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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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Distribution of seismic stations and earthquakes in Ethiopia during 2001–2009.(a) Temporary seismic stations deployed in Ethiopia plotted on SRTM digital elevation model. Grey circles are from the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) during October 2001–February 2003. White squares are from the Boina Broadband Network operational during October 2005–September 2006. Grey triangles are 'SEARIFT' seismic stations operating during March 2007–October 2009. White stars are 'Afar Consortium' seismic stations deployed during October 2007–October 2009. BOOE and ALEE are seismic stations Boina and Ado'Ale, respectively, deployed in the Dabbahu magmatic segment (D). Solid black lines are Oligo-Miocene age border faults. Solid red shapes are Quaternary-Recent magmatic segments. The Main Ethiopian rift (MER), Red Sea rift (RSR) and Gulf of Aden (GA) rift form the three arms of the Afar Triple Junction. Dashed-red lines show zones of submarine seafloor spreading. The MER terminates at the Tendaho-Goba'ad Discontinuity (TGD), shown by a black dashed line. (b) Distribution of local seismicity recorded during the operation of the named temporary seismic stations. Motions of the Arabian and Somalian plates relative to a fixed Nubian plate are shown as grey arrows.
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f1: Distribution of seismic stations and earthquakes in Ethiopia during 2001–2009.(a) Temporary seismic stations deployed in Ethiopia plotted on SRTM digital elevation model. Grey circles are from the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) during October 2001–February 2003. White squares are from the Boina Broadband Network operational during October 2005–September 2006. Grey triangles are 'SEARIFT' seismic stations operating during March 2007–October 2009. White stars are 'Afar Consortium' seismic stations deployed during October 2007–October 2009. BOOE and ALEE are seismic stations Boina and Ado'Ale, respectively, deployed in the Dabbahu magmatic segment (D). Solid black lines are Oligo-Miocene age border faults. Solid red shapes are Quaternary-Recent magmatic segments. The Main Ethiopian rift (MER), Red Sea rift (RSR) and Gulf of Aden (GA) rift form the three arms of the Afar Triple Junction. Dashed-red lines show zones of submarine seafloor spreading. The MER terminates at the Tendaho-Goba'ad Discontinuity (TGD), shown by a black dashed line. (b) Distribution of local seismicity recorded during the operation of the named temporary seismic stations. Motions of the Arabian and Somalian plates relative to a fixed Nubian plate are shown as grey arrows.

Mentions: The region comprises the three rift arms of the Afar Triple Junction that formed during the past ∼30 Myrs from the rifting of Africa and Arabia within a Paleogene flood basalt province13 (Fig. 1). The topographic depression is flanked by large offset, seismically active border faults initially formed during plate stretching in Oligo-Miocene times14. Since <4 Myr ago, however, volcanism and dyke-induced normal faulting have localized to ∼15-km-wide, ∼60-km-long axial volcanic ranges with aligned chains of basaltic cones and fissural flows1215. These Quaternary-Recent magmatic segments (for example, Dabbahu segment) are similar in size, morphology, structure and spacing to the second-order (∼50 km-long) non-transform segments of a slow-spreading mid-ocean ridge1215 (Fig. 1). The highly extended and intruded crust beneath most of the Afar Depression is ∼18–25 km thick, contrasting the ∼35–45-km-thick crust of the relatively unfaulted adjacent plateaux16. Kinematic models constrained by Global Positioning System (GPS) data suggest ∼5 mm per year of ∼N100°E opening occurs in the Main Ethiopian rift (MER)1719 and ∼20 mm per year of ∼N50°E extension across the Red Sea rift (RSR)1819 (Fig. 1). Structural data14 and kinematic models based on geodetic data19 suggest the Quaternary-Recent magmatic segments of the MER extend as far north as the Tendaho-Goba'ad Discontinuity (TGD; Fig. 1). Rifting in Ethiopia currently occurs above a broad, slow seismic velocity anomaly in the mantle, interpreted as a region of anomalously high temperature and partial melt that is caused primarily by the combined effects of the African superswell and ongoing lithospheric thinning2021.


