Upper mantle deformation and seismic anisotropy in continental rifts

We investigate the seismic anisotropy signature of the continental rifting process. Several sources of anisotropy are considered: the lithospheric def ormation, the asthenospheric flow and the occurrence of oriented melt-pocke ts in the asthenospheric mantle. Our results show that contrasted anisotrop y patterns should be associated with the various conceptual models of rifti ng. Thus seismic anisotropy measurements may allow one to discriminate betw een these models. Anisotropy measurements in the Rio Grande, East-African a nd Rhine rifts suggest that these rifts formed by a transtensional deformat ion of the lithospheric mantle rather than by homogeneous extension of the lithosphere. Alignment of melt-lenses in the asthenospheric wedge may also account for a significant part of the seismic anisotropy recorded in the in ternal domains of these rifts. (C) 2000 Elsevier Science Ltd. All rights re served.