Je ´ro ˆme Bascou

Guilhem Barruol

Alain Vauchez

David Mainprice

Marcos Egydio-Silva

EBSD-measured lattice-preferred orientations and seismic properties of eclogites

Keywords: Electron backscatter diffraction (EBSD), Lattice-preferred orientation (LPO), Omphacite deformation, Numerical models, Seismic anisotropy, Reflectivity

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 selfconsistent 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.

Introduction

High-pressure rocks in collisional orogens are often regarded as relicts of subduction zones. Their detection is therefore fundamental in the understanding of the earth's dynamic. In particular, the pres-ence of eclogite bodies within the continental lithosphere could explain high velocity patches detected in seismic tomography models or bright seismic reflectors on deep seismic reflection profiles [START_REF] Butler | Thrust tectonics, deep structure and crustal subduction in the Alps and Himalayas[END_REF][START_REF] Laubscher | The problem of the Moho in the Alps[END_REF][START_REF] Nicolas | ECORS -CROP traverse and the deep structure of the western Alps: a synthesis[END_REF][START_REF] Austrheim | Eclogite formation and dynamics of crustal roots under continental collision zones[END_REF][START_REF] Warner | Seismic reflections from the mantle represent relict subduction zones within the continental lithosphere[END_REF]. Laboratory measurements of P-and S-wave velocities (e.g. [START_REF] Fountain | Eclogite -facies shear zones -deep crustal reflectors?[END_REF][START_REF] Kern | Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust[END_REF] show that eclogites are characterized by P-wave velocities around 8.0 km/s, which are much faster than the average Pwave velocities of other crustal rock (e.g. Barruol and Mainprice, 1993a;[START_REF] Christensen | Seismic velocity structure and composition of the continental crust: a global review[END_REF]. These velocities are however similar to the average velocity in the uppermost mantle, i.e., 8.09 km/s for the Pn-average velocity.

Possible effects of seismic anisotropy on the reflectivity of eclogite bodies in contact with peridotite have been pointed out by [START_REF] Mauler | Pwave anisotropy in eclogites and relationship to the omphacite crystallographic fabric[END_REF]. Unfortunately, if peridotite seismic anisotropy and its relationship with olivine lattice-preferred orientation is widely documented [START_REF] Babuska | Elasticity and anisotropy of dunite and bronzitite[END_REF][START_REF] Nicolas | Mechanism of flow in naturally and experimentally deformed peridotites[END_REF][START_REF] Nicolas | Formation of anisotropy in upper mantle peridotites -a review[END_REF][START_REF] Ismaı ¨l | An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy[END_REF], it is not the case of eclogitic rocks seismic properties. Eclogites seismic anisotropy is also expected to be controlled by the anisotropy of the elastic properties of omphacite single-crystal [START_REF] Bhagat | Single-crystal elastic properties of omphacite -C2/c by Brillouin spectroscopy[END_REF], but the relationship between the omphacite LPO, the deformation mechanisms and the resulting seismic properties is poorly documented [START_REF] Godard | Deformation-induced clinopyroxene from eclogites[END_REF].

The overall aim of this paper is therefore to increase our knowledge on the eclogite physical properties. To access this point, (1) we quantify the crystallographic orientations of omphacite and garnet in eclogites sampled from various localities (Alps, Norway, Mali and eastern China) that display different types and intensity of deformation; (2) we discuss the deformation mechanisms that produce the measured omphacite LPO. The lack of experimental data on omphacite polycrystals deformation stimulates us to perform numerical models of LPO development by dislocation glide in order to constrain the relationship between LPO and slip systems activity;

(3) we compute the seismic properties (P and S velocities) of the selected eclogites from their measured LPO; and (4) we finally discuss the reflectivity of horizontal interfaces involving eclogite, crustal and mantle rocks.

Geological settings and textures description

Alpe Arami eclogites (Alps)

The Alpe Arami peridotite massif is located within the Lepontine gneisses, north from the Insubric Line (Fig. 1). This lens-shaped peridotite massif, 1-km long, is surrounded by eclogite boudins partly converted to an amphibolite assemblage [START_REF] Mo ¨ckel | Structural petrology of the garnet -peridotite of Alpe Arami (Ticino, Switzerland)[END_REF][START_REF] Ernst | Mineralogic study of eclogitic rocks from Alpe Arami, Lepontine Alps, southern Switzerland[END_REF]. Thermobarometric investigations [START_REF] Heinrich | Eclogite facies regional metamorphism of hydrous mafic rocks in the Central Alpine Adula Nappe[END_REF] suggest P -T metamorphic conditions in the range 1.8 -3.5 GPa and 750 -900 °C.

Sample AB.E is petrologicaly representative of the eclogitic event. It contains 60% of omphacite, 30% of garnet and rutile, ca -amphibole and kyanite as accessory minerals. This rock is characterized by a penetrative foliation and a lineation marked by a shape-preferred orientation of omphacite crystals. The largest omphacite crystals are more than 4-mm long (Fig. 2a). The garnet crystals, weakly elongated, are roughly 800-mm long in average and contain inclusions of omphacite.

Mt Mucrone eclogites (Alps)

In the Monte Mucrone area (Fig. 1), glaucophanebearing eclogitic boudins are embedded in the Sesia -Lanzo zone micaschists. These eclogites are characterized by a good preservation of the high-pressure mineralogical assemblages [START_REF] Lardeaux | Relations entre le me ´tamorphisme et la de ´formation dans la zone de Sesia -Lanzo (Alpes occidentales) et le proble `me de l'e ´clogitisation de la crou ˆte continentale[END_REF][START_REF] Rubie | A thermal -tectonic model for high-pressure metamorphism and deformation in the Sesia Zone, Western Alps[END_REF] and might represent a slice of Austroalpine continental crust subducted during the cretaceous Alpine collision (e.g. [START_REF] Compagnoni | The Sesia -Lanzo zone: a slice of continental crust with alpine high pressure -low temperature assemblage in the Western Italien Alps[END_REF]. The metamorphic conditions are estimated to be around 1.4 -1.6 GPa and 500-560 °C [START_REF] Lardeaux | Relations entre le me ´tamorphisme et la de ´formation dans la zone de Sesia -Lanzo (Alpes occidentales) et le proble `me de l'e ´clogitisation de la crou ˆte continentale[END_REF].

Sample eclo 01 is composed of omphacite (55%), garnet (25%), zoisite (10%) and minor phengite, glaucophane and rutile. The microstructure is characterized by a bimodal distribution of omphacite grain size with large porphyroclasts (> 2 mm) surrounded by fine-grained recrystallized omphacite. The largest grains show undulose extinction and subgrain boundaries (Fig. 2b). Another class of omphacite crystals is made of elongated grains about 2-mm long that are free of sub-structure. The foliation is marked by zoisite-rich layers and the elongation of omphacite porphyroclasts.

Monviso eclogites (Alps)

The Monviso meta-ophiolitic complex is located in the Western Alps (Fig. 1). Eclogitic metamorphism is related to the subduction of an oceanic crust during the Alpine orogeny (e.g. [START_REF] Caby | Structure, me ´tamorphisme et mode `le d'e ´volution tectonique des Alpes occidentales[END_REF]. The pressure and temperature conditions of the eclogitic metamorphism have been estimated to be 2.4 GPa and 620 ± 50 °C [START_REF] Messiga | Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy)[END_REF].

Sample g4129 (65% omphacite, 20% garnet, 8% rutile) was collected within the highly deformed, eclogitic metagabbros outcropping in the Lago Superior Unit [START_REF] Philippot | Chemical -microstructural changes in eclogitic shear zones, Monviso (Western Alps), as indicators of strain history, mechanisms and scales of mass transfer[END_REF]. This sample is characterized by a mylonitic foliation formed by fine ( < 40 mm), recrystallized omphacite grains. Some large omphacite grains (>1 mm; Fig. 2c) are remnants of the original texture; they display undulose extinction and subgrain boundaries. Garnet grains are small (500 mm) and rounded. Fine layers of elongated and fragmented rutile grains underline the foliation and the lineation.

Siviez -Mischabel nappe eclogites (Alps)

These eclogites are located in the polymetamorphic basement of the Penninic Siviez -Mischabel nappe (Valais, Switzerland; Fig. 1). Eclogitization was not generated during Alpine orogeny but is supposed to have occurred during Paleozoic times at a temperature around 650 °C and a pressure above 1.5 GPa [START_REF] The ´lin | Eclogites of Paleozoic or early Alpine age in the basement of the Penninic Siviez -Mischabel nappe, Wallis, Switzerland[END_REF]. Lately, the high-pressure mineral assem- blage has been partially retrogressed in the amphibolite-and greenschist-facies.

Sample g3638 was collected in the core of a decametric-scale boudin enclosed in retrogressed gneisses. Its modal composition is primarily omphacite (65%) and garnet (20%). White mica, zoisite and kyanite are present as accessory components. In thin section (Fig. 2d), elongated crystals of omphacite as well as mica, zoisite and rutile underline the foliation.

Omphacite grains are sub-automorphous and about 500-mm long. Garnet grains are fractured and rounded.

Bergen Arc eclogites (Norway)

The eclogites of series G (three samples) have been collected on Holsnøy Island (Western Norway; Fig. 1), within the Hundskjeften shear zone that crosscut granulite facies anorthositic complex of the Bergen Arc. This complex formed during the Proterozoic Grenvillian orogeny (i.e., 1 Ga) was subsequently strongly deformed and partially eclogitized during the Caledonian orogeny [START_REF] Austrheim | Shear deformation and eclogite formation within granulite facies anorthosites of the Bergen Arcs, western Norway[END_REF][START_REF] Austrheim | Eclogitization of the lower crustal granulites by fluid migration through shear zones[END_REF]. The pressure and temperature conditions of eclogitization were estimated to be around 1.8 GPa and 700 -800 °C [START_REF] Austrheim | Shear deformation and eclogite formation within granulite facies anorthosites of the Bergen Arcs, western Norway[END_REF].

The three samples of the series G are fine grained and display similar modal content. Thus, we only present one of those samples (G3) in Fig. 2e; it is composed of omphacite (65%), garnet (25%) and minor phengite, zoisite and kyanite. Sub-automorphous omphacite crystals, 200 -500-mm long, mark the foliation. Garnet generally is rounded and generally millimetric in size. The other mineralogical phases are strongly elongated in the foliation.

Western Gneiss Region eclogites (Norway)

These eclogites are located within the Western Gneiss Region (WGR; Fig. 1), north of the Nordfjord -Sogn Detachment. This detachment was active during the late to post orogenic extension of the Caledonides and is thought to have played an important role in the exhumation of the WGR [START_REF] Andersen | Extensional tectonics in the Caledonides of southern Norway, an overview[END_REF].

Sample B6 is sampled from the core of an eclogite boudin that was retrogressed in the amphibolite metamorphic facies. In the core of this boudin, eclogite facies parageneses are still preserved. B6 contains 55% omphacite, 35% garnet plus minor quartz, phengite and rutile. Omphacite grains up to 2-mm long display a weak symplectization. A shape-preferred orientation of omphacite crystals marks the foliation and lineation. Garnet is inclusion-rich and millimetric in size (Fig. 2f).

Sample Norv1 (not in-situ) was collected in the Nordfjord area (Fig. 1). Mineral composition is omphacite (60%), garnet (30%), with kyanite, zoisite, phengite and quartz as minor components. This sample is coarse-grained with omphacite grains > 4 mm and large garnets (> 3 mm; Fig. 2g) showing abundant amphibole and quartz inclusions.

Gourma eclogites (Mali)

Samples S522 and S509 represent eclogitic metasediments from the internal nappes of the Gourma area, northern Mali (Fig. 1). These eclogites are thought to represent sediments of a passive continental margin subducted during the closure of a pre-Pan-African ocean. They might be exhumed during the Pan-African collision of the West African craton with an arc and/or with the Hoggar -Iforas continental margin [START_REF] Caby | Precambrian coesite from northern Mali: first record and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt[END_REF]. The eclogitic metamorphism occurred at pressure above 2.7 GPa and temperature ranging between 700 and 750 °C [START_REF] Caby | Precambrian coesite from northern Mali: first record and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt[END_REF].

In the calc-silicate sample S522 (Fig. 2h), coesite relics in omphacite have been described by [START_REF] Caby | Precambrian coesite from northern Mali: first record and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt[END_REF]. Omphacite represents the major mineral phase (70%). The other phases are garnet, quartz, ankerite and accessory rutile and phengite. A penetrative foliation is defined by the shape fabric of tabular omphacite crystals (> 1.5 mm). Garnet crystals are millimetric in size and rounded in shape. A finegrained matrix contains omphacite grains of about 400 mm, quartz and ankerite.

Sample S509 was collected in a metabasic sill within micaschists. This sample is characterized by a mineralogical layering of omphacite-rich ( + quartz) and omphacite-poor, garnet-rich layers. In the omphacite-rich layers, omphacite displays evidence of an overgrowth with megacrysts above 1.5 cm in length containing abundant quartz inclusions. In omphacitepoor, garnet-rich layers, the grain size of omphacite and garnet is relatively homogeneous (1-2 mm).

Sulu eclogites (eastern China)

Dabie and Sulu areas are considered as the eastern extension of the Quinling -Dabishan orogenic belt, which results from a collision between the North and South China Blocks (Fig. 1). The occurrence of coesite in the Dabie and Sulu gneissic and eclogitic terranes indicates a subduction of the South China continental crust down to more than 100 km (e.g. [START_REF] Faure | Where is the North China -South China block boundary in eastern China?[END_REF]. In the Sulu terrain, the P, T conditions were evaluated at ca. 700 -890 °C and P > 2.8 GPa [START_REF] Zhang | Petrology of ultrahigh-pressure rocks from the southern Su -Lu region, eastern China[END_REF].

Studied eclogites were sampled in the southern part of the Sulu terrain. The less retromorphozed sample (SL50) is made of omphacite (70%), garnet (20%), quartz (7%) and minor phengite, epidote, rutile and kyanite. Elongated crystals of omphacite (about 1 Â 3 mm in size) underline the foliation and lineation. Garnet grains of about 0.8 mm are equidimensional and quartz grains up to 1-mm long are strongly deformed.

Electron backscatter diffraction (EBSD) measurements

Experimental setup and indexing procedure

Omphacite and garnet lattice-preferred orientations (LPO) were measured on a scanning electron microscope JEOL JSM 5600 by electron backscatter diffraction techniques (EBSD; [START_REF] Lloyd | Crystallographic textures[END_REF][START_REF] Adams | Orientation imaging: the emergence of a new microscopy[END_REF][START_REF] Dingley | Electron backscatter diffraction and orientation imaging microscopy[END_REF]. This technique is based on automatic analysis of diffraction patterns. These patterns composed of Kikuchi bands are generated by interaction of a vertical incident electron beam with a flat crystal surface. We used polished thin sections about 10 cm 2 and tilted at 70°from the horizontal within the microscope chamber. A phosphor screen is located close to the thin section to collect the backscattered electrons and to emit a photonic image of the EBSD pattern. A low-light, high-resolution camera acquires this photonic image, which is then processed and indexed using the CHANNEL + software [START_REF] Schmidt | Computer-aided determination of crystal-lattice orientation from electron-channeling patterns in the SEM[END_REF]. At each point of measurement, the Euler angles (j1,f,j2) characterizing the lattice orientation of the crystal are determined and stored together with the nature of the mineral. The whole procedure (pattern acquisition, image freezing, band detection, indexing and result backup) can be carried out automatically. The relative precision of crystal orientations measured from electron backscattering patterns is better than 1° [START_REF] Krieger Lassen | The relative precision of crystal orientations measured from electron backscattering patterns[END_REF].

In order to improve the quality of the diffraction patterns, relief on the thin section is minimized by a careful polishing. The polishing procedure ends with a 1/4-mm diamond paste followed by a SYTON chemical polishing. In addition, charging effect were prevented by a slight platinum coating. EBSP were acquired at accelerating voltage of 15 kV and a working distance of about 40 mm.

Crystal orientation is determined by comparing the observed pattern of Kikuchi bands with those simulated for a pre-defined crystallographic structure. For each investigated mineral phase, one has therefore to generate first the crystallographic structure file (Lau ¨e group, unit cell parameters, atoms position within the unit cell) and calculate the corresponding database file containing the reflectors suitable for the best indexing. For garnet, we used a set of the 64 strongest reflections bands calculated from pyrope structure of cell edge 11.459 A ˚. For omphacite, we used a set of the 73 strongest Kikuchi bands calculated using the crystallographic data of a Norway omphacite [START_REF] Clark | Crystal -chemical characterization of clinopyroxenes based on eight new structure refinements[END_REF]. The weak impact on the reflection of the C2/c to P2/n ordering structure transformation [START_REF] Mauler | Identification of EBSD patterns in a monoclinic solid-state solution series: example of omphacite[END_REF] in the omphacite compositional range, allows using a single omphacite hkl-file. Lattice-preferred orientations were measured in automatic mode when no electronic charging artifacts affected the pattern quality. Otherwise, we used a manual procedure. During automatic measurements, we used discriminators in order to reject suspicious indexings. These discriminators give a control on the pattern quality (band contrast and band slope parameters), but also on the indexation velocity (search rounds parameter) and on the quality of the agreement between detected and simulated Kikuchi bands (Mean Angular Deviation parameter). Data that did not fulfill the parameters were rejected. The diffraction pattern of mineral other than omphacite and garnet, as for example, kyanite, systematically produced a zero solution. For all the samples, the whole surface of the thin sections was investigated. Finally, the stage motorization allows a stepby-step measurement on a predefined grid. We choose a step size of the same order as the omphacite grain size of the studied sample (see Table 1), except for the sample Norv1, whose grain size was too large. In most cases, the relative small size of the mean grains relative to the thin section area secures a good sampling statistic in spite of a possible oversampling of the largest grains. In ''manual'' procedure, only the step size along the Y direction was imposed, whereas along the X direction, each new pattern was indexed by the operator avoiding duplicate measurements in the same grains (Table 1 summarizes all methodological information). For each sample, both omphacite and garnet were indexed. If omphacite displays significant latticepreferred orientations, it is not the case for garnet that is almost randomly oriented. For this reason, we concentrate our following microstructural analyze on omphacite.

Omphacite lattice-preferred orientations

Omphacite LPO is generally measured from thin section cut parallel to the structural (XZ)-plane, i.e., normal to the foliation and parallel to the lineation. The well-marked mineralogical foliation and lineation characterizing our samples ensures a good precision in the determination of the orientation of the structural X-, Y-, and Z-axis (X = lineation; Z = pole of the foliation). The measured omphacite LPO (Figs. 3, 4 and5) are presented on equal area, lower hemisphere projections in the structural (XZ ) reference frame. Foliation is vertical E -W and the lineation is horizontal in this plane. Only sample Norv1 was not cut parallel the (XZ )-plane and LPO of this sample was rotated a posteriori for easier comparison with the others samples.

Omphacite LPO (Figs. 3,4 and 5) are characterized by strong concentrations of (010)-poles and [001]axes normal to the foliation and parallel to the lineation, respectively. [100]-axes and (110)-poles are poorly oriented. However, (110)-poles tend to concentrate in a girdle perpendicular to the lineation with a maximum at ca. 60°to the foliation. [100]-axes display a maximum of concentration close to the Ystructural axis (i.e., normal to the lineation within the foliation) is often observed.

