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Magmatic Rifting and Active Volcanism (2016)

Geological Society (London, U.K.) ; Wright, T. J. ; Ayele, A. ; [et al.]

London : The Geological Society

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Description / Table of Contents: A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a ten-day period, c. 2.5 km3 of magma were intruded along a 60 km-long dyke separating the Arabian and Nubian plates. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. This activity led to a renewed international focus on the role of magmatism in rifting, with major international collaborative projects working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This work has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.

Pages: Online-Ressource (VI, 374 Seiten)

ISBN: 9781862397293

URL: http://sp.lyellcollection.org/content/420/1

Language: English

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Reassessment of source parameters for three major earthquakes in the East African rift system from historical seismograms and bulletins (2006)

Ayele, A. ; Kulhánek, O.

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Publication Date: 2020-11-20

Description: Source parameters for three majo earthquakes in the East African rift are re-computed from historical seismograms and bulletins. The main shock and the largest foreshock of the August 25, 1906 earthquake sequence in the main Ethiopian rift are re-located on the eastern shoulder of the rift segment.The magnitude of the main shock is estimated to be 6.5 (Mw) from spectral analysis. The December 13, 1910 earthquake in the Rukwa rift (Western Tanzania) indicated a significant strike-slip component from teleseismcs body-waveform inversion for fault mechanism and seismic moment. The January 6, 1928 earthquake in the Gregory rift (Kenya) showed a multiple rupture process and unusually long duration for a size of 6.6(Mw). The May 20, 1990 earthquake in Southern Sudan, mentioned merely for the sake of comparison, is the largest of all instrumentally recorded events in the East African rift system. Despite the fact that the mode of deformation in the continental rift is predominantly of extensional nature, the three largest earthquakes known to occur in the circum-Tanzanian craton have shallow focal depths and significant strike-slip component in their fault mechanisms. This and similar works will enrich the database for seismic hazard assessment in East Africa.

Description: JCR Journal

Description: open

Keywords: East African rift system ; fault mechenism ; seismic hazard ; source parameters ; 04. Solid Earth::04.06. Seismology::04.06.05. Historical seismology

Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)

Type: article

Format: 6402324 bytes

Format: application/pdf

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Mantle upwellings, melt migration and the rifting of Africa: insights from seismic anisotropy (2007)

Kendall, J.-M. ; Pilidou, S. ; Keir, D. ; [et al.]

In: Geological Society Special Publication 259: 55-72.

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Publication Date: 2007-10-08

Description: The rifting of continents and eventual formation of ocean basins is a fundamental component of plate tectonics, yet the mechanism for break-up is poorly understood. The East African Rift System (EARS) is an ideal place to study this process as it captures the initiation of a rift in the south through to incipient oceanic spreading in north-eastern Ethiopia. Measurements of seismic anisotropy can be used to test models of rifting. Here we summarize observations of anisotropy beneath the EARS from local and teleseismic body-waves and azimuthal variations in surface-wave velocities. Special attention is given to the Ethiopian part of the rift where the recent EAGLE project has provided a detailed image of anisotropy in the portion of the Ethiopian Rift that spans the transition from continental rifting to incipient oceanic spreading. Analyses of regional surface-waves show sub-lithospheric fast shear-waves coherently oriented in a north-eastward direction from southern Kenya to the Red Sea. This parallels the trend of the deeper African superplume, which originates at the core-mantle boundary beneath southern Africa and rises towards the base of the lithosphere beneath Afar. The pattern of shear-wave anisotropy is more variable above depths of 150 km. Analyses of splitting in teleseismic phases (SKS) and local shear-waves within the rift valley consistently show rift-parallel orientations. The magnitude of the splitting correlates with the degree of magmatism and the polarizations of the shear-waves align with magmatic segmentation along the rift valley. Analysis of surface-wave propagation across the rift valley confirms that anisotropy in the uppermost 75 km is primarily due to melt alignment. Away from the rift valley, the anisotropy agrees reasonably well within the pre-existing Pan-African lithospheric fabric. An exception is the region beneath the Ethiopian plateau, where the anisotropy is variable and may correspond to pre-existing fabric and ongoing melt-migration processes. These observations support models of magma-assisted rifting, rather than those of simple mechanical stretching. Upwellings, which most probably originate from the larger superplume, thermally erode the lithosphere along sites of pre-existing weaknesses or topographic highs. Decompression leads to magmatism and dyke injection that weakens the lithosphere enough for rifting and the strain appears to be localized to plate boundaries, rather than wider zones of deformation.

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A multidisciplinary study of the final episode of the Manda Hararo dyke sequence, Ethiopia, and implications for trends in volcanism during the rifting cycle (2015)

Barnie, T. D., Keir, D., Hamling, I., Hofmann, B., Belachew, M., Carn, S., Eastwell, D., Hammond, J. O. S., Ayele, A., Oppenheimer, C., Wright, T.

Geological Society (of London)

In: Special Publications / Geological Society London

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Publication Date: 2015-05-14

Description: The sequence of dyke intrusions between 2005 and 2010 in the Manda Hararo rift segment, Ethiopia, provided an opportunity to test conceptual models of continental rifting. Based on trends up to dyke 13 in the sequence, it was anticipated that, should magma supply continue, dykes would shorten in length and eruptions would increase in size and decrease in distance from the segment centre as extensional stress was progressively released. In this paper we revisit these predictions by presenting a comprehensive overview of the May 2010 dyke and fissure eruption, the 14th and last in the sequence, from InSAR, seismicity, satellite thermal data, ultraviolet SO 2 retrievals and multiple LiDAR surveys. We find the dyke is longer than other eruptive dykes in the sequence, propagating in two directions from the segment centre, but otherwise fairly typical in terms of opening, propagation speed and geodetic and seismic moment. However, though the eruption is located closer to the segment centre, it is much smaller than previous events. We interpret this as indicating that either the Manda Hararo rifting event was magma limited, or that extensional stress varies north and south of the segment centre.

Print ISSN: 0305-8719

Electronic ISSN: 2041-4927

Topics: Geosciences

Published by Geological Society (of London)

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Magmatic rifting and active volcanism: introduction (2016)

Wright, T. J., Ayele, A., Ferguson, D., Kidane, T., Vye-Brown, C.

Geological Society (of London)

In: Special Publications / Geological Society London

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Publication Date: 2016-07-08

Description: A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a 10-day period, c. 2.5 km 3 of magma were intruded into the upper crust along a 60 km-long dyke separating the Arabian and Nubian plates. There was an intense seismic swarm and a small rhyolitic eruption; extension of up to 10 m occurred across the rift segment. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. The activity in Afar led to a renewed international focus on the role of magmatism in rifting, with major collaborative projects involving researchers from Ethiopia, the UK, the USA, France, Italy and New Zealand working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This renewed focus on magmatism and its role in rifting has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.

Print ISSN: 0305-8719

Electronic ISSN: 2041-4927

Topics: Geosciences

Published by Geological Society (of London)

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Seismicity of the central Afar rift and implications for Tendaho dam hazards (2015)

Ayele, A., Ebinger, C. J., van Alstyne, C., Keir, D., Nixon, C. W., Belachew, M., Hammond, J. O. S.

Geological Society (of London)

In: Special Publications / Geological Society London

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Publication Date: 2015-06-04

Description: Temporary broadband seismic networks deployed from 2007 to 2011 around the Afar triple junction of the East African Rift System provide insights into seismicity patterns of the actively deforming crust around the 1.86 km 3 impounded lake system behind the Tendaho dam. The observed seismicity correlates well with the active magmatic centres around central Afar. The area around the dam site is characterized by a network of intersecting NNE- and NW-trending faults. Seismicity clusters observed in the specified time interval indicate that both fault sets are active and are potential sources of seismogenic hazards. The dam neighbourhood is naturally active and it is a challenge to associate the observed seismic activity to either a change in magmato-tectonic conditions or attribute it to the influence of reservoir load. It is evident that the dam region experiences high levels of seismic and volcano-tectonic unrest, regardless of the origin of the activity. The spatial overlap of narrow zones of crustal seismicity and upper mantle low velocity zones observed in S-wave tomography models suggests that melt production zones guide the distribution of strain during continental rupture. Given its volcanically and seismically active setting, the Tendaho dam site and the surrounding region require continuous monitoring for the safety of downstream populations and development infrastructures in the Afar National Regional State of Ethiopia.

Print ISSN: 0305-8719

Electronic ISSN: 2041-4927

Topics: Geosciences

Published by Geological Society (of London)

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Mantle upwelling and initiation of rift segmentation beneath the Afar Depression (2013)

Hammond, J. O. S., Kendall, J.- M., Stuart, G. W., Ebinger, C. J., Bastow, I. D., Keir, D., Ayele, A., Belachew, M., Goitom, B., Ogubazghi, G., Wright, T. J.

Geological Society of America (GSA)

In: Geology

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Publication Date: 2013-05-21

Description: The Afar Depression, at the northern end of the East African Rift, is the only place on land where the transition from a plume-induced continental breakup to seafloor spreading is active today. New images of seismic velocity structure, based on exceptional new data sets, show that the mantle plume that initiated rifting in Africa is absent beneath Afar today. The images are dominated by a major low-velocity feature at ~75 km depth closely mimicking the abrupt changes in rift axis orientation seen at the surface. This is likely associated with passive upwelling beneath the rift. Additional focused low-velocity anomalies show that small diapiric upwellings are present beneath major off-axis volcanoes. These multiple melting sources can explain the wide range of geochemical signatures seen in Afar. These images suggest that passive upwelling beneath Afar marks the initiation of rift segmentation as continental breakup progresses to seafloor spreading.

Print ISSN: 0091-7613

Electronic ISSN: 1943-2682

Topics: Geosciences

Published by Geological Society of America (GSA)

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Local Earthquake Magnitude Scale and b-Value for the Danakil Region of Northern Afar (2017)

Illsley-Kemp, F., Keir, D., Bull, J. M., Ayele, A., Hammond, J. O. S., Kendall, J.- M., Gallacher, R. J., Gernon, T., Goitom, B.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA)

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Publication Date: 2017-03-23

Description: 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 =log( A )–log( A 0 )+ C . The resultant distance correction term is given by –log( A 0 )=1.274336log( r /17)–0.000273( r –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.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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