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Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression.

Keir D, Belachew M, Ebinger CJ, Kendall JM, Hammond JO, Stuart GW, Ayele A, Rowland JV - Nat Commun (2011)

Bottom Line: Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup.The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres.The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. d.keir@soton.ac.uk

ABSTRACT
Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture.

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Profiles of shear-wave splitting parameters across the rift valleys in Ethiopia.The across-rift profiles show variation in splitting parameters at stations located 100 km either side of profiles x–x′ for the MER and z–z′ across Afar. For both the MER (a, b) and RSR (c, d), note the dramatic increase in magnitude of anisotropy synchronous with rotation of fast direction into the Quaternary-Recent magmatic segments. The top two panels show topography along the displayed profiles.
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f4: Profiles of shear-wave splitting parameters across the rift valleys in Ethiopia.The across-rift profiles show variation in splitting parameters at stations located 100 km either side of profiles x–x′ for the MER and z–z′ across Afar. For both the MER (a, b) and RSR (c, d), note the dramatic increase in magnitude of anisotropy synchronous with rotation of fast direction into the Quaternary-Recent magmatic segments. The top two panels show topography along the displayed profiles.

Mentions: In the Quaternary-Recent magmatic segments of the MER and Afar, the dominant strike of extensional features such as faults and aligned cones is perpendicular to rift-opening direction323334, resulting in observed φ that is perpendicular to the inferred SHmin. At stations in the MER, φ varies between ∼N–NE, with a modal strike of ∼NNE, orthogonal to the ∼N105°E extension direction (Figs 3 and 4). In Afar, at stations east of the TGD and therefore in the Gulf of Aden and RSRs, φ is ∼NNW–NW, approximately parallel to the strike of faults and aligned volcanic cones here, and orthogonal to the direction of extension (Figs 3 and 4). We observe a systematic clockwise rotation of φ from ∼NW in the Asal rift of Djibouti to ∼NNW in northern Afar, a rotation that closely mimics along-strike variation in rift opening determined using GPS measurements19. A striking feature in the data is the marked similarity between φ and the strike of elongate clusters of seismicity in the three rifts of the Afar triple junction (Fig. 1b). This, along with MER parallel ∼NE-NNE oriented φ at stations located within Afar but west of the TGD lend support to structural measurements14 and kinematic models19 that suggest the TGD currently partitions most extension related to the separation of the Arabian Plate from Africa, and that between the Somalian and Nubian Plates in the East African rift.


Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression.

Keir D, Belachew M, Ebinger CJ, Kendall JM, Hammond JO, Stuart GW, Ayele A, Rowland JV - Nat Commun (2011)

Profiles of shear-wave splitting parameters across the rift valleys in Ethiopia.The across-rift profiles show variation in splitting parameters at stations located 100 km either side of profiles x–x′ for the MER and z–z′ across Afar. For both the MER (a, b) and RSR (c, d), note the dramatic increase in magnitude of anisotropy synchronous with rotation of fast direction into the Quaternary-Recent magmatic segments. The top two panels show topography along the displayed profiles.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3104516&req=5

f4: Profiles of shear-wave splitting parameters across the rift valleys in Ethiopia.The across-rift profiles show variation in splitting parameters at stations located 100 km either side of profiles x–x′ for the MER and z–z′ across Afar. For both the MER (a, b) and RSR (c, d), note the dramatic increase in magnitude of anisotropy synchronous with rotation of fast direction into the Quaternary-Recent magmatic segments. The top two panels show topography along the displayed profiles.
Mentions: In the Quaternary-Recent magmatic segments of the MER and Afar, the dominant strike of extensional features such as faults and aligned cones is perpendicular to rift-opening direction323334, resulting in observed φ that is perpendicular to the inferred SHmin. At stations in the MER, φ varies between ∼N–NE, with a modal strike of ∼NNE, orthogonal to the ∼N105°E extension direction (Figs 3 and 4). In Afar, at stations east of the TGD and therefore in the Gulf of Aden and RSRs, φ is ∼NNW–NW, approximately parallel to the strike of faults and aligned volcanic cones here, and orthogonal to the direction of extension (Figs 3 and 4). We observe a systematic clockwise rotation of φ from ∼NW in the Asal rift of Djibouti to ∼NNW in northern Afar, a rotation that closely mimics along-strike variation in rift opening determined using GPS measurements19. A striking feature in the data is the marked similarity between φ and the strike of elongate clusters of seismicity in the three rifts of the Afar triple junction (Fig. 1b). This, along with MER parallel ∼NE-NNE oriented φ at stations located within Afar but west of the TGD lend support to structural measurements14 and kinematic models19 that suggest the TGD currently partitions most extension related to the separation of the Arabian Plate from Africa, and that between the Somalian and Nubian Plates in the East African rift.

Bottom Line: Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup.The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres.The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. d.keir@soton.ac.uk

ABSTRACT
Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture.

Show MeSH
Related in: MedlinePlus