Besides these general features, LPO variations are observed from sample to sample. Sample AB.E from Alpe Arami (Fig. 3) displays the best concentration of (010)-poles close to the Z-axis. A weak but significant obliquity (about 7°) is observed between the maximum of density of [001]-axes and the lineation. For this sample, three thin sections in the XZ plane were manually measured on a 5-axis universal stage mounted on an optical microscope (Fig. 4). The LPO measured from EBSD technique and from a 5-axis universal stage are very similar. Especially, we observe for both techniques an obliquity between the maximum concentration of [001]-axes and the line- ation. Discrepancies between LPO patterns may be related partly to lower precision of measurement derived from U-stage optical technique (which is expected to be about 5°) relative to the EBSD technique ( < 1°; [START_REF] Mainprice | Individual orientation measurements in quartz polycrystals: advantages and limitations for texture and petrophysical property determinations[END_REF][START_REF] Krieger Lassen | The relative precision of crystal orientations measured from electron backscattering patterns[END_REF]. Moreover, the higher concentration of omphacite LPOs derived from universal stage measurements may be due to grain selection. Indeed, due to the monoclinic symmetry of omphacite crystals, only the grains showing a cleavage plane, such as {110}, can be measured at the universal stage. Samples eclo 01, g4129, g3638 (Fig. 3), B6, Norv1, S509 and SL50 (Fig. 5) are characterized by a strong [001] maximum sub-parallel to the lineation. These samples (except g4129) present a slight obliquity between the maximum of [001]-axes concentration and the lineation. The concentration of (010)poles is weaker than one [001]-axis and (010)-poles tend to form a girdle perpendicular to the lineation. Moreover, (010)-poles of samples g4129, B6 and Norv1 are grouped in two maxima: the strongest maximum is close to the Z-axis and the weakest is close to the Y-axis. Also, sample SL50 shows two maxima located between the Z-and Y-axis: the strongest is close to the Z-axis and the weakest is at ca. 45°f rom the Z-axis. Sample eclo 01 shows (010)-poles located between the Z-and Y-axis and (110)-poles close to the Z-axis, whereas for the other samples the maximum of density of (010)-poles stands close to the Z-axis and the maximum of ( 110) is located between the Z-and Y-axis. Sample Norv1 displays [100]-axes close to Y with a weaker sub-maximum at ca. 20°f rom Z. However, the larger concentration of LPO for Norv1 may be due to oversampling of the largest omphacite crystals. Sample S509 is characterized by a clustering of [001]-axes in the foliation at ca. 20°from X.

[100]-axes show a weak maximum of density normal to the foliation. ( 010)-poles and (110)-poles tend to form a girdle perpendicular to the lineation with a density maximum close to the Z-axis.

The three samples of the series G (we only present G3 in Fig. 5) from Norway present similar LPO characterized by weak density maxima. The maximum of (010)-poles concentration is normal to the Fig. 6. Omphacite LPO simulated using an anisotropic viscoplastic self-consistent (VPSC) model in simple shear and axial compression [START_REF] Bascou | Plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites[END_REF]. These LPOs are developed for an equivalent strain of 1. Equal area projection, lower hemisphere. Contours 1, 2, 3%. . . by 1% area. Full line: foliation (XY plane) is vertical and lineation (X ) is horizontal. Dotted line: shear direction (g = 1.73). N: Number of grains. DM: maximum density. foliation and the [001]-axes tend to form a girdle in the foliation plane.

Sample S522 from Gourma area (Fig. 5) is characterized by [001]-axes scattered in the foliation plane and (010)-poles strongly concentrated close to Z. (110)-poles and [100]-axes display weak preferred orientations. However, (110)-poles present a weak maximum near the normal to the foliation.

Omphacite LPO significance

Variations in omphacite LPO are generally related to the strain regime [START_REF] Helmstaedt | Petrofabric studies of eclogite, spinel -websterite, and spinel -lherzolite xenoliths from kimberlite-bearing breccia pipes in southeastern Utah and northeastern Arizona[END_REF][START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF][START_REF] Godard | Deformation-induced clinopyroxene from eclogites[END_REF][START_REF] Abalos | Omphacite fabric variation in the Cabo Ortegal eclogite (NW Spain): relationship with strain symmetry during high-pressure deformation[END_REF]. Omphacite LPO of samples AB.E, eclo 01, g4129, g3638, B6, Norv1, S509 and SL50 are similar to the L-type or LS-type LPO pattern defined by [START_REF] Helmstaedt | Petrofabric studies of eclogite, spinel -websterite, and spinel -lherzolite xenoliths from kimberlite-bearing breccia pipes in southeastern Utah and northeastern Arizona[END_REF] as resulting from constriction strain. On the other hand, samples S522 and G3 display LPO similar to the S-type or SL-type, interpreted as resulting from flattening [START_REF] Helmstaedt | Petrofabric studies of eclogite, spinel -websterite, and spinel -lherzolite xenoliths from kimberlite-bearing breccia pipes in southeastern Utah and northeastern Arizona[END_REF][START_REF] Godard | Deformation-induced clinopyroxene from eclogites[END_REF][START_REF] Abalos | Omphacite fabric variation in the Cabo Ortegal eclogite (NW Spain): relationship with strain symmetry during high-pressure deformation[END_REF]. The asymmetric omphacite LPO observed for samples AB.E, g3638 (Fig. 3) and samples B6, Norv1 and SL50 may result from a noncoaxial deformation and the obliquity between the LPO and the X, Y and Z structural axes could be related to the sense of shear [START_REF] Bouchez | Fabric asymmetry and shear sense in movement zones[END_REF][START_REF] Mainprice | Development of shape and lattice preferred orientations: application to the seismic anisotropy of the lower crust[END_REF]. Asymmetric omphacite LPO were already described by [START_REF] Boundy | Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes[END_REF] within eclogitized metagabbro dykes from the Bergen Arc, and by [START_REF] Abalos | Omphacite fabric variation in the Cabo Ortegal eclogite (NW Spain): relationship with strain symmetry during high-pressure deformation[END_REF] who used this obliquity to deduce the sense of shear.

In plastically deformed rocks, the LPO development due to intracrystalline slip is well established. For most minerals (e.g. olivine), when one slip system is predominant during deformation, the slip direction and the slip plane tend to coincide with the flow direction and the flow plane, respectively [START_REF] Mainprice | Development of shape and lattice preferred orientations: application to the seismic anisotropy of the lower crust[END_REF]. Thus, the strong preferred orientation of [001]-axes parallel to the lineation and (010)poles perpendicular to the foliation that characterizes the omphacite LPO in naturally deformed eclogites, suggest a dominant activation of the [001] (010) slip system. However, dislocations associated with this slip system have rarely been observed in naturally deformed omphacite. The dominant slip systems identified from TEM observations are 1/2h110i{11 ¯0}, [001] {110} and [001] (100) [START_REF] Van Roermund | The dislocation substructures of naturally deformed omphacites[END_REF][START_REF] Van Roermund | Petrofabrics and microstructures of omphacites in a high temperature eclogite from the Swedish Caledonides[END_REF][START_REF] Buatier | Deformation and recrystallization mechanisms in naturally deformed omphacites from the Sesia -Lanzo zone; geophysical consequences[END_REF][START_REF] Godard | Deformation-induced clinopyroxene from eclogites[END_REF].

In order to better understand the deformation mechanisms that could have produced the omphacite LPO, we compared the LPO of naturally deformed omphacite with those simulated by numerical models [START_REF] Bascou | Plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites[END_REF]. In polycrystal plasticity models, like the anisotropic viscoplastic self-consistent model (VPSC) developed by [START_REF] Lebensohn | A self-consistent anisotropic approach for the simulation of plastic deformation and texture development of polycrystals: application to zirconion alloys[END_REF] and applied to mantle rocks by [START_REF] Tommasi | Viscoplastic self-consistent and equilibrium-based modeling of olivine lattice preferred orientations: implications for upper mantle seismic anisotropy[END_REF], the LPO evolution is directly related to the slip systems activity. Fig. 6 displays some modeled omphacite LPO developed in simple shear and axial compression. The relative strength (Critical Resolved Shear Stresses) of the various omphacite slip systems (Table 2) are derived from naturally deformed omphacites TEM observations of dislocations (lowest CRSS values have been affected to those systems whose dislocations are the most frequently observed).

Simulated LPO display general characteristics similar to those observed in the LPO measured in naturally deformed eclogites. In particular, modeled omphacite LPO developed in simple shear deformation (g = 1.73) reproduces well LPO observed for The various potential slip systems considered come from diopside deformation experiments [START_REF] Raterron | High temperature deformation of diopside IV: predominance of {110} glide above 1000 °C[END_REF]). a Critical Resolved Shear Stress. For this model, lowest CRSS values have been affected to those systems whose dislocations are the most frequently observed in the TEM analyses of omphacite naturally deformed (see text). sample AB.E (Fig. 3). Likewise, omphacite LPO predicted to develop under axial compression reproduces well LPO observed for sample S522 (Fig. 5). Interestingly, a strong preferred orientation of the (010)-poles is developed during these simulations even when the [001] (010) slip system is not activated.

The good agreement between the omphacite LPO measured in naturally deformed eclogites and the LPO simulated using a self-consistent polycrystal plasticity model suggests that omphacite LPO may be generated by dominant slip on the 1/2h110i{11 ¯0}, [001] {110} and [001] (100) systems. These models may reconcile the interpretation of the measured omphacite LPO in eclogites and TEM observations of dislocations in naturally deformed omphacite. From our measurements, we propose that most of the omphacite LPOs could have developed through plastic deformation during noncoaxial strain under eclogitic conditions. These LPOs are therefore related to the tectonic history of eclogites.

Eclogite seismic properties

Seismic properties calculation

Seismic velocities are calculated from the Christoffel equation [START_REF] Crosson | Voigt and Reuss prediction of anisotropic elasticity of dunite[END_REF]) that takes into account each single crystal elastic stiffness matrix, the crystals orientation and density. The knowledge of the single crystal elastic constants and the LPO of each rock-forming mineral allows to determine the 6 Â 6 elastic stiffness matrix characterizing the elastic behavior of each mineral phase [START_REF] Mainprice | A FORTRAN program to calculate seismic anisotropy from the lattice preferred orientation of minerals[END_REF]. The rock elastic stiffness matrix is computed taking into account the elastic matrix of each mineral phase according to its modal proportion in the rock. In the calculation, we only take into account the major mineral phases of eclogitic metamorphism (i.e., omphacite, garnet, and epidote). For sample SL50 from the Sulu terrain, we also considered the contribution of quartz in the seismic properties. Seismic calculations were carried out using the single crystal elastic constants measured at room temperature and atmospheric pressure by [START_REF] Bhagat | Single-crystal elastic properties of omphacite -C2/c by Brillouin spectroscopy[END_REF] for omphacite (30% diopside, 58% jadeite), [START_REF] Babuska | Elastic properties of garnet solid-solution series[END_REF] for garnet (40% pyrope, 40% almandin, 20% grossular) and [START_REF] Mcskimin | Elastic moduli of quartz versus hydrostatic pressure at 25°and À 195[END_REF] for quartz. For sample eclo 01 containing 10% of epidote, we used the epidote elastic constants reported by [START_REF] Ryzhova | The elastic properties of rock-forming minerals V: additional data on sillicates[END_REF] but we assumed this mineral as randomly oriented.

