Vauchez, A., and Tommasi, A., 2003, Wrench faults down to the asthenosphere: Geological and geophysical evidenceand thermo-mechanical effects, in Storti, F., Holdsworth, R.E., and Salvini, F., eds., Intraplate Strike-SlipDeformation Belts: London, Geological Society of London, Special Publication 210, p. 15-34.

Wrench faults down to the asthenosphere: Geological andgeophysical evidence and thermo-mechanical effects

Alain Vauchez and AndréaTommasiLaboratoire de Tectonophysique, Université de Montpellier II et CNRS

Pl. Eugène Bataillon, F-34095 Montpellier cedex 5, France

vauchez@dstu.univ-montp2.fr; andrea.tommasi@dstu.univ-montp2.fr

Abstract:We review a set of geological and geophysical observations that strongly support a coherent

deformation of the entire lithosphere in major intracontinental wrench faults. Tectonic studies of wrenchfaults eroded down to the middle to lower crust show that, even in cases in which the lower to middle crust ispartially melted, strain remains localized (although less efficiently) in transcurrent shear zones. Seismicprofiling as well as seismic tomography and magnetotelluric soundings provide strong argument in favor ofmajor wrench faults crosscutting the Moho and deforming the upper mantle. Pn velocity anisotropy, shear-wave splitting and electric conductivity anisotropy measurements over major wrench faults and intranspressional domains support that a wrench fault fabric exists over most or even the entire lithospherethickness. These seismic and electrical anisotropies are generated by a crystallographic preferredorientation of olivine and pyroxenes developed in the mantle during the fault activity, which is frozen in thelithospheric mantle when the deformation stops. The preservation of such a "wrench fault-type" fabric withinthe upper mantle may have major effects on the subsequent tectono-thermal behavior of continents, becauseolivine is mechanically and thermally anisotropic. Indeed, the association of numerical models andlaboratory data on textured mantle rocks strongly suggests that the orogenic continental lithosphere is ananisotropic medium with regards to its stiffness and to heat diffusion. This anisotropy may explain thefrequent reactivation, at the continents scale, of ancient lithospheric-scale wrench faults and transpressionalbelts during subsequent tectonic events.

Introduction:Horizontal displacements in transcurrent

faults represent one of the fundamental modesof accommodation of deformation in thecrust. It is quite obvious that transcurrentfaults generated at transform plate boundaries,like the San Andreas Fault in California or theAlpine fault in New Zealand crosscut theentire lithosphere. It is however less clearwhether intracontinental strike-slip faultsystems generated in active margins or incollisional domains are only crustal structuresor are rooted in the upper mantle. Thepenetration of a "wrench-fault type" tectonicfabric (i.e., a vertical flow plane associatedwith a horizontal flow direction) deep into theupper mantle may have major geodynamicimplications, since it would generate ananisotropy of the mechanical and thermalproperties of the lithospheric mantle and,hence, modify the large-scale rheological

behavior of continental plates duringsubsequent tectonic events (Tommasi et al.,2001; Tommasi & Vauchez, 2001).

Assuming that major, i.e., continental-scale, strike-slip faults observed today at thesurface continue down to the base of thelithosphere implies a strong mechanicalcoupling between the various rheologicallayers of the lithosphere. This raises thequestion of the mechanical properties of thehot middle to lower crust. Strain localizationshould remain efficient enough to allow thedevelopment of strike-slip faults zones at thislevel. In addition, rheological contrastsbetween the lower crust and the upper mantleshould remain moderate; otherwise the lowercrust would behave as a horizontal decouplinglevel in which upper crustal wrench faultswould root. These issues have been alreadyaddressed in a large number of studies on therheological stratification of the continentallithosphere (e.g., Meissner et al., 2002;

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Molnar, 1988; Ranalli & Murphy, 1987;Vauchez et al., 1997), but experimental dataon the rheology of lower crustal materials isso limited that these studies are notconclusive.

In this paper, in order to evaluate howdeep a coherent "transcurrent fabric" maypenetrate, we analyze direct observationsfrom surface geology, which are of courserestricted to the crust, and indirectinformation from geophysics andgeochemistry that gives a hint on thecrust/mantle coupling. We consider evidencefrom active and fossil tectonic domains anddiscuss observations from both individualshear zones and broad transpressive domains.The review of this broad dataset suggests thatmajor wrench faults do crosscut the entirelithosphere. This leads us to discuss the effectof these lithospheric-scale wrench faults onthe thermo-mechanical evolution ofcontinental plates.

Transcurrent shear zones and strainlocalization in a hot middle to lower crust

If major transcurrent faults were rooted intothe crust, the wrench deformation in the uppercrust must be decoupled from the mantle flow.Decoupling between crustal and mantledeformations is supposed to be favored in themiddle to lower crust (especially in regionsdisplaying high geothermal gradients) by the lowstiffness of crustal material at high homologoustemperature (T/Tm, with Tm != !meltingtemperature). It would be marked by rooting ofthe strike-slip faults into this low stiffness layer,and therefore by a listric shape of the fault inorder to accommodate the transition from avertical to a horizontal flow plane.

In this section, we examine a set ofcontinental-scale transcurrent faults eroded toincreasingly deeper levels from the middle to thelower crust. In all these cases, during transcurrentdeformation, the crustal levels exposed todaywere submitted to high temperatures and evenpartial melting. These levels represent former lowviscosity layers into which crustal-scale strike-slipfaults might have rooted.

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Figure 1: The high-temperature Borborema shear zone system of Northeastern Brazil (Vauchez et al., 1995). (a) Sketch mapshowing the complex pattern of transcurrent faults formed during the Neoproterozoic orogeny: (1) Neoproterozoic granitoids, (2)Mid- and Late Proterozoic sedimentary basins, (3) Mesozoic sedimentary basins, (4) Neoproterozoic high-temperature shear zones,and (5) Neoproterozoic low-temperature shear zones. (b) and (c) are two Landsat images showing segments of two major high-temperature wrench faults: the Patos and the West Pernambuco shear zones, respectively. Gray lines in (b) mark the shear zonelimits

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In northeastern Brazil, the Neoproterozoicprovince of Borborema displays a complexnetwork of wrench faults (Figure 1) that areseveral hundred kilometers long and up to 30 kmwide (e.g., Vauchez et al., 1995). Satellite imageshighlight a clear textural contrast between theshear zones and the country rock. This contrast ismostly due to the transition from a predominantlow-angle metamorphic foliation outside the shearzones to a steeply dipping mylonitic foliationwithin the shear zones. At the satellite imagescale, the boundaries of the fault zones appearusually rather sharp, although in the field acontinuous transition from the external flat-lyingfoliation to the internal steeply dipping foliation(half "flower-structure") is observed where nosubsequent reactivation concealed the originalrelationships. Mylonites outcropping in the shearzones were formed at depths of 16-18 km(P!≤!500 MPa) and at high temperature (>650°C).

Under these conditions, the protoliths of themylonites (metasediments, pre-kinematicintrusives, felsic gneisses from the basement)were partially melted and the resulting rock isindeed a migmatitic mylonite. At thesetemperature conditions, felsic rocks are expectedto display low viscosity, which will be furtherdecreased by partial melting. Nevertheless, evenwhen the degree of melting is rather high, thefoliation in the shear zones remains consistentlysteeply dipping and bears a shallow-dippingstretching lineation (Figure 2). Shear-senseindicators developed in the partially meltedmylonites consistently support dextral wrenching(Figure 3). Evidence of downward decrease of thefoliation dip, suggesting rooting of the faults, hasnever been reported. On the contrary, a largevolume of mantle-derived magmas, especiallydiorites, was emplaced as syn- or late-kinematicdikes (Figure 4) and/or elongated plutons withinthe shear zones (Neves et al., 2000; Vauchez etal., 1995); this strongly suggests that the faultswere connected to a partially melted upper mantle.

The Neoproterozoic Mozambique belt inMadagascar and East Africa is also characterizedby the development of a large network of wrenchfaults (Figure 5) at ca. 530-500 Ma (Martelat etal., 2000). The present-day level of exposureshows rocks that were 20 to 30 km deep duringthe deformation [0.5 to 1.1 GPa; \Martelat, 2000#4157; Pili, 1997 #3960]. At these depths,deformation took place at temperatures >750°C.The major shear zones in this domain are typicallyseveral hundred kilometers long and up to 40 kmwide. Numerous minor ductile wrench faultsformed under similar P-T conditions are alsodocumented. The tectonic fabric in the shear

Figure 2: High-temperature vertical foliation (S) andhorizontal mineral stretching lineation (L) in a mylonitefrom the Borborema shear zone system. Deformation in thisfelsic mylonite occurred at T>600°C. Location on Fig.1a.Scale bar is 0.5m.

Figure 3: Migmatitic mylonite from the WestPernambuco shear zone (see location on Figure 1a).Downward view. Intense shearing occurred along asubvertical foliation in a partially melted crust. Whitelayers are leucocratic neosome. Arrows indicatedextral shear.

Figure 4: Diorite dikes injected in a porphyriticgranodiorite emplaced in the Pernambuco shear zone(location on figure 1a). Dikes were emplaced within thetranscurrent shear zone and deformed before completesolidification. No evidence of solid-state deformation hasbeen observed

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zones is typical of ductile strike-slip faults: thefoliation is steeply dipping, the mineral-stretchinglineation is sub-horizontal and consistent shear-sense criteria are observed. Outside the shearzones, the granulites that form the country-rockdisplay a low-angle foliation and the fabric ismetamorphic-migmatitic rather than mylonitic.

According to Martelat et al. (2000), thedeformation regime in the southern Mozambiquebelt was transpressional and the deformation waspartitioned; transcurrent shearing was localizedwithin the vertical shear zones and large-scalefolding accommodated transverse shortening.Through a regional scale investigation of the C-and O-isotopes compositions of carbonates frommarbles and metabasites, Pili et al. (1997b) haveshown that CO2 in the major wrench faults of thenetwork has a mantle origin. This suggests thatthese major faults were connected to the mantle.On the other hand, in minor shear zones and inmetamorphic rocks outside the shear zones, CO2has a crustal isotopic signature. In the sameregion, Pili et al. (1997a) documented asystematic association of a short wavelengthpositive gravity anomaly to major strike-slip shearzones that also supports a deep rooting of themajor wrench faults of the Mozambique belt. Thisanomaly was interpreted as due to a shallowercrust-mantle boundary beneath the faults. Such anupward deflection of the Moho might result fromthinning of the crust in response to the intensestretching associated with simple shear in the faultzones (Pili et al., 1997a).

In NE-Brazil as well as in the MadagascarNeoproterozoic belts, strain localization in

transcurrent shear zones is observed even atcrustal levels where synkinematic temperatureswere high enough to induce partial melting. Thewidth of the fault zones is extremely large (severaltens of kilometers) compared to typical widths ofshear zones developed under lower temperatureconditions (cm to hundred m). This points outthat, at these high temperatures, strain localizationwas less efficient and strain was distributed over alarger volume of rocks than it is usually observedin upper crustal shear zones. Rocks within theshear zones display a high-temperature myloniticfabric largely due to dislocation creep assisted byvery effective diffusional processes (in particulargrain boundary migration), and consistent shearcriteria. In addition, petrological and geochemicalobservations strongly suggest that fluidspercolated from the mantle into the crust alongthese major shear zones, and therefore that thefaults were continuous through the upper mantle.

"Moho" faults vs. lithospheric faultsThe observations presented above

strongly support that major transcurrent faults donot root in some intracrustal decoupling level, butrather crosscut the entire crust and are, in someway, connected to the upper mantle. Theseobservations are however not sufficient toevaluate whether those faults are rooted at thecrust-mantle interface or penetrate deeply into theupper mantle. Clear evidence supporting thatmajor wrench faults crosscut the Moho andpenetrate deeply into the upper mantle isnevertheless obtained by combining various

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techniques of geophysical exploration of thelithosphere. Evidence may be subdivided in twogroups. Seismic profiling, magnetotelluricsoundings, and seismic tomography have imaged"Moho faults" (Diaconescu et al., 1997), i.e.,discontinuities crosscutting the Moho beneathseveral wrench faults observed at the surface. Onthe other hand, electric conductivity anisotropyevidenced in magnetotelluric soundings,azimuthal anisotropy of Pn velocities, and S-waves splitting are directly related to the tectonicfabric of the upper mantle and support that thelithospheric mantle was deformed in majorwrench faults.

Electric conductivity anisotropy in the uppermantle is interpreted as due to a preferredorientation of graphite films elongated along thefoliation (Mareschal et al., 1995) or to ananisotropic electrical conductivity in a "wet"mantle due to the anisotropy of H+ diffusion in theolivine crystal (Mackwell & Kohlstedt, 1990;Simpson, 2001). In both cases, a "wrench-faulttype fabric" (i.e., a steeply-dipping flow plane, orfoliation, containing a subhorizontal flowdirection, or lineation) within the mantle wouldgenerate a higher conductivity parallel to the traceof the wrench fault observed at the surface.

Seismic anisotropy in the upper mantle, which

may be characterized by measurement of anazimuthal anisotropy of Pn velocities or by thesplitting of teleseismic S-waves, results from thelattice preferred orientation (LPO) of rock-forming minerals during high-temperaturedeformation by dislocation creep. Wrench-faulting within the lithospheric mantle wouldgenerate a LPO of olivine, the dominant mineralphase in mantle peridotites, characterized by aconcentration of [100] axes close to the lineation(i.e., subhorizontal) and of [010] axes normal tothe foliation plane [Figure 6; \Tommasi, 1999#3215]. Olivine is elastically anisotropic. Thus ifdeformation produces coherent olivine LPOs atthe scale of tens of km in the upper mantle, it alsoresults in anisotropic seismic properties(Mainprice & Silver, 1993; Nicolas &Christensen, 1987; Silver et al., 1999). P-wavesthat propagate either parallel to the maximum of[100] or [010] axes of olivine in the mantle arerespectively the fastest and the slowest. On theother hand, S-waves propagating through adeformed upper mantle split into two quasi-Swaves polarized in orthogonal planes; the fastestone is polarized in a plane containing both themaximum concentration of olivine [100] axis andthe propagation direction. The delay time betweenthe arrivals of the two split waves is proportionalto both the length of wave propagation path withinthe deformed layer and the propagation directionrelative to the structural fabric; the largest S-waves splitting is observed for waves thatpropagate at low angles to the maximum of [001]axis. A wrench-fault fabric in the mantle wouldtherefore be evidenced (Figure 6) by a fastpropagation of P-waves (in particular, horizontallypropagating Pn waves) parallel to the fault and apolarization of the fast split S-wave in a planecontaining both the direction of propagation of thewave and the lineation, i.e., parallel to the faultdirection for waves having an almost verticalincidence (such as SKS, SKKS, PKS...). It is alsoin this case that the birefringence will be thelargest, leading to relatively large time lagsbetween the arrivals of the fast and slow split S-waves. Indeed, SKS splitting data above transformboundaries, such as the Caribbean or the Alpinefault in New Zealand, systematically display fastshear waves polarized parallel to the transformdirection and delay times significantly larger than1s, which imply that the entire lithospheredeformed in a strike-slip regime (Klosko et al.,1999; Russo et al., 1996).

These techniques "probe" the uppermantle fabric with different spatial resolutionsand depth sensitivities. Magnetotelluric

Figure 6: Cartoon illustrating the concept of lithosphericfault, in which crustal fault zones broaden downward and tendto coalesce forming a broad shear zone that cuts across theentire lithospheric mantle. It displays the tectonic fabricassociated with the fault within the crust and the mantle, thecrystallographic fabric of olivine expected to develop in themantle section of such a fault zone (oriented in the structuralframework of the fault: X=lineation and Z=normal to thefoliation), and the splitting of a polarized incoming shear wavethat propagates across a lithospheric mantle displaying a"wrench fault type" fabric. A seismic station locate above sucha lithospheric shear zone will record a fast shear wavepolarized parallel to the shear zone trend (X direction) andstrong delay times (>1s).

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soundings using a large spectrum ofmeasurement frequencies allow an evaluationof the electrical conductivity anisotropy fromthe crust to the asthenospheric mantle.However, MT data depend on both anisotropyand heterogeneity of electrical conductivity,and reliable anisotropy determinations mayonly be obtained when high-quality, long-period MT transfer functions are available andlateral conductivity gradients are small(Simpson, 2001). Pn waves sample theuppermost mantle (3-5 km beneath theMoho), but the measured velocities depend onboth the anisotropy and the heterogeneity (intemperature and composition) along the wavepath. Teleseismic S-waves splitting providesreliable evidence of seismic anisotropy with avery good spatial resolution (ca. 50 km), butthese measurements integrate all anisotropiccontributions along the wave path (which isroughly vertical from the core-mantleboundary to the surface for the mostcommonly used SKS waves). Theirassociation should therefore allow us to betterconstrain the structural fabric of the uppermantle. Indeed, comparison of electricconductivity anisotropy determined bymagnetotelluric soundings and S-wavessplitting measurements shows that thedirection of largest conductivity and the fastsplit S-wave polarization plane are oftenalmost parallel (Barruol et al., 1997b;Simpson, 2001; Wannamaker et al., 1996) or

make a slight, but consistent angle (Mareschalet al., 1995). Ji et al. (1996) interpreted thisslight obliquity as representing the obliquitybetween the foliation and the shear plane inshear zones.

To investigate how deep a "wrenchfault fabric" may penetrate into the uppermantle, we analyze geophysical data forseveral ancient or active wrench faults andtranspressional belts. In each case,transcurrent displacement, either in a singlefault or in a broader domain of transpressionaldeformation, is supported by surface geology.

Transcurrent shear zones

Recently, Pollitz et al. (Pollitz et al.,2000; Pollitz et al., 2001) using a combinationof GPS and synthetic aperture radar (InSAR)data, have shown that the deformation in theyears following the 1992-Landers and 1999-Hector Mine major earthquakes in the Mojavedesert (California-USA) was about 3 timesgreater than before the earthquakes. Thisinterseismic velocity field supports a rightlateral displacement parallel to the SanAndreas transform fault system (Figure 7).According to these authors, the visco-elasticrelaxation of the lower crust and upper mantlewas the dominant postseismic process; thisrequires that the lower crust acted as acoherent stress guide coupling the upper crustwith the upper mantle (Pollitz et al., 2001).

Figure 7: (a) Structural sketch displaying the active faultsin California. (b) Shear-wave splitting in western California fromHartog et al. (2001). Anisotropy beneath the westernmoststations, i.e., those above the San Andreas Fault system, resultsfrom the superposition of two anisotropic layers. The upper layer,which corresponds to the lithospheric mantle, is characterized bya polarization of the fast shear wave (black bars) in a planeparallel to the San Andreas Fault system and a delay time close toor even higher than1s. The easternmost stations display a simpleranisotropy pattern (gray bars) that may be accounted for by asingle anisotropic layer with a roughly E-W flow direction. Asimilar flow direction is inferred for the lower anisotropic layer(gray bars) in the westernmost California. (c) Horizontal velocityfield showing the contemporary interseismic deformation acrosssouthern California (relative to a group of GPS and VLBI stationson the stable North American Plate). Geodetic data includeGlobal Positioning System (GPS), Very Long BaselineInterferometry (VLBI), and Electro-optical DistanceMeasurement (EDM) obtained by the Crustal DeformationWorking Group of the Southern California Earthquake Centerduring the past three decades. Error ellipses are regions of 95%confidence. Release 2, 1998, available at: http://www.scecdc.scec.org:3128/group_e/release.v2

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These conclusions are consistent with thosedrawn from the analysis of the seismic anisotropymeasured across the San Andreas Fault systemslightly north of the Landers and Hector Mineearthquakes area (Hartog & Schwartz, 2001;Ozalaybey & Savage, 1995; Silver & Savage,1994). The shear waves splitting parametersretrieved from a large number of recordsconsistently suggest two layers of anisotropywithin the upper mantle (Figure 7). The upperlayer, which corresponds to the lithosphericmantle, is characterized by a polarization of thefast split shear wave parallel to the San AndreasFault system. This suggests that the lithosphericmantle has a tectonic fabric consistent with thecrustal fabric, i.e., a steeply dipping foliationbearing a subhorizontal lineation. Both geodeticand seismologic observations therefore convergetoward a coherent deformation of the entirelithosphere.

