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

Abstract

The Main Ethiopian Rift is part of the East African Rift with its unique geological setting as an active continental breakup zone. The Main Ethiopian Rift includes a number of understudied active volcanoes with potentially high risks for this densely populated part of Ethiopia. Using newly recorded (2016) magnetotelluric data along a 110 km long transect crossing the whole rift, we present a regional 2‐D model of electrical resistivity of the crust. The derived model endorses a previous study that drew the surprising conclusion that there was no highly conductive region associated with a magma chamber directly under the central rift volcano Aluto. This has implications for the estimation of the amount of magma present, its water content, and the storage conditions, as the volcano is actively deforming and results from seismicity and CO2 degassing studies all indicate magma storage at about 5 km depth. Additionally, the existence of a strong conductor under the Silti Debre Zeyt Fault Zone approximately 40 km to the northwest of the rift center is confirmed. It is located with a slight offset to the Butajira volcanic field, which hosts a number of scoria cones at the boundary between the NW plateau and the rift. The magnetotelluric model reveals different electrical structures below the eastern and western rift shoulders. The western border is characterized by a sharp lateral contrast between the resistive plateau and the more conductive rift bottom, whereas the eastern flank shows a subhorizontal layered sequence of volcanic deposits and a smooth transition toward the shoulder.

Crustal Magmatism and Anisotropy Beneath the Arabian Shield—A Cautionary Tale

“…The lack of regional context presents additional challenges. This is made strikingly clear by the more regional studies of Whaler and Hautot () and Hübert et al (), which model 100‐ to 200‐km‐long profiles extending outside a magmatic rift segment and into tectonically stable crust. Models from both these studies image the most intense conductive features outside the active rift segment, calling into question the magmatic interpretations almost ubiquitously given to conductors within an active volcanic environment.…”

“…MT studies of the East African Rift system, one arm of the Afar triple junction (Figure a), differ considerably, with investigations of segments of the Main Ethiopian Rift system alternately advocating for large volumes of upper‐crustal melt (e.g., Didana et al, ; Samrock et al, ) or a complete absence of crustal melt (e.g., Hübert et al, ; Samrock et al, ). Many such studies are based upon short profiles (<50 km) or small arrays located almost entirely within active magmatic rift segments (e.g., Desissa et al, ; Johnson et al, ; Samrock et al, ).…”

Mixing and Crystal Scavenging in the Main Ethiopian Rift Revealed by Trace Element Systematics in Feldspars and Glasses

“…Magnetotelluric surveys, used for inferring the Earth's subsurface electrical conductivity, have been conducted at several sites along the EARS (Desissa et al, 2013;Hübert et al, 2018;Johnson et al, 2015;Samrock et al, 2015;Whaler & Hautot, 2006). Resistivity is sensitive to fluid content, allowing the method to identify the presence of partial melt beneath volcanoes (Desissa et al, 2013;Johnson et al, 2015).…”

“…Crystal-rich mushes may be disrupted and disaggregated prior to and during eruptions, leading to the accumulation of a range of diverse crystals in the magma, for example, Fish Canyon Tuff (Bachmann et al, 2002), Shiveluch Volcano (Humphreys et al, 2008), Mount Hood (Cooper & Kent., 2014), and Yellowstone (Wotzlaw et al, 2014). Added to these geochemical lines of evidence, geophysical imaging of the upper crust has often failed to detect large bodies of melt beneath active volcanoes (e.g., Hübert et al, 2018;Manzella et al, 2004;Samrock et al, 2015). This has led to suggestions that, in some circumstances, magmas may be stored as melt-poor mush, which is expected to have a low electrical conductivity and low V p /V s ratio (Chu et al, 2010;Miller & Smith, 1999;Steck et al, 1998;Zandt et al, 2003).…”

The Role of Global/Regional Earth Conductivity Models in Natural Geomagnetic Hazard Mitigation

“…We should also note that the electrical conductivity of Africa in Alekseev et al (2015) is not inferred from magnetotelluric data [(as suggested by De Villiers et al (2016)] but rather combined from the distribution of sediments and a default lithospheric electrical conductivity value. By contrast, geothermal fields in Ethiopia have been extensively studied with magnetotellurics, see e.g., Whaler and Hautot (2006), Desissa et al (2013), Didana et al (2015), Samrock et al (2015), Hübert et al (2018) among others, consistently recovering three orders of magnitude variations in electrical conductivity throughout the Earth's crust (and sometimes lithosphere).…”