Maggy Heintz
email: heintz@dstu.univ-montp2.fr

Alain Vauchez
email: vauchez@dstu.univ-montp2.fr

Marcelo Assumpc °a ‹o
email: marcelo@iag.usp.br

Guilhem Barruol
email: barruol@upf.pf

Marcos Egydio-Silva

Jeune Equipe

Shear wave splitting in SE Brazil: an e¡ect of active or fossil upper mantle £ow, or both? §

Keywords: seismic anisotropy, orogenic lithosphere, SE Brazil, crust^mantle coupling, lithospheric-scale faults, shear wave splitting

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

Introduction

The tectonic behavior of the continental upper mantle during orogenesis still remains poorly understood. The question of the mechanical coupling/uncoupling of the crust and the mantle during the deformation of the continental lithosphere represents a key issue to improve our understanding of the geodynamics of continents. Shear wave splitting measurements provide an opportunity to investigate the upper mantle structure (see, e.g., [START_REF] Silver | Seismic anisotropy beneath the continents: probing the depths of geology[END_REF][START_REF] Savage | Seismic anisotropy and mantle deformation: what have we learned from shear wave splitting?[END_REF] for review), because the splitting parameters retrieved from the data are closely related to the crystallographic fabric developed due to past or present mantle £ow [3^6]. Comparison of core shear wave polarization anisotropy and surface geology may therefore provide insights into the coherence of the deformation across the lithosphere in orogenic domains.

Shear wave splitting measurements have been performed in southeastern Brazil over an area of ca. 100 000 km 2 that encompasses several tectonic domains of contrasted age and structure : the southern termination of the Sa ‹o Francisco craton, surrounded by the Ribeira and Brasilia mobile belts, both partially buried southwestward under the Parana ¤ large igneous province. The southernĉ entral Ribeira belt is of special interest since a network of lithospheric-scale wrench faults [START_REF] Hasui | The Ribeira folded belt[END_REF][START_REF] Egydio-Silva | High-temperature deformation in the Neoproterozoic transpressional Ribeira belt, southeast Brazil[END_REF] dominates its tectonic fabric. An e⁄cient crustm antle coupling might occur in major intracontinental transcurrent faults resulting in a polarization of the fast split shear wave parallel to the trend of the faults [START_REF] Vauchez | Wrench faults down to the asthenosphere: geological and geophysical evidence and thermo-mechanical e¡ects[END_REF]. In addition, receiver functions analysis [START_REF] Assumpc °a ‹o | Crustal thickness in SE brazilian shield by receiver function analysis: implications for isostatic compensation[END_REF] suggests that the eastern margin of the continent was thinned during the South Atlantic initial rifting, 140 Myr ago, raising the question of the e¡ect of this event on the fabric and anisotropy of the old continental lithosphere.

Seismic anisotropy and the splitting of teleseismic shear waves are commonly observed in continental areas. The anisotropy of rock elastic properties may have various origins (see, e.g., [START_REF] Babuska | Seismic Anisotropy in the Earth[END_REF]). There is, however, a consensus that anisotropy observed in teleseismic shear waves mostly results from the lattice-preferred orientation (LPO) of rock-forming minerals in the mantle [START_REF] Silver | Seismic anisotropy beneath the continents: probing the depths of geology[END_REF][START_REF] Vinnik | Golbal patterns of azimuthal anisotropy and deformations in the continental mantle[END_REF][START_REF] Mainprice | The seismic anisotropy of the Earth's mantle: from single crystal to polycrystal[END_REF]. The seismic anisotropy recorded at the Earth's surface represents a vertically integrated e¡ect of anisotropy from the core^mantle boundary to the surface. It is thus impossible to directly determine the depth of the anisotropy source. For vertically propagating shear waves, although small contributions from the DQ layer [START_REF] Kendall | Investigating causes of DQ anisotropy[END_REF], the lower mantle [START_REF] Wookey | Mid-mantle deformation inferred from seismic anisotropy[END_REF] and the crust [15^17] cannot be ruled out, the main source of the observed seismic anisotropy is restricted between the 410 km olivine^spinel phase transition and the Moho [START_REF] Silver | Seismic anisotropy beneath the continents: probing the depths of geology[END_REF][START_REF] Savage | Seismic anisotropy and mantle deformation: what have we learned from shear wave splitting?[END_REF][START_REF] Mainprice | Interpretation of SKS-waves using samples from the subcontinental lithosphere[END_REF]. Surface wave tomographic models including anisotropic parameters provide, however, tighter constraints on the depth of anisotropy. For instance, an automated Rayleigh waveform inversion has shown that most of the anisotropy is restricted within the ¢rst 200 km beneath the South American continent [START_REF] Heintz | Seismic anisotropy and surface wave tomography of South America[END_REF].

In the upper mantle, the most abundant and easily deformable mineral is olivine, whose LPO is probably responsible for the anisotropy observed in teleseismic waves. Two models have been proposed: either olivine LPO results from the active deformation of the asthenospheric mantle that accommodates absolute plate motion [START_REF] Vinnik | Golbal patterns of azimuthal anisotropy and deformations in the continental mantle[END_REF][START_REF] Bormann | Upper mantle anisotropy beneath Central Europe from SKS wave splitting: e¡ects of absolute plate motion and lithosphere-asthenosphere boundary topography?[END_REF], or it developed during the past deformation, then was 'frozen' in the lithosphere during post-tectonic thermal relaxation [1,21^25]. Lithospheric mantle xenoliths systematically display an olivine LPO [START_REF] Ben-Ismail | The Kaapvaal craton seismic anisotropy: petrophysical analyses of upper mantle kimberlite nodules[END_REF], suggesting that anisotropy is a pervasive and ubiquitous feature of the lithospheric mantle that can be preserved over a very long period. On the other hand, numerical models of the sublithospheric mantle deformation that accommodates plate motion show that a LPO, and thus an elastic anisotropy, can be generated in the asthenosphere [START_REF] Tommasi | Seismic anisotropy in ocean basins: Resistive drag of the sublithospheric mantle?[END_REF].

When a polarized S-wave propagates across an anisotropic medium (e.g. a textured upper mantle), it is split into two quasi S-waves that are polarized in two orthogonal planes and propagate at di¡erent velocities. Shear wave splitting is therefore characterized by two parameters : P, the orientation of the plane in which the fast split wave is polarized, and dt, the delay between the arrival times of the fast and slow split waves. Petrophysical studies have shown that P is a proxy of the orientation of the [100] axis of olivine (assumed to be statistically parallel to the £ow direction [START_REF] Ben-Ismail | An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy[END_REF]). The delay time depends on the intrinsic anisotropy, the thickness of the anisotropic layer, the orientation of the ray path with respect to the elastic matrix of the anisotropic medium, and the vertical coherence of the mantle fabric. The relatively simple relationships between splitting parameters and the crystallographic fabric of mantle rocks have made shear wave splitting a useful tool to investigate upper mantle deformation [START_REF] Silver | Mantle structural geology from seismic anisotropy[END_REF]. In continental areas, shear wave splitting measure-ment is commonly performed on teleseismic S-waves such as SKS, SKKS or PKS phases. These phases are generated from a P-to-S conversion at the core^mantle boundary. They are thus polarized along the radial direction and arrive at the station with a nearly vertical incidence. The presence of energy on the transverse component together with an elliptical particles motion in the horizontal plane indicate that S-waves propagated across an anisotropic medium on their way up to the surface.

Geological setting

The geology of southeastern Brazil spans from the Archean to the Cenozoic (Fig. 1). The Southern Sa ‹o Francisco craton comprises 2.7^3.3-Gyrold terrains and was stabilized at ca. 1.5 Gyr. It is surrounded by two slightly diachronic Neoproterozoic belts: (1) at the southwestern edge of the craton, the NW^SE-trending southern Brasilia belt, which results from the convergence of the Amazon and Sa ‹o Francisco cratons, and was stabilized at ca. 620 Myr, and (2) the Ribeira^Arac °ua| ¤ belt, which stretches along the eastern edge of the craton. The Ribeira^Arac °ua| ¤, and its African counterpart, the West-Congo orogen [START_REF] Trompette | 500 Ma): Pan-African-Brasiliano Aggregation of South America and Africa[END_REF], formed during the ¢nal amalgamation stage of western Gondwana (580^540 Myr), then split during the South Atlantic rifting (140 Myr).

The structural trend of the Ribeira^Arac °ua| ¤ belt changes at ca. 21 ‡S latitude, from NNE or NS within the northern domain, to ENE in the southern domain. This curvature is spatially correlated to the southern termination of the Sa ‹o Francisco craton. The change in structural trend is associated with a major variation in the tectonic style along the belt. Indeed, the northern domain is dominated by thrusting towards the craton whereas transpressional wrench faulting dominates in the central domain.

In between the southern Brasilia and Ribeira mobile belts lies the 'Cunha de Guaxupe '' (or Guaxupe ' syntaxis), a giant allochton that involves a composite section of Neoproterozoic deep continental crust, equivalent to the Brasilia belt. It has been interpreted as the root of a magmatic arc, possibly connected with the Rio de la Plata/ Parana ¤ craton. The southeastern boundary of the Cunha de Guaxupe ' was reworked by ductile wrench faults belonging to the Ribeira belt [START_REF] Camposneto | Terrane accretion and upward extrusion of high-pressure granulites in the Neoproterozoic nappes of Southeast Brazil: petrologic and structural constraints[END_REF].

West of 49 ‡W, the Ribeira and Brasilia belts are buried under the Parana ¤ basin, which represents an intracontinental large igneous province resulting from the upwelling of the Tristan da Cunha plume at ca. 135 Myr. Recent tomographic models performed by VanDecar et al. [START_REF] Vandecar | Assumpc °a ‹o, Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces[END_REF] and Schimmel et al. [START_REF] Schimmel | Upper mantle seismic velocity structure beneath SE Brazil from P-and S-wave travel time inversion[END_REF] imaged a cylindrical-shaped low-velocity anomaly, extending from 200 to at least 600 km deep beneath the Parana ¤ basin. This low-velocity anomaly was interpreted as the fossil conduit of the Tristan da Cunha plume. As the continent has migrated more than 3000 km westward since the opening of the South Atlantic ocean, VanDecar et al. [START_REF] Vandecar | Assumpc °a ‹o, Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces[END_REF] proposed that the South American continental lithosphere and the underlying upper mantle moved coherently since the Atlantic opening.

Data acquisition and processing

Three-component intermediate-and broadband portable stations from the Institute of Astronomy and Geophysics (Sa ‹o Paulo, Brazil), and from the French lithoscope and broad-band networks were deployed at 39 sites from the Parana ¤ basin to the coastal Ribeira belt, and northward to Brasilia (Fig. 2). This experiment was designed to test and complement the preliminary results obtained by James and Assumpc °a ‹o [START_REF] James | Tectonic implications of S-wave anisotropy beneath SE Brazil[END_REF]. In this paper, we focus on the measurements performed in the Ribeira and southern Brasilia belts. Sixty-nine events (see Table 1 2 ) at teleseismic distances in the range of 85^150 ‡, with a magnitude (Mw) v 5.5, and a good signal-to-noise ratio were selected using the Preliminary Determination of Epicenters from the National Earthquake Information Center. Due to both the location of the study area relative to the seismogenic zones and the event selection parameters, good recordings of core shear waves were only obtained over a rather narrow range of backazimuths (Fig. 3). Most events used for splitting analysis came from Fid-ji^Tonga^Kermadec and Papoua^New Guinea regions.

Shear wave splitting measurements were made using Silver and Chan's algorithm [START_REF] Silver | Shear wave splitting and subcontinental mantle deformation[END_REF], which consists in minimizing the energy on the transverse component, by rotating and time shifting the traces (e.g. see Fig. 4 2 ). The energy on the transverse component is automatically evaluated for many candidate values of P and dt (increments of 1 ‡ and 0.05 s respectively) to retrieve the P and dt pair that better removes anisotropy. This method assumes the seismic anisotropy to be homogeneous in a single horizontal layer.

The in£uence of time windowing and ¢ltering (Butterworth pass-band ¢lter, lower limit = 0.03 Hz and upper limit varying from 0.2 to 0.9 Hz, with an increment of 0.1 Hz) on the results was systematically checked. Individual measurements are sorted as 'good', 'fair' or 'poor', with respect to the four quality criteria de¢ned by Barruol et al. [START_REF] Barruol | Seismic anisotropy in the eastern United States: deep structure of a complex continental plate[END_REF]: (1) quality of the initial signal (signal-tonoise ratio and possible interference with direct S-wave), (2) ellipticity of particle motion in the horizontal plane when anisotropy is present, (3) linearization of particle motion by anisotropy removal, and (4) waveform coherence between the fast and slow split shear waves. Measurements satisfying the four criteria, producing similar pulse shapes and linear particles motion after correction along with fairly small error ellipses, were rated as 'good', while those verifying only three criteria were rated as 'fair'. Only 'good' and 'fair' measurements were retained for interpretation.

Silver and Chan's method [START_REF] Silver | Shear wave splitting and subcontinental mantle deformation[END_REF] has been extended by Wolfe and Silver [START_REF] Wolfe | Seismic anisotropy of oceanic upper mantle: shear wave splitting methodologies and observations[END_REF] to ¢nd the P and dt pair that better removes the anisotropy in a data set composed of several events recorded at a single station. This formulation also assumes a single anisotropic layer, and it is well suited for events azimuthally well distributed, i.e. when the initial polarization of the phases varies. Estimates are usually improved and the 95% con¢dence domain signi¢cantly reduced. In our case, as backazimuthal coverage is not homogeneous, this method, hereinafter referred to as the 'multipleevent method', was used for stations with relatively homogeneous splitting parameters, to determine an average value of P and dt from 'good' and 'fair' measurements.

Results

After severe selection based on the teleseismic distance, magnitude, and signal-to-noise ratio, between 1 and 20 measurements per station have been retained (Table 2). Results are summarized in Fig. 5A (individual shear wave splitting measurements) and Fig. 5B (null measurements and results of the multiple-event method).

Splitting parameters display consistent variations along the Ribeira belt and can be sorted into three domains, corresponding to: (1) the northern, thrust-dominated part of the Ribeira belt, (2) the central, wrench fault-dominated domain, and (3) the southernmost part, characterized by a shallower deformation and lower grade metamorphism.

At the transition between the northern and central domains, splitting parameters were retrieved at three stations: CRG, NATB and LJM. At CRG, 15 measurements have been made on 12 events with di¡erent backazimuths (between 142.4 ‡ and 234.9 ‡) and a good signal-to-noise ratio. None of these measurements show evidence of energy on the transverse component (null measurements; see Fig. 6 2 ). A 'null' measurement may be due either to an absence of anisotropy or to an initial polarization of the incoming shear wave parallel or orthogonal (N320 ‡E or N050 ‡E) to the fast anisotropic direction. Considering the backazimuthal coverage of the data used at this station, the results suggest an apparent isotropy.

NATB, LJM, JFOB and BARB display a rather large dispersion of P and dt with two main P directions : VENE and VWNW, respectively slightly oblique and almost normal to the structural trend. At NATB, for instance, two measurements performed on two distinct events give well-constrained (good to very good) but sig-ni¢cantly di¡erent results: P = 361 þ 10 ‡ and dt = 1.1 þ 0.25 s on event 9 (backazimuth = 358.2 ‡), and P = 85 þ 7 ‡ and dt = 0.8 þ 0.1 s on event 16 (backazimuth = 228.5 ‡). These results might suggest either a dipping anisotropic structure or multiple anisotropic layers. However, the poor backazimuthal coverage impedes further testing of this hypothesis. Considering the complexity of the results at those stations, we avoided computing the average value using the multiple-event method and retained both dominant P directions.

In the central Ribeira belt, stations located close to the large-scale dextral wrench faults show polarization directions of the fast split shear wave ranging between N060 ‡E and N080 ‡E. These directions are close to the structural trend of the Ribeira belt (N065 ‡E). They are also close to the direction of the APM as determined using HS3-Nuvel 1A (N253 ‡E) [START_REF] Gripp | Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model[END_REF] and signi¢cantly oblique on the APM determined using Wang and Wang's model (N283 ‡E) [START_REF] Wang | Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame[END_REF].

Delay times measured at IGAB and ALP, respectively located on the Igarata and Alem Paraiba wrench faults, are exceptionally large. Individual splitting measurements performed on six events, with similar backazimuth recorded at IGAB, yield values ranging between 1.95 þ 0.3 s and 3.55 þ 0.18 s (Fig. 7). These values result from high-quality measurements and are regarded as reliable. Measurements have been made on both SKS and SKKS phases for event 39 and show delay time of respectively 3.55 þ 0.20 s and 3.40 þ 0.45 s. This good agreement supports the idea that anisotropy is acquired along the segment of the path below the station that is common to both waves. An average value of 2.40 þ 0.30 s has been computed for dt on four events using the multiple-event method. This seems a reasonable estimate for the anisotropy at IGAB. For the ALP station, the delay time obtained for individual measurements varies between 1.5 and 2.5 s, and an average value of 1.92 þ 0.18 s was obtained using two events and three phases. In the southern Ribeira belt, the splitting parameters P and dt retrieved at station JUQB are respectively in the range of N82^101 ‡E and 0.81 .55 s, and represent a middle term between those obtained at IGAB (PVN54^74 ‡E; dtV1.95^3.55 s), and at RSTB (PVN96^123 ‡E; dtV0.65^2.8 s). Splitting parameters retrieved at the GEO-SCOPE station SPB are in the range of N811

12 ‡E for P and 1.15^1.95 s for dt. An average value of P = 86 þ 1.5 ‡ and dt = 1.45 þ 0.08 s was computed using seven good-quality events. The P directions retrieved at those stations suggest a rotation of the polarization plane of the fast split wave between the central and southern parts of the Ribeira belt.

In the southern Brasilia belt, measurements performed at FURB, FRMB [START_REF] James | Tectonic implications of S-wave anisotropy beneath SE Brazil[END_REF] and BSCB are characterized by consistent P and dt values. The polarization plane of the fast split shear waves trends VN125 ‡E, i.e. parallel to the tectonic grain of the belt. The delay times measured are around 1.0 s.

A sharp change in the direction of the polarization plane of the fast S-wave occurs between the Ribeira and Brasilia belts (Fig. 8). Measurements performed for the same event (39) at two stations, located in the Ribeira (JFOB) and Brasilia belts (BSCB) (Fig. 8) give P = 90 þ 5 ‡ at JFOB and 363 þ 6 ‡ at BSCB. This di¡erence is therefore not due to the use of di¡erent subsets of data and re£ects short wavelength variations in the upper mantle structure.

In the Cunha de Guaxupe ', south of the Brasilia belt, a single measurement performed at SJM gives P = N138 ‡E þ 2 ‡ and dt = 2.0 þ 0.3 s. AREB reaches a null result (possible directions of anisotropy: N320 ‡E or N050 ‡E) in agreement with measurements at SJM, FURB, FRMB and BSCB. Although only few measurements were performed at SJM and AREB, the good consistency with the results obtained at the stations in the southern Brasilia belt suggests a similar mantle structure beneath both the southern Brasilia belt and the Guaxupe ' allochton.

Measurements performed at CAMB, close to the Atlantic coast, yield P ranging from 46 ‡ to 89 ‡, with two values near east^west and low dt (0.5^0.9 s). The low dt that characterizes this station might suggest that the orogenic lithosphere was a¡ected by the Atlantic rifting.

As far as possible, for each event measurements were made on both SKS and SKKS phases. In most cases, similar splitting parameters were retrieved for the two phases. For a few events, however, the splitting parameters retrieved from the SKS and SKKS phases display a signi¢cant discrepancy (see Fig. 9 2 ). For event 63 recorded at JFOB, both phases are characterized by a good signal-to-noise ratio. Measurements using the SKS phase (see Fig. 9A 2 ) yield P = 78 þ 9 ‡, and dt = 0.6 þ 0.1 s, whereas for the SKKS phase (see Fig. 5. Measured directions of polarization of the fast split wave. (A) Good (black) and fair (gray) measurements. The length of each line is proportional to the delay time. (B) Null measurements associated with the results of the multipleevent method (gray bold lines, with the lengths of lines proportional to the delay time). Crosses denote absence of splitting. Each branch is either parallel or perpendicular to the backazimuth of the incoming waves and represents a possible direction of anisotropy. Fig. 9B 2 ) P = 97 þ 8 ‡ and dt = 1.35 þ 0.25 s. Both measurements are of good quality and appear as equally reliable. The source of this discrepancy should be either that di¡erent anisotropies have been recorded by both waves along the segments of their ray paths that are not common or that the di¡erence in incidence angle for SKS and SKKS beneath the stations results in sampling an apparently di¡erent anisotropy. This latter case is especially expected for the dipping mantle fabric.

Discussion

Because the relationships between seismic anisotropy and the development of LPO of minerals in upper mantle rocks are quite straightforward, shear wave splitting may be interpreted in terms of tectonic fabric within the upper mantle. The poor vertical resolution of shear wave splitting techniques, however, requires discussing whether a lithospheric or an asthenospheric source, or a combination of both, may account for the observed anisotropy.

Asthenospheric origin of the anisotropy

Simple asthenospheric £ow due to the decoupling between a '£at' lithospheric plate and the underlying mantle [START_REF] Vinnik | Golbal patterns of azimuthal anisotropy and deformations in the continental mantle[END_REF] would result in splitting parameters coherent in both P and dt over geological domains that di¡er in both age and structure. APM determined for southeastern Brazil trends N253 ‡E using the HS3-Nuvel 1A model [START_REF] Gripp | Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model[END_REF], a direction close to the structural trend of the Ribeira belt (N065 ‡E), and N283 ‡E using the T22A model [START_REF] Wang | Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame[END_REF]. An asthenospheric origin of the anisotropy could thus account for the pattern of anisotropy in the Ribeira belt. But short spatial-scale variations of splitting parameters observed over the entire region, especially between the southern Brasilia and Ribeira belts (Fig. 8), does not favor a model of simple asthenospheric £ow.

Recent surface [START_REF] Heintz | Seismic anisotropy and surface wave tomography of South America[END_REF][START_REF] Silveira | Anisotropic tomography of the Atlantic ocean from Rayleigh surface waves[END_REF][START_REF] Silveira | Anisotropic tomography of the Atlantic Ocean[END_REF] and body [START_REF] Vanderlee | Upper mantle S velocity structure of central and western South America[END_REF] wave tomographic models have shown that the Sa ‹o Francisco craton has a thicker lithosphere than the surrounding domains. This topography of the lithosphere^asthenosphere boundary might de£ect the asthenospheric £ow [START_REF] Bormann | Upper mantle anisotropy beneath Central Europe from SKS wave splitting: e¡ects of absolute plate motion and lithosphere-asthenosphere boundary topography?[END_REF]. This model was suggested by Barruol et al. [START_REF] Barruol | Seismic anisotropy in the eastern United States: deep structure of a complex continental plate[END_REF] as a possible explanation for shear wave splitting observations in the eastern United States. As in southeastern Brazil, the APM in the eastern United States is roughly parallel to orogenic belts (the Grenville and Appalachians belts) that bound the North American craton. Although a lithospheric source for seismic anisotropy is likely, especially considering the short wavelength variations in splitting parameters, a contribution from the asthenospheric mantle £owing around the cratonic root cannot be ruled out. Fouch et al. [START_REF] Fouch | Shear wave splitting, continental keels, and patterns of mantle £ow[END_REF] have modeled the asthenospheric £ow around a cylindrical cratonic keel including a divot to better reproduce the shape of the North American craton as revealed by tomographic studies [START_REF] Vanderlee | Upper mantle S velocity structure of North America[END_REF]. From the modeled £ow ¢eld, they suggested that anisotropy generated in the subcontinental upper mantle is de£ected around cratonic keels and that variations in the directions of the polarization plane of the fast split shear waves may occur over short distances where the boundary of the cratonic keel displays reentrants and promontories.

The Fouch et al's [START_REF] Fouch | Shear wave splitting, continental keels, and patterns of mantle £ow[END_REF] model is, however, not directly transposable to SE Brazil. In contrast with the modeled sti¡ domain, the Sa ‹o Francisco craton has a NS elongated shape. The APM determined using the HS3-Nuvel 1A model [START_REF] Gripp | Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model[END_REF] is N253 ‡E and thus oblique with respect to the long axis of the craton.

Considering this situation, an asthenospheric £ow may be expected at the southeastern tip of the craton that parallels the structural trend of the Ribeira belt, and thus, might explain the observed anisotropy. This model, however, fails to explain the anisotropy pattern in the southernmost Brasilia belt, since this would require a sharp curvature of the £ow direction, hardly compatible with mantle rheology [START_REF] Rabinowicz | A rolling mill e¡ect in asthenosphere beneath oceanic spreading centers[END_REF]. Wang and Wang's [START_REF] Wang | Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame[END_REF] model suggests a South American APMV N283 ‡E. Considering this value, a de£ected asthenospheric £ow around the southern tip of the craton might explain the pattern of anisotropy in the southernmost Brasilia belt, but not within the Ribeira belt. Moreover, asthenospheric £ow alone fails to explain short-scale dt variations and large dt values recorded at stations located above the lithospheric-scale shear zones within the Ribeira belt.

Lithospheric origin of the anisotropy

There is a conspicuous correlation between the orientation of the fast split shear wave polarization plane and the structural trend of both the Brasilia and Ribeira belts, respectively trending N125 ‡E and N065 ‡E. This correlation may be regarded as a reliable evidence that shear wave splitting results from anisotropy frozen within the lithosphere since the last orogenic event (Neoproterozoic orogeny, ca. 600 Myr). In the Ribeira belt, this agreement is particularly good for stations close to the major wrench fault, but an obliquity with the structural trend is observed in the northern and southern domains. This discrepancy may result either from variations in the tectonic fabric frozen within the lithospheric mantle (thrust-type in the northern and southern domains, wrench fault-type in the central domain) or from a combination of anisotropies from various sources (lithospheric vs. asthenospheric) variable along-strike. Delay times usually observed in southeastern Brazil are close to the worldwide average of 1 s [START_REF] Silver | Seismic anisotropy beneath the continents: probing the depths of geology[END_REF]. However, two stations located above the lithospheric-scale strike-slip faults yield dt among the largest ever observed. Average dt retrieved at ALP, located on the Alem Paraiba fault, and at IGAB, located on the Igarata fault, are respectively 1.92 þ 0.18 s and 2.40 þ 0.30 s. Interpretation of such high delay times requires either a much larger intrinsic anisotropy of mantle rocks than commonly described in petrophysical studies, or an anisotropic layer thicker than the lithosphere. Crustal anisotropy may account for delay times up to 0.2^0.3 s [START_REF] Mcnamara | Shear wave anisotropy beneath the Tibetan Plateau[END_REF][START_REF] Herquel | Anisotropy and crustal thickness of Northern-Tibet. New constraints for tectonic modelling[END_REF], consistent with values predicted from rock physics [START_REF] Herquel | Anisotropy and crustal thickness of Northern-Tibet. New constraints for tectonic modelling[END_REF]. According to Barruol and Mainprice [START_REF] Barruol | A quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic SKS waves[END_REF], maximum delay times up to 0.2 s per 10 km of pervasively struc-tured crust might be expected. Major wrench faults in the Ribeira belt are marked by wide (up to 10 km) mylonitic structures with pervasive vertical foliations and subhorizontal lineations [START_REF] Egydio-Silva | High-temperature deformation in the Neoproterozoic transpressional Ribeira belt, southeast Brazil[END_REF]. Assuming these faults crosscut the entire crust, the maximum crustal contribution to splitting would be V0.6 s. Then, a delay time of 1.8 s has to be generated in the upper mantle.

Tomographic models [START_REF] Heintz | Seismic anisotropy and surface wave tomography of South America[END_REF] suggest a normal lithosphere thickness (V100 km) beneath the belts surrounding the Sa ‹o Francisco craton. Estimates of shear wave anisotropy for continental lithospheric mantle rocks usually range from 3% to 10% for waves propagating in a direction nearly normal to the lineation in the foliation plane [START_REF] Mainprice | Interpretation of SKS-waves using samples from the subcontinental lithosphere[END_REF][START_REF] Ben-Ismail | The Kaapvaal craton seismic anisotropy: petrophysical analyses of upper mantle kimberlite nodules[END_REF]. Assuming a seismic anisotropy of 5%, a value regarded as reasonable for a 'normal' upper Fig. 8. Shear wave splitting measurements performed on the same event (event 39, see Table 1 2 ) recorded at BSCB and JFOB respectively located in the Brasilia and Ribeira belts. mantle, the anisotropic layer would be 170 km thick to explain a delay time of 1.8 s, i.e. a thickness that exceeds the lithosphere thickness and would suggest a sublithospheric contribution. Then, two interpretations are possible : either the frozen fabric in the lithosphere and the APM-related fabric in the asthenosphere are close enough to mimic a single anisotropic layer, or the lithospheric and asthenospheric fabrics result from a single event, and no decoupling occurred between the lithosphere and the asthenosphere since Neoproterozoic times. Considering that the HS3-Nuvel 1A APM is close to the structural trend of the Ribeira belt, the ¢rst hypothesis is favored. On the other hand, body waves tomography of southeastern Brazil [START_REF] Vandecar | Assumpc °a ‹o, Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces[END_REF][START_REF] Schimmel | Upper mantle seismic velocity structure beneath SE Brazil from P-and S-wave travel time inversion[END_REF] imaged a cylindrical low-velocity anomaly extending beneath the Parana ¤ large igneous province from 200 to at least 600 km. This anomaly, interpreted as the fossil signature of the Tristan da Cunha plume, is regarded as a strong argument supporting a coherent motion of the whole upper mantle since at least 130 Myr [START_REF] Vandecar | Assumpc °a ‹o, Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces[END_REF]. Coupling of the lithospheric and sublithospheric mantle would result in an absence of sublithospheric deformation due to current day plate motion.

Assuming the seismic anisotropy entirely generated into the lithosphere, an average S-wave seismic anisotropy of 11% would be required to generate dt in the range 1.5^2 s in a 70^80-km-thick lithospheric mantle. Although these values are quite large compared with the 5% considered previously, they are still realistic. Ben Ismail and Mainprice [START_REF] Ben-Ismail | An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy[END_REF] have reported S-wave anisotropies larger than 10% and up to 15%. These values were, however, calculated for pure olivine aggregates and should be slightly reduced to take into account the e¡ect of 20^30% of pyroxenes C Fig. 10. Apparent P (A,C) and dt (B,D) vs. backazimuth of the incoming wave at station TRRB, modeled using two layers of anisotropy with the values of P 2 and dt 2 in the bottom layer representing: (A) the direction of APM determined using the HS3-Nuvel 1A model, N253 ‡E [START_REF] Silver | Shear wave splitting and subcontinental mantle deformation[END_REF] and 0.8 s; (B) the direction of APM determined using the T22A model [START_REF] Wolfe | Seismic anisotropy of oceanic upper mantle: shear wave splitting methodologies and observations[END_REF] and 0.8 s. In both models, the fast S-wave polarization in the upper layer is parallel to the structural trend of the Ribeira belt (P 1 = 65 ‡, dt 1 = 1 s). in lherzolites. Major wrench faults such as the Alem Paraiba and Igarata faults represent structures in which strain is localized and a strong LPO is expected to develop in the lithospheric mantle.

For several stations located along the Ribeira belt, P, although retrieved from good-quality data, varies between N060 ‡E and N080 ‡E. Under the assumptions that the anisotropic medium possesses a hexagonal symmetry with a horizontal axis, and that anisotropy is restricted to a single homogeneous layer, one should expect no large azimuthal variation in the splitting parameters retrieved at a single station from S-waves propagating with a near-vertical incidence. The observed variation may result from laterally varying anisotropy, dipping axis of symmetry or presence of several anisotropic layers. The similarity of splitting parameters obtained at two nearby stations (e.g. ALP and TRRB) precludes short-scale variations of anisotropy.