The omphacite single crystal seismic properties are presented in Fig. 7. The maximum P-wave velocities (9.37 km/s) is at ca. 30°from the [001] direction which is also a direction of fast propagation (9.0 km/ s). A girdle of lower velocity contains the (010)-pole. The P-wave anisotropy (AVp) defined as 100 Â [Vp max À Vp min)/((Vp max + Vp min)/2)] is rather high (20.4%) for the omphacite. The S-wave anisotropy is defined for each propagation direction as 100 Â [Vs 1 À Vs 2 )/((Vs 1 + Vs 2 )/2)]. The S-wave anisotropy is characterized by a complex pattern with three zones of high anisotropy (> 10%) containing the (010)-pole and [001]-axis. The orientation of the fast S-wave polarization plane has rather complex relationship with the crystallographic axis.

Eclogite seismic properties

The seismic properties of the selected eclogites are summarized in Table 3 and the 3D results are shown in equal area projection (Fig. 8). The P-wave velocity patterns of eclogite samples are rather similar. The fast P-wave velocity (Vp max) ranges from 8.51 to 8.75 km/s and its propagation direction is generally close to the X-axis. The low velocity (Vp min) ranges from 8.39 to 8.62 km/s and is roughly in a direction close to the Z structural axis. The direction of Vp max and Vp min are directly related to the omphacite single crystal properties and the observed LPO. The [001]-axis (a fast velocity direction) is indeed preferentially oriented close to X and the (010)-pole (a low velocity direction) is statistically oriented close to Z. The P-wave anisotropy is weak, generally less than 3%.

The S-wave anisotropy is also very weak (less than 2%) and characterized by complex patterns. However, most of the samples (excepted S522) show directions of highest anisotropy concentrated in four zones at 45°between X, Y and Z. Although the orientation of the fast S-wave polarization plane depends strongly on the orientation of the incoming S-wave with respect to the structural fabric, polarization planes tend to be parallel to the foliation.

For sample SL50, we calculated the seismic properties of omphacite aggregate (100% omphacite) and velocities calculation, ignoring the contribution of quartz (75% omphacite, 25% garnet; Table 3). Despite a high seismic anisotropy of omphacite single crystal and a significant omphacite LPO, the seismic anisotropy of omphacite aggregate is very weak (AVp = 1.90% and AVs max = 2.52%). The contribution of garnet (25%) increases the P-and S-waves velocities but reduces the overall seismic anisotropy (AVp = 1.30% and AVs max = 1.80%). Vp//Z: P-wave velocity normal to the foliation; AVp: P-wave anisotropy; omph/grt: modal proportion of omphacite and garnet used for the calculation. Calculations are performed using the Voigt average. For sample SL50, also are calculated the seismic properties ignoring the contribution of quartz (75% omphacite, 25% garnet) and the seismic properties of omphacite aggregate (100% omphacite). a Isotropic epidote (10%) has been taken into account to calculate the seismic properties. b Quartz (10%) has been taken into account to the seismic properties.

The effect of quartz is to decrease the P-and Swaves velocities and slightly increase the P-and Swave anisotropies. The average seismic properties and the corresponding elastic constants of the 10 selected samples have been calculated (Fig. 9 and Table 4). This mean sample is almost isotropic, Pwave anisotropy is 1.4% and S-wave anisotropy is less than 1%.

Comparison between calculated and measured eclogite seismic velocities

In their compilation of laboratory measurements in continental crustal rocks, [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: a global review[END_REF] have highlighted the narrow range of P-wave velocity variation in eclogite facies rocks compared to others major metamorphic rocks. For these authors, mean P-wave velocities in eclogites range between 7.9 and 8.5 km/s at pressure corresponding to a depth of 20 km. Compared to these measured velocities, our calculated P-wave velocities are slightly faster, around 8.6 km/s. As for the P-waves, the average calculated S-wave velocities (around 4.9 km/s) are higher than seismic velocities, which are measured around 4.5 km/s by [START_REF] Kern | Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust[END_REF] and by [START_REF] Manghnani | Compressional and shear wave velocities in granulite facies rocks and eclogites to 10 kbar[END_REF]. Such discrepancy may be related to the presence of minor phases, grains boundaries, microcracks, and to alteration of the main minerals whose effects are not taken into account in the calculations, and may affect velocity measurements. Compositional layering may also induce significant differences between calculations and measurements [START_REF] Mauler | Pwave anisotropy in eclogites and relationship to the omphacite crystallographic fabric[END_REF]. In addition, we calculated velocities with the Voigt average that usually gives an upper bound for seismic velocities [START_REF] Crosson | Voigt and Reuss prediction of anisotropic elasticity of dunite[END_REF][START_REF] Mainprice | Methods of calculating petrophysical properties from lattice preferred orientation data[END_REF]. Sample G3 is interesting since direct laboratory measurements were performed by [START_REF] Fountain | Eclogite -facies shear zones -deep crustal reflectors?[END_REF] on samples collected in the same area. The calculated mean P-wave velocity for this sample is 8.6 km/s, close to the fastest velocities, about 8.5 km/s, measured at 600 MPa confining pressure in the eclogite from this locality [START_REF] Fountain | Eclogite -facies shear zones -deep crustal reflectors?[END_REF]. Both calculated and measured Vp anisotropy are weak, respectively, about 2% and 3% in average. Moreover, calculated and measured Pwave velocities display roughly the same anisotropy pattern characterized by fast velocities parallel to the lineation, and low velocities normal to the foliation [START_REF] Manghnani | Compressional and shear wave velocities in granulite facies rocks and eclogites to 10 kbar[END_REF][START_REF] Fountain | Eclogite -facies shear zones -deep crustal reflectors?[END_REF][START_REF] Kern | Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust[END_REF][START_REF] Mauler | Pwave anisotropy in eclogites and relationship to the omphacite crystallographic fabric[END_REF]. This suggests that, in eclogite, the P-wave velocity pattern is primarily controlled by omphacite LPO.

Discussion

Although the primary goal of this paper is to characterize the 3D seismic properties of eclogite rocks, a potential use of these data is to discuss the detectability of eclogite bodies within the crust and the mantle. Our calculations show that eclogites are systematically quasi isotropic. This absence of anisotropy implies that eclogites are poor candidates to generate any detectable shear wave splitting that could be used to indirectly detect eclogites bodies. Although the nondetection of a geophysical signal (here the splitting of shear wave ) is always hard to interpret in a field experiment, eclogite bodies are characterized by high density, high seismic velocities and very small seismic anisotropies. Since upper mantle rocks are generally much more anisotropic, this isotropy is the main criterion that may distinguish peridotite bodies from eclogites.

On the basis of geothermal and seismic considerations, possible eclogite/peridotite contact at the base of the continental crust has been suggested by several authors (e.g. [START_REF] Furlong | Continental crust underplatting: thermal considerations and seismic-petrologic consequences[END_REF][START_REF] Griffin | Is the continental Moho the crust-mantle boundary?[END_REF]. Moreover, eclogite layers, as relicts of subduction zones, may be preserved within the crust or the mantle. Preservation of these layers depends on the protolith composition and on the thermal gradient. If large eclogite layers escape from the thermal reactivation, the interface eclogite/crustal rocks could generate bright seismic reflectors that might be misinterpreted as a Moho discontinuity.

In order to quantify the reflectivity of such eclogites in contact with crustal or mantle rocks, we present simple geometric models, taking into account the seismic properties issued from our calculations. We evaluate the reflectivity of simple lithological interfaces between eclogite and various lithologies (Table 5). The reflection coefficient (Rc) is calculated for a seismic wave arriving at vertical incidence on a horizontal interface. For this simplest case, a simple relation allows to calculate the reflection coefficient: Rc=[Vp(//Z) 1 r 1 À Vp(//Z) 2 r 2 ]/[Vp(//Z) 1 r 1 + Vp(// Z) 2 r 2 ]. To evaluate the influence of seismic anisotropy, we determined in a first calculation the reflec-tion coefficient, taking into account the anisotropic seismic properties of the rocks, and in a second calculation, the isotropic properties of the rocks. In the anisotropic case, Rc calculation is carried out using the P-wave velocity in a direction normal to the foliation. The seismic velocities of typical crustal lithologies (gneiss, kinzigite, gabbro, diorite and stronalite) were taken from the literature [START_REF] Barruol | 3D seismic study of a ductile shear zone from laboratory and petrofabric data (Saint Barthe ´le ´my Massif, Northern Pyre ´ne ´es, France)[END_REF]Barruol and Mainprice, 1993a,b). The ''mean crust'' (plagioclase 35%, quartz 15%, biotite 7%, clinopyroxene 8%, amphibole 15%, K-feldspar 20% (isotropic)) corresponds to the average crustal composition given by [START_REF] Anderson | Theory of the Earth[END_REF]. We also investigated the reflectivity of an eclogite body embedded within the mantle. The seismic properties of this upper mantle rock correspond to those of a continental lherzolite sampled in the Ronda massif [START_REF] Vauchez | Seismic properties of an asthenospherized lithospheric mantle: constraints from lattice pre-ferred orientations in peridotite from the Ronda Massif[END_REF]. We selected this lherzolite sample on its typical porphyroclastic texture and LPO and on its modal composition (65% olivine, 20% orthopyroxene, 10% clinopyroxene and 2% spinel). Seismic Pwave properties of this peridotite are characterized by a maximum velocity (8.55 km/s) parallel to the lineation, a minimum velocity (8.09 km/s) normal to the foliation and by a P-wave anisotropy of 5.5%. We summarize in Table 5 the calculated Rc for the various interfaces. The eclogite/crustal rocks interfaces as well as the peridotite/''mean crust'' interface systematically display very high reflection coefficient (higher than 0.1). This strong reflectivity is primarily due to the density contrast between eclogite (density of 3487 kg/ m 3 on average) or peridotite (density of 3310 kg/m 3 ) and the crustal rocks (mean density of 2700 kg/m 3 ). Reflectivity of the eclogite/kinzigite interface as well as eclogite/''main crust''/interface is slightly higher when the seismic anisotropy of the rocks is taken into account. This is due to the fact that these rock types are characterized by high velocities parallel to the foliation and by lower velocities for directions at high angle from the foliation. Therefore, in the isotropic case, the velocity along the Z structural directions is higher than in the anisotropic case, and the Rc slightly lower. On the other hand, the eclogite/peridotite interface is characterized by a much weaker reflection coefficient (Rc = 0.06). Such reflection magnitude could be almost impossible to detect, particularly at a large depth by seismic reflection techniques. This interface becomes even less reflective when the mantle seismic anisotropy is not taken into account. Such difference between an anisotropic (Rc = 0.06) and isotropic medium (Rc = 0.04) suggests that the eclogite/''upper mantle'' reflectivity could be sensitive to the direction of the seismic wave propagation.

The main result of these simple calculations is that an eclogite body embedded in the crust would generate very strong seismic reflections, which could help to identify eclogite bodies but which may be misinterpreted in terms of Moho discontinuity. On the other hand, our calculations show that an eclogite body within the upper mantle would generate weak seismic reflection and might therefore remain almost invisible to vertical seismic reflection techniques. Moreover, the Vp/Vs ratio and the corresponding Poisson ratio s [START_REF] Christensen | Poisson's ratio and crustal seismology[END_REF] calculated for eclogite ((Vp/Vs) iso = 1.74) and for the Ronda lherzolite ((Vp/Vs) iso = 1.71) are very close and therefore not discriminatory.

Conclusion

Although the studied eclogites come from orogenic domains of contrasted age and geodynamic evolution, garnet and omphacite LPO patterns display similar main characteristics. 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 roughly normal to the foliation. We suggest that these LPOs formed in relation with the deformation of eclogites through dislocation creep. This statement is supported by a numerical model result, which shows that omphacite lattice-preferred orientation can develop through plastic deformation by dominant activation of 1/2h110i{11 ¯0}, [001] (110) and [001] (100) slip systems, in agreement with TEM observations of naturally deformed omphacite. LPO of omphacite may therefore help to constrain the tectonic processes active during the high-pressure deformation of eclogites.

Seismic properties of eclogites are mostly governed by omphacite LPO. As variations of these LPO in our dataset are rather limited, the resulting seismic properties are almost similar. Although seismic anisotropy of P-waves is weak, the fast propagation is generally parallel to the rock lineation and the low velocity propagation is normal to the foliation. For P-waves, the relationship between seismic properties and the LPO is straightforward: the fast propagation direction is associated to the concentration of [001]-axes parallel to the lineation, and the slow propagation direction is associated to a concentration of (010)-poles perpendicular to the foliation. The Pand S-wave anisotropies are very weak, less than 2%. The S-wave maximum polarization anisotropy is preferentially concentrated at 45°between the X, Y and Z-structural axes. Orientation of the fast S-wave polarization plane is poorly correlated with the structural framework but tends to be parallel to the foliation. The velocity patterns and the anisotropy magnitude are in rather good agreement with laboratory-derived seismic velocities. Average P-wave velocities (8.6 km/s) and S-wave velocities (4.9 km/ s) are similar to the highest velocities measured in eclogites. Part of the difference between calculated and measured velocities may be related to minor mineral phases and microfracturing, which are not taken into account in our calculation.

Finally, estimation of reflection coefficient highlights the fact that an eclogite body within the upper mantle would be difficult to detect by seismic reflection techniques, whereas eclogite bodies embedded within the crust would generate very strong seismic reflections. The eclogite isotropy might be a distinctive parameter since peridotites are characterized by a large seismic anisotropy, whereas the intrinsic seismic anisotropy of eclogite is usually very weak.

Fig. 1 .

 1 Fig. 1. Schematic maps showing the geographical and geological setting of the studied eclogite samples. (I) Alps region: AB.E, Alpe Arami massif; eclo 01, Monte Mucrone; g4129, Monviso Complex; g3638 Siviez -Mischabel nappe. 1 = Helvetic basement; 2 = Penninic realm undifferentiated; 3 = Piemonte zone (Penninic); 4 = Monte Rosa, Gran Paradiso and Dora Maira nappes (Penninic basement); 5 = Sesia Zone and Dent Blanche nappe; 6 = South Alpine units. (II) Norway area: G3 (Holsnøy island), Bergen Arc; B6 and Norv.1, Western Gneiss Region (WGR). (III) Mali area: S522 and S509, Gourma basin. (IV) Dabie and Sulu areas: SL50, Sulu terrain. NCB = North China Block, and SCB = South China Block.

Fig. 2 .

 2 Fig. 2. Photomicrographs showing typical microstructures of eclogite samples. (a) Sample AB.E; (b) sample eclo 01; (c) sample g4129; (d) sample g3638; (e) sample G3; (f) sample B6; (g) sample Norv1; (h) sample S522. Thin sections in the XZ structural plane except g; g contains X-axis and makes an angle of 60°with the XY plane. Polarized light.

Fig. 3 .

 3 Fig. 3. Omphacite LPO of Alpine eclogites measured using EBSD technique. Equal area projection, lower hemisphere. Contours 1, 2, 3%. . . by 1% area. Foliation (XY plane; full line) is vertical and lineation (X ) is horizontal in this plane. N: number of measurements. DM: maximum density.

Fig. 5 .

 5 Fig. 5. Omphacite LPO of Norway and Gourma eclogite samples. EBSD measurements. Equal area projection, lower hemisphere. Contours 1, 2, 3%. . . by 1% area. Foliation (XYplane; full line) is vertical and lineation (X ) is horizontal in this plane. N: number of measurements. DM: maximum density.

Fig. 7 .

 7 Fig. 7. Seismic properties of the omphacite single crystal. Equal area projection, lower hemisphere.

Fig. 8 .

 8 Fig. 8. Seismic properties of eclogite samples. Equal area projection in the structural reference frame X, Y, Z, (lower hemisphere).

Fig. 9 .

 9 Fig. 9. Average seismic properties from the 10 eclogite samples studied in this work. Equal area projection in the structural reference frame X, Y, Z, (lower hemisphere).

Table 1 P

 1 , T conditions of eclogitic metamorphism, analytic data and grain size for the studied samples

	Samples	Sample location	P (GPA), T (°C) peak	Mean	EBSD	Step (mm)	Mean grain size (mm)
				reference	procedure
	Alpine eclogites
	AB.E	Alpe Arami	1.8 -3.5, 750 -900	[1]	manual	y: 1000	omph: 4000 Â 1000; grt: 800
	Eclo 01	Mt. Mucrone	1.3 -1.6, 600 -650	[2]	automatic	x,y: 500	omph: 3000 Â 1000 (porph);
							< 1000 (recr); grt: 1000
	g4129	Monviso	> 2.4, 570 -670	[3]	manual	y: 500	omph: 2000 Â 500 (porph);
							< 40 (recr); grt: 500 Â 300
	g3638	Siviez -Mischabel	> 1.5, 620 -680	[4]	automatic	x,y: 500	omph: 500 Â 200, matrix < 300;
							grt: 500
	Norway eclogites
	G1	Bergen Arcs	> 1.8, 700 -750	[5]	manual	y: 500	omph: 500 Â 50; grt: 15000
	G2				automatic	x,y: 300	omph1: 1000 Â 300,
							omph2: 200; grt: 2000
	G3				automatic	x,y: 400	omph: 500 Â 200;
							grt: 1000 Â 1500
	Western gneiss region
	B6	Nordfjord area			manual		omph: 3 000 Â 1000; grt: 2000
	Norv.1	(Verpeneset )	1.4 -1.6, 650 -700	[6]	automatic	x,y: 1000	omph > 4000; grt > 3000
	Gourma eclogites
	S522		> 2.7, 700 -750	[7]	manual	y: 1000	omph: 1500 Â 800 and
							400 Â 200; grt < 1000
	S509		> 2.7, 700 -750	[7]	manual	y: 1000	omph > 1000; grt:1000
	Sulu eclogites
	SL50		> 2.8, 700 -890	[8]	automatic	x,y: 1000	omph: 3000 Â 1000; grt < 1000

Omph: omphacite; grt: garnet; porph: porphyroclasts; recr: recrystallized grains. Figs. 3 and 5 give the number of measurement (N ) of omphacite per sample. [1] Heinrich (1986); [2] Lardeaux et al. (1982); [3] Messiga et al. (1999); [4] The ´lin et al. (1990); [5] Austrheim and Griffin (1985); [6] Krogh (1982); [7] Caby (1994); [8] Zhang et al. (1995).

Table 4

 4 Elastic constants (Cij) of the average of the 10 eclogite samples

	Modulus	Cij (GPa)
	C 11	254.57
	C 12	86.80
	C 13	86.83
	C 14	À 0.16
	C 15	0.12
	C 16	À 0.30
	C 22	261.43
	C 23	87.77
	C 24	0.37
	C 25	0.01
	C 26	À 0.11
	C 33	256.07
	C 34	0.07
	C 35	0.05
	C 36	À 0.08
	C 44	84.70
	C 45	À 0.02
	C 46	À 0.02
	C 55	84.29
	C 56	À 0.11
	C 66	84.31
	Density of this mean aggregate of eclogite: d = 3487 kg/m 3 .

Table 5

 5 Absolute reflection coefficients |Rc| calculated from anisotropic Pwave velocity and from isotropic P-wave velocity The P-wave propagation is assumed to be vertical and the foliation horizontal. For eclogite (eclo), Vp is the average of velocities normal to foliation. In the anisotropic case Vp = 8.56 km/s, and in the isotropic case Vp = 8.6 km/s. References for the others rocks: 1 Barruol and Mainprice (1993a); 2[START_REF] Barruol | 3D seismic study of a ductile shear zone from laboratory and petrofabric data (Saint Barthe ´le ´my Massif, Northern Pyre ´ne ´es, France)[END_REF];3 Barruol and Mainprice (1993b);4 Vauchez and Garrido (2001).

	Interfaces	jRcj coefficient	|Rc| coefficient
		for the anisotropic	for the isotropic
		medium	medium
	Eclo/gneiss 2	0.30	0.30
	Eclo/kinzigite 1	0.28	0.26
	Eclo/magmatic gabbro 1	0.19	0.20
	Eclo/diorite 1	0.20	0.20
	Eclo/stronalite 1	0.14	0.14
	Eclo/mean crust 3	0.26	0.25
	Eclo/peridotite 4	0.06	0.04
	Mean crust/peridotite	0.2	0.2

Acknowledgements

We thank R. Caby, B. Ildefonse, M. Faure and L. Labrousse for providing the eclogite samples. Thanks to C. Nevado who made tens of wonderful thin sections and EBSD polishing that allowed this study. R. Caby and A. Tommasi are gratefully acknowledged for fruitful discussions. We thank A. Mauler and an anonymous reviewer for constructive comments on the manuscript. This work benefited from a grant from CNRS/INSU program ''Inte ´rieur de la Terre, Exhumation des roches profondes''; contribution INSU no. 282. The crystal orientation measurements were made using the Laboratoire de Tectonophysique EBSD/ SEM system funded by grants from CNRS/INSU, Universite ´Montpellier II, the NSF project #EAR-9526840 ''Anatomy of an Archean craton'' and ISTEEM. J.B. benefited from a PhD fellowship from the CAPES-Brazil.


A Vauchez

G Barruol

Seismic anisotropy, structures and geodynamics of continents Shear-wave splitting in the Appalachians and the Pyrenees: importance of the inherited tectonic fabric of the lithosphere

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.

Shear-wave splitting on continents

During the last decade, shear-wave splitting has been measured in many different continental areas to characterize the seismic anisotropy of continents. Although various possible sources have been recog nized, there is a good consensus that the best candi date to generate teleseismic shear-wave splitting is the upper mantle (Vinnik et al., 1984, l 989a,b;[START_REF] Nicolas | Formation of anisotropy in upper mantle peridotites-a review[END_REF][START_REF] Silver | Implications for continental structure and evolution from seismic anisotropy[END_REF][START_REF] Babuska | Models of seismic anisotropy in the deep continental lithosphere[END_REF]Mainprice and Silver, 1993). Mantle rocks (xenoliths, peridotite bodies or ophiolites) systematically display a significant anisotropy [START_REF] Nicolas | Formation of anisotropy in upper mantle peridotites-a review[END_REF]Main-price and Silver, 1993). This anisotropy results from the good crystallographic preferred orientation of rock-forming minerais and from the single crystal anisotropie properties, especially of olivine, which is the primary upper-mantle minerai. This suggests that splitting of teleseismic shear waves is mostly related to the structure of the upper mantle, either owing to active (asthenospheric) or 'frozen' (lithospheric) mantle flow.

From the wealth of data accumulated, it appears that a common situation is a fast splitting polariza tion direction subparallel to the tectonic grain of the studied area (e.g. [START_REF] Silver | Shear wave splitting and subcontinental mantle deformation[END_REF][START_REF] Bormann | Teleseismic shear-wave splitting and deformations in central Europe[END_REF][START_REF] James | Seismic studies of continental lithosphere beneath SE Brazil[END_REF]. Various, sometimes divergent, interpretations have been proposed to ac count for this observation. A much debated question is the depth range in which splitting of shear waves occurs. Does this anisotropy correspond to a • frozen' lithospheric structure acquired during ancient oro genic events (e.g. [START_REF] Silver | Implications for continental structure and evolution from seismic anisotropy[END_REF] or does it correspond to a present-day deformation of the as thenosphere by resistive drag beneath the moving rigid lithosphere (e.g. [START_REF] Bormann | Teleseismic shear-wave splitting and deformations in central Europe[END_REF])? In the hypothesis of a lithospheric origin of shear-wave splitting, for realistic lithosphere thicknesses, the intrinsic seismic anisotropy observed in mantle rocks may account for the recorded delay times only if the frozen tlow plane (foliation) is steeply dipping and flow direction Oineation) shallowly plunging (Mainprice et al., 1993). Various tectonic situations may explain such a fabric in the upper mantle. [START_REF] Silver | Implications for continental structure and evolution from seismic anisotropy[END_REF] suggested that in collision zones a preferred orientation of olivine b-axes in mantle rocks parallel to the bulk direction of shortening may result from a deformation of the lithospheric mantle by pure shear. [START_REF] Vauchez | Mountain building: strike parallel displacements and mantle anisotropy[END_REF] favoured simple shear through strike-slip faulting of the entire lithosphere. More recently, [START_REF] Nicolas | Why fast polarization directions of SKS seis mic waves are parallel to mountain belts?[END_REF] suggested that during pre-orogenic extension, mantle flow par allel to rift walls occurred and entailed a pre-oro genic lithospheric fabric that was retained and largely influenced the deformation of the lithosphere during subsequent orogeny.

One of the main difficulties in deriving an accu rate interpretation of shear-wave splitting is that the tectonic evolution of the lithosphere in many of the studied areas is poorly known. To avoid this prob lem, we examine splitting data from the Appalachi ans (eastem USA) and the Pyrenees (southem Eu rope). These two orogenic belts are of special inter est they have been ex:tensively studied and their geodynamic evolution is well constrained by geolog ical and geophysical data. In both orogens, a belt parallel fast shear-wave direction has been deter mined at most stations. In this paper, we consider the various hypotheses that may explain the measured seismic anisotropy. This highlights that, after an early orogenic event responsible for the main conti nent assembly in the two considered domains (i.e. the Grenville and the Hercynian orogenies for east em North America and southem Europe, respec tively ), successive geodynamic events have gener ated structures with similar trends and thus may have contributed in the development of the recorded anisotropy. Finally, in the Iight of the mantle fabric suggested by shear-wave splitting, we discuss the possible causes of structural inheritance, especially during the break-up of continents.

Shear-wave splitting in the Appalachians and the Pyrenees

Splitting of teleseismic SKS, SKKS and PKS waves was recently measured in the Appalachians (Barruol et al., 1994) and in the Pyrenees [START_REF] Souriau | Anisotropy beneath the North Pyrenean fault[END_REF][START_REF] Barruol | Anisotropy beneath the Pyre nees range from teleseismic shear wave splitting[END_REF]. Splitting parameters <t> (direction of polarization of the fast split shear wave) and ôt (the time lag between the arriva] of the two split shear waves) were obtained using the algorithm described by Sil ver and Chan (1991).

In the Appalachians, data were recorded using three stations (DTMR, CSMR, BCMR) of a portable three-component, broad-band seismic network from the Carnegie Institution of Washington, which ran for 18 months during the period l 990-1992. Data from permanent stations in the eastem USA were also used to measure the splitting of teleseismic shear waves: 6 years of data were analysed for the Global Digital Seismographic Network (GDSN) Sta tion SCP, 2 years for the Incorporated Research Insti tute for Seismology (IRIS) Stations HRV and CCM, 4 years for the GEOSCOPE Station WFM and 2 years for al! the other stations from the US National Seismic Network (USNSN). In the Appalachians, values of ôt typically around l.O s are observed and <f> is usually close to the strike of the regional foliation (N050-N060) for most of the stations lo cated on the extemal units in the central part of the belt or in the Grenville belt, west of the Appalachi ans front (RSCP). In the interna! domain, we ob tained contrasting results at stations located in the Piedmont, which represents a lithospheric unit ac creted to the North American Iithosphere during the Appalachian orogeny: the fast polarization direction is oriented roughly N090 at WFM, HRV and LSCT, and no detectable splitting was found at DTMR and CEH. Results at MIAR, in the Ouachitas Mountains, which represent the continuation of the Appalachians to the SW, display a fast split wave oriented N080 parallel to the trend of the belt in Arkansas. At WMOK, in the Wichita Mountains, the fast polariza tion direction is oriented N120, parallel to the Amar illo-Wichita uplift. CCM and FVM, two stations located on the pre-Grenvillian basement, display a fast direction oriented around N040.

Data in the Pyrenees were recorded during a 5 months deployment of three-component portable stations across the belt from the European plate to the Iberian plate and by Station NElO from the Network of Autonomously Recording Stations (NARS) array [START_REF] Oost | The NARS array: a seismic experiment in western Europe[END_REF]. This experiment showed that the three stations located on the Iberian plate display large delay times (1.3-1.5 s) and uniform fast polarization directions (NlOOE), whereas the stations located north of the North Pyrenean Fault are characterized by smaller delay times (0.5-1.0 s) and varying fast polarization orientation [START_REF] Barruol | Anisotropy beneath the Pyre nees range from teleseismic shear wave splitting[END_REF]. New preliminary results in the east ern Pyrenees display smaller variations of ôt through the North Pyrenean Fault but confirm the uniform <p direction (NlOO) across the belt.

In bath areas, for most stations <p is found to be close to the structural trend of the belt (Fig. 1) and ôt, although variable, is usually around or higher than 1 s. Possible sources for the observed anisotropy will be successively considered in the light of the geodynamic evolution of the Appalachians and the Pyrenees.

.1. Absolute plate motion

Considering that for eastem North America the absolute plate motion (APM; [START_REF] Gordon | Present plate motions and plate boundaries[END_REF] is close to the fast polarization direction of SKS waves, it may be argued that the source of the anisotropy lies in the asthenosphere flowing beneath the continent. This interpretation, however, requires similar fast polarization directions at nearby stations. The results obtained in the eastem USA (Barruol et al., 1994) do not fulfil this condition: the azimuth of the fast split shear wave is significantly oblique to the APM direction (N070) at WFM and HRV (N090), CBM and FVM (N40), or WMOK (N l20), for instance (Fig. l(a)), and no splitting has been detected at DTM and CEH. Moreover, these directions of fast wave correlate well with regional-scale Iithospheric structures.

In the Pyrenees also it is difficult to link the splitting characteristics with plate motion. The cur rent motion of western Europe is slow [START_REF] Minster | Present-day plate motion[END_REF][START_REF] Gripp | Current plate velocities relative to the hotspots incorporating the Nuvel-1 global plate motion mode![END_REF] and this makes more difficult an accurate determination of APM. However, recent models of plate motions (see the review in [START_REF] Gordon | Present plate motions and plate boundaries[END_REF], suggest that western Eurasia is moving either northeastward or very slowly southwestward depending on the reference frame. In any case, there is a significant obliquity (about 50 °) between the direction of the fast shear wave and the APM. Moreover, the recorded anisotropy parameters ( <p and ô t) display significant variations in a few tens of kilometres (Fig. l(b)), which are in disagree ment with the APM hypotheses [START_REF] Barruol | Anisotropy beneath the Pyre nees range from teleseismic shear wave splitting[END_REF]. [START_REF] Barruol | Anisotropy beneath the Pyre nees range from teleseismic shear wave splitting[END_REF] showed that in the central Pyrenees, the North Pyrenean Fault, which is interpreted as the boundary between the Iberian and European plates, also forms a limit between two groups of anisotropy parameters, and that the varia tion of ôt is well correlated with the variation of the thickness of the lithosphere beneath the stations de rived from studies of teleseismic P residuals (Poupinet et al., 1992). From these arguments, they suggested that the source of the measured anisotropy is primarily located in the lithospheric mantle.

Lithospheric shear zanes

In the internai domain of the Appalachians, the continental collision responsible for the formation of the belt resulted in an orogen-parallel displacement of lithospheric domains accommodated by continen tal-scale transcurrent shear zones parallel to the structural trend [START_REF] Vauchez | Southward extrusion tectonics during Carboniferous Africa-North America collision[END_REF][START_REF] Vauchez | Orogen-parallel tangential motion in the late Devonian-early Carboniferous southem Appalachians intemides[END_REF][START_REF] Hinze | The magnetic anomaly map of North America; a new tool for regional geologic mapping[END_REF]. Regarding the Pyrenees, the early Cretaceous rotation of Iberia relative to Eurasia was accommodated by transcurrent motion along the North Pyrenean Fault (e.g. [START_REF] Choukroune | Tectonic evolution of the Pyrenees[END_REF] and references therein). A possible explanation for the splitting characteristics is therefore to consider that the lithosphere beneath the stations bas a fabric owing to the deformation associated with transcur rent lithospheric shear zones (Vauchez and Nicolas,I 991 ). Although this interpretation may account for the splitting characteristics in the internai domains, where transcurrent faulting dominates (e.g. [START_REF] Choukroune | Tectonique des plaques et Pyrenées: sur le fonctionnement de la faille transformante nord-pyrénéenne; comparaison avec les modèles actuels[END_REF] and [START_REF] Vauchez | Southward extrusion tectonics during Carboniferous Africa-North America collision[END_REF], for the Pyrenees and the Appalachians, re- spectively), it does not hold for the belt-parallel fast shear waves in the extemal domains of the belts. Seismic reflection profiles across the Appalachians [START_REF] Cook | Thin-skinned tectonics in the crystalline southern Appalachians; COCORP seismic-reflection profiling of the Blue Ridge and Piedmont[END_REF] and the Pyrenees [START_REF] Choukroune | The ECORS Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt[END_REF][START_REF] Choukroune | Tectonic evolution of the Pyrenees[END_REF] strongly suggest that only a faint deformation, limited to the shallow crust, affected the lithosphere of the extemal domains (Fig. 2). Consequently, the seismic anisotropy recorded in these extemal do mains (for instance, at SCP in the Appalachians and at AVL in the Pyrenees) is unlikely to be related to a pervasive fabric of the lithosphere developed during the formation of the belts.

Pre-orogenic rifting

Pre-orogenic rift-parallel mantle flow [START_REF] Nicolas | Why fast polarization directions of SKS seis mic waves are parallel to mountain belts?[END_REF] may account, at least locally, for the litho spheric fabric in the two considered belts. In the Appalachians, the eastem margin of the North Amer ican plate (Laurentia) was formed during an Early Palaeozoic episode of rifting. This resulted in the opening of the prolo-Atlantic (Iapetus) Ocean, and, according to Dalziel et al. (1994) ( 1989) for the crust, and by [START_REF] Mattauer | Une autre interprétation du profil ECORS Pyrénées[END_REF] for the lithospheric mantle, of the ECORS seismic profile. In both cases, the external domain of the belt appears to have escaped pervasive reworking.

and its subsequent closure during the formation of the Appalachians is especially supported by the pres ence of an ultramafic belt [START_REF] Hess | Serpentinites, orogeny, and epeirogeny[END_REF] that stretches ail along the belt and in which ophiolites have been recognized (e.g. [START_REF] Drake | The Piney Branch complex-a metamorphosed fragment of the central Ap palachian ophiolite in northern Virginia[END_REF]. The ori entation of the initial rift (Fig. 3) was close from the trend of the present-day Appalachians belt (e.g. [START_REF] Thomas | Evolution of Appalachian salients and recesses from reentrants and promontories in the continental margin[END_REF][START_REF] Thomas | The Appalachian-Ouachita rifted margin of southeastem North America[END_REF]. Seisrnic profiling (e.g. [START_REF] Cook | Thin-skinned tectonics in the crystalline southern Appalachians; COCORP seismic-reflection profiling of the Blue Ridge and Piedmont[END_REF][START_REF] Owens | Seismic evidence for an ancien! rift beneath the Cumberland Plateau, Tennessee: a detailed analysis of broadband teleseismic P waveforms[END_REF] as well as gravity studies suggest that subsidiary rifts, formed during disruption of the eastem North American continent, have been preserved in the buried North American basement.

In the Pyrenees, lithospheric thinning occurred beneath the North Pyrenean Zone (the palaeo European margin). Pull-apart basins formed during the Albian-Cenomanian displacement of the Iberian Peninsula along the North Pyrenean Fault, related to the opening of the Bay of Biscay in the northem Atlantic (e.g. [START_REF] Choukroune | Tectonic evolution of the Pyrenees[END_REF] and references therein). These basins (a possible reconstruction is shown Fig. 4) are clearly associated with a thin crust and lithosphere (a proposed cross-section of the sys tem at that time is shown in Fig. 2(b), top). Evidence from gravity (Tome et al., 1989), seismic refraction [START_REF] Gallart | Crustal structure of the Paleozoic Axial Zone of the Pyrenees and the transition of the North Pyrenean Zone[END_REF] and seismic reflection (Choukroune and ECORS Pyrenees Team, 1989) suggests a thick Iberian crust and a thin crust be neath the European margin. Teleseismic P-residual studies (Poupinet et al., 1992) are compatible with a thinner lithosphere beneath the North Pyrenean Zone than beneath the Iberian plate. The presence of nu merous lherzolite massifs in the North Pyrenean Zone emplaced in sedimentary rocks together with high-temperature metamorphism [START_REF] Golberg | High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)[END_REF]) also argue for an extremely thinned lithosphere beneath the North Pyrenean Zone during the rotation of Iberia [START_REF] Vielzeuf | Crustal splitting and the emplacement of the Pyrenean lherzolites and granulites[END_REF]. The evidence for lithospheric thinning is re stricted to the North Pyrenean Zone. Rift-parallel mantle flow therefore may be invoked to explain the seismic anisotropy recorded on the palaeo-European margin but fails to explain data from the southern domain of the belt.

Post-orogenic rifting

Rifting subsequent to an orogeny may also have contributed to the fabric of the lithosphere responsi ble for the recorded anisotropy. In the eastern North American lithosphere, a clear parallelism between the Appalachians' structural trend, the continent mar gin, and the main magnetic anomalies that mark the growth of the ocean (Fig. 5) highlights that the initial rift propagated southward along the internai domain of the belt. Evidence of local deformation of the lithosphere related to the Atlantic Ocean opening is common across the belt. Triassic rifts, elongated parallel to the Appalachians' structural grain, have been formed in the eastern domain, from the Pied mont to the continental margin. Moreover, several occurrences of rocks with kimberlite affinity have been emplaced in the external domain (Blue Ridge) and even beyond the tectonic front of the belt, in the Cumberland plateau. Their emplacement bas been interpreted as an asthenospheric upwelling occurring during the early stages of the North Atlantic rifting [START_REF] Taylor | Kimberlitic magmatism in the eastem United States: relationships to mid-Atlantic tectonism[END_REF]. This suggests that the deformation field associated with the opening of the ocean pene trated far inland. It remains difficult, however, to believe that rift-parallel mantle flow may have per vasively affected the lithospheric mantle structure over such a large area. In the case of the Pyrenees, this process should be ruled out, as this area did not undergo post-orogenic rifting.

lnherited lithospheric fabric

An alternative explanation for the anisotropy recorded in the external domain of both the Ap palachians and the Pyrenees is to consider that the fabric of the lithospheric mantle responsible for the splitting of shear waves did not develop during the Appalachian and the Pyrenean orogenies, but during an earlier tectonic episode.

The two considered belts lie on the edge of continents that have been largely shaped during an earlier, major event of continent assembly: the Fig. 6. Schematic map showing the parallelism between the Appalachian-Ouachita orogen and the Grenville belt in eastern North America. Modified from [START_REF] Hoffman | Precambrian geology and tectonic history of North America[END_REF].

Grenvillian orogeny for the Appalachians and the Hercynian orogeny for the Pyrenees. Considering that during these orogenies a strip of continental lithosphere exceeding I 000 km in width was deeply deformed, it may be expected that a pervasive litho spheric fabric was developed. It is, in fact, during these events that the eastern North American and the western European continents were built.

The NE-SW-trending Appalachian belt and east-west-trending Ouachitas Mountains are consis tently parallel with the Grenville belt (Fig. 6 ) , which wraps around the old cratonic nucleus of North America. In the Canadian branch of the Grenville belt, the main thrust faults and limits between the tectono-metamorphic zones trend northeastward [START_REF] Hoffman | Precambrian geology and tectonic history of North America[END_REF] . Southward, exposures are scarce, but magnetic and gravimetric anomaly patterns char acteristic of the different domains of the belt defined in the northern area allow us to identify buried terranes and extrapolate their limits [START_REF] Hinze | The magnetic anomaly map of North America; a new tool for regional geologic mapping[END_REF]. This leads us to suggest that the seismic anisotropy recorded at the stations located in the external Appalachians, on the sedimentary nappes overthrusting the Grenvillian basement, is the signa ture of a Grenvillian lithospheric structure. That would be the case, for instance, for SCP, MCWV, MYNC, CSMR and BLA, which display fast split shear waves oriented NOS0-070.

