Magma imaged magnetotellurically beneath an active and an inactive magmatic segment in Afar, Ethiopia
N. E. Johnson, K. A. Whaler, S. Hautot, S. Fisseha, M. Desissa, G. J. K. Dawes, 2016. "Magma imaged magnetotellurically beneath an active and an inactive magmatic segment in Afar, Ethiopia", Magmatic Rifting and Active Volcanism, T. J. Wright, A. Ayele, D. J. Ferguson, T. Kidane, C. Vye-Brown
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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 two-dimensional subsurface resistivity modelling criteria. Profiles across the segments had well-defined 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 km3) 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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Magmatic Rifting and Active Volcanism
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.