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

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 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.

The Electrical Structure of the Central Main Ethiopian Rift as Imaged by Magnetotellurics: Implications for Magma Storage and Pathways

“…From the dimensionality analysis and very small tipper values, we conclude that there are no indicators for off-profile conductors in the data, so we consider the absence of a conductive anomaly at the proposed depth of the magma chamber as a robust feature in our model and not an MT resolution issue. This is in stark contrast to the situation elsewhere in the MER and Afar, where the presence of magma is associated with an electrically conducting subsurface (e.g., Didana et al, 2015;Johnson et al, 2015;Whaler & Hautot, 2006). This first order contradiction between our MT results and other geophysical and geochemical observations gives new insights into the storage of melt under an on-axis volcano in the CMER.…”

“…Previous surveys have highlighted the capability of MT to quantify the amount of magma present in the subsurface. Desissa et al () and Johnson et al () estimated magma volumes below the Manda‐Hararo magmatic segment in Afar based on the observation of very high conductivities using models that link electrical conductivity to melt fractions. Further south in the Main Ethiopian Rift, Whaler and Hautot () found that the subsurface was significantly less conductive than in Afar but that there were still conductive features located below the rift axis as well as near the rift flanks.…”

Using Ambient Noise to Image the Northern East African Rift

“…This is most clearly evident at the Dabbahu and Hararo magmatic segments, the location of the 2005–2010 Dabbahu dyking event (Ebinger et al, ; Grandin et al, ; Illsley‐Kemp et al, ). Previous studies also find slow velocities (Korostelev et al, ; Stork et al, ) and high conductivities (Desissa et al, ; Johnson et al, ), which have been used to estimate a 500‐km 3 melt body with 13% melt beneath this region associated with the dyking event (Desissa et al, ). Slow velocities coupled with the recent dyking event suggest we image melt associated with the event.…”

Magmatic rifting and active volcanism: introduction

“…Again, many different disciplines have been applied, including petrology and geochronology (Ferguson 2011;Ferguson et al 2013a;Field et al 2013;Medynski et al 2013Medynski et al , 2015, magnetotellurics (Desissa et al 2013;Johnson et al 2015), seismology Hammond & Kendall 2016), geodesy (Grandin et al 2010a;Pagli et al 2012) and combinations of techniques (Field et al 2012). Collectively, these studies show that the Afar crust contains large quantities of partial melt and they give insights into how it is stored.…”

“…This book includes seven studies that document different aspects of magmadominated rifting in Afar. Johnson et al (2015) used broadband magnetotelluric observations to compare the conductivity structure beneath the active Dabbahu segment of the Arabia -Nubia plate boundary with that beneath the currently inactive Hararo segment. As the dominant cause of high conductivity anomalies below the shallow crust is melt, they used these observations to quantify the amount of melt present beneath each rift segment.…”