The Himalayan orogen provides someof the best examples of active wrench faultsin an intracontinental setting. These faultshave accommodated large lateraldisplacements associated with the India-Asiacollision (e.g., Tapponnier et al., 1986). Themain faults of the system have been mappedover hundreds of kilometers and arecommonly several kilometers wide. The RedRiver fault, for instance, was recognized over1000!km from Tibet to the Gulf of Tonkin.Pham et al. (1995) have performed a 70!kmlong magnetotelluric profile across the RedRiver fault system in North Vietnam (Yen Bairegion). In this area, the Red River system isformed by three parallel transcurrent faults, afew tens of kilometers apart (Tapponnier etal., 1990). This MT survey (Figure 8) showsthat: 1) each fault is characterized by a highconductivity zone down to the uppermostmantle, 2) the Sông Hông fault, the mainbranch of the Red River fault system,separates two lithospheric domains presentingcontrasted electrical properties, and 3) a largeconductivity anisotropy is observed in boththe crust and the uppermost mantle; thedirection of highest conductivity isconsistently parallel to the strike of the faults.This anisotropy is consistent with a steeplydipping foliation within the uppermost mantleas well as in the entire crust.

In Tibet , se ismic anisot ropymeasurements have been performed aboveand in the vicinity of two other well-knowmajor wrench faults, the Altyn Tagh and theKunlun faults. These faults, several hundredsof kilometers long (1800 km for the AltynTagh fault), have accommodated severalhundreds km of lateral escape during theIndia-Eurasia collision (e.g., Tapponnier etal., 1986). Wittlinger et al (1998) haveperformed a seismic tomography study of anarea where the Altyn Tagh fault juxtaposesPrecambrian basement with the Qailamsedimentary basin. This tomography shows asoutheastern domain characterized by low-velocity perturbations in contrast with anorthwestern domain where high-velocity

Figure 8: Magnetotelluric soundings from Pham et al.(1995) across the Red River Fault system. (a) MTgeoelectrical section obtained by 2D numerical modelingshowing marked resistivity contrasts between domainsseparated by the faults. Each bloc is characterized by itslongitudinal (i.e., parallel to the strike of the faults) andtransverse (in brackets) resistivities (in Wm). Low-resistivity domains beneath each branch of the fault aredisplayed in light gray. Conductive zones in the lowercrust and uppermost mantle are displayed in medium anddark gray, respectively. (b) MT sounding curves showinga pronounced variation in apparent resistivity betweenthe transverse (normal to the strike of the faults) andlongitudinal direction in both the crust and the uppermostmantle. The highest conductivity is parallel to the strike ofthe faults, a result in good agreement with a "wrench faulttype" fabric in the uppermost mantle

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perturbations dominate. The limit betweenthese domains is marked by a low-velocityanomaly located just beneath the Altyn Taghfault (Figure 9a). From these resultsWittlinger et al. (1998) have suggested thatthe Altyn Tagh fault in the mantle is ca. 40km wide and is continuous down to a depth of140 km at least. In addition, shear-wavesplitting measurements above the Altyn Taghfault (Herquel et al., 1999) show fast splitshear waves polarized in a plane parallel tothe trend of the fault and delay times betweenthe fast and slow S-waves arrivals of ca. 1s.Such delay times require a thickness ofanisotropic mantle of ca. 100 km, inagreement with the values of fault penetrationinferred from seismic tomography (Figure9b). Shear-wave splitting measurement aboveand across the Kunlun fault (Herquel et al.,1999; McNamara et al., 1994) have reached

similar results. Approaching the Kunlun faultzone the orientation of the fast S-wavepolarization plane progressively rotates intoparallelism with the trend of the fault,suggesting a shear strain gradient and anupper mantle fabric similar to that in the crust.The 2s of delay time measured above theKunlun fault requires a thickness ofanisotropic material >200km, assuming asteeply dipping flow plane and asubhorizontal flow direction, thus larger thanthe lithosphere thickness. This suggests thatthe asthenosphere fabric also contributes tothe recorded anisotropy and deformssomewhat coherently with the lithosphere. Similar observations also characterize ancientwrench faults whose fabric was frozen intothe lithospheric mantle at the end of theorogenic evolution. The Great Glen – WallsBoundary fault (GGWBF) is a major wrenchfault that belongs to a more complex faultarray developed in the northern segment ofthe Caledonian belt between 428 and 390 Ma(e.g., Stewart et al., 1999). Two segments ofthe initial fault are exposed: the Great Glenfault in Scotland and the Walls Boundaryfault in the Shetland Islands. Paleomagneticreconstructions suggest that several hundredkilometers of sinistral strike-slip displacement

Figure 10: Shear-wave splitting in northern UK fromHelffrich (1995). Initials (e.g., MCD, LRW…) represent thename of the stations. APM is the Absolute Plate Motion inthe hot-spot framework calculated using Morgan andMorgan's model (see, Barruol et al., 1997a). Thick grayline north of the Shetland Islands marks the location of theUNST deep seismic reflection profile displayed in Fig. 11.

Figure 9: Mantle structure beneath the Altyn Tagh andKunlun active faults in Tibet. (A) Cross section displaying themain geological structures and the P-wave velocity structureacross the Altyn Tagh fault system (Wittlinger et al., 1996).Light gray and dark gray colors correspond to the crust andmantle, respectively. Lighter shades in both layers indicatedomains of lower P-wave velocities. (B) Compilation of shear-wave splitting measurements across the Kunlun and AltynTagh faults from Herquel et al. (1999). Both faults arecharacterized by a fast split shear wave polarized parallel tothe trend of the fault, contrasting significantly with theanisotropy pattern away from the faults.

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have been accommodated along this fault.Shear-wave splitting has been measured(Helffrich, 1995) at stations close to theGGWBF in Scotland (Figure 10; stationMCD) and in the Shetland Islands (Figure 10;station LRW). In both stations, the fast splitshear wave is polarized in a plane parallel tothe trace of the fault and a delay time of 0.94and 0.53 s is observed between the arrivals ofthe two SKS waves for MCD and LRW,respectively. The fast S-wave polarizationdirection close to the fault is significantlyoblique to the fast polarization directionmeasured at other stations in the BritishCaledonides (Barruol et al., 1997a).Interestingly, several seismic profilesperformed across the GGWBF, in Scotland aswell as in the Shetland Islands (e.g.,Klemperer & Hobbs, 1991; Klemperer et al.,1991; McGeary, 1989), show topography anda change in the seismic expression of theMoho tightly associated with the trace of theGGWBF at the surface (Figure 11). Thesefeatures have been interpreted as due to thefault crosscutting the Moho and bounding twoinitially remote domains that show contrastedseismic responses. This interpretation is ingood agreement with shear-wave splittingmeasurements. Altogether these resultsstrongly suggest that the GGWBF, rather thanbeing rooted in some crustal decoupling level(McBride, 1995), is a lithospheric fault thatcrosscuts the Moho and penetrates deeply intothe upper mantle.

The well-known South Armorican ShearZone (SASZ) in Brittany, France, is a majorintracontinental transcurrent fault formedduring the Hercynian orogeny. Surface

geology evidence of strain localization andstrike-slip displacement has been reported in alarge number of papers (e.g., Berthé et al.,1979; Jegouzo, 1980). The fault is locatednorth of the high-pressure domain that marksthe trace of the suture between two collidedcontinents. A seismic velocity model of thestructure of the lithosphere down to 200kmbeneath Brittany has been obtained through arecent passive seismology experiment (Granet

Figure 11: Deep seismic reflection profile across theShetland platform (McGeary, 1989). M1, M2, M3 indicateMoho reflectors. D refers to diffraction hyperbolae.

Figure 12: Deep lithospheric structure beneath the Southand North Armorican shear zones (SASZ and NASZ,respectively) in Brittany, West of France. (a) P-velocitymodel from Judenherc et al. (in press) showing that theSASZ separates a northern domain characterized by highseismic velocities from a southern domain, where lowvelocities dominate. High P-wave velocities below thelithosphere (below 90 km) are interpreted as representing afossil slab. (b) Shear-wave splitting measurements.Approaching the SASZ, the fast split shear wavepolarization turns parallel to the trend of the fault,suggesting a coherent tectonic fabric in both the crust andthe mantle. In contrast, shear-wave splitting measurementsabove the NASZ do not show fast shear waves polarizedparallel to the fault trend, suggesting that this latter is acrustal fault.

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et al., 2000; Judenherc, 2000). P-wavevelocity perturbation models show a markedcontrast between two domains (Figure 12):the northeastern domain is characterized by apositive velocity anomaly, whereas thesouthwestern domain displays negativeanomalies. The limit between these twodomains coincides with the trace of the SASZand is observed down to the base of thelithosphere. In addition, the direction of fastpropagation of Pn waves and the direction ofthe polarization plane of the fast split shearwave are consistently parallel to the trend ofthe SASZ. The delay time between the fastand slow split shear waves at stations close tothe SASZ is consistently larger than 1s, alsosuggesting that the entire lithosphere displaysa "wrench-fault type" fabric (Judenherc,2000). These results are very consistent andaltogether suggest that the South ArmoricanShear Zone crosscuts the entire lithosphere.

Combined magnetotelluric (MT) andseismic anisotropy measurements (Figure 13)have been recently performed in the vicinityof the Proterozoic Great Slave Lake shearzone (GSLSZ; e.g., Hanmer et al. 1992), innorthwestern Canada (Wu et al., 2002). ThisNE-SW trending dextral wrench fault is 25km wide and its magnetic expression can becorrelated over 1300 km. This study providedinteresting insights on the lithospheric

structures associated with this major wrenchfault: 1) The fault is associated with a crustal-scale resistive zone which is coincident with amagnetic low, 2) The resistivity structure inthe lower crust to upper mantle isapproximately 2D with a geoelectric strikeN60°E parallel to the large-scale trend of theGSLSZ, and 3) There is a close parallelismbetween the orientation the fast split shearwave polarization plane and the geoelectricstrike retrieved from long-period MTmeasurements. This similarity of seismic andelectric conductivity anisotropies suggeststhat they both have an origin related to thewrench fault fabric of the lithospheric mantlebeneath the GSLSZ.

Transpressional orogenic domainsOften, orogenic domains as a whole

have been submitted to a transpressionaldeformation characterized by the associationof thrusting normal to the belt and lateralescape accommodated by transcurrent faultingparallel to the belt. Recently, Meissner et al.(2002) using Pn anisotropy measurementshave shown that in such domains theuppermost (sub-Moho) mantle ischaracterized by a fast propagation of P-waves parallel to the trend of the belt,pointing to a flow fabric in the uppermostmantle dominated by the lateral escape oflithospheric blocks. Shear-wave splittingmeasurements in active and fossil orogenicareas also record orogen-parallel flowdirections in the upper mantle (e.g., Savage,1999; Silver et al., 1999; Vauchez & Nicolas,1991). Fast shear waves are polarized parallelto the trend of the transpressional belts, evenin domains where crustal deformation isessentially accommodated by thrusting, anddelay times frequently ≥ 1s indicate that this"wrench fault type" flow fabric affects theentire lithospheric mantle.

Figure 13: Comparison of magnetic field data, MT high-conductivity strikes for the period band of 20-500s, and SKSfast directions for the Great Slave shear zone (Wu et al.2002). H and L refer to magnetic highs and lows,respectively. Delay times between the arrivals of the twosplit SKS waves are of 1.1-1.5 s.

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Taiwan is currently deforming inresponse to the oblique convergence betweenthe Philippines and the Eurasian plates. As aresult, the crust displays evidence of atranspressive deformation and strain ispartitioned between thrusting and wrenchfaulting normal and parallel to the belt,respectively (e.g., Huang et al., 2000;Lallemand et al., 2001). Shear-wave splittingmeasurements by Rau et al. (2000) displaynevertheless a coherent pattern over the entireTaiwan Island (Figure 14). S-waves generatedin the Benioff zone by local earthquakes thatprobe the mantle above the subduction zone,are split. The fast shear wave is polarizedparallel to the tectonic grain and delay timesare up to 2s. These observations suggest thatthe upper mantle beneath Taiwan has ahomogeneous transcurrent /transpressionalfabric due to northward tectonic escape, i.e., atransport direction parallel to the activeorogen.

