SEISMIC ANISOTROPY AND MANTLE FLOW BENEATH THE BAIKAL RIFT-ZONE

SEISMIC studies have shown that continental rifts such as Lake Baikal and the Great Rift Valley of East Africa are like midocean rifts in th at they lie above broad regions of asthenospheric upwarp of much great er extent than the surface expression of rifting(1-4). The direction o f mantle flow in such regions ran be investigated using the seismic an isotropy created by flow-induced orientation of mantle olivine crystal s(5-8). Seismic studies of the Mid-Atlantic Ridge have revealed upwell ing mantle flow beneath the ridge and flow normal to the ridge axis on either side(8-10). Here we present results from an array of seismic s tations across the Baikal rift zone in southern Siberia. The splitting in arrival times of SKS seismic waves indicates that the upper mantle beneath the rift zone is anisotropic, with the fast direction (which reflects the direction of mantle how) being horizontal and normal to t he rift axis. This suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid-ocea n rifts. This may help to elucidate the processes involved in continen tal rifting.