SKS SPLITTING BENEATH CONTINENTAL RIFT ZONES

We present measurements of SKS splitting at 28 digital seismic station s and 35 analog stations in the Baikal rift zone, Siberia, and adjacen t areas, and at 17 stations in the East African Rift in Kenya and comp are them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone a re distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjace nt Siberian platform and northern Mongolian fold belt, only the rift-o rthogonal fast direction is observed. In southcentral Mongolia, the do minant fast direction changes to rift-parallel again, although a small number of measurements are still rift-orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriente d on average NNE, that is, rotated clockwise from the N-S trending rif t. All three rifts are underlain by low-velocity upper mantle as deter mined from teleseismic tomography. Rift-related mantle flow provides a plausible interpretation for-the rift-orthogonal fast directions. The rift-parallel fast directions near the rift axes can be interpreted b y oriented magmatic cracks in the mantle or small-scale mantle convect ion with rift-parallel flow. The agreement between stress estimates an d corresponding crack orientations lends some weight to the suggestion that the rift-parallel fast directions are caused by oriented magmati c cracks.