Variations in melt emplacement beneath the northern East African Rift from radial anisotropy
Variations in melt emplacement beneath the northern East African Rift from radial anisotropy
Where and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26 s period and then inverted for shear velocity. V
SV is 0.15 ± 0.03 km/s lower than V
SH from 5-30 km depth on average. V
SH>V
SV across most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where V
SV>V
SH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state.
East African Rift, Rayleigh and Love waves, ambient noise, radial anisotropy, surface waves, tomography
Chambers, Emma L.
bedffcf1-a971-4568-823f-56daa2e95a74
Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830
Rychert, Catherine
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Keir, Derek
5616f81f-bf1b-4678-a167-3160b5647c65
1 November 2021
Chambers, Emma L.
bedffcf1-a971-4568-823f-56daa2e95a74
Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830
Rychert, Catherine
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Keir, Derek
5616f81f-bf1b-4678-a167-3160b5647c65
Chambers, Emma L., Harmon, Nicholas, Rychert, Catherine and Keir, Derek
(2021)
Variations in melt emplacement beneath the northern East African Rift from radial anisotropy.
Earth and Planetary Science Letters, 573, [117150].
(doi:10.1016/j.epsl.2021.117150).
Abstract
Where and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26 s period and then inverted for shear velocity. V
SV is 0.15 ± 0.03 km/s lower than V
SH from 5-30 km depth on average. V
SH>V
SV across most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where V
SV>V
SH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state.
Text
EChambers_Radial_anisotropy_2021_EPSL
- Accepted Manuscript
More information
Accepted/In Press date: 5 August 2021
e-pub ahead of print date: 1 September 2021
Published date: 1 November 2021
Additional Information:
Funding Information:
E.L.C is funded through NERC studentship NE/L002531/1 . D.K. is supported by NERC grant NE/L013932 and by MIUR through PRIN grant 2017P9AT72 . C.A.R. and N.H. acknowledge funding from the Natural Environment Research Council ( NE/M003507/1 and NE/K010654/1 ) and the European Research Council ( GA 638665 ). All data needed to generate these models are freely available from the IRIS Data Management Center (IRISDMC; https://service.iris.edu/fdsnws/dataselect/1/ ). IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-126168 . We thank SEIS-UK for use of the instruments and their computing facilities. The facilities of SEIS-UK are supported by the Natural Environment Research Council (NERC) under agreement R8/H10/64.F . The final velocity and anisotropy model is available at https://doi.org/10.5258/SOTON/D1689 .
Publisher Copyright:
© 2021 Elsevier B.V.
Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.
Keywords:
East African Rift, Rayleigh and Love waves, ambient noise, radial anisotropy, surface waves, tomography
Identifiers
Local EPrints ID: 451140
URI: http://eprints.soton.ac.uk/id/eprint/451140
ISSN: 0012-821X
PURE UUID: ffca01c8-7f4f-4e1e-8560-db2d0371a069
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Date deposited: 14 Sep 2021 15:15
Last modified: 17 Mar 2024 06:49
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Author:
Emma L. Chambers
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