Crustal resistivity structures beneath the western flank across the Central Main Ethiopian Rift: Its implications for partial melt and pathways

Abstract

The Wonji Fault Belt (WFB) and the Silti Debre Zeit Fault Zone (SDFZ) are two linear belts in the Central Main Ethiopian Rift (CMER) where recent magmatic and tectonic activities have been concentrated. To examine deep resistivity patterns, magma storage, and its path beneath the Earth's crust across the CMER, the researchers used a two-dimensional (2D) inversion model of magnetotelluric data. The time series magnetotelluric data ranging from 0.003125 to 1450.