Why do continents break-up parallel to ancient orogenic belts? A. Vauchez*, G. Barruol and A. Tommasi? Lahoraroire de Tectonophysique, C'niverritC de Montpellier II & CNRS, F-35095 44ontpelher C~dexOF, Francc

ABSTRACT The frequently observed parallelism between rifts and the pre- assembly, a pervasive deformation of the lithosphere induces a existing orogenic fabric of continents suggests that the inherited lattice-preferred orientation of olivine in mantle rocks. Later on, tectonic fabric of the lithosphere influences the rupture of this crystallographic fabric is 'frozen-in' and represents the main continents. We propose that the existence of a pervasive fabric in source of shear wave splitting. This olivine fabric may entail a the lithospheric mantle induces an anisotropic strength in the mechanical anisotropy in the lithospheric mantle. During lithosphere, that guides the propagation of continental rifts. subsequent tectonic events, especially during rifting, Subcrustal mantle mechanical anisotropy is supported by (i) the mechanical anisotropy may control the tectonic behaviour of anisotropicstrength of olivine, (ii) an ubiquitous tectonic fabric in the lithosphere. exposed mantle rocks, and (iii) measurements of seismic and electrical anisotropy. During major episodes of continent Terra Nova, 9, 62-66, 1997

Mozambique belt and terminates From Florida to the Barents Sea, the Rifting parallel to orogenic belts northward into the Afar. The western North Atlantic ocean opening occurred Ocean-opening through rifting and branch follows the Ubendian belt that parallel to the Hercynian and Caledo- continent break-up is frequently related curves from W-SE to NE-SW. Theu- nian orogens (Fig. 2). In castern North to the occurrence of hotspots. There IS, nissen et a/. (1996) showed that: (i) the America, parallelism between the Ap- however, a discrepancy between hot- Ubendian belt. south and west of the palachians. Triassic rifts. the continent spots acting as pinpoint sources of heat Tanzania craton, is a wide and steep margin, and the main magnetic anoma- and the linear extent of rifts over thou- shear belt resulting from Palaeo- and lies that mark the Atlantic opening sands of kilometres. Moreover rifts tend Neoproterozoic tectonic events, (ii) the indicates that the initial rift propagated to parallel pre-existing orogenic fab- west Tanzania rift developed during along the internal domain of the belt. rics, e.g. North Atlantic rift (Wilson, Phanerozoic times through multiphase South of the West .4frican craton, the 1966), Rio Grande rift (Olsen et a/., reactivation of the Precambrian steeply Atlantic rift formed parallel to Neopro- 1987), North-east China rift (Ma and dipping fabric, and (iii) fault geometry terozoic (southern Senegal, Sierra Leone) Wu, IY87), Bai'kal rift (Delvaux et a/., and kinematics are complex and point and Hercynian belts (northern Senegal, 1995). East African rift (Ring, 1994), toward a SE-NW propagation of the rift Mauritania). North of the craton, rifting West African rift (e.g. Fairhead and in a dextral transtensional strain regime. at = 200 Ma reactivated the Hercynian Binks, 1991), Cape Graben (Burke, belt ofMorocco (plque and Lavillc, 1996). 1976), and Eastern Brazilian rift (Chang South of the North American continent. ef al., 1992). From these observations it formation of oceanic lithosphere in the was suggested that structural inheri- Gulf of Mexico was preceded by the tance influences rift propagation. This dcvclopment of triassic extensional ba- paper propounds that the source of sins elongated parallel to the Ouachitas structural inheritance lies in the fabric mountains (Worral and Snelson, 1989). of the lithospheric mantle developed Gondwana Cretaceous break-up be- during major orogenic episodes. tween South America and Africa is The East African rift and the North and South Atlantic oceans provide spectacular examples of parallelism be- tween rifts and older orogenic belts. The East African rift system (Fig. I), > 2000 km long and still active, formed mostly during the Cenozoic in a shield rtable since the Neoproterozoic (Ring, 1994). The southern part of the system trends almost N-S, parallel to the Mo- zambique belt. Northward, the rift sys- Ocean 20" tem splits into two branches that wrap around the Tanzania craton. The east- ern branch remains parallel to the

Correspondence: A. Vauchez, E-mail: vau- Fig. 1 Schematic map (modified from [email protected] fr Ring, 1994) illustrating the parallelism Fig. 2 Schematic map of circum-North talso at Lab. Petrologie et Tectonique. between normal faults in the Fast African Atlantic Caledonian-Hercynian belt\ he- Universite Lyon I & CNRS, France Rift and basement structural fabric forc North Atlantic lifting

62 c 1997 Blackwell Science Ltd Terra Nova, Vol 9, No. 2, 62-66 Continents break-up parallel to orogenic belts A. Vauchez et a/...... related to the upwelling of the Tristan belt along the N-S-trending Neopro- tends to impede propagation of rifts da Cunha and St. Helena plumes. The terozoic Dahomeyide belt, but the main into cratonic nuclei, and their bound- initial rift system, however, propagated continental breakup occurred through ary with the surrounding lithosphere over more than 3000 km between and wrench faulting between the St. Helena may represent a preferential site for beyond these structures, following the plume and the North Atlantic spread- extensional deformation to localize. Malmesbury-Gariep, Dom Feliciano, ing ridge. This Cretaceous wrench tec- This model is certainly valid for rifts Kaoko, Ribeira and West Congolian tonics (Fairhead and Binks, 1991) formed along old cratonic nuclei as the Neoproterozoic belts (Fig. 3a). The reactivated Neoproterozoic transconti- Bdikal and the Kenya rifts, although it parallelism observed between the South nental E-W-shear zones. remains difficult to evaluate the respec- Atlantic rift system and the continental tive Contribution of rheological hetero- geneity and structural inheritance since fabric is frequently striking since basins Rift propagation and rheologicat follow the curvatures of these orogenic usually orogenic belts wrap around the heterogeneity of the continental cratons and rifts are parallel to both the belts. At the southern tip of Africa, for lithosphere instance, the initial rift bends parallel to craton boundary and the surrounding the Gariep-Malmesbury then the Cape Various interpretations have been pro- belts. Moreover, rheological heteroge- fold belts where it became transten- posed to explain the relationship be- neities fail to explain all cases of rifts sional (Burke, 1976). Along the eastern tween continental lithospheric rifts and developed in an ancient continental coast of Brazil, Chang et al. (1992) the tectonic fabric of continents. Rheo- lithospherc, since in many places rifts showed that the east Brazilian rift is logical calculations (e.g. Dunbar and formed far from any old cratonic nu- remarkably parallel to basement struc- Sawyer, 1989) show that an increase in cleus (e.g. the southern East African tures (Fig. 3b). Even where it propa- crustal thickness can halve the total Rift or the South Atlantic rift). gated in the SBo Francisco craton, the strength of the continental lithosphere south Atlantic rift followed the steep and thus favour localization of rifting. Inherited lithospheric mantle tectonic fabric and large-scale shear In active orogens, crustal thickness is fabric: clues from seismic and zones of the 2Gyr-old Itabuna belt expected to vary and to culminate close electrical anisotropies and Salvador-Esplanada belt of un- to suture zones that represent good known age. Burke (1976) emphasized candidates for the localization of sub- Upper mantle xenoliths and peridotite that grabens oblique to the tectonic sequent rifting. For rifting beginning massifs usually display a well-defined fabric of the continents rapidly aborted. shortly after the end of an orogeny (e.g. crystallographic fabric due to past de- The central Atlantic is more complex North Atlantic in the Appalachians/ formation of the lithosphere (e.g. Mer- since the ocean curves from N-S to Mauritanidesorogen), variation in crustal cier and Nicolas, 1975). Since upper almost E-W and crosscuts various geo- thickness across the belt is likely and mantle rock-forming minerals (espe- tectonic domains. Bonatti (1996) sug- may result in lateral variation of the total cially olivine) are seismically anisotro- gests that a cold equatorial belt impeded strength of the lithosphere and favour pic, their crystallographic preferred the propagation of the southern Atlan- the localization of the rupture process. orientation results in a significant seis- tic rift northward. An aborted branch Local lithospheric weakness asso- mic anisotropy of mantle rocks (e.g. of the rift propagated north of this cold ciated with crustal roots does not satis- Nicolas and Christensen, 1987), and is factorily explain rift propagation within regarded as the main source of teleseis- an old and stable shield (e.g. South mic shear wave splitting (Silver, 1996). Atlantic or East African rifts). The time This supports the notion that shear lag between the last orogeny and rift wave splitting is mostly related to the initiation is of several hundreds of Myr, tectonic fabric of the upper mantle, a duration long enough for large-scale either due to active (asthenospheric) excess crustal thickness to be removed or ‘frozen’ (lithospheric) mantle flow. by surface erosion, isostatic compensa- Short-scale spatial variations of split- tion or gravitational collapse. The grav- ting parameters on continents and a ity map of Brazil, for instance, does not good correlation with surface geology show any anomaly suggesting a residual (e.g. Barruol er al., 1997) point to a crustal root beneath late Proterozoic dominant contribution of continental orogens (Ussami et al., 1993), and it may lithospheric mantle fabric to the split- reasonably be assumed that this was ting of shear waves. already the case during the Cretaceous Most commonly, the fast split shear when the South Atlantic rifting initiated. wave is polarized subparallel to the Rheological heterogeneity of the tectonic grain of the orogens (e.g. Sil- Fig. 3 Schematic map of circum-South ver, 1996). The difference in arrival Atlantic Neo-Proterozoic belts (insert) continental lithosphere may also arise and of eastern Brazil showing the paralle- from old cratonic nuclei incorporated time between the fast and slow waves lism between the Neoproterozoic belts into continents (e.g. Tommasi et al., is usually around 1 s and may excep- and the Cenozoic rifts formed along the 1995). Even several hundreds of Myr tionally exceed 2 s. Considering the continent margin. Normal faults from after continent assembly, old cratonic intrinsic anisotropy of mantle xeno- Chang ef al. (1992). SFC, Slo Francisco nuclei display a ‘colder’geotherm and a liths, such time lags require that the craton; RPC, Rio de la Plata craton; R, thicker lithosphere than the surround- waves propagated through at least a Ribeira belt. ing terranes. Their stiffer lithosphere 100 km-thick layer of anisotropic man-

Q1997 Blackwell Science Ltd 63 Continents break-up parallel to orogenic belts A. Vauchez et a/. Terra Nova, Vol 9, No 2, 62-66

tle characterized by a preferred orienta- belts lie on the edge of continents that the break-up between Europe and I ber- tion of the [010] and [lo01 axes of olivine, were mostly built during the Grenvil- ia. with the initial rupture occurring respectively, perpendicular and close to lian orogeny for the eastern US and the parallel to the Hercynian belt in the the strike of the belt (Mainprice and Hercynian orogeny for the western futurc sitc of the Pyrenees. Subscqucntly. Silver. 1993). Thcsc mcasurements hint Europe. Considering that, during these pull-apart basins elongated parallel to at the existence of a well-developed episodes of continent assembly. strips the plate boundary formed = 100 Ma crystallographic fabric over the entire of continental lithosphere > 1000 km north of thc NPF. Finally. N-S con- thickness of the lithospheric mantle. wide were deeply deformed. it may be vergence of Iberia and Europa. mainly Coupled shear wave splitting and expected that a pervasive fabric was during the Eocene, led to the formation magnetotclluric experiments tend to developed over thc ciitirc lithosphere. of the Pyrenees which also parallels the support a common lithospheric origin Inherited lithospheric fabric had a older hercynian grain. of seismic and electrical anisotropies. clear influence on the evolution ofeast- Electrical conductivity anisotropy is ern North America. The NE-trending Mechanical anisotropy of the oftcn interpreted as resulting from the Appalachian belt and EW-trending lithospheric mantle existence of conducting graphite films Ouachitas Mountains are consistently along grain boundaries, with the high- parallel with the Grenville belt. In its The existence of a penetraLive fabric est conductivity parallcl to the foliation Canadian branch. thc main thrust faults within the lithospheric mantle suggests trend (e.g Sknechal ef a/., 1996). The and limits of tectono-metamorphic zones an alternative explanation for continental depth at which electrical anisotropy of the Grenville belt trend northeast- break-up parallel to orogenic belts. If occurs is related to the frequency range ward (Hoffman. 1989). Southward, ex- the strength of individual crystals in in which it is observed, and is therefore posures are scarce. but magnetic and tnantle rocks is anisotropic. a strong. approximately determinable. Senechal gravimetric anomalies characteristic of lattice-preferred orientation of rock- el ul. (1996), for instance, carried out the belt domains allow identification of forming minerals may entail a mechan- shear wave splitting measurements and buried terranes and of their limits. Post- ical anisotropy of upper mantle rocks magnetotelluric soundings along a trans- Grenvillian structures systematically and therefore of the lithosphere. cct in the Canadian shield: consistcnt formcd parallcl to the Grcnvillc bclt. In Metals, ceramics or ice often display a directions of anisotropy and measured the Upper Proterozoic- Lower Palaeozoic mechanical anisotropy, i.e. their strength electrical anisotropy originating be- (i.e. about 400 Myr after the Cirenvdle depends on the orientation of the ap- tween 50 and 150 km, suggested that orogeny), the lithosphere was rifted par- plied force. Rocks may also display such seismic anisotropy originated from a allel to the belt, then a passive margm a mechanical anisotropy (c.g. Siricys. well-defined fabric in the lithospheric formed. After a period of ocean opcning. 1982): under tension. thcir strcngth is upper mantle. Preliminary results from the eastern American margn turned con- generally lower for extension normal a magnetotelluric experiments in the vergent, underwent oblique accretion of than parallel to the foliation plane. This eastern US (Ogawa et al., 1996) also terranes and finally collision with Africa, behaviour is especially clear during point to a subcrustal electrical anisotropy resulting in the building of the Appala- elastic and brittle deformation. but it with the highest conductivity along the chian-Ouachitas system that fits almost also occurs during ductile deformation. strike of the Appalachians, a result con- exactly the Grenville belt. Then a similar Microscopic studies of deformed sistent with shear wave splitting data. cvolution recurred (Wilson, 1966) with pcridotitcs and cxpcrimcntal axial com- Shear wave splitting measurements in the opening of the North Atlantic ocean pression of olivine crystals in different the Appalachians (Barruol rt d.,1997) along the Appalachian internal domain. crystallographic orientations provide and the Pyrenees (Barruol and Souriau. Reconstitution of the Variscan belt evidence that: (i) few slip systems are 1995) show (Fig. 4) that most stations also highlights parallelism bctwccn the availablc to accommodate plastic dc- located in the external tectonic domains, Pyrenees and pre-existing Hercynian formation. and (ii) these systems dis- or even beyond the front of the belts, are structures. The tectonic rabric of the play a significantly different resistance charactcrized by fast split waves polar- Pyrenees axial zone is mostly Hercy- to flow. The relative strength of the ized parallel to the belts. This observa- nian in age, and parallels the belt (e.g. different slip systems in olivine depends tion cannot be explained by a pervasive Zwart. 1986). This led us to suggest that strongly on temperature. At high-tem- deformation of the lithosphere during the the roughly E-W fast polarization di- peratures, the (010)[100] system is at Appalachian or the Pyrenean orogenies rections found in the southern Pyrenees least two times weaker than any other only. since seismic reflection profiles (Fig. 4) may be generated by an inher- system and should accommodate most across the Appalachians (Cook er d., ited Hercynian lithosphcric fabric (Bar- of the strain (Durham and Goctzc, 1977: 1979) and the Pyrenees (Choukroune, ruol and Souriau, 1995; Vauche7 and Bai rt a/., 1991; Fig. 5). This observa- 1992) strongly suggest that deforma- Barruol, 1996). A strong mechanical ef- tion agrees with microstructural obser- tion only affected the shallow crust of fect of this inherited fabric is suggested vations of naturally deformed peridotites the cxternal domains of these belts. The from the subsequent geodynamic evo- (e.g. Nicolas and Pokier, 1976). which most likely explanation is that the fab- lution of the pyrenean domain. Iberia clearly indicate that olivine deforms ric of the lithospheric mantle responsi- displacement relative to Eurasia along predominantly by glide on the (010)[100] ble for shear wave splitting was not the North Pyrenean Fault (NPF) started slip system under upper mantle condi- formed during the Appalachian or the when the Atlantic rift, following the tions. Under experimental conditions, Pyrenean orogenies, but was inherited curvature of the Hercynian belt, wrapped a transition from this high temperature from an earlier orogenic episode (Vau- around Iberia and produced the open- [ 1001 glide to low-temperature [OOl] glide chez and Barruol, 1996). Thcsc two ing of the bay of Biscay. This triggcrcd occurs at c. 1000 "C. Howcvcr. at nat-

64 I 1997 Blackwell Science Ltd Terra Nova, Vol 9, No. 2, 62-66 Continents break-up parallel to orogenic belts A. Vauchez eta/.

Fig. 4 Teleseismic shear wave splitting in (a) the Appalachians (Barruol et a/.,1997) and (b) the Pyrenees. Anisotropy data in the Central Pyrenees from Barruoland Souriau (1995); in the Eastern Pyrenees the map shows preliminary results from a portable experiment by the Universite de Montpellier I1 and the Observatoire Midi Pyrenees of Toulouse. Black lines show directions of fast split wave polarization; size of grey circles (a) or squares (b) are proportional to the time lag between fast and slow waves. Abbreviations are for station locations. Grey areas in (b) represent exposed Hercynian massifs.

preferred orientation may also be ap- pervasive crystallographic/tectonicfabric. proached through a self-consistent nu- Anisotropic strength of the lithosphere merical model (Lebensohn and Tome, may favour break-up of continents par- 1993); for a dunite, strength ratios larger allel to existing fabric even if the exten- than 2 are obtained depending on the sion direction is not normal to the orientation of the stresses relative to the structural trend of the belt. In this latter crystallographic fabric. Thus if, as sug- case, rifting will be characterized by a gested from shear wave splitting mea- transtensional deformation associating surements, the continental lithosphere extension and strike-slip faulting. is characterized by a steep foliation and Rifts tend to propagate where the a low-angle lineation, preferential or- energy required for lithosphere break- ientation of the weak (010)[100] slip up is minimum. Because cratonic roots system should induce reactivation of are stiffer than surrounding terranes, the initial fabric involving a strike-slip rifts will generally bypass these do- component. This theoretical considera- mains and localize in mobile belts I I tion is consistent with the large propor- around them. When rifting occurs ra- .o 100 tion of earthquakes with a significant pidly after an orogenic crisis, the me- Stress (MPa) component of strike-slip faulting that chanical anisotropy of the mantle may Fig. 5 Stress-strain rate plot for dry occur within continental rift zones be enhanced by thermal heterogeneities olivine single crystals compressed in three (Doser and Yardwood, 1991). and lithosphere strength variation due different directions ([11O]c, [101]c, [Ol l]c) to local crustal thickening. All these processes have convergent effects. Me- relative to the crystallographic structure. Conclusion (After Durham and Goetze, 1997.) chanical anisotropy, however, should Evidence from xenoliths and peridotite be preserved as long as temperature ural strain rates this transition occurs at massifs, shear wave splitting and mag- conditions do not allow a significant lower temperatures. Thus we might ex- netotelluric soundings strongly support modification (by annealing for instance) pect that upper mantle rocks displaying that the lithospheric mantle beneath of the fabric of mantle rocks to occur; it a crystallographic preferred orientation continents retains a penetrative fabric may therefore satisfactorily account for of olivine will be mechanically aniso- developed during past orogenies. In the the observed break-up of continents along tropic. Recent experimental axial com- Pyrenees or the eastern US, seismic aniso- their main structural trend several hun- pression ofdunite under high temperature tropy studies together with the tectonic dreds of Myr after the last orogeny. and pressure (Chopra, pers. comm. 1995; history of these continental domains sug- Wendt and Mainprice in prep.) confirm gest that break-up occurred preferen- this hypothesis: the strength of pretex- tially along a mantle fabric formed during Acknowledgements tured dunites depends on the orienta- the Grenvillian or the Hercynian orogeny. This paper is a contribution to International tion of the foliation with the lowest Considering that olivine crystals dis- Commission of the Lithosphere program strength and the most ductde behaviour play a significant mechanical anisotropy, 111-5. We are indebted to A. Souriau who is for compression at 45" to the foliation. we suggest that after a main episode of co-owner of seismic data in the Eastern The mechanical behaviour of an ag- continent assembly, the lithosphere dis- Pyrenees. We also thank D. Mainprice and gregate displaying an initial lattice- plays a mechanical anisotropy due to a R. Meissner for valuable discussions. el997 Blackwell Science Ltd 65 Continents break-up parallel to orogenic belts A. Vauchez eta/. Terra Nova, Vol 9, No. 2, 6266

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66 ( 1997 Blackwell Science Ltd