HR: 1340h
AN: T43B-1405    [Abstracts]
TI: Upper Mantle Seismic Structure of the Northern Ethiopian Rift - a Region of Incipient Continental Breakup
AU: Kendall, J
EM: gljmk@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: * Bastow, I D
EM: bastow@seis.sc.edu
AF: Department of Geological Sciences, University of South Carolina 701 Sumter Street, Columbia, SC 29201 United States
AU: Stuart, G W
EM: graham@earth.leeds.ac.uk
AF: School of Earth and the Environment, University of Leeds Otley Road, Leeds, LS2 9JT United Kingdom
AU: Ebinger, C J
EM: c.ebinger@gl.rhul.ac.uk
AF: Department of Geology, Royal Holloway University of London, Egham, Surrey, London, TW20 0EX United Kingdom
AU: Ayele, A
EM: atalay@geobs.aau.edu.et
AF: Geophysical Observatory, Addis Ababa University Addis Ababa, Addis Abbaba, 1176 Ethiopia
AB: The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction and captures the transition from continental rifting in East Africa to oceanic sea-floor spreading in Afar. As rifting proceeds to sea-floor spreading, it is expected that fault controlled (mechanical) segmentation associated with the continental rift setting will be superseded by asthenospheric and magmatic processes, which are more prevalent in oceanic settings. The EAGLE passive networks consisted of 79 broadband seismometers over an area 250x350km centered on the Boset magmatic segment 70km SE of Addis Ababa, in the center of the rift. Analysis of absolute travel-time residuals from the stations shows that the northern Ethiopian rift is one of the most seismically slow regions on Earth. We have imaged upper-mantle structure between depths of 40-300km depth using teleseismic tomographic inversion of P and S wave travel-time data. Our tomographic inversions image a 75km-wide tabular low velocity zone (Vp=-1.5%, Vs=-4%) between 40-300km depth, interpreted as upwelling in the upper mantle beneath the less evolved southern part of the rift. At depths of ~100km, north of 9 degrees N this anomaly broadens towards the oceanic spreading centre of Afar where it may be connected to deeper low-velocity structure. Along axis, the low velocity upwelling beneath the rift is segmented, with low velocity material in the uppermost 100 km always offset to whichever side of the rift has the highest rift flank topography. Lateral velocity contrasts and comparison of P and S travel-times suggests high temperatures and partial melt in the upper-mantle beneath the rift. Furthermore, the travel-time data indicate that the melt beneath the study area exists in elongate inclusions - consistent with interpretations of SKS shear wave splitting data. We propose, that magma emplacement has occurred within the lithosphere, thereby allowing rifting in an area where large-scale tectonic forces may be too small for amagmatic rifting to occur.
DE: 7205 Continental crust (1219)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 8108 Continental tectonics: compressional
DE: 9305 Africa
SC: Tectonophysics [T]
MN: Fall Meeting 2005