Vp anisotropy (7.6 per cent). Amphibole also strongly influences the rock birefringence patterns. For both kind of rocks, the foliation is highly birefringent and the fast polarized shear wave is systematically oriented parallel to the foliation. We show that the number of mineral phases in the rock strongly controls the anisotropy. The seismic anisotropy has a complex role in the P-wave reflectivity. Compared to the isotropic case, anisotropy enhances the reflection coefficient for about 60 per cent of the possible lithological interfaces. For 40 per cent of the interfaces, the reflection coefficient is much lower when one considers the medium as anisotropic.
ISSN: 0956-540X

hal-01389122
http://hal.univ-reunion.fr/hal-01389122
http://hal.univ-reunion.fr/hal-01389122/document
http://hal.univ-reunion.fr/hal-01389122/file/Barruol_3D_velocities_GJI1993.pdf
DOI : 10.1111/j.1365-246X.1993.tb01519.x

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
The frequently observed parallelism between rifts and the pre­ existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents. We propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere, that guides the propagation of continental rifts. Subcrustal mantle mechanical anisotropy is supported by (i) the anisotropie strength of olivine, (ii) an ubiquitous tectonic fabric in exposed mantle rocks, and (iii) measurements of seismic and electrical anisotropy. During major episodes of continent Rifting parallel to orogenic belts Ocean-opening through rifting and continent break-up is frequently related to the occurrence ofhotspots. There is, howevcr, a discrepancy between hot­ spots acting as pin point sources of heat and the linear extent of rifts over thou­ sands ofkilometres. Moreover rifts tend to parallel pre-existing orogenic fab­
ISSN: 0954-4879

hal-01389721
http://hal.univ-reunion.fr/hal-01389721
http://hal.univ-reunion.fr/hal-01389721/document
http://hal.univ-reunion.fr/hal-01389721/file/Vauchez_TerraNova1997_hal.pdf
DOI : 10.1111/j.1365-3121.1997.tb00003.x

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
In order to constrain interpretations of seismic reflection records more effectively, the seismic properties of a middle crustal section exposed in the Saint Barthélémy Massif have been determined. The massif, transected by a 200 m thick shear zone has been systematically sampled for density measurements and modal analysis has been performed in order to define the spatial variations of physical properties. Seismic velocities (Vp, V5, shear wave birefringence), have been measured on five representative samples to 600 MPa and 600°C simultaneously in the three structural directions (X, Y and Z). For two samples, the expep.mental data have been compared with calculated values, based on petrofabric analyses. The Lattice Preferred Orientation (LPO) is determined using universal stage, electron channelling microscopy and neutron diffraction goniometry. Using the experimental and calculated velocity data, we establish a lateral homogeneous anisotropie model.
ISSN: 0954-4879

hal-01389722
http://hal.univ-reunion.fr/hal-01389722
http://hal.univ-reunion.fr/hal-01389722/document
http://hal.univ-reunion.fr/hal-01389722/file/Barruol_aniso_St_Barthelemy_TerraNova1992_hal.pdf
DOI : 10.1111/j.1365-3121.1992.tb00451.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We investigated the structure of the upper mantle beneath southeastern Brazil using teleseismic shear wave splitting measurements. Measurements were performed on seismic data recorded in the Ribeira and Brasilia Neoproterozoic belts, which wrap around the southern termination of the São Francisco craton and disappear westward under the Paraná basin. In the northern Ribeira belt, dominated by thrust tectonics, the fast shear wave polarization planes trend on average N080°E, whereas in the central domain, dominated by strike-slip tectonics, fast shear waves are polarized parallel to the structural trend (N065°E). Stations located above the main transcurrent fault display large delay times (> 2.5 s). Such values, among the largest in the world, require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. Within the southern Brasilia belt, fast split shear waves are polarized parallel to the structural trend of the belt, at a high angle from the APM. Although part of our data set strongly favors an origin of anisotropy related to a fabric frozen in the lithospheric mantle since the Neoproterozoic, a contribution of the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters.
ISSN: 0012-821X

hal-01389116
http://hal.univ-reunion.fr/hal-01389116
http://hal.univ-reunion.fr/hal-01389116/document
http://hal.univ-reunion.fr/hal-01389116/file/Heintz_SKS_Brazil_EPSL2003_hal.pdf
DOI : 10.1016/S0012-821X(03)00163-8

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the VSV structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent averagecross-sectionforthePacificOceanlithosphereandfoundthattheincreaseinVSV with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average VSV trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.
ISSN: 0956-540X

hal-00105949
https://hal.archives-ouvertes.fr/hal-00105949
https://hal.archives-ouvertes.fr/hal-00105949/document
https://hal.archives-ouvertes.fr/hal-00105949/file/Maggi_Pacif_tomo_GJI2006.pdf
DOI : 10.1111/j.1365-246X.2006.03037.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The seismic (elastic) properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics. This modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic SKS-waves. Mafic rocks such as anorthosite or pyroxene-bearing gabbros have complex S-waves properties, i.e. with small shear-wave anisotropies (< 3%) and without clear relationships between the fast shear-wave polarization plane orientations and the fabric. On the other hand foliated felsic rocks such as gneisses or schists and mafic rocks such as amphibole-bearing gabbros are strongly birefringent (anisotropies greater than 5%) and exhibit clear relationships with the structure. Biotite and amphibole preferred orientations clearly play a major rote in shear-wave splitting in crustal rocks. We show strong correlations between the orientation of the foliation and the amplitude of the delay time. Maximum delay time is systematically observed for waves propagating parallel to the foliation. The fast shear-wave is furthermore polarized parallel to the foliation plane. We calculate a delay time of about 0.1-0.2 s per 10 km of crustal rocks.
ISSN: 0031-9201

hal-01389713
http://hal.univ-reunion.fr/hal-01389713
http://hal.univ-reunion.fr/hal-01389713/document
http://hal.univ-reunion.fr/hal-01389713/file/Barruol_SKS_crust_PEPI1993_hal.pdf
DOI : 10.1016/0031-9201(93)90161-2

Éditeur(s) : HAL CCSD
Résumé : International audience
Seismic anisotropy is a ubiquitous feature of the subcontinental mantle. This can be inferred both from direct seismic observations of shear wave splitting from teleseismic shear waves, as well as the petrofabric analyses of mantle nodules from kimberlite pipes. The anisotropy is principally due to the strain-induced lattice preferred orientation (LPO) of olivine. The combined use of these mantle samples, deformation experiments on olivine, and numerical modeling of LPO, provides a critical framework for making inferences about mantle deformation from observed seismic anisotropy. In most cases there is a close correspondence between mantle deformation derived from seismic observations of anisotropy, and crustal deformation, from the Archean to the present. This implies that the mantle plays a major, if not dominant role in continental deformation. No clear evidence is found for a continental asthenospheric decoupling zone, suggesting that continents are probably coupled to general mantle circulation.
Mantle Petrology: Field Observations and high Pressure experimentations: a Tribute to Francis R. (Joe) Boyd

hal-01391514
http://hal.univ-reunion.fr/hal-01391514
http://hal.univ-reunion.fr/hal-01391514/document
http://hal.univ-reunion.fr/hal-01391514/file/Silver_et_al_BoydVolume 1999_hal.pdf

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
A dense network of seismic stations has been deployed on the Kaapvaal craton (South Africa) to investigate the upper mantle seismic structures. In order to bring independent petrophysical constraints, we analyze a direct sampling of the cratonic upper mantle and determine the seismic properties of 48 mantle nodules brought up to the Earth's surface by kimberlite eruptions. Seismic properties of these nodules are calculated from the olivine and pyroxene crystal preferred orientations and the single crystal elastic constants. Despite variations in the nodules compositions, microstructures and crystallographic preferred orientations, seismic anisotropy is rather homogeneous throughout the craton. Mean S-wave anisotropy is weak (2.64 %), which is compatible with the small measured SKS wave splitting (mean delay time of 0.62 s).
ISSN: 0094-8276

hal-01389119
http://hal.univ-reunion.fr/hal-01389119
http://hal.univ-reunion.fr/hal-01389119/document
http://hal.univ-reunion.fr/hal-01389119/file/BenIsmail_GRL_kaapvaal_GRL2001.pdf
DOI : 10.1029/2000GL012419

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
Splitting of teleseismic shear waves has been measured in the Appalachians (eastern USA) and the Pyrenees (western Europe) using data recorded by permanent and portable stations. From a comparison of the results, it appears that an interpretation of the recorded seismic anisotropy in terms of geodynamics is not straightforward. Successive geodynamic events have generated structures that may have resulted in a similar pattern of mantle flow and that therefore may have contributed in the development of the recorded anisotropy. Combining geological and geophysical arguments, it appears that the mantle anisotropy measured across the Appalachians and the Pyrenees may not be systematically Appalachian or Pyrenean in age but may be mainly due to a lithospheric structure formed during earlier major tectonic events, i.e. the Grenvillian and the Hercynian orogenies, respectively. We suggest that during major episodes of continent assembly, a pervasive tectonic fabric is developed in the lithospheric mantle. In the subsequent evolution of the continent, this fabric may induce a significant mechanical anisotropy that will drastically influence the mechanical behaviour of the Iithosphere when submitted to new tectonic events.
ISSN: 0031-9201

hal-01389717
http://hal.univ-reunion.fr/hal-01389717
http://hal.univ-reunion.fr/hal-01389717/document
http://hal.univ-reunion.fr/hal-01389717/file/Vauchez_Barruol-PEPI96_hal.pdf
DOI : 10.1016/0031-9201(95)03125-1

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The anisotropic single crystal seismic properties are reviewed in the light of recent experimental and theoretical determinations. Although considerable progress has been made on the determination of single crystal properties, data are still lacking, particularly for the temperature derivatives of transition zone and lower mantle phases. The common types of LPO of olivine, opx and cpx are presented together with their associated seismic properties. It is emphasized that simple seismic symmetry pattern of upper mantle rocks are a direct result of the interaction of olivine, opx and cpx. Using the standard structural frame (X lineation, Z pole to foliation) for typical LPOs of olivine, opx and cpx have the maximum Vp parallel to X, in the XZ plane and parallel to Y respectively. Destructive interference occurs between these minerals and hence P-wave anisotropy should be sensitive to the aggregate composition. For shear wave splitting (dVs) typical olivine and opx LPOs result in similar patterns with the maximum dVs in the YZ plane and the fast split shear wave (Vs1) polarized parallel to the foliation. A typical cpx LPO on the other hand produces destructive interference as the max dVs is close to X. By comparison with experiments and numerical simulations, it is estimated that upper mantle samples have an olivine LPO strength which recorded shear strain gamma of between 0.25 and 2.0. Pyrolite and piclogite models are compared with global transverse isotropic models. The slowly reducing P-wave anisotropy in the first 200 km can be explained by a model with constant composition and LPO strength. The sharp decrease in the observed anisotropy in the global models cannot be explained by the transformation of opx to cpx at 300 km, it is proposed that this decrease is due to a reduction in LPO strength from 200 to 350 km at the base of the lithosphere.
Earth's Deep Interior: Mineral Physics and Tomography From the Atomic Scale to the Global Scale

ISBN : 9780875909752
hal-01391524
http://hal.univ-reunion.fr/hal-01391524
http://hal.univ-reunion.fr/hal-01391524/document
http://hal.univ-reunion.fr/hal-01391524/file/Mainprice_AGUmonog_DM_GB_WBI_2000.pdf