HR: 11:50h
AN: T22B-07    [Abstracts]
TI: Lithospheric Deformation Beneath the Ethiopian Plateau
AU: * Weeraratne, D S
EM: weeraratne@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Solomon, S C
EM: scs@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Nyblade, A A
EM: andy@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: The interruption of the Ethiopian Plateau by the Main Ethiopian Rift (MER) is a rare example of deformation in continental lithosphere. We investigate the lithospheric stability of the Ethiopian Plateau through the study of Rayleigh waves of 18-91 s period recorded by 30 stations of the Ethiopian Broadband Seismic Experiment. We use a tomographic inversion method that takes advantage of a high volume of crossing wave paths and accounts for deviations from great-circle paths through a finite-frequency approximation that includes the effects of scattering in the incoming wavefield. Preliminary inversion results indicate that phase velocities increase with period from 3.4 km/s at 20 s to 3.8 km/s at 91 s. In maps of phase velocity, a narrow low-velocity anomaly, approximately 150 km wide and elongated in a NE-SW direction, is observed beneath the NE-trending MER at all periods. This low-velocity anomaly appears to shift steadily to the east with increasing period. Phase velocities to the SE and NW of the MER are ~6% higher than within the rift. Azimuthal anisotropy, resolved only at periods below 40 s, is small at 0.7%, but the direction of fast velocity displays a constant NNE azimuth. This finding is in agreement with the NNE fast direction and moderate delay times from shear-wave splitting studies in the area and suggests a shallow crustal or upper mantle source for the anisotropy, likely a signature of rift tectonics such as aligned melt pockets but perhaps also influenced by older Mozambique accretionary sutures. Inversion of phase velocity for shear-wave velocity structure indicates that mantle velocities associated with the Ethiopian lithosphere are relatively high to 110 km depth but do not exceed 4.3 km/s. By comparison, velocities and lithospheric thicknesses of undisturbed cratonic lithosphere elsewhere reach 4.7 km/s and 200-250 km, respectively. The combined observation of anomalously low velocities within the rift zone and lower than average seismic velocities in the lid suggest that the Ethiopian lithosphere has been subject to significant deformation possibly from exposure to present or past mantle upwelling, as well as from rifting.
DE: 7218 Lithosphere (1236)
DE: 7270 Tomography (6982, 8180)
DE: 8103 Continental cratons
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005