Upper mantle temperature and the onset of extension and break-up in Afar, Africa

Armitage, John; Ferguson, David; Goes, Saskia; Hammond, James; Calais, E.; Rychert, C.; Harmon, Nicholas

doi:10.1016/j.epsl.2015.02.039

Cited by 55 publications

(68 citation statements)

References 68 publications

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“…Tomographic studies in northern Afar also show a region of low seismic velocity from 50 to 400 km depth, which is interpreted as a region of elevated temperature and partial melt Stork et al, 2013;Civiero et al, 2015). This interpretation is further corroborated by modeling geochemical data that suggest an elevated mantle temperature of 1450°C (Ferguson et al, 2013;Armitage et al, 2015). The elevated mantle temperature will promote partial melting, even at relatively low rates of extension.…”

Section: Geological Settingmentioning

confidence: 72%

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Illsley‐Kemp¹,

Keir²,

Bull³

et al. 2017

Bulletin of the Seismological Society of America

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The Danakil region of northern Afar is an area of ongoing seismic and volcanic activity caused by the final stages of continental breakup. To improve the quantification of seismicity, we developed a calibrated local earthquake magnitude scale. The accurate calculation of earthquake magnitudes allows the estimation of b-values and maximum magnitudes, both of which are essential for seismic-hazard analysis. Earthquake data collected between February 2011 and February 2013 on 11 three-component broadband seismometers were analyzed. A total of 4275 earthquakes were recorded over hypocentral distances ranging from 0 to 400 km. A total of 32,904 zero-to-peak amplitude measurements (A) were measured on the seismometer's horizontal components and were incorporated into a direct linear inversion that solved for all individual local earthquake magnitudes (M L ), 22 station correction factors (C), and 2 distance-dependent factors (n, K) in the equation M L logA− logA 0 C. The resultant distance correction term is given by − logA 0 1:274336 logr=17 − 0:000273r − 17 2. This distance correction term suggests that attenuation in the upper and mid-crust of northern Afar is relatively high, consistent with the presence of magmatic intrusions and partial melt. In contrast, attenuation in the lower crust and uppermost mantle is anomalously low, interpreted to be caused by a high melt fraction causing attenuation to occur outside the seismic frequency band. The calculated station corrections serve to reduce the M L residuals significantly but do not show a correlation with regional geology. The cumulative seismicity rate produces a b-value of 0:9 0:06, which is higher than most regions of continental rifting yet lower than values recorded at midocean ridges, further supporting the hypothesis that northern Afar is transitioning to seafloor spreading.Electronic Supplement: List of all local earthquakes used in the study with calculated local magnitudes and associated magnitude error.

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Section: Geological Settingmentioning

confidence: 72%

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Illsley‐Kemp¹,

Keir²,

Bull³

et al. 2017

Bulletin of the Seismological Society of America

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“…High upper crustal seismic anisotropy values are attributed to melt pockets in cracks orientated parallel to the rift segment axes . Through a combination of numerical modelling and geochemistry, Armitage et al (2015) provided strong evidence that lithospheric melt is necessary to explain certain features of the seismic data. Crustal receiver functions typically have high v P /v S values .…”

Section: Discussionmentioning

confidence: 99%

Magma imaged magnetotellurically beneath an active and an inactive magmatic segment in Afar, Ethiopia

Johnson

Whaler

Hautot

et al. 2015

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This paper presents broadband magnetotelluric data collected along profiles over two magmatic segments comprising part of the subaerial Red Sea arm of the Afar triple junction. One of these segments has been active since late 2005 and the other segment is currently inactive. After robust processing and galvanic distortion analysis, we found that the data passed the twodimensional subsurface resistivity modelling criteria. Profiles across the segments had welldefined geoelectrical strike directions parallel to the local rift axes. Data from the northern end of the active segment had a more ambiguous strike oblique to the profile and rift axes, but the direction did not have a severe impact on the deduced model. All three models displayed prominent zones of low resistivity, interpreted as arising from magma and partial melt. Petrological information has been used to constrain the resistivity of the parent melt and hence to estimate the melt fractions from the bulk resistivity. The total amount of melt estimated beneath the profile crossing the active segment (c. 500 km 3 ) is approximately an order of magnitude greater than that beneath the profile crossing the currently inactive rift. This implies that the availability of magma is at least one factor affecting whether a rift segment is active.

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“…Again, the transition from the pan-African lithosphere to the rifted region is very sharp at the base of the plate. Based on a self-consistent model of mantle flow, Armitage et al (2015) argued that the warmed lithosphere is thinned to ,50 km beneath the rift, but the origin of the melt-related discontinuity at deeper depths is still unclear. Both the Moho and the lithosphereasthenosphere boundary observations show that strain is localized at the rift flanks in a narrow region and do not support ideas of a broad, distributed stretching of the plate, as suggested by the tectonic stretching model.…”

Section: Geophysical Signature Of Lithospheric Meltmentioning

confidence: 99%

Why is Africa rifting?

Kendall

Lithgow‐Bertelloni

2016

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Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model. Gold Open Access:This article is published under the terms of the CC-BY 3.0 license.

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Upper mantle temperature and the onset of extension and break-up in Afar, Africa

Cited by 55 publications

References 68 publications

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Magma imaged magnetotellurically beneath an active and an inactive magmatic segment in Afar, Ethiopia

Why is Africa rifting?

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