The reconstruction of the Variscan belt [START_REF] Matte | Accretionary history and crustal evolution of the V ariscan belt in western Europe[END_REF] also displays a good parallelism between the Hercynian structures and the Pyrenees belt. Most of the pervasive structures in the Axial Zone of the Pyrenees are Hercynian in age and oriented parallel to the belt (e.g. [START_REF] Zwart | The Variscan geology of the Pyrenees[END_REF] . [START_REF] Carreras | The geological significance of the western termination of the Mérens fault at Port Vell ( Central Pyrenees)[END_REF] , for instance, showed that the main Hercynian structures are oriented parallel to the North Pyrenean Fault in the eastern Pyrenees. In the central Pyre nees, Hercynian regional foliation trends east-west and is steeply dipping [START_REF] Zwart | The Variscan geology of the Pyrenees[END_REF] . [START_REF] Vissers | Variscan extension in the Pyrenees[END_REF] found a consistent pattern of shallowly plunging Hercynian Iineations oriented parallel to the belt throughout the axial zone. These observations lead us to suggest that the roughly east-west fast direc tions found south of the North Pyrenean Fault may be inherited from a Hercynian Iithospheric structure. There is, furthermore, a striking similarity between the splitting parameters obtained in the Iberian Pyre nees and those obtained at the Toledo station in central Spain (Vinnik et al., 1989b;Silver and Chan, I 991 ) , which also give roughly east-west fast waves parallel to the Hercynian crustal fabric.

Evolution of continents and tectonic inheritance

Considering the evolution of the eastern margin of the North American continent, it appears that after the Grenville orogeny ( 1.3-1.0 Ga ) ail structures, including the Appalachian belt and the pre-and post-Appalachian rifts, formed parallel to the struc tural grain of the Grenville belt. Between 0.8 Ga and 0.6 Ga (i.e. more than 0.5 Ga after the Grenville orogeny ) episodic rifting resulted in the break-up of the Gondwana supercontinent parallel to the strike of the belt and in the formation of a passive margin. At the end of a period of ocean opening, the east American margin became convergent, and underwent the accretion of several exotic terranes and finally a collision with Africa, resulting in the building of a new belt (the Appalachians-Ouachitas system ) that fits the older one almost exactly. Tuen a similar evolution started again with the break-up of the amalgamated continent along the Appalachians inter nai domain and the opening of the North Atlantic Ocean.

A similar observation is made in the Pyrenees during the opening of the Bay of Biscay 120-80 m.y. ago. The initial break-up between Europe and Iberia was certainly dominantly transcurrent (see review by [START_REF] Choukroune | Tectonic evolution of the Pyrenees[END_REF]) : the North Pyrenean Fault formed parallel to the strike of the Hercynian belt, and Iberia began to translate southeastward along the fault. During the Upper Cretaceous, narrow basins elongated parallel to the trend of the North Pyrenean Fault, and therefore to the Hercynian belt, have been active. Finally, the convergence between lberia and Europe resulted in the building of the Pyrenees with a structural trend parallel to the Hercynian grain.

These two examples highlight the importance of structural inheritance in the evolution of continents. Although this has already been stressed by many workers (e.g. [START_REF] Wilson | Did the Atlantic close and then re-open?[END_REF][START_REF] Vinck | Preferential rifting of continents: a source of displaced terranes[END_REF]Dunbar andSawyer, 1988, 1989 ) , and several expia nations of this behaviour have been suggested, the mechanism through which tectonic inheritance arises still remains unclear. For eastern North America, a rheological heterogeneity of the lithosphere owing to the presence of the old cratonic nucleus with a cold and thick lithosphere could be invoked to explain continent break-up parallel to the boundary of the old craton. This explanation, however, remains un likely because the rupture did not occur at the inter face between the two domains, but far eastward within the internai domain of the Grenville belt. Furthermore, in the Pyrenees, the absence of an old nucleus precludes an influence of rheological hetero geneities in the break-up between Europe and Iberia.

The Grenvillian and Hercynian orogenies were responsible for the main assembly of the continental lithosphere of eastern North America and western Europe, and resulted in a tectonic fabric consistent over thousands of kilometres. The characteristics of shear-wave splitting in the external domain of the Appalachian and the Pyrenean belts, i.e. the ten dency for the fast shear wave to be polarized in a plane parallel to the structural grain of the Grenville and the Hercynian belts and time lags frequently exceeding 1 s, probably denote a pervasive structural fabric over the entire thickness of the lithospheric mantle. We suggest an alternative explanation from our observations: the source of the tectonic inheri tance in the break-up of the Grenvillian and the Hercynian lithosphere could be a mechanical anisotropy of the lithospheric mantle of orogenic areas caused by a well-developed fabric within the lithospheric mantle. The Grenvillian or Hercynian lithospheric fabric may have been substantially re worked in the internai domain of the belts during the Appalachian or the Pyrenean orogeny, but remained almost undeformed beneath the external domain. This is in agreement with a width of the orogens consider ably narrower at the level of the lithospheric mantle than at the surface.

Conclusions

The growing body of data available on the split ting of teleseismic shear waves on continents sup ports the idea that the main source of anisotropy lies in the lithosphere. A consistent parallelism between the orientation of the fast split shear wave and the trend of the continental-scale tectonic structures of the considered region is frequently observed, and this hints that the anisotropie structure responsible for shear-wave splitting is probably the frozen deforma tion fabric of the lithosphere formed during past orogenic events. Shear-wave splitting within the de formed asthenosphere beneath continental plates may also contribute, but cannot explain all the evidence, except in a few areas.

Interpreting shear-wave splitting in terms of geo dynamic processes, however, is not straightforward. In the Appalachians and the Pyrenees, where tele seismic shear-wave splitting is characterized by a fast wave dominantly parallel to the structural trend of the belt, successive events have generated struc tures that have similar trends and are probably asso ciated with similar mantle flow patterns. These struc tures may have therefore contributed in the develop ment of the recorded anisotropy, although their re spective contribution is difficult to recognize. In both belts, splitting characteristics are probably related in large part to the pre-orogenic structure of the litho sphere developed during an earlier, major event. During subsequent orogenic events, the lithospheres of eastern North America and western Europe, re spectively built during the Grenvillian and the Her cynian orogenies, seem to have largely escaped per vasive reworking, except in the internai domains of the Appalachians and the Pyrenees, and in the do main where the initial rifting of the Atlantic Ocean occurred. The evolution of these two areas during subsequent deformations, rifting and splitting of con tinents, as well as accretion of oceanic arcs and microplates and continent-continent collision, is characterized by the development of new tectonic structures that match the initial fabric of the Iitho sphere formed during the main event of continent assembly.

Structural inheritance in the break-up of conti nents is an obvious process in many places through out the world at least since the Neoproterozoic. The rupture of the continental lithosphere appears to propagate along the pre-existing orogenic fabric of the continents. Considering that shear-wave splitting parameters are in agreement with a steeply dipping fossil flow plane (foliation) and a shallowly plunging flow direction Oineation) in the lithospheric mantle, we propose that structural inheritance arises through mechanical anisotropy of the lithospheric mantle ow ing to a pervasive inherited fabric.

  Fig. I. Shear-wave splitting in the Appalachians (a) and the Pyrenees (b) (after Samarcq ( 1995) and Barruol and Souriau (1995)). Circle (a) and square (b) size is proportional to the delay lime, and the fast polarization directions are indicated by the black lines. (In the Appalachians, note the absence of anisotropy at CEH and DTMR.) In the Pyrenees, shaded areas indicate exposure of the Hercynian basement. NPF, North Pyrenean Fault.

Fig. 2 .

 2 Fig. 2. Interpretative geological cross-sections in the southern Appalachians (a ) and the Pyrenees (b ) . (a) is an interpretation of the crustal structures of the COCORP seismic profile by[START_REF] Hatcher | Tectonic implications of re gional aeromagnetic and gravity data from the southern Ap palachians[END_REF] and (b ) is a compilation of the interpretations by[START_REF] Choukroune | The ECORS Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt[END_REF] for the crust, and by[START_REF] Mattauer | Une autre interprétation du profil ECORS Pyrénées[END_REF] for the lithospheric mantle, of the ECORS seismic profile. In both cases, the external domain of the belt appears to have escaped pervasive reworking.

Fig. 3 .

 3 Fig. 3. Map showing the initial rift system of the Iapetus Ocean (Upper Proterozoic-Lower Palaeozoic) in eastem North America. Salients and recesses of the Appalachians are correlated with the geometry of the ridge-transform system. After Thomas (1991).

Fig. 4 .

 4 Fig. 4. Possible reconstruction of the geometry of the pull-apart basins formed during the displacement of the lberian plate along the North Pyrenean Fault during the middle Cretaceous. Insert shows a schematic section across a pull-apart basin. After Choukroune ( 1992).

-

  Rift basins, exposed -Rift basins, inferred D Mesozoic-Cenozoic basins -Exposed Paleozoic orogenic belt . Magnetic anomaly . . ...

Fig. 5 .

 5 Fig. 5. Post-orogenic rift-basins formed on the margin of eastem North America in relation to the opening of the Atlantic Ocean (simplified from Sheridan, 1989). Kimberlite occurrences in the externat domain of the belt are also shown. The east coast magnetic anomal y delineates approximately the boundary between the transitional crust and the oceanic crust. (Note the good parallelism between the trend of the Appalachian belt, the Triassic and Mesozoic rifts and the magnetic anomaly.)

Acknowledgements

This work benefited from many fruitful discus sions with K. Burke, D. Mainprice, A. Nicolas, A. Souriau, P. Silver and A. Tommasi. We are indebted to G. Ranalli and M. Daignières for helpful reviews. We acknowledge financial support from CNRS-INSU (Action Tomographie), International Lithosphere Programme, and CNRS-NSF collaboration pro gramme.