HR: 1340h
AN: T33A-1347    [Abstracts]
TI: Evidence of Melt-Induced Seismic Anisotropy and Magma Assisted Rifting in the North Ethiopian Rift
AU: * Pilidou, S
EM: sylvana@earth.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT United Kingdom
AU: Kendall, J
EM: kendall@earth.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT United Kingdom
AU: Stuart, G
EM: graham@earth.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT United Kingdom
AU: Bastow, I
EM: i.bastow@earth.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT United Kingdom
AB: The complex process of the transition from continental to oceanic rifting remains poorly understood. The Northern Ethiopian Rift, being free from interference from other tectonic processes, is an ideal place to study the continental breakup process. With this in mind, the recent EAGLE experiment saw the deployment of 79 broadband seismometers over an area 250-350km centred on the Northern Ethiopian Rift. We investigate the signature of crustal and upper-mantle anisotropy in these data as it provides insights into rifting processes. Recent studies employing shear-wave splitting techniques provide strong and consistent evidence for a rift-parallel (NNE-SSW) fast anisotropic direction beneath the rift. The detailed characteristics of these observations imply a single anisotropic layer confined to the upper 100km. Surface-wave tomography shows that the fast rift-parallel directions persist to a depth of 400km beneath a broader area surrounding the rift. These observations eliminate a number of plausible causes of anisotropy including plate motion drag, radial mantle-flow in the Afar plume head, mantle flow perpendicular to the rift induced by the rifting process, or pre-existing frozen-in crystallographic fabric. The observed anisotropy is more likely to be caused by {\sl either} channeled horizontal mantle-flow along the rift axis, which would cause the lattice preferred orientation (LPO) of olivine with the fast $\alpha$-axes paralleling the rift, {\sl or} the presence of rift-aligned melt-filled pockets (MFP) in the mantle. % SKS-splitting results show that the distribution of the fast anisotropic orientation mimics closely the distribution of strain and magmatism in the rift, implying MFP-induced anisotropy. % However, the techniques employed by the studies to date do not provide a means of conclusively separating the two candidate causes of anisotropy. The speeds of horizontally propagating $S_{\rm v}$ and $S_{\rm h}$ waves vary in similar fashions with azimuth for LPO- and MFP-induced anisotropy ( $S_{\rm v} \propto \cos({\theta})$, $S_{\rm h} \propto \cos({2\theta})$, where $\theta$ is the azimuth measured from the rift axis). However, the the relative change in the two shear-wave velocities is distinctive for LPO- and MFP-induced anisotropy. This provides a powerful tool for distinguishing between the two candidate causes of anisotropy. % We present strong evidence for MFP-induced anisotropy beneath the rift in the depth range of 20--60~km, by showing that the azimuthal variation of the speeds of $S_{\rm v}$ and $S_{\rm h}$ waves propagating horizontally through the rift area is in good agreement with predicted anisotropy models for vertical rift-parallel melt-filled dykes.We obtain shear-velocity models by inverting the group and phase velocity dispersion experienced by a number of local and teleseismic Rayleigh and Love waves with total or inter-station propagation paths crossing the rift area with a variety of azimuths. By only using highly coherent waves for the phase-velocity measurements and by applying a phase-matching filtering technique in the group velocity extraction procedure we ensure that our measurements are free from bias introduced by scattering and noise and that our results are reliable and robust.
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
DE: 1734 Seismology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting