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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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Orientation and magnitude of crustal shear-wave splitting measurements in the Ethiopian and Afar rifts.(a) Local earthquake shear-wave splitting measurements in Ethiopia. The arrows are oriented parallel to the fast-polarization direction (φ) and the arrow length is scaled by delay time (δt). Measurements from Keir et al.25 from the MER are in blue, and an average of measurements from the Asal rift in Djibouti are black. New splitting measurements from our study are green. (b) Estimate of magnitude and direction of anisotropy derived from the raw S-wave splitting measurements displayed in a. The arrows are oriented parallel to the fast anisotropic direction and the arrow lengths are scaled to % anisotropy, assuming that S-wave splitting is accrued along the full ray-path length. In the MER, crustal anisotropy is particularly high and in the Boset-Kone (BK) magmatic segment where Quaternary-Recent volcanism is particularly voluminous. The profiles displayed in Figure 4 are x–x′ and z–z′.
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f3: Orientation and magnitude of crustal shear-wave splitting measurements in the Ethiopian and Afar rifts.(a) Local earthquake shear-wave splitting measurements in Ethiopia. The arrows are oriented parallel to the fast-polarization direction (φ) and the arrow length is scaled by delay time (δt). Measurements from Keir et al.25 from the MER are in blue, and an average of measurements from the Asal rift in Djibouti are black. New splitting measurements from our study are green. (b) Estimate of magnitude and direction of anisotropy derived from the raw S-wave splitting measurements displayed in a. The arrows are oriented parallel to the fast anisotropic direction and the arrow lengths are scaled to % anisotropy, assuming that S-wave splitting is accrued along the full ray-path length. In the MER, crustal anisotropy is particularly high and in the Boset-Kone (BK) magmatic segment where Quaternary-Recent volcanism is particularly voluminous. The profiles displayed in Figure 4 are x–x′ and z–z′.

Mentions: We conduct S-wave splitting analysis on 78 seismograms from 34 seismic stations in Ethiopia to constrain the polarization direction of the fast S-wave (φ) and the time delay between the fast and slow S-waves (δt) (Supplementary Fig. S1). The seismograms are derived from local earthquakes in Ethiopia occurring at a depth of 4–22 km (Supplementary Table S1). More than one S-wave splitting measurement is available at a number of the seismic stations (Fig. 2). In these cases, δt increases nearly linearly with ray-path distance between the hypocentre and seismic station, a pattern most clearly seen for the 6 splitting measurements at BOOE and for the 30 splitting measurements at ALEE (Fig. 2, Supplementary Figs S2 and S3). This strongly suggests that S-wave splitting is incrementally accrued along the full ray-path length, and we therefore normalize δt measurements by deriving percent (%) S-wave anisotropy beneath each station assuming uniformly distributed anisotropy through the crust. In addition, the 30 measurements from station ALEE have sufficient range in backazimuth to allow us to invert the data set for fractures parameters22. The splitting parameters are best-fit by a simple homogeneous fracture-induced model with a fracture strike of 145±10° and fracture density of 0.063±0.015 (Supplementary Fig. S4). At ALEE, the average φ and percent anisotropy are 147° and 5%, respectively. For the other stations with more than one splitting measurement, φ shows little variation between measurements, and we therefore plot and interpret the average measured fast direction with the 2-sigma error estimate within ±12° (Fig. 3, Supplementary Table S2). We see no evidence for temporal changes in splitting parameters at any of our stations as seen near active volcanoes with shallow magma chambers elsewhere2324 (Supplementary Fig. S3).


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)

Orientation and magnitude of crustal shear-wave splitting measurements in the Ethiopian and Afar rifts.(a) Local earthquake shear-wave splitting measurements in Ethiopia. The arrows are oriented parallel to the fast-polarization direction (φ) and the arrow length is scaled by delay time (δt). Measurements from Keir et al.25 from the MER are in blue, and an average of measurements from the Asal rift in Djibouti are black. New splitting measurements from our study are green. (b) Estimate of magnitude and direction of anisotropy derived from the raw S-wave splitting measurements displayed in a. The arrows are oriented parallel to the fast anisotropic direction and the arrow lengths are scaled to % anisotropy, assuming that S-wave splitting is accrued along the full ray-path length. In the MER, crustal anisotropy is particularly high and in the Boset-Kone (BK) magmatic segment where Quaternary-Recent volcanism is particularly voluminous. The profiles displayed in Figure 4 are x–x′ and z–z′.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Orientation and magnitude of crustal shear-wave splitting measurements in the Ethiopian and Afar rifts.(a) Local earthquake shear-wave splitting measurements in Ethiopia. The arrows are oriented parallel to the fast-polarization direction (φ) and the arrow length is scaled by delay time (δt). Measurements from Keir et al.25 from the MER are in blue, and an average of measurements from the Asal rift in Djibouti are black. New splitting measurements from our study are green. (b) Estimate of magnitude and direction of anisotropy derived from the raw S-wave splitting measurements displayed in a. The arrows are oriented parallel to the fast anisotropic direction and the arrow lengths are scaled to % anisotropy, assuming that S-wave splitting is accrued along the full ray-path length. In the MER, crustal anisotropy is particularly high and in the Boset-Kone (BK) magmatic segment where Quaternary-Recent volcanism is particularly voluminous. The profiles displayed in Figure 4 are x–x′ and z–z′.
Mentions: We conduct S-wave splitting analysis on 78 seismograms from 34 seismic stations in Ethiopia to constrain the polarization direction of the fast S-wave (φ) and the time delay between the fast and slow S-waves (δt) (Supplementary Fig. S1). The seismograms are derived from local earthquakes in Ethiopia occurring at a depth of 4–22 km (Supplementary Table S1). More than one S-wave splitting measurement is available at a number of the seismic stations (Fig. 2). In these cases, δt increases nearly linearly with ray-path distance between the hypocentre and seismic station, a pattern most clearly seen for the 6 splitting measurements at BOOE and for the 30 splitting measurements at ALEE (Fig. 2, Supplementary Figs S2 and S3). This strongly suggests that S-wave splitting is incrementally accrued along the full ray-path length, and we therefore normalize δt measurements by deriving percent (%) S-wave anisotropy beneath each station assuming uniformly distributed anisotropy through the crust. In addition, the 30 measurements from station ALEE have sufficient range in backazimuth to allow us to invert the data set for fractures parameters22. The splitting parameters are best-fit by a simple homogeneous fracture-induced model with a fracture strike of 145±10° and fracture density of 0.063±0.015 (Supplementary Fig. S4). At ALEE, the average φ and percent anisotropy are 147° and 5%, respectively. For the other stations with more than one splitting measurement, φ shows little variation between measurements, and we therefore plot and interpret the average measured fast direction with the 2-sigma error estimate within ±12° (Fig. 3, Supplementary Table S2). We see no evidence for temporal changes in splitting parameters at any of our stations as seen near active volcanoes with shallow magma chambers elsewhere2324 (Supplementary Fig. S3).

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