No abstract

Seismic anisotropy is thought to result from the strain-induced lattice-preferred orientation of mantle minerals, especially olivine, owing to shear waves propagating faster along the a-axis of olivine crystals than along the other axes. This anisotropy results in birefringence, or 'shear-wave splitting', which has been investigated in numerous studies. Although olivine is also anisotropic with respect to electrical conductivity (with the a-axis being most conductive), few studies of the electrical anisotropy of the upper mantle have been undertaken, and these have been limited to relatively shallow depths in the lithospheric upper mantle. Theoretical models of mantle flow have been used to infer that, for progressive simple shear imparted by the motion of an overriding tectonic plate, the a-axes of olivine crystals should align themselves parallel to the direction of plate motion. Here, however, we show that a significant discrepancy exists between the electromagnetic strike of the mantle below Australia and the direction of present-day absolute plate motion. We infer from this discrepancy that the a-axes of olivine crystals are not aligned with the direction of the present-day plate motion of Australia, indicating resistance to deformation of the mantle by plate motion.

Upper mantle electrical conductivities can be explained by hydrogen diffusivity in hydrous olivine. Diffusivity enhances the conductivity of olivine anisotropically, making the a axis the most conductive of the three axes. Therefore, the hypothesis that plate motion induces lattice-preferred orientation of olivine can be tested with the use of long-period electromagnetic array measurements. Here, we compared electrical anisotropies below the slow-moving Fennoscandian and fast-moving Australian plates. The degree of olivine alignment is greater in the mantle below the Fennoscandian plate than below the Australian plate. This finding may indicate that convection rather than plate motion is the dominant deformation mechanism.

Abstract. A generic, three-dimensional (3-D) model has been developed which explains the threedimensionality exhibited by magnetovariational (MV) and magnetotelluric (MT) data from southern Kenya. In this model, observed variations in electromagnetic strike with period and location, impedance phase splitting, and peaks in tipper magnitude are all understood in terms of two regionally two-dimensional (2-D) structures striking NW-SE and N-S, respectively. The observed period and location dependence of the electromagnetic strike may arise as an indirect consequence of a rotating stress field, with regional-scale structures formed at different stages of the stress-strain history of Kenya being preserved as conductive lineaments. These conductive structures are not all confined to the upper crust. Thus, whereas stress data provide constraints on rifting at the upper crustal scale, the MT impedance tensor data provide constraints at lithospheric scales. The constraints and resolution provided by the MV and MT data have been rigorously investigated using 3-D forward modeling. Decoupling of the period and site dependence of electromagnetic strike aids resolution and constraint of conductors, rendering attempts to fix an average strike in space and frequency inexpedient. An anomalous apparent "strike" at the center of the Rift Valley reflects neither the N-S strike of the riff, nor the NW-SE striking shear fabric, but is shown to be a virtual strike arising as a result of coupling between the respective strike directions. The NW-SE trending conductivity anomaly straddles the rift and both its flanks, extends to at least middle to lower crustal depths, and appears substantially more electrically anomalous than the rift itself. A hypothesis that melt exists in the mantle directly below the rift at latitude 1.8øS is not supported by the MT data.