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Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift

Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift
Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift
In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P-wave velocity model of the mid-crust and present the first 3-D Vp/Vs model of the region. Our models show high P-wave velocities (6.5 km s?1) beneath the axis of the rift at a depth of 12–25 km. The presence of high Vp/Vs ratios (1.81–1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp/Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ?1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.
tomography, earthquake source observations, seismic tomography, crustal structure, frica
0956-540X
1033-1048
Daly, E.
1f12d65f-2f85-4401-a1a0-ebe537da1a0c
Keir, D.
5616f81f-bf1b-4678-a167-3160b5647c65
Ebinger, C.J.
aedfe44b-76f6-4882-9522-0079fba9feda
Stuart, G.W.
42655128-f55d-42a7-afd8-7a2fca1c01a3
Bastow, I.D.
fa3f4d3f-8ec5-4dee-8de5-57d987d8d7ed
Ayele, A.
2a0d90e9-a4be-4484-9301-5c4df46f9407
Daly, E.
1f12d65f-2f85-4401-a1a0-ebe537da1a0c
Keir, D.
5616f81f-bf1b-4678-a167-3160b5647c65
Ebinger, C.J.
aedfe44b-76f6-4882-9522-0079fba9feda
Stuart, G.W.
42655128-f55d-42a7-afd8-7a2fca1c01a3
Bastow, I.D.
fa3f4d3f-8ec5-4dee-8de5-57d987d8d7ed
Ayele, A.
2a0d90e9-a4be-4484-9301-5c4df46f9407

Daly, E., Keir, D., Ebinger, C.J., Stuart, G.W., Bastow, I.D. and Ayele, A. (2008) Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift. Geophysical Journal International, 172 (3), 1033-1048. (doi:10.1111/j.1365-246X.2007.03682.x).

Record type: Article

Abstract

In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P-wave velocity model of the mid-crust and present the first 3-D Vp/Vs model of the region. Our models show high P-wave velocities (6.5 km s?1) beneath the axis of the rift at a depth of 12–25 km. The presence of high Vp/Vs ratios (1.81–1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp/Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ?1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.

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More information

Published date: 2008
Keywords: tomography, earthquake source observations, seismic tomography, crustal structure, frica

Identifiers

Local EPrints ID: 175151
URI: http://eprints.soton.ac.uk/id/eprint/175151
ISSN: 0956-540X
PURE UUID: 8b1a0fbc-a098-4a18-8f64-476e0ca39ec1
ORCID for D. Keir: ORCID iD orcid.org/0000-0001-8787-8446

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Date deposited: 22 Feb 2011 17:08
Last modified: 14 Mar 2024 02:57

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Contributors

Author: E. Daly
Author: D. Keir ORCID iD
Author: C.J. Ebinger
Author: G.W. Stuart
Author: I.D. Bastow
Author: A. Ayele

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