HR: 0830h
AN: S51C-0064    [PDF]
TI: Shear-Wave Splitting due to Rifting and Precambrian Accretion of Ethiopia
AU: * Gashawbeza, E
EM: ewenet1@stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305-2215 United States
AU: Keranen, K
EM: kkeranen@stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305-2215 United States
AU: Nyblade, A
EM: andy@geosc.psu.edu
AF: Department of Geosciences, Pennsylvania State University, University Park, PA 16802 United States
AU: Klemperer, S L
EM: sklemp@stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305-2215 United States
AU: Walker, K
EM: ktwalker@geo.stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305-2215 United States
AB: We have utilized a dataset from the broadband seismic experiment of Nyblade and Langston (EOS v.83 p. 405, 2002) for a shear-wave splitting analysis in Ethiopia. A total of twenty-five broadband seismic stations, widely distributed in various physiographic regions in Ethiopia, were used. Six stations were installed on the southeastern plateau, twelve stations on the western plateau, and seven stations inside the Rift Valley, which runs northeast-southwest and separates the western and eastern highlands. The distribution of the broadband stations in the present study spans a broad region and allows us to compare the results of shear-wave splitting analysis inside the rift and on the rift-bounding plateaus. Previous shear-wave splitting results in Kenya, located south of Ethiopia, show fast polarization azimuths sub-parallel to the strike of the rift which are interpreted to be the result of vertical magma-filled cracks in the lithosphere opening in the direction perpendicular to the extension direction (Gao et al., 1997; Barruol and Ismail, 2001). However, this orientation is also perpendicular to the collision direction for the Mozambique belt and thus consistent with the fast azimuth being the result of fossilized anisotropy in the Precambrian lithosphere. Results from an SKS splitting analysis in Ethiopia (Maguire et al., EOS 2003 in press; Kendall et al., this session) show the orientation of the fast polarization azimuth within the Main Ethiopian Rift to be approximately NNE-SSW. Similar to the results from Kenya, this is parallel to the volcanic centers in the rift and perpendicular to the geodetically determined opening direction. Splitting directions on the rift shoulders are closer to NE, parallel to both the rift-bounding faults (perpendicular to the inferred average Neogene opening direction) and to the Precambrian accretionary structures. Our preliminary result from Addis Ababa suggests a fast azimuth of N23E, sub-parallel to the rift orientation in agreement with previous results by other workers. Further south near Arbaminch, results suggest an azimuth of N6E possibly due to the shift of rift orientation from NE-SW to nearly N-S in this region. Stations 250 km NW of the rift axis (Debre Markos) and 250 km SE of the rift axis (Goba) both show more north-easterly trends (N37E and N21E, respectively). Because of the distance of these stations from the rift, we suggest that this NE fast splitting direction is more likely related to fossilized anisotropy in the Precambrian lithosphere than to Neogene rifting.
DE: 7218 Lithosphere and upper mantle
DE: 8109 Continental tectonics--extensional (0905)
DE: 9305 Africa
SC: Seismology [S]
MN: 2003 Fall Meeting