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)

Distribution of seismic stations and earthquakes in Ethiopia during 2001–2009.(a) Temporary seismic stations deployed in Ethiopia plotted on SRTM digital elevation model. Grey circles are from the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) during October 2001–February 2003. White squares are from the Boina Broadband Network operational during October 2005–September 2006. Grey triangles are 'SEARIFT' seismic stations operating during March 2007–October 2009. White stars are 'Afar Consortium' seismic stations deployed during October 2007–October 2009. BOOE and ALEE are seismic stations Boina and Ado'Ale, respectively, deployed in the Dabbahu magmatic segment (D). Solid black lines are Oligo-Miocene age border faults. Solid red shapes are Quaternary-Recent magmatic segments. The Main Ethiopian rift (MER), Red Sea rift (RSR) and Gulf of Aden (GA) rift form the three arms of the Afar Triple Junction. Dashed-red lines show zones of submarine seafloor spreading. The MER terminates at the Tendaho-Goba'ad Discontinuity (TGD), shown by a black dashed line. (b) Distribution of local seismicity recorded during the operation of the named temporary seismic stations. Motions of the Arabian and Somalian plates relative to a fixed Nubian plate are shown as grey arrows.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3104516&req=5

f1: Distribution of seismic stations and earthquakes in Ethiopia during 2001–2009.(a) Temporary seismic stations deployed in Ethiopia plotted on SRTM digital elevation model. Grey circles are from the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) during October 2001–February 2003. White squares are from the Boina Broadband Network operational during October 2005–September 2006. Grey triangles are 'SEARIFT' seismic stations operating during March 2007–October 2009. White stars are 'Afar Consortium' seismic stations deployed during October 2007–October 2009. BOOE and ALEE are seismic stations Boina and Ado'Ale, respectively, deployed in the Dabbahu magmatic segment (D). Solid black lines are Oligo-Miocene age border faults. Solid red shapes are Quaternary-Recent magmatic segments. The Main Ethiopian rift (MER), Red Sea rift (RSR) and Gulf of Aden (GA) rift form the three arms of the Afar Triple Junction. Dashed-red lines show zones of submarine seafloor spreading. The MER terminates at the Tendaho-Goba'ad Discontinuity (TGD), shown by a black dashed line. (b) Distribution of local seismicity recorded during the operation of the named temporary seismic stations. Motions of the Arabian and Somalian plates relative to a fixed Nubian plate are shown as grey arrows.
Mentions: The region comprises the three rift arms of the Afar Triple Junction that formed during the past ∼30 Myrs from the rifting of Africa and Arabia within a Paleogene flood basalt province13 (Fig. 1). The topographic depression is flanked by large offset, seismically active border faults initially formed during plate stretching in Oligo-Miocene times14. Since <4 Myr ago, however, volcanism and dyke-induced normal faulting have localized to ∼15-km-wide, ∼60-km-long axial volcanic ranges with aligned chains of basaltic cones and fissural flows1215. These Quaternary-Recent magmatic segments (for example, Dabbahu segment) are similar in size, morphology, structure and spacing to the second-order (∼50 km-long) non-transform segments of a slow-spreading mid-ocean ridge1215 (Fig. 1). The highly extended and intruded crust beneath most of the Afar Depression is ∼18–25 km thick, contrasting the ∼35–45-km-thick crust of the relatively unfaulted adjacent plateaux16. Kinematic models constrained by Global Positioning System (GPS) data suggest ∼5 mm per year of ∼N100°E opening occurs in the Main Ethiopian rift (MER)1719 and ∼20 mm per year of ∼N50°E extension across the Red Sea rift (RSR)1819 (Fig. 1). Structural data14 and kinematic models based on geodetic data19 suggest the Quaternary-Recent magmatic segments of the MER extend as far north as the Tendaho-Goba'ad Discontinuity (TGD; Fig. 1). Rifting in Ethiopia currently occurs above a broad, slow seismic velocity anomaly in the mantle, interpreted as a region of anomalously high temperature and partial melt that is caused primarily by the combined effects of the African superswell and ongoing lithospheric thinning2021.

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