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

Abstract

Abstract 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 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.