Deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy. Traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region, thus providing a signature of the deformation field at depth. In this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle. The finite-strain fields of the passive upwelling model versus the buoyancy-enhanced upwelling model are quite different. This suggests that mineral aggregates deform differently in the two models, thus developing seismic signatures that are distinguishable. Numerical estimates of the corresponding mineral 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 stress. The computed grain deformation reflects a balance between stress equilibrium, for the aggregate as a whole, and strain continuity between neighbouring grains within the aggregate. This approach enables a direct link to be made between the model flow fields and the resulting texture development. Given these mineral orientation distributions, elastic parameters are calculated and wavefronts are propagated through the anisotropic structure. Traveltimes for teleseismic body waves are computed using ray theory, and amplitudes are estimated for an across-axis profile extending 100 km from the ridge axis. Relative P-wave residuals of up to 1 s are predicted for the buoyant model with on-axis arrivals being earliest, since near-vertical velocities are fastest beneath 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 continue to increase slowly as distance from the ridge increases. S-wave splitting of almost a second is predicted for the buoyant model, whereas less than a half-second of splitting is determined for the passive model.
We explore global variations in D″ shear wave structure by examining 12 years of long‐period Global Digital Seismograph Network (GDSN) data to identify reflected phases from the D″ layer in the lowermost mantle. We restrict our search to epicentral distances between 63° and 74° where a precritical D″ reflection will lie between the S and ScS arrivals. With GDSN long‐period records a D″ signal at these ranges is generally obscured by the stronger neighboring phases, so to isolate the D″ reflections we develop a technique for stripping away the interfering S and ScS waveforms. We interpret travel time variations in the observed SdS phases in terms of variations in D″ thickness and generate maps of inferred D″ thickness for areas of good seismic coverage. The regions of observed D″ reflections are beneath Australasia, north central Asia, the Arctic, Alaska, and central America. We find considerable variation in D″ structure in these regions, a result consistent with previous studies which have found evidence for lateral heterogeneity in D″. On average, the estimated D″ thickness is 260 km, but there is a fairly uniform distribution of thicknesses between 150 km and 350 km. A possible correlation is observed between D″ thickness and regions of predicted strong horizontal mantle flow.
Recent observational evidence of upwelling‐mantle anisotropy at a slow spreading center has motivated the modeling of teleseismic arrivals at mid‐ocean ridges. The models consider a variety of types of anisotropy and heterogeneity where the emphasis is to ascertain whether or not travel‐times can be used to discriminate between the existence of partial melt and anisotropy. Two mechanisms for anisotropy in the upwelling asthenosphere are considered: one due to the preferential alignment of the fast axes of olivine crystals in the direction of mantle flow and the other due to the preferential alignment of cracks that feed melt towards the spreading axis. The results indicate that P‐waves are most sensitive to even modest amounts of flowinduced asthenospheric anisotropy, while S‐waves are most sensitive to the presence of mantle melt. Multiple S‐wave arrivals are predicted for many models, most notably the ones with anisotropy due to crack‐alignment where very large S‐wave separations develop. The model which best fits existing data requires a higher degree of crystal‐alignment anisotropy in the upwelling‐asthenosphere than in the lithosphere. This effect has been predicted in studies of the evolution of crystal‐alignment anisotropy in polycrystalline aggregates.
Seismic energy propagating through the mantle beneath an oceanic spreading centre develops a signature due both to the subaxial deformation field and to the presence of melt in the upwelling zone. Deformation of peridotite during mantle flow results in strong preferred orientation of olivine and significant seismic anisotropy in the upper 100 km of the mantle. Linked numerical models of flow, texture development and seismic velocity structure predict that regions of high anisotropy will characterize the subaxial region, particularly at slow-spreading mid-ocean ridges. In addition to mineral texture effects, the presence of basaltic melt can cause travel-time anomalies, the nature of which depend on the geometry, orientation and concentration of the melt. In order to illustrate the resolution of subaxial structure that future seismic experiments can hope to achieve, we investigate the teleseismic signature of a series of spreading centre models in which the mantle viscosity and melt geometry are varied. The P-wave travel times are not very sensitive to the geometry and orientation of melt inclusions, whether distributed in tubules or thin ellipsoidal inclusions. Travel time delays of 0.1-0.4 s are predicted for the melt distribution models tested. The P-wave effects of mineral texture dominate in the combined melt-plus-texture models. Thus, buoyancy-enhanced upwelling at a slow spreading ridge is characterized by 0.7-1.0 s early P-wave arrival times in a narrow axial region, while the models of plate-drivenonly flow predicts smaller advances (less than 0.5 s) over a broader region. In general S-wave travel times are more sensitive to the melt and show more obvious differences between melt present as tubules as opposed to thin disks, especially if a preferred disk orientation exists. Mineral texture and the preferred alignment of melt inclusions will both produce shear-wave splitting, our models predict as much as 4 s splitting in some cases.
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