Spatial Variations in Lithospheric Structure and Anisotropy Across the Northern Main Ethiopian Rift
Abstract
The magmatic Northern Main Ethiopian Rift (NMER) is believed to represent the transitional stage between continental and oceanic rifting, with recent studies from the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE) project supporting models of magma assisted rifting. 97 broadband seismometers were deployed across the northern Main Ethiopian Rift at a nominal spacing of 5 km and the resulting data have been analyzed using shear-wave splitting and receiver function methods. Split shear waves are analysed from SKS and SKKS arrivals in two distinct period bands (2-5 and 5-14 s) along the cross-rift profile. The orientations of the fast shear-wave for both period bands are broadly rift-parallel with a systematic rotation northwards toward the rift valley from each end of the profile. Significant variation in both the fast-to-slow shear-wave lag time and the orientation of the fast shear-wave is present over short length scales in both the short- and long-period bands. Lag times vary from 1.0 to 2.5 s with maximum splitting occurring beneath the rift and beneath the southeast rift flank. Comparison of lag times for the short- and long-period bands reveals an average of ~ 0.3 s less splitting for the short-period band on the northwestern rift flank which we interpret in terms of depth-dependent anisotropy. Up to 2.5 s of lag is observed in both period bands beneath the rift, indicating that the dominant anisotropy mechanism is confined to upper lithosphere. Models of anisotropy are tested to explain the results in terms of the preferred alignment of melt pockets and/or pre-existing lithospheric fabric. The shear-wave splitting results are combined with and compared to receiver function results that provide an estimation of the Moho depth and Vp/Vs ratio at each station along the cross-rift profile. Together, these analyses allow the identification of regions of melt across the rift that in turn enable an assessment of the extent and role of melt in continental break-up that is dominated by magmatic processes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.T51E..05C
- Keywords:
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- 7205 Continental crust (1219);
- 7208 Mantle (1212;
- 1213;
- 8124);
- 7218 Lithosphere (1236);
- 8109 Continental tectonics: extensional (0905);
- 9305 Africa