Silver and Savage [START_REF] Silver | The interpretation of shearwave splitting parameters in the presence of two anisotropic layers[END_REF] have shown that when two anisotropic layers are present beneath a station, splitting parameters measured assuming a single anisotropic layer will display coherent azimuthal variations. We performed two-layer modeling in order to explain the dependence of the values with the backazimuth. Systematic testing of all possible solutions would generate a large amount of models di⁄cult to handle. We therefore restricted our models to situations having a geological meaning.

Modeling has been performed for several stations located in the northern part of the Ribeira belt, but only results obtained at stations TRRB and JFOB are presented. The most popular model of two-layers anisotropy is to consider that the lower layer corresponds to sublithospheric mantle and the upper layer to the 'orogenic' lithosphere. Two di¡erent situations have then been modeled for stations TRRB and JFOB (Figs. 10 and11) with the anisotropy in the lower layer correspond- ing either to the HS3-Nuvel 1A [START_REF] Gripp | Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model[END_REF] or to the T22A [START_REF] Wang | Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame[END_REF] APM (P 2 = 253 ‡ or P 2 = 283 ‡ respectively). In both models, the fast S-wave polarization in the upper layer is parallel to the structural trend of the Ribeira belt (P 1 = 65 ‡). The delay times chosen for each layer have been calculated considering 5% of intrinsic anisotropy and a 100km-thick upper layer (dt 1 = 1 s). For the lower layer, we check the in£uence of the delay time on the ¢tting of the model with the data. Increasing the lower-layer anisotropy has slight e¡ects on the apparent P distribution and only shifts the apparent dt towards larger values. Only models with dt 2 = 0.8 s are shown in this paper. In model 1, P 1 and P 2 are almost parallel. There is thus a good vertical coherence between both anisotropic layers and the apparent polarization of the fast split S-wave remains close to 70 ‡. This model does not account for the data dispersion observed at stations TRRB and JFOB. Model 2 (Fig. 10B) also failed to simulate the observed variations at both stations.

Ozalaybey and Savage [START_REF] Ozalaybey | Double-layer anisotropy resolved from S phases[END_REF] have shown that when teleseismic S-waves propagate across two seismically anisotropic layer, the apparent splitting parameters display a large variability for waves with backazimuth close to the fast or slow axis of the bottom layer. As the mean backazimuth for events arriving from the Tonga and recorded at TRRB is ca. N220 ‡E, this direction was used as anisotropy direction in the lower layer (P 2 = 220 ‡, dt 2 = 0.8 s for the lower and P 1 = 65 ‡, dt 1 = 1 s for the upper layer). This model results in a large variation of the apparent splitting parameters for backazimuths around 220 ‡ that ¢t well with splitting parameters observed at TRRB and JFOB (Fig. 11). If P 1 and P 2 are inverted, the ¢t with the data obtained at TRRB and JFOB is slightly deteriorated. However, the di¡erence between these two models should only be evaluated by using data with a better backazimuthal coverage.

Although these models have been performed on a data set with a poor backazimuthal coverage, they suggest that a contribution of the lithospheric and the asthenospheric mantle cannot be ruled out. Models with an upper-layer anisotropy corresponding to the structural trend of the Ribeira belt and a lower-layer anisotropy corresponding to the HS3-Nuvel 1A [START_REF] Gripp | Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model[END_REF] or the T22A [START_REF] Wang | Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame[END_REF] APM failed to satisfactorily ¢t the observed splitting parameters. The best ¢t was obtained with a lower-layer anisotropy close to the main backazimuth of the core shear waves used in this study. This direction, however, has no clear geological meaning and this suggests that the dispersion of the direction of the fast split shear wave polarization observed in the central Ribeira belt re£ects a greater complexity than the simple two-layer models. Schimmel et al. [START_REF] Schimmel | Upper mantle seismic velocity structure beneath SE Brazil from P-and S-wave travel time inversion[END_REF], for instance, have shown that body wave velocities are abnormally high beneath the central Ribeira belt and this may suggest a complex lithospheric structure at depth (fossil slab?).

Conclusion

Shear wave splitting parameters retrieved at stations deployed in southeastern Brazil around the southern termination of the Sa ‹o Francisco craton are organized in a coherent pattern. Southwest of the craton, in the Brasilia belt, the fast split S-waves are coherently polarized in a direction VN125 ‡E, i.e. parallel to the structural trend of the belt. Southeast of the craton, the fast split S-waves are polarized in a plane trending N0600 80 ‡E, parallel to the Ribeira belt.

A model of asthenospheric £ow around the southern termination of the craton may explain part of the data set, but fails to account for both the anisotropy pattern in Ribeira and Brasilia belts. A network of large-scale wrench faults characterizes the central Ribeira belt. The orientation of the fast split shear wave polarization plane is parallel to the trend of the faults, and delay times measured at nearby stations are among the largest ever observed (average of 2.4 s). Such large delay times require either 11% of intrinsic anisotropy for an average lithospheric thickness of 100 km, or an asthenospheric contribution. In this latter case, anisotropy within the sublithospheric mantle should be close to the anisotropy within the lithospheric mantle implying either (1) a parallelism of the anisotropies respectively frozen in the lithosphere and generated in the asthe-nosphere due to the ongoing deformation that accommodates plate motion, or (2) the existence of a single tectonic fabric in the lithosphere and the asthenosphere, meaning that no decoupling occurred since Neoproterozoic times.

The ¢rst hypothesis is supported by the parallelism between the HS3-Nuvel 1A APM and the structural trend of the Ribeira belt but not by the results of the two-layer models. However, these models are poorly constrained due to the limited backazimuthal coverage of the events used in this study. The second hypothesis is in agreement with the suggestion by VanDecar et al. [START_REF] Vandecar | Assumpc °a ‹o, Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces[END_REF] and Schimmel et al. [START_REF] Schimmel | Upper mantle seismic velocity structure beneath SE Brazil from P-and S-wave travel time inversion[END_REF] that no decoupling occurred within the upper mantle beneath southeastern South America since the opening of the Atlantic Ocean. This suggestion, inferred from the observation of a low-velocity anomaly beneath the Parana ¤ basin interpreted as the fossil conduit of the Tristan da Cunha plume, implies that no deformation would be produced in the asthenospheric mantle to accommodate the absolute plate motion of South America.

In any case, it seems di⁄cult to interpret the pattern of anisotropy obtained in southeastern Brazil without considering a contribution of both the lithospheric and sublithospheric mantle. A strong signature of the major wrench faults in the observed anisotropy is a conclusion di⁄cult to avoid. The good corrrelation of the orientation of the polarization plane of the fast S-wave with the structural trend of the Brasilia belt also favors a lithospheric origin. On the other hand, a contribution of the sublithospheric mantle is likely since it may explain the dispersion of the splitting parameters and also part of the large dt recorded above lithospheric-scale shear zone. However, simple lithosphere^asthenosphere models fail to account for the bulk of the anisotropy pattern, and this suggests that the deep structure beneath southeastern Brazil is more complex than suspected.

Fig. 1 .

 1 Fig. 1. Cartoon showing the main geological subdivisions of southeastern Brazil.

Fig. 3 .

 3 Fig.3. Distribution of epicenters of events used for this study.

Fig. 2 .

 2 Fig. 2. Location of the 39 instrumented sites. Dashed line = main geological subdivision. Black squares = stations for which results are presented in this paper; the black star represent SPB (Sa ‹o Paulo) permanent GEOSCOPE station to the south, and BDFB (Brasilia) permanent GTSN station to the north.

Fig. 7 .

 7 Fig.7. Examples of splitting measurements at station IGAB. (A) Event 39 (see Table12 : azimuth = 129.7 ‡, backazimuth = 231.2 ‡): P = 70 þ 3 ‡, and dt = 3.4 þ 0.45 s, on phase SKKS. (B) Event 39 (see Table12 : azimuth = 129.7 ‡, backazimuth = 231.2 ‡): P = 67 þ 1.5 ‡, and dt = 3.55 þ 0.25 s, on phase SKS. (C) Event 42 (see Table12 : azimuth = 150.2 ‡, backazimuth = 212.6 ‡): P = 64 þ 2 ‡, and dt = 3.2 þ 0.20 s.

  Fig.8. Shear wave splitting measurements performed on the same event (event 39, see Table12 ) recorded at BSCB and JFOB respectively located in the Brasilia and Ribeira belts. (A) Station BSCB (azimuth = 128.7 ‡, backazimuth = 231.2 ‡): P = 363 þ 6 ‡ and dt = 1.00 þ 0.18 s. (B) Station JFOB (azimuth = 130.2 ‡, backazimuth = 229.9 ‡): P = 90 þ 5 ‡ and dt = 0.95 þ 0.10 s.

CFig. 11 .

 11 Apparent P and dt vs. backazimuth of the incoming wave at stations TRRB (A,B) and JFOB (C,D), modeled using two layers of anisotropy (upper layer: P 1 = 65 ‡, dt 1 = 1 s; lower layer: P 2 = 220 ‡, dt 2 = 0.8 s).

Table 2

 2 Station information and averaged splitting results (M.E.P. = multiple-event parameters)

	Station		Event	P ( ‡)	Error ( ‡)	dt (s)	Error (s)	Phase	Quality	M.E.P.
										P ( ‡)	dt (s)
										Error ( þ ‡)	Error ( þ s)
	ALP		39	81	6	2.016	0.352	SKKS	G	72	3
	Lat. ( ‡)	321.880	39	63	3	2.528	0.416	SKS	G	1.92	0.18
	Long. ( ‡)	342.664	56	67	4.5	1.664	0.192	SKKS	G
	AREB		14	372	22.5	0.5	2.225	SKS	GN
	Lat. ( ‡)	321.362	14	367	6	1.35	0.275	SKKS	G
	Long. ( ‡)	346.123	15	327	4	4	0.375	SKS	G
			16	347	8.5	1.7	0.725	SKS	G
			16	352	6.5	2.1	0.6	SKKS	G
			17	56	22.5	0.35	2.15	PKS	GN
			18	340	22.5	2.4	3.175	SKKS	FN
			19	342	22.5	1.5	2.725	SKKS	FN
			20	23	2.5	4	3.975	SKKS	GN
			21	41	22.5	2.5	3.225	SKS	GN
			22	337	22.5	4	4.025	SKKS	GN
			23	16	22.5	1.75	2.85	SKS	FN
			24	340	5.5	2.8	3.425	SKS	GN
			24	343	22.5	0.95	2.5	SKKS	GN
			25	348	22.5	1.35	2.65	SKS	GN
			25	352	22.5	1.25	1.25	SKKS	FN
			28	347	22.5	1.45	1.225	PKS	GN
			31	344	5.5	1.55	0.55	SKS	GN
			31	345	22.5	1.4	2.675	SKKS	FN
			32	9	22.5	1.4	2.675	SKKS	FN
			35	359	22.5	0.55	1.325	PKS	GN
			36	358	22.5	0.5	0.75	PKS	GN
			37	374	22.5	0.45	2.2	SKKS	FN
			39	343	22.5	2.15	3.05	SKS	FN
			40	352	22.5	2.3	3.175	SKKS	FN
			40	355	6	2.3	3.125	SKS	FN
	ATDB		2	375	22.5	0.85	1.95	SKKS	FN	157	0.80
	Lat. ( ‡)	321.290	6	354	8	1.65	0.625	SKS	F
	Long. ( ‡)	342.861	9	313	3.5	2.45	0.7	SKKS	G
			11	332	22.5	1.8	1.35	SKKS	FN
			12	380	22	0.75	1.65	SKKS	FN
			67	381	22.5	0.875	2.188	SKS	FN
			68	374	14.5	0.575	0.163	SKS	G
			68	378	8	1.125	0.15	SKKS	G
	BARB		25	373	21.5	0.75	0.425	SKS	F
	Lat. ( ‡)	321.221	25	61	7	1.6	0.525	SKKS	G
	Long. ( ‡)	343.800	26	66	16	1.85	0.8	SKKS	F
			42	360	22.5	1.1	1.75	SKKS	F
			46	381	13	0.8	0.2	SKKS	G
			51	363	2.5	1.5	0.15	SKS	G
			52	369	6.5	1.1	0.225	SKKS	G
			56	36	13	0.95	0.375	SKKS	G
	BRSB		5	337	22.5	3.2	3.625	SKS	FN	82	0.80
	Lat. ( ‡)	322.535	7	337	22.5	3.2	3.625	SKS	FN	5.5	0.1
	Long. ( ‡)	345.585	7	56	22.5	0.9	2.475	SKKS	FN
			7	383	17	1.2	0.475	SKS	F
			10	67	22.5	1.4	2.675	SKS	FN
			14	65	3.5	0.95	0.15	SKS	G
			14	80	16.5	1.15	0.4	SKKS	G
			16	87	16.5	0.75	0.25	SKS	G
			16	381	19.5	0.6	0.45	SKKS	F
			25	365	10	1.25	0.375	SKKS	G
			25	366	12.5	1.2	0.325	SKS	F
	BSCB		25	355	22.5	0.65	2.35	SKS	F	357	1.05
	Lat. ( ‡)	320.999	25	356	3.5	1.55	0.275	SKKS	G	2.5	0.13
	Long. ( ‡)	344.764	30	344	22.5	2.3	3.175	SKS	FN
			30	367	17.5	1	0.425	SKKS	F
			31	355	5	1.1	0.2	SKS	G
			31	355	10.5	1.2	0.375	SKKS	G
			33	344	2.5	3.75	3.9	SKS	FN

Table 2 (

 2 Continued).

	Station		Event	P ( ‡)	Error ( ‡)	dt (s)	Error (s)	Phase	Quality	M.E.P.
										P ( ‡)	dt (s)
										Error ( þ ‡)	Error ( þ s)
			33	345	2.5	4	1.4	SKKS	FN
			35	379	19.5	0.55	0.275	PKS	G
			36	344	2	1.65	0.45	SKKS	G
			39	363	6	1	0.18	SKKS	F
			39	369	22.5	0.5	0.825	SKS	F
			40	353	2	3.5	3.335	SKS	F
			40	355	22.5	2.5	3.275	SKKS	FN
			42	377	22.5	0.95	0.95	SKKS	FN
	CAMB		38	23	2	4	0.5	SKKS	FN	355	0.85
	Lat. ( ‡)	321.784	39	52	22.5	2.4	1.95	SKS	FN	7.5	0.23
	Long. ( ‡)	341.429	39	89	11.5	0.5	0.125	SKKS	G
			40	46	4	2.25	3.15	SKS	F
			40	51	22.5	1.25	2.65	SKKS	FN
			42	85	19.5	0.9	0.525	SKKS	F
	CRG		40	342	2	4	0.336	SKS	GN
	Lat. ( ‡)	320.750	40	51	22.5	0.384	2.208	SKKS	GN
	Long. ( ‡)	342.000	41	333	6	2.272	3.12	SKS	FN
			42	376	22.5	0.608	0.608	SKKS	FN
			43	43	22.5	0.768	2.4	SKKS	FN
			45	347	1.5	2.176	0.176	SKKS	FN
			50	327	22.5	0.8	0.608	SKKS	FN
			54	376	1	1.504	0.176	SKS	GN
			55	47	1.5	4	3.984	SKS	FN
			56	373	22.5	2.112	3.04	SKKS	N
			57	328	5	1.312	0.224	SKKS	F
			58	329	0.5	2.56	0.288	SKS	FN
			58	325	2	3.552	0.464	SKKS	FN
			59	68	4	0.992	0.176	SKS	GN
			59	350	3.5	1.312	0.336	SKKS	G
	FRMB									124	0.95
	Lat. ( ‡)	320.488								7	0.28
	Long. ( ‡)	345.642
	FURB									126	0.85
	Lat. ( ‡)	320.678								12	0.4
	Long. ( ‡)	346.278
	IGAB		65	54	3.5	4	0.575	SKS	F	71	2.4
	Lat. ( ‡)	323.253	37	69	7.5	1.95	0.3	SKKS	G	3	0.3
	Long. ( ‡)	346.117	39	67	1.5	3.55	0.25	SKS	G
			39	70	3	3.4	0.45	SKKS	G
			42	64	2	3.2	0.2	SKKS	F
			48	59	3.5	4	0.625	SKS	F
			65	74	4.5	2.5	0.3	SKKS	F
			68	69	5	3.6	0.8	SKKS	F
			68	65	2.5	4	0.575	SKS	F
	JFOB		29	88	16	1.15	0.4	SKKS	F
	Lat. ( ‡)	321.728	32	371	6	2.1	0.675	SKKS	F
	Long. ( ‡)	343.326	33	350	10	1.7	0.625	SKS	FN
			33	79	18	1.2	0.475	SKKS	F
			36	83	15.5	1.05	0.35	SKKS	F
			38	19	2.5	2.25	0.6	SKKS	GN
			39	62	2	1.95	0.3	SKS	G
			39	90	5	0.95	0.10	SKKS	G
			40	62	1.65	1.1	0.5	SKS	F
			40	49	6.5	1.65	0.575	SKKS	F
			52	63	8	3.25	0.825	SKKS	F
			56	59	10	1.25	0.225	SKKS	G
			61	70	13.5	0.95	0.375	SKS	F
			63	78	9	0.6	0.125	SKS	G
			63	383	8	1.35	0.25	SKKS	F
	JUQB		58	82	6	1.55	0.3	SKS	G	386	0.85
	Lat. ( ‡)	324.093	58	382	8	1.15	0.175	SKS	G	2	0.05
	Long. ( ‡)	347.716	63	90	2.5	0.8	0.05	SKS	G
			64	379	2	0.8	0.05	SKKS	G
			66	77	8	1.45	0.275	SKS	G

Table 2 (

 2 Continued).

	Station		Event	P ( ‡)	Error ( ‡)	dt (s)	Error (s)	Phase	Quality	M.E.P.
										P ( ‡)	dt (s)
										Error ( þ ‡)	Error ( þ s)
			68	82	14	1.3	0.525	SKKS	F
			68	85	4.5	1.4	0.225	SKS	G
			69	388	8	1.25	0.20	SKS	F
	LJM		52	79	21	0.736	0.368	SKS	F
	Lat. ( ‡)	321.290	44	368	1	2.112	0.096	SKS	G
	Long. ( ‡)	342.053	47	20	2.5	1.6	0.16	SKKS	G
			49	65	3	2.208	0.4	SKS	G
			58	362	14	0.6	0.16	SKS	G
			58	76	14	0.7	0.24	SKKS	G
			59	84	5	1.1	0.16	SKKS	G
			60	79	11	1.4	0.32	SKKS	F
	NATB		3	376	15.5	0.7	0.225	SKKS	F
	Lat. ( ‡)	321.055	7	77	17	0.45	0.3	SKS	F
	Long. ( ‡)	342.004	7	88	15	1	0.4	SKKS	F
			9	361	10	1.1	0.25	SKKS	G
			10	56	22.5	2.85	3.4	SKS	FN
			16	67	22.5	0.75	1.1	SKS	F
			16	85	7	0.8	0.1	SKKS	G
	RSTB		64	365	3.5	0.65	0.05	SKKS	G	379	0.85
	Lat. ( ‡)	324.651	65	357	6	2.8	1	SKKS	F	5	0.13
	Long. ( ‡)	349.034	65	384	3.5	1.15	0.075	SKS	G
	SJM									138	2
	Lat. ( ‡)	321.931								2	0.3
	Long. ( ‡)	345.963
	SPB		8	384	4.5	1.95	0.225	SKS	F	86	1.45
	Lat. ( ‡)	323.592	8	379	9	1.35	0.275	SKKS	F	1.5	0.08
	Long. ( ‡)	347.432	31	382	6.5	1.45	0.2	SKKS	G
			31	89	1	1.45	0.05	SKS	G
			32	384	10	1.75	0.5	SKKS	F
			33	368	11	1.7	0.45	SKS	F
			34	81	4	1.45	0.175	PKS	F
			35	89	4	1.35	0.125	PKS	G
			36	377	9	1.4	0.275	SKKS	G
			39	89	5	1.35	0.15	SKS	G
			39	377	3.5	1.7	0.15	SKKS	G
			52	81	21.5	1.8	1.1	SKS	F
			53	384	4	1.7	0.35	SKKS	F
			62	388	11	1.45	0.425	SKS	F
			63	379	4.5	1.6	0.175	SKKS	G
			63	89	5	1.15	0.15	SKS	G
	TRRB		2	69	13	3.2	0.8	SKKS	F	78	1.4
	Lat. ( ‡)	322.154	4	71	11.5	1.65	0.9	SKKS	F	1.5	0.1
	Long. ( ‡)	343.195	5	64	4	3.45	0.7	SKS	G
			7	66	5	1.65	0.25	SKS	G
			7	82	5	2.2	0.25	SKKS	G
			9	73	22.5	0.95	0.975	SKKS	F
			13	81	15.5	0.85	0.35	SKS	G
			13	80	7.5	1.75	0.275	SKKS	G
			16	76	10.5	1.15	0.275	SKS	G
			16	90	11.5	1.45	0.3	SKKS	F
			24	70	2	1.9	0.225	SKS	G
			24	86	20	0.95	0.475	SKKS	F
			27	76	11	1.2	0.275	SKS	F
			27	81	10.5	1.55	0.325	SKKS	G
	VABB		1	74	4	2.2	0.4	SKS	F	86	1.45
	Lat. ( ‡)	323.002	1	349	4	2.55	0.625	SKKS	F	1.5	0.08
	Long. ( ‡)	346.966	3	388	7	1.35	0.3	SKKS	F
			9	69	22.5	1.75	2.85	SKS	GN
			14	387	3	1.55	0.1	SKKS	G
			14	75	2	1.7	0.125	SKS	G
			16	385	5	1.45	0.15	SKKS	G
			16	84	4.5	1.5	0.2	SKS	G

Table 1

 1

2

: azimuth = 129.7 ‡, backazimuth = 231.2 ‡): P = 70 þ 3 ‡, and dt = 3.4 þ 0.45 s, on phase SKKS. (B) Event 39 (see Table

1

2 : azimuth = 129.7 ‡, backazimuth = 231.2 ‡): P = 67 þ 1.5 ‡, and dt = 3.55 þ 0.25 s, on phase SKS. (C) Event 42 (see Table

1

2 : azimuth = 150.2 ‡, backazimuth = 212.6 ‡): P = 64 þ 2 ‡, and dt = 3.2 þ 0.20 s.

See the online version of this article.

Acknowledgements

This work bene¢ted from ¢nancial support from CNPq/CNRS collaborative program (pro-cesso no. 910144/98-2 and PICS763), CAPES/CO-FECUB collaborative program (no. 287/99-II) and CNRS (Programme Interieur de la Terre). Support from the INSU lithoscope and broadband networks and from the French Embassy in Brazilia is acknowledged. We are indebted to S. Chevrot, V. Levin and an anonymous reviewer for their helpful comments, to J. 'Ze '' Roberto Barbosa for ¢eld, laboratory and administrative assistance and to A. Tommasi for ¢eld assistance and fruitful discussions. [BARD]


Nicolas Peyrot
email: nicolas.peyrot@univ-reunion.fr

Biomechanical constraints associated with walking in obese individuals

Nicolas Peyrot

Laboratoire CURAPS-DIMPS, Faculte ´ des Sciences de l'Homme et de l'Environnement, Universite ´ de la Re ´union, Le Tampon, La Re ´union, France Invited Commentary on: 'Gait and increased body weight (potential implications for musculoskeletal disease)', Sheehan and Gormley.

Obesity is partly attributable to a low level of physical activity leading to insufficient total daily energy expenditure. Accordingly, increasing levels of physical activity is one of the key challenges in the prevention and treatment of obesity. [START_REF] Hill | Role of physical activity in preventing and treating obesity[END_REF] To date, studies report only modest adherence to physical activity in obese individuals. [START_REF] King | Can we identify who will adhere to long-term physical activity? Signal detection methodology as a potential aid to clinical decision making[END_REF][START_REF] Ekkekakis | Exercise does not feel the same when you are overweight: the impact of self-selected and imposed intensity on affect and exertion[END_REF] These persistently low levels of daily physical activity may be attributed to physiological and biomechanical factors that limit gait performance. [START_REF] Wearing | The biomechanics of restricted movement in adult obesity[END_REF][START_REF] Hills | The biomechanics of adiposity -structural and functional limitations of obesity and implications for movement[END_REF][START_REF] Nantel | Physical activity and obesity: biomechanical and physiological key concepts[END_REF] For instance, walking is one of the most convenient daily physical activities recommended for increasing energy expenditure. However, obese individuals encounter some limitations during walking such as modifications in the gait pattern, higher net metabolic rate (gross minus standing, W/kg), and pain due to alteration of the musculoskeletal system. [START_REF] Wearing | The biomechanics of restricted movement in adult obesity[END_REF][START_REF] Hills | The biomechanics of adiposity -structural and functional limitations of obesity and implications for movement[END_REF][START_REF] Nantel | Physical activity and obesity: biomechanical and physiological key concepts[END_REF][START_REF] Malatesta | Mechanical external work and recovery at preferred walking speed in obese subjects[END_REF][START_REF] Browning | Pound for pound: working out how obesity influences the energetics of walking[END_REF][START_REF] Sheehan | Gait and increased body weight (potential implications for musculoskeletal disease)[END_REF][START_REF] Peyrot | Do mechanical gait parameters explain the higher metabolic cost of walking in obese adolescents?[END_REF][START_REF] Runhaar | A systematic review on changed biomechanics of lower extremities in obese individuals: a possible role in development of osteoarthritis[END_REF] As a consequence of the interaction of these characteristics, walking may be a critical source of biomechanical loads that link obesity and musculoskeletal pathology, such as lower-extremity osteoarthritis, rheumatoid arthritis, low back pain, foot pathologies, or altered skeletal alignment. [START_REF] Hills | The biomechanics of adiposity -structural and functional limitations of obesity and implications for movement[END_REF][START_REF] Sheehan | Gait and increased body weight (potential implications for musculoskeletal disease)[END_REF][START_REF] Runhaar | A systematic review on changed biomechanics of lower extremities in obese individuals: a possible role in development of osteoarthritis[END_REF][START_REF] Krul | Musculoskeletal problems in overweight and obese children[END_REF][START_REF] Browning | Effects of obesity on the biomechanics of walking at different speeds[END_REF] In a recent of the journal of Physical Therapy Reviews, the article by Sheehan and Gormley 9 stresses the importance of understanding how obesity affects the biomechanical load involved in walking. This could provide appropriate individualized prescription in order to prevent the onset and progression of musculoskeletal diseases. To this end, the authors present the differences in biomechanical gait parameters between normal-weight and obese individuals. In obese children and adults, an increased body weight leads to major modification in the gait pattern. [START_REF] Nantel | Physical activity and obesity: biomechanical and physiological key concepts[END_REF][START_REF] Malatesta | Mechanical external work and recovery at preferred walking speed in obese subjects[END_REF][START_REF] Sheehan | Gait and increased body weight (potential implications for musculoskeletal disease)[END_REF][START_REF] Peyrot | Do mechanical gait parameters explain the higher metabolic cost of walking in obese adolescents?[END_REF][START_REF] Runhaar | A systematic review on changed biomechanics of lower extremities in obese individuals: a possible role in development of osteoarthritis[END_REF][START_REF] Browning | Effects of obesity on the biomechanics of walking at different speeds[END_REF][START_REF] Shultz | Childhood obesity and walking: guidelines and challenges[END_REF] Obese individuals adopt a slower preferred walking speed, and spend more time in stance and double-limb support, and less time in swing at all walking speeds. Moreover, obese individuals walk with greater step width. These specific temporal-spatial parameters and the reduced preferred walking speed may be a locomotor strategy to improve gait stability and to prevent falls.

Lower-extremity joint biomechanics is also affected by increased body weight during walking. The association between increased body weight and kinematic adaptations at the hip, knee, and ankle in all three planes of motion (i.e. frontal, sagittal, and transverse) leads to higher joint moments and thus to an increased load across the lower-extremity joints. For instance, Browning and Kram [START_REF] Browning | Effects of obesity on the biomechanics of walking at different speeds[END_REF] have shown that in the frontal plane, a high external knee adduction moment increases the compressive load on the medial compartment of the knee in obese adults. This kneejoint load across the tibiofemoral compartment has been shown to be positively correlated with osteoarthritis disease severity and progression. [START_REF] Miyazaki | Dynamic load at baseline can predict radiographic disease progression in medial compartment knee osteoarthritis[END_REF] Moreover, the peak external knee moment has been shown to increase with walking speed. [START_REF] Browning | Effects of obesity on the biomechanics of walking at different speeds[END_REF] Browning and Kram 13 therefore proposed that an obese adult would need to walk at about 1.1 m/second (close to their preferred walking speed) to have a biomechanically equivalent joint load as normal-weight adults walking at 1.4 m/second. Further, Ehlen et al. [START_REF] Ehlen | Energetics and biomechanics of inclined treadmill walking in obese adults[END_REF] pointed out that moderate obese individuals have to walk faster than their preferred walking speed to increase exercise intensity and to follow physical activity guidelines. However, lower-extremity joint loads and the associated risk of musculoskeletal injuries or disease increase with walking speed. To limit this negative effect of increased level walking speed on joint loads, the authors proposed to test whether uphill walking at a slower speed is an alternative form of moderate intensity exercise that may reduce joint loading. They concluded that walking using a treadmill incline (6-9u) at a relatively slow speed (,0.75 m/second) is an effective strategy to reduce load across the lowerextremity joints while providing adequate cardiovascular stimulus for weight management. [START_REF] Ehlen | Energetics and biomechanics of inclined treadmill walking in obese adults[END_REF] Moreover, walking at slower speeds and moderate inclines may improve the adherence and the activity time due to a reduced perceived exertion of effort. These studies therefore contribute to the support of appropriate individualized prescription in obese individuals to reduce biomechanical load involved in walking and prevent the onset and progression of musculoskeletal diseases.

Interestingly, Sheehan and Gormley 9 report that the reduced muscular strength of the lower limbs in obese individuals could lead to gait adaptations, such as a more erect posture during walking. Hills et al. [START_REF] Hills | The biomechanics of adiposity -structural and functional limitations of obesity and implications for movement[END_REF] also suggested that the reduced muscular strength can impair motor function and limit obese individuals' ability to perform activities of daily living. Further, results of recent studies suggest that the lower relative muscle strength in obese individuals could be partly responsible for the higher net metabolic rate. [START_REF] Hunter | Increased strength and decreased flexibility are related to reduced oxygen cost of walking[END_REF][START_REF] Peyrot | Why does walking economy improve after weight loss in obese adolescents?[END_REF] Future research should assess the effects of biomechanical properties of the lower-extremity muscles on gait performance in obese individuals. This work could have important practical applications for clinicians in the design of appropriate interventions, because strength training could be recommended in obese individuals to improve walking economy, motor function, and gait pattern.

In conclusion, the combination of excess body weight and altered gait biomechanics in obese individuals increases the load applied to the weight-bearing joints during walking. These greater lower-extremity joint loads could be related to the onset of musculoskeletal disorders. For exercise prescription in obese individuals, strategies like walking at a relatively slow speed up a moderate incline should be considered to prevent the onset and progression of musculoskeletal disorders. Further biomechanical research is warranted to assess the source of biomechanical loads and musculoskeletal pain involved in walking and to provide appropriate individualized exercise prescriptions in obese individuals.