HR: 0800h
AN: T41E-1262    [Abstracts]
TI: Crustal Structure From EAGLE Teleseismic and Gravity Studies Across the Northern Main Ethiopian Rift
AU: * Cornwell, D G
EM: dgc2@le.ac.uk
AF: Department of Geology, University of Leicester, Leicester, LE1 7RH United Kingdom
AU: Mackenzie, G D
AF: Department of Geology, University of Leicester, Leicester, LE1 7RH United Kingdom
AU: Kendall, J M
AF: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Denton, P
AF: Department of Geology, University of Leicester, Leicester, LE1 7RH United Kingdom
AU: Maguire, P K
AF: Department of Geology, University of Leicester, Leicester, LE1 7RH United Kingdom
AU: England, R W
AF: Department of Geology, University of Leicester, Leicester, LE1 7RH United Kingdom
AB: We present new teleseismic earthquake and gravity data that forms part of the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE). 97 broadband seismometers were deployed along a profile across the northern Main Ethiopian Rift (EAGLE line 1) for two months at a nominal spacing of 5 km and the recorded waveforms are being analyzed for travel-time residuals, shear-wave splitting and crust / upper mantle P-to-S wave conversions using receiver functions. Teleseismic P-wave residuals have been calculated with respect to published Earth seismic velocity models and a consistent regional delay of 5 s is observed. Superimposed upon the regional delay are significant along-line residual variations of 1.0 to 1.5 s indicating a highly heterogeneous crust and upper mantle velocity structure. A smooth transition to relatively early arrivals is observed over 50 km of the southeast rift flank which could be explained by crustal thinning, elevated P-wave velocities or a combination of both. Analysis of SKS and SKKS arrivals indicates split shear wave delays of 1.0 to 2.5 s across the rift with maximum splitting occurring beneath the southeast flank. The orientation of the fast shear-wave is broadly rift-parallel but in detail it exhibits a systematic rotation northwards toward the rift valley from each end of the profile. There is significant variation of both the fast-to-slow shear-wave lag time and the orientation of the fast shear-wave over short length scales, suggesting that at least part of the cause of the variation in shear-wave polarizations is crustal. 72 new gravity stations provided Bouguer anomaly (BA) values with $\sim$5 km station spacing along the same profile. The stations were located using a differential GPS technique and accurately terrain corrected so that the final BA precision is less than 0.5 mgal. The most prominent feature observed is an asymmetric positive anomaly with maximum amplitude of 70 mgal near the rift axis. The latest EAGLE cross-rift seismic refraction model is used as a starting model for 2.5D gravity anomaly modelling that provides important constrains on the number and location of upper crustal mafic intrusive bodies, the requirement for magmatic underplating and changes in upper mantle density beneath the rift.
DE: 9305 Africa
DE: 8109 Continental tectonics--extensional (0905)
DE: 7205 Continental crust (1242)
DE: 1219 Local gravity anomalies and crustal structure
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting