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Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments

Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments
Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments
Active deformation within the northern part of the Main Ethiopian Rift (MER) occurs within approximately 60 km-long, 20 km-wide ‘magmatic segments’ that lie within the 80 km-wide rift valley. Geophysical data reveal that the crust beneath the <1.9 Ma magmatic segments has been heavily intruded; magmatic segments accommodate strain via both magma intrusion and faulting. We undertake field and remote sensing analyses of faults and eruptive centres in the magmatic segments to estimate the relative proportion of strain accommodated by faulting and magma intrusion and the kinematics of Quaternary faults. Up to half the ?10 km-long normal faults within the Boset-Kone and Fantale-Dofen magmatic segments have eruptive centres or extrusive lavas along their length. Comparison of the deformation field of the largest Quaternary fault and an elastic half-space dislocation model indicates a down-dip length of 10 km, coincident with the seismogenic layer thickness and the top of the seismically imaged mafic intrusions. These relations suggest that Quaternary faults are primarily driven by magma intrusion into the mid- to upper crust, which triggers faulting and dyke intrusion into the brittle upper crust. The active volcanoes of Boset, Fantale and Dofen all have elliptical shapes with their long axes in the direction N105, consistent with extension direction derived from earthquake focal mechanisms. Calderas show natural strains ranging from around 0.30 for Boset, 0.55 for Fantale, and 0.94 for Dofen. These values give extension strain rates of the order of 0.3 microstrain per year, comparable to geodetic models. Structural analyses reveal no evidence for transcurrent faults linking right-stepping magmatic segments. Instead, the tips of magmatic segments overlap, thereby accommodating strain transfer. The intimate relationship between faulting and magmatism in the northern MER is strikingly similar to that of slow-spreading mid-ocean ridges, but without the hard linkage zones of transform faults.
0305-8719
143-163
Casey, M.
c4b63f72-5c46-422f-964a-1b0440df8caa
Ebinger, C.
8d61032b-66e9-4a84-b2b9-450cb898ce92
Keir, D.
5616f81f-bf1b-4678-a167-3160b5647c65
Gloaguen, R.
5d62a377-c474-402b-bcec-b315ccc04b46
Mohamed, F.
ec709bef-7102-4e5d-8143-7e38cc9840dd
Casey, M.
c4b63f72-5c46-422f-964a-1b0440df8caa
Ebinger, C.
8d61032b-66e9-4a84-b2b9-450cb898ce92
Keir, D.
5616f81f-bf1b-4678-a167-3160b5647c65
Gloaguen, R.
5d62a377-c474-402b-bcec-b315ccc04b46
Mohamed, F.
ec709bef-7102-4e5d-8143-7e38cc9840dd

Casey, M., Ebinger, C., Keir, D., Gloaguen, R. and Mohamed, F. (2006) Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments. Geological Society London Special Publications, 259, 143-163. (doi:10.1144/GSL.SP.2006.259.01.13).

Record type: Article

Abstract

Active deformation within the northern part of the Main Ethiopian Rift (MER) occurs within approximately 60 km-long, 20 km-wide ‘magmatic segments’ that lie within the 80 km-wide rift valley. Geophysical data reveal that the crust beneath the <1.9 Ma magmatic segments has been heavily intruded; magmatic segments accommodate strain via both magma intrusion and faulting. We undertake field and remote sensing analyses of faults and eruptive centres in the magmatic segments to estimate the relative proportion of strain accommodated by faulting and magma intrusion and the kinematics of Quaternary faults. Up to half the ?10 km-long normal faults within the Boset-Kone and Fantale-Dofen magmatic segments have eruptive centres or extrusive lavas along their length. Comparison of the deformation field of the largest Quaternary fault and an elastic half-space dislocation model indicates a down-dip length of 10 km, coincident with the seismogenic layer thickness and the top of the seismically imaged mafic intrusions. These relations suggest that Quaternary faults are primarily driven by magma intrusion into the mid- to upper crust, which triggers faulting and dyke intrusion into the brittle upper crust. The active volcanoes of Boset, Fantale and Dofen all have elliptical shapes with their long axes in the direction N105, consistent with extension direction derived from earthquake focal mechanisms. Calderas show natural strains ranging from around 0.30 for Boset, 0.55 for Fantale, and 0.94 for Dofen. These values give extension strain rates of the order of 0.3 microstrain per year, comparable to geodetic models. Structural analyses reveal no evidence for transcurrent faults linking right-stepping magmatic segments. Instead, the tips of magmatic segments overlap, thereby accommodating strain transfer. The intimate relationship between faulting and magmatism in the northern MER is strikingly similar to that of slow-spreading mid-ocean ridges, but without the hard linkage zones of transform faults.

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Published date: 2006
Organisations: Geology & Geophysics

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Local EPrints ID: 350996
URI: http://eprints.soton.ac.uk/id/eprint/350996
ISSN: 0305-8719
PURE UUID: 90dbe81d-439d-47c3-b7aa-d3848c865c16
ORCID for D. Keir: ORCID iD orcid.org/0000-0001-8787-8446

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Date deposited: 12 Apr 2013 15:24
Last modified: 17 Dec 2019 01:40

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Author: M. Casey
Author: C. Ebinger
Author: D. Keir ORCID iD
Author: R. Gloaguen
Author: F. Mohamed

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