HR: 08:00h
AN: T41G-01    [Abstracts]
TI: Upper Mantle Seismic Structure of the Northern Ethiopian Rift - a Region of Incipient Continental Breakup
AU: * Bastow, I D
EM: ibastow@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Otley Road, Leeds, LS2 9JT United Kingdom
AU: Stuart, G W
EM: graham@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Otley Road, Leeds, LS2 9JT United Kingdom
AU: Kendall, M J
EM: kendall@earth.leeds.ac.uk
AF: School of Earth Sciences, 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, 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it is expected that fault controlled (mechanical) segmentation associated with the continental rift setting will be superceded by asthenospheric and magmatic processes, which are more prevalent in oceanic settings. The EAGLE passive networks consisted of 79 broadband seismometers over an area 250x250km centred on the Boset magmatic segment 70km SE of Addis Ababa, in the centre of the rift. We image upper-mantle structure between depths of 40-300km depth using teleseismic tomographic inversion of P and S wave travel-time data. Crustal structure, unresolvable in our inversions, is accounted for using an a priori crustal model which we derive from receiver function analyses of Moho P-to-S converted phases and from the results of recent EAGLE refraction experiments in Ethiopia. Analysis of relative arrival-time residuals shows that the rift flanks are asymmetric with arrivals associated with the south-eastern Somalian Plate faster ($\sim$0.65s for the P waves; $\sim$2s for the S waves) than the north-western Nubian Plate. 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 $\sim$100km, north of 9 degrees N this anomaly broadens towards the oceanic spreading centre of Afar where it appears to 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. We propose, that magma emplacement has occured within the lithosphere, thereby allowing rifting in an area where large scale tectonic forces may be too small for amagmatic rifting to occur.
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