Continental rifting parallel to ancient collisional belts: an effect of the mechanical anisotropy of the lithospheric mantle

Analysis of major rift systems suggests that the preexisting structure of t he lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Contin ental rifts often display a clear component of strike-slip deformation, in particular in the early rifting stage. Moreover, although the close tempora l and spatial association between flood basalt eruption and continental bre akup suggests that mantle plumes play an important role in the rifting proc ess, there is a paradox between the pinpoint thermal and stress perturbatio n generated by an upwelling mantle plume and the planar geometry of rifts. These observations suggest that the deformation of the lithosphere, especia lly during rifting, is controlled by its preexisting structure. On the othe r hand, (1) the plasticity anisotropy of olivine single crystal and aggrega tes, (2) the strong crystallographic orientation of olivine observed in man tle xenoliths and Iherzolite massifs, and (3) seismic anisotropy data, whic h require a tectonic fabric in the upper mantle coherent over large areas, suggest that preservation within the lithospheric mantle of a lattice prefe rred orientation (LPO) of olivine crystals may induce a large-scale mechani cal anisotropy of the lithospheric mantle. We use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the conti nental breakup process. We assess the deformation of an anisotropic contine ntal lithosphere in response to an axi-symmetric tensional stress field pro duced by an upwelling mantle plume by calculating the deformation of textur ed olivine polycrystals representative of the lithospheric mantle at differ ent positions above a plume head. Model results show that a LPO-induced mec hanical anisotropy of the lithospheric mantle may result in directional sof tening, leading to heterogeneous deformation. During continental rifting, t his mechanical anisotropy may induce strain localisation in domains where e xtensional stress is oblique (30-55 degrees) to the preexisting mantle fabr ic. This directional softening associated with olivine LPO frozen in the li thospheric mantle may also guide the propagation of the initial instability , that will follow the preexisting structural trend. The preexisting mantle fabric also controls the deformation regime, imposing a strong strike-slip shear component. A LPO-induced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting (s tructural inheritance), the plume-rift paradox, and the onset of transtensi on within continental rifts. (C) 2001 Elsevier Science B.V. All rights rese rved.