The Neoproterozoic Ribeira orogenicbelt of Southeastern Brazil formed during thefinal amalgamation of Gondwana between580 and 540 Ma (Egydio-Silva et al., 2002).The southern and central domains of the beltwere subjected to an oblique convergencebetween the South American and Africanprotocontinents (Figure 15a). This resulted indevelopment of numerous dextral wrenchfaults, hundreds of kilometers long and up to10 kilometers wide, oriented parallel orslightly oblique to the belt. In the centraldomain, the current level of erosion (17-20km) shows mylonites that formed at hightemperature (T>800°C) and continued todeform during a slow cooling down to ca.740°C. Southward, the erosion level is moresuperficial and the shear zones are marked bymylonites formed under amphibolite faciesmetamorphic conditions (Vauchez et al.,1994). The wrench faults reworked a slightlyolder low-angle foliation due to thrustingtoward the South American protocontinent.During the late orogenic stages, both orogen-normal thrusting and orogen-parallel wrenchfaulting occurred. As a whole, the southern-

central Ribeira belt represents atranspressional orogenic segment about100km wide and almost 1000 km long(Trompette, 1994). Shear-wave splittingmeasurements performed over the southernbranch of the Ribeira belt (Heintz et al., 2000)have yielded a coherent pattern characterizedby a polarization of the fast S-wave in adirection parallel to the orogenic grain (Figure15b), suggesting that the bulk volume oflithosphere in the transpressional domain hasa "wrench fault-type fabric". Larger delaytimes between the fast and slow shear wavesarrivals (up to 2.5s) have usually beenretrieved from data recorded above or close tothe main shear zones, suggesting that strainwas not homogeneously accommodated butwas somewhat localized in the main shearzones.

Figure 14: Deep structure beneath the active Taiwanorogen. A: simplified map showing the geodynamicsituation of the Taiwan orogen (from Lallemand et al.,2001). B : Shear-wave splitting measurements (Rau et al.,2000) using S waves from local earthquakes andteleseismic ScS.

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The Pyrenees in Western Europe(Figure 16) formed during the Mesozoic dueto displacement of Iberia relative to Eurasia.This motion, generated by the opening of theAtlantic ocean between North America andIberia, was mainly accommodated along theNorth Pyrenean fault (e.g., Choukroune,1992). At first, the deformation regime wastranstensive and several pull-apart basinsformed. Then, during the final stages of theevolution it became transpressive and finallycompressive. Indeed, the North Pyrenean

fault, i.e., the rupture between Iberia andEurasia, reactivated an older, pervasivetranspressive fabric formed during the latestages of the Hercynian orogeny (e.g.,Bouchez & Gleizes, 1995; Vauchez &Barruol, 1996). Shear-wave splittingmeasurements performed across the Pyreneesand adjacent areas revealed a very consistentpattern of anisotropy (Barruol et al., 1998).The fast shear wave polarization plane isusually oriented parallel to the belt, and thedelay between the fast and slow S-wave

Figure 16: Shear-wave splitting in the Pyrenees and adjacent areas. (a) Sketch map of the main hercynian structural directionsin the Pyrenees and adjacent regions. NPF is for the North Pyrenean Fault and SASZ for the South Armorican Shear Zone (seeFigure 12). The relative position of Iberia relative to Europa is the current position. (b) Shear-wave splitting measurements inthe Pyrenees (Barruol et al., 1998). At each location, the size of the circle is proportional to the delay time that is usually >1sand the line indicates the polarization of the fast split shear wave.

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arrivals is larger than 1s, even beyond theMesozoic Pyrenees belt. Pn anisotropymeasurements (Judenherc et al., 1999) are ingood agreement with S-wave splittingmeasurements; the fast propagation directionof Pn is also parallel to theHercynian/Pyrenean tectonic fabric,suggesting that the entire lithosphere beneaththe probed area has a coherent "wrench faulttype" fabric.

The analysis of the seismic anisotropydata for the active orogen of Taiwan, theNeoproterozoic Ribeira belt and theHercynian/Alpine Pyrenean belt leads tosimilar conclusions. S-waves splitting resultsare consistent with seismic anisotropy modelsin which the lithospheric mantle deforms byhomogeneous transpression, instead of thepartitioned mode displayed by the crust(Tommasi et al., 1999). However, the tectonicfabric of the mantle does not correspond tothe classical transpression as defined bySanderson and Marchini (1984, i.e., with avertical stretching), but rather to lengthening-thinning shear (i.e., plane transpression,Tikoff & Fossen, 1999; Tommasi et al.,1999). This deformation regime involvessimultaneous shortening normal andstretching parallel to the trend of the belt andresults in a lateral escape of the lithosphericmantle. This may explain why observation ofa seismic anisotropy coherent with orogen-normal thrusting at the scale of the lithosphereis so scarce (e.g., Silver, 1996).

Lithospheric wrench faults: Thermo-mechanical effects

The various examples presented aboveconverge toward a model of major wrenchfaults deeply rooted into the upper mantle.Especially seismic tomography and shear-wave splitting observations support that thefault fabric affects the entire lithospherethickness. The width of the domain presentinga "wrench fault-type" fabric likely rangesbetween several tens of kilometers for a singlefault to several hundreds of kilometers for atranspressional domain involving varioustranscurrent and thrust faults. Moreover,seismic anisotropy observations using long-period data such as SKS waves imply that the

olivine lattice preferred orientation associatedwith this "wrench fault-type" fabric,characterized by horizontal [100] axes andvertical (010) planes, both parallel to the faulttrace, is coherent at scales ≥ 50 km.

On the other hand, the olivine crystaldoes not only display an anisotropic elasticity,which leads to the observed seismicanisotropy. The plastic deformation andthermal diffusivities of olivine also are highlyanisotropic (Bai et al., 1991; Chai et al., 1996;Durham & Goetze, 1977; Kobayashi, 1974).Thus if major wrench faults are characterizedby a coherent olivine lattice preferredorientation that affects the entire lithosphereover domains several hundreds (or thousandsin the case of a transpressional belt) ofkilometers long and tens (or hundreds) ofkilometers wide, these domains might also bethe source of a large-scale mechanical andthermal anisotropy within the continentallithosphere that may influence the thermo-mechanical behavior of the plate duringsubsequent tectonic events.

Strain-induced mechanical anisotropy of thecontinental lithosphereExperimental deformation of olivine singlecrystals under different orientations relative toits crystallographic lattice shows that olivinehas only three independent slip systems andthat these systems display significantlydifferent strength or critical resolved shearstress (CRSS) values (Bai et al., 1991;Durham & Goetze, 1977). Under high-temperature conditions, the (010)[100] slipsystem displays the lowest critical resolvedshear stress; this means that, compared to theother possible slip systems, for a given stressit is able to accommodate the largest slip rate,or, conversely, that it requires the lowestbuilt-up resolved shear stress to accommodatea given strain rate. In other words, fordeformation in the dislocation creep regime,which is expected to prevail in thelithospheric mantle in active areas, olivinedisplays an anisotropic viscosity.

In a lithospheric wrench fault, theweakest (010)[100] slip system is orientedparallel to the fault, i.e. the olivine crystals arepreferentially oriented with the (010) plane

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subvertical and the [100] axis horizontal,parallel to the shear direction. The question iswhether the anisotropic mechanical behaviorof the olivine single crystal combined withsuch a LPO coherent over large scales in thelithospheric mantle may result, at the scale ofthe lithospheric mantle, in an anisotropy ofviscosity large enough to influence thedeformation of the lithosphere duringsubsequent tectonic solicitations.

Tommasi and Vauchez (2001) used apolycrystal plasticity model to investigate theeffect of a pervasive "wrench fault-type"fabric frozen in the lithospheric mantle on thecontinental breakup process. In this work, thedeformation of an anisotropic continentallithosphere in response to an axi-symmetrictensional stress field produced by anupwelling mantle plume was evaluated bycalculating the deformation of texturedolivine polycrystals representative of thelithospheric mantle at different positionsabove a plume head (Figure 17). Thesemodels show that an LPO-inducedmechanical anisotropy of the lithosphericmantle may result in directional softening,leading to heterogeneous deformation.Reactivation of the inherited crystallographicfabric, which is favored by tensional stressesoblique to its trend, is characterized by higherstrain rates than other deformation regimes.

The reactivation of the pre-existingfabric also results in higher strain rates thanthose accommodated by an isotropic mantlein similar conditions. During continentalrifting, this mechanical anisotropy may thusinduce strain localization in domains whereextensional stress is oblique (30-60°) to thepre-existing mantle fabric. The directionalsoftening associated with olivine LPO frozenin the lithospheric mantle may also guide thepropagation of the initial instability that willfollow the pre-existing structural trend. Theinherited mantle fabric also controls thedeformation regime, imposing a strong strike-slip shear component to the deformation. AnLPO-induced mechanical anisotropy maytherefore explain both the systematicreactivation of ancient collisional belts during

rifting (structural inheritance) and the onset oftranstension within continental rifts.

Figure 17. Predicted deformation of a lithospheredisplaying a wrench fault-type fabric above a mantle plume(Tommasi & Vauchez, 2001). (a) Strain rate (Von Misesequivalent strain rate, normalized relative to the isotropicbehavior) as a function of the orientation of the radialtensional stress relative to the [100] axis maximum of thepreexisting LPO for points above the plume head peripheryfor three models with different initial LPOs. (b) Normaland shear components of the strain rate tensor (normalizedby the Von Mises equivalent strain rate displayed by anisotropic polycrystal) for the model in which the initialLPO is the model aggregate. The reference frame isdefined relative to the preexisting mantle fabric: X isparallel to the [100] axis maximum, i.e., parallel to thepreexisting structural trend, Y is normal to the preexistingshear plane, and Z is vertical. Positive normal strain ratesdenote extension and negative ones, shortening. Grayregion marks orientations that may trigger strainlocalization

15

These results, obtained for a specificgeodynamic case, can be extended to a moregeneral situation. In major strike-slip faultsand transcurrent/transpressional orogenicdomains, the inherited fabric of thelithospheric mantle should induce adirectional softening, with the consequencethat this fabric should be preferentiallyreactivated. Development of new structuresoblique to the preexisting shear zones shouldonly be observed when the new tectonicsolicitations (either distensive or compressive,Figure 18) are normal or parallel to theinherited foliation, i.e., when no shear stressesare applied parallel to the inherited fabric. Inmost cases, reactivation will occur throughtranstension or transpression, and the relativeproportion of simple and pure shear dependson the obliquity of the stress axes relatively tothe inherited fabric.

The crustal fabric in lithospheric-scaleshear zones also contributes to this

mechanical anisotropy. Indeed, localizeddeformation in the middle and lower crustgives rise to strong LPOs. Crustal minerals,in particular micas that are important phasesin mylonites, display a still strongermechanical anisotropy than olivine; theirlayered structure results in plasticdeformation accommodated by glide on the(001) plane only. In addition, strengthvariations in polymineralic crustal rocksoften gives rise to a mm- to cm-scalecompositional layering parallel to the shearzone that, at a larger scale, also contributes toa directional weakening and reactivation ofthe shear zone. Finally, grain-size reductionassociated with shearing in the upper/middlecrust may result in an isotropic strain-softening within the shear zone; at thesedepths, the shear zone will thus act as aplanar weak heterogeneity localizing thesubsequent deformation.

Repeated reactivations of majortranscurrent shear zones or domains duringlong periods of time and the necessity for thecause of this persistence to be in thelithospheric mantle have been recognizedlong ago (e.g., Watterson, 1975). Manyexamples of such reactivation in variousgeodynamic environments are available in theliterature. Tommasi and Vauchez (2001) havealready discussed those related to thereactivation of lithospheric-scale shear zoneor transpressional belts during continentalrifting. So we will focus on one of the bestillustrations of the reactivation of a collisionalwrench fault as a transform boundary: thedevelopment of the Newfoundland-Azores-Gibraltar transform plate boundary at thenorthern edge of the central Atlantic Oceanduring the Early Mesozoic (Figure 19). TheNewfoundland-Azores-Gibraltar fault zoneformed a major

Figure 18 . Compressional deformation of a lithospheredisplaying a wrench fault-type fabric. Calculated strain rates(Von Mises equivalent strain rate, normalized relative to theisotropic behavior) are displayed as a function of the orientationof the imposed shortening relative to the (010) plane maximum ofthe preexisting LPO.

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Hercynian dextral strike-slip faultzone that offsets the Appalachians orogenicfront in Newfoundland (Keppie, 1989).During the final stages of the Appalachian-Variscan convergence, this faultaccommodated the relative displacementbetween the Iberian and North African blocks.This fault subsequently played a major role onthe Central Atlantic initial rifting, limiting oneof the promontories of the North Americanstable margin. Indeed, the opening of thecentral Atlantic Ocean took place almostsimultaneously from Florida to theNewfoundland-Azores-Gibraltar transform(the first Central Atlantic magnetic anomaly,M25 is identified along this entire segment(Owen, 1983)), but further northwardpropagation of the Central Atlantic leading toseparation between Eurasia and NorthAmerica did not occur until Late Cretaceoustime. From mid-Jurassic to Late Cretaceoustime, the Newfoundland-Azores-Gibraltartransform connected the Central Atlantic andthe Thetys oceanic basins, accommodatingthe differential motion between Africa andEurope.

Thermal conductivity anisotropyHeat transfer is a key process

controlling the Earth's dynamics, sincetemperature is a major parameter controllingthe rheological behavior of both crustal andmantle rocks. Thermal conductivity in bothmantle and crust is usually assumed to beisotropic. Yet, experimental data shows that,at ambient conditions, the dominant mineralphases in the crust and upper mantle display alarge anisotropy of thermal diffusivity. Inolivine, for instance, heat conduction parallelto the [100] crystallographic axis is 1.5 timesfaster than parallel to the [010] axis (Chai etal., 1996). Quartz and micas, the mainconstituents of crustal mylonites also displaya strongly anisotropic thermal conductivity,with the highest and lowest conductivitiesparallel to the [001] axis and within the (001)plane, respectively (Clauser & Huenges,1995).

This thermal anisotropy is also observedat the rock scale. Recent studies combiningpetrophysical modeling and thermaldiffusivity measurements on upper mantlerocks (Tommasi et al., 2001) show that adeformation-induced olivine LPO may resultin a significant thermal diffusivity anisotropyin the uppermost mantle: heat transportparallel to the olivine [100] axesconcentration (flow direction) is up to 30%faster than normal to the flow plane ([010]concentration). Moreover, in the studiedtemperature range (300 to 1250 °K), thethermal diffusivity anisotropy does notdepend on temperature, suggesting it might bepreserved even at higher temperaturescorresponding to asthenospheric conditions.Seismic anisotropy data, like those presentedin the previous sections, indicate that majorwrench faults are characterized by a coherentolivine lattice preferred orientation thataffects the entire lithosphere over domainsseveral hundred (or thousand in the case of atranspressional belt) of kilometers long andtens (or hundreds) of kilometers wide. This"wrench fault type" fabric should thereforeinduce a large-scale thermal diffusivityanisotropy in the lithospheric mantle,characterized by faster heat conduction within

Figure 19: Fit of the Central and North Atlantic oceanshowing that the initial rift in the central domainpropagated parallel to the Hercynian orogen and that theNew Foundland-Açores-Gibraltar hercynian wrench faultwas reactivated in the Mesozoic as a transform faulttransferring extension from the Central Atlantic basin tothe Thetys basin.

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the shear zone parallel to the shear directionand slower conduction normal to the shearzone.

A similar thermal anisotropy should bepresent in the crustal section of a lithosphericshear zone. Laboratory measurements ofthermal conductivity of gneisses drilled in theKTB borehole show up to 40% of anisotropy(Buntebarth, 1991). In these samples, whichdisplay mineralogical compositions (quartz,micas, and feldspars) and microstructuressimilar to those of high-temperature mylonitesin the Borborema, Ribeira, and Madagascarshear zones, heat conduction parallel to thefoliation plane is on average 1.2 times fasterthan normal to it. A weaker anisotropy isobserved within the foliation plane, with thehighest conductivity measured parallel to thelineation. Comparison between measuredthermal conductivities and those predicted bypetrophysical modeling suggests that,similarly to the mechanical anisotropy, themajor contributions to the gneisses thermalconductivity anisotropy stems from the strongLPO of micas and quartz (Siegesmund, 1994).

Existence of a large-scale, strain-induced thermal anisotropy in the uppermantle implies that the temperaturedistribution, rheology, and, hence, the uppermantle dynamics depend on its deformationhistory. Olivine orientations frozen in thecontinental lithosphere may modify plume-lithosphere interactions for instance.Enhanced thermal diffusivity alonglithospheric-scale wrench zones, i.e., parallelto the olivine [100] preferred orientation, maylead to anisotropic heating of the lithosphereabove a mantle plume, favoring thereactivation of these structures duringcontinental break-up (Tommasi & Vauchez,2001; Vauchez et al., 1999; Vauchez et al.,1998). Such a control of the pre-existinglithospheric structure on the propagation of athermal anomaly may be inferred, forinstance, from tomographic images of theEast African rift in Kenya (Achauer & krisp-group, 1994). In these images, the low-velocity seismic anomalies display two maintrends: a N-S trend, parallel to the surfaceexpression of the East African rift, and a NW-SE trend following Neoproterozoic structures

that were reactivated during the Mesozoic togive rise to the Anza rift.

ConclusionGeolog ica l and geophys ica l

observations in active and fossil orogenicbelts converge to support that major wrenchfaults are rooted into the upper mantle. Hugetranscurrent shear zones (several hundreds ofkilometers long and a few tens of kilometerswide) in Brazil and Madagascar have beeneroded down to levels where deformation wasaccommodated under high temperatureconditions (650°C to >800°C) in partiallymelted rocks. It is remarkable that under thesehigh-temperature and, hence, low-viscosityconditions, which were highly favorable todevelopment of a decoupling level, noevidence of rooting of these shear zones hasbeen observed; on contrary, strain was stilllocalized in wide transcurrent shear zones.Seismic profiling, seismic tomography, Pnazimuthal anisotropy and magnetotelluricsoundings also support that several majorwrench faults crosscut the Moho discontinuityand penetrate the uppermost mantle. Inaddition, shear-wave splitting measurementsand electric conductivity anisotropy abovemajor strike-slip faults are in agreement witha "wrench fault type" mantle fabric coherentacross most or even the totality of thelithosphere thickness. Indeed, transform faultboundaries as the San Andreas Fault, forwhich a connection with the mantle isrequired, display geophysical characteristicssimilar to those of the main intracontinentalfaults, either active or fossil. A similarconclusion is reached for transpressionalorogenic domains deforming in response tooblique convergence/collision.

The existence of a "wrench fault-type"fabric into the continental mantle, besidesinducing anisotropic elastic and electricalproperties, may result in the development of adirectional softening and an anisotropicconduction of heat in the continental mantle.These anisotropic properties probablyinfluence the large-scale tectonic behavior ofthe continents. Reactivation of the inheritedmantle fabric represents in most cases themost economic behavior in terms of energy.

18

Only in very specific situations (solicitationorthogonal or parallel to the ancient fabric),the pre-existing fabric of the lithosphericmantle will not be reactivated. Preferentialpropagation of continental break-up parallelto ancient orogenic belts as well as thesystematic reactivation of major wrench faultslikely result from both a directional softeningand an anisotropic heat transfer due towrench-type olivine-preferred orientationsfrozen in the continental mantle.

Finally, the work by Pollitz et al.(Pollitz et al., 2000; Pollitz et al., 2001) thatsuggests that the mantle beneath activewrench faults deform coherently with thecrust and, in some way, determines theinterseismic characteristics of the fault raisesthe question of the effect of the mechanicalanisotropy of the lithospheric mantle on thedynamics of active faults. Characteristics ofthe fault like the slip rate, the stress buildingrate and therefore the magnitude and therecurrence of earthquakes could be affectedby a lower stiffness of the mantle in a specificdirection.

Acknowledgements: J. M. Lardeaux andJ. E. Martelat provided the map and images ofthe Madagascar shear zones and M. Granetthe seismological results on the Armoricanmassif. We thank C. Teyssier and L. Burlinifor constructive reviews.

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