HR: 12:05h
AN: S51F-07    [PDF]
TI: Melt migration and mantle anisotropy beneath the Ethiopian rift
AU: * Kendall, J
EM: m.kendall@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Stuart, G
EM: g.stuart@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Bastow, I
EM: i.bastow@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Ebinger, C
EM: c.ebinger@gl.rhul.ac.uk
AF: Royal Holloway - University of London, Department of Geology, Egham, TW20 0EX United Kingdom
AB: The EAGLE broadband experiment aims to study the crust and upper-mantle structure in the northern Ethiopian rift, a region of transition from continental to oceanic rifting. 30 broadband seismometers have been deployed for a period of 16 months over an area of 250 $km^2$, centered on the Nazret volcanic zone. These stations are complimented by data from a 3-month deployment of 50 instruments concentrated in the rift valley. We present analysis of upper-mantle anisotropy from measurements of shear-wave splitting in SKS, SKKS and PKS phases. The concentrated station coverage and nearly 500 splitting measurements provide a detailed picture of mantle anisotropy in a rifting environment. In general the results are of high quality with low error estimates. The highest splitting ($> 2$ secs) occurs on the rift flanks, where the strain partitioning is expected to be highest. There is an asymmetry in results from either side of the rift (i.e., the Ethiopia plateau vs. the Somalian plate). Within the Rift there is a consistent increase in delay times towards the north and Afar, from 1.0 secs in the south to 1.6 secs in the north. Outside the Rift, the polarisations of the fast shear-wave lie in a NE--SW rift--parallel trend and do not align with the direction of absolute plate motion. Within the Rift the orientations swing to more northerly azimuths, following magmatic segments and faulting patterns. The increase in rift splitting times northwards correlates with the amount of magma within the system. Large variations in magnitude of splitting or orientation of the shear-wave over short distances suggests that the source of the anisotropy is quite shallow ($<100 km$). Cumulatively, these results suggest that the anisotropy is associated with melt migration beneath the rift. To explain these results we appeal to recent deformation experiments of Holtzman et al. (2003) which show anisotropy in partially molten rocks being due to a combination of preferred melt inclusion alignment, layering and olivine crystal alignment.
DE: 7218 Lithosphere and upper mantle
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting