Why do continents break-up parallel to ancient orogenic belts?

The frequently observed parallelism between rifts and the pre­ existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents. We propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere, that guides the propagation of continental rifts. Subcrustal mantle mechanical anisotropy is supported by (i) the anisotropie strength of olivine, (ii) an ubiquitous tectonic fabric in exposed mantle rocks, and (iii) measurements of seismic and electrical anisotropy. During major episodes of continent Rifting parallel to orogenic belts Ocean-opening through rifting and continent break-up is frequently related to the occurrence ofhotspots. There is, howevcr, a discrepancy between hot­ spots acting as pin point sources of heat and the linear extent of rifts over thou­ sands ofkilometres. Moreover rifts tend to parallel pre-existing orogenic fab­ rics, e.g. North Atlantic rift (Wilson, 1966), Rio Grande rift (Olsen et al., 1987), North-east China rift (Ma and Wu, 1987), Baïkal rift (Delvaux et al., 1995), East African rift (Ring, 1994), West African rift (e.g. Fairhead and Binks, 1991), Cape Graben (Burke, 1976), and Eastern Brazilian rift (Chang et al., 1992). From these observations it was suggested that structural inheri­ tance influences rift propagation. This paper propounds that the source of structural inheritance lies in the fabric of the lithospheric mantle developed during major orogenic episodes. 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. 1 ), > 2000 km long and still active, formed mostly during the Cenozoic in a shield stable 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 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 chez(a)dstu.univ-montp2 fr talso at Lab. Petrologie et Tectonique. Cniversité Lyon I & CNRS, France assembly, a pervasive deformation of the lithosphere induces a lattice-preferred orientation of olivine in mantle rocks. Later on, this crystallographic fabric is 'frozen-in' and represents the main source of shear wave splitting. This olivine fabric may entail a mechanical anisotropy in the lithospheric mantle. During subsequent tectonic events, especially during rifting, mechanical anisotropy may contrai the tectonic behaviour of the lithosphere. Mozambique belt and terminates northward into the Afar. The western branch follows the Ubendian belt that curves from NW-SE to NE-SW. Theu­ nissen et al. (1996) showed that: (i) the Ubendian belt, south and west of the Tanzania craton, is a wide and steep shear belt resulting from Palaeoand Neoproterozoic tectonic events, (ii) the west Tanzania rift developed during Phanerozoic times through multiphase reactivation of the Precambrian steeply dipping fabric, and (iii) fault geometry and kinematics are complex and point toward a SE-NW propagation of the rift in a dextral transtensional strain regime.