TELESEISMIC IMAGING OF SUBAXIAL FLOW AT MIDOCEAN RIDGES - TRAVEL-TIMEEFFECTS OF ANISOTROPIC MINERAL TEXTURE IN THE MANTLE

Deformation of peridotite caused by mantle how beneath an oceanic spre ading centre can result in the development of seismic anisotropy. Trav eltime anomalies and shear-wave splitting will develop as seismic ener gy propagates through such an anisotropic region, thus providing a sig nature of the deformation field at depth. In this study we investigate the nature of deformation associated with mantle upwelling for two mo dels of flow in the upper 100 km of the mantle. The finite-strain fiel ds of the passive upwelling model versus the buoyancy-enhanced upwelli ng model are quite different. This suggests that mineral aggregates de form differently in the two models, thus developing seismic signatures that are distinguishable. Numerical estimates of the corresponding mi neral textures are made using polycrystal theory for olivine with four operative slip systems. The activation of a slip system is determined for each grain on the basis of the local critical resolved shear stre ss, The computed grain deformation reflects a balance between stress e quilibrium, for the aggregate as a whole, and strain continuity betwee n neighbouring grains within the aggregate. This approach enables a di rect link to be made between the model flow fields and the resulting t exture development. Given these mineral orientation distributions, ela stic parameters are calculated and wavefronts are propagated through t he anisotropic structure, Traveltimes for teleseismic body waves are c omputed using ray theory, and amplitudes are estimated for an across-a xis profile extending 100 km from the ridge axis. Relative P-wave resi duals of up to 1 s are predicted for the buoyant model with on-axis ar rivals being earliest, since near-vertical velocities are fastest bene ath the axis. On-axis P-wave arrivals for the passive model are half a second earlier than arrivals 60 km off-axis, and relative delays cont inue to increase slowly as distance from the ridge increases, S-wave s plitting of almost a second is predicted for the buoyant model, wherea s less than a half-second of splitting is determined for the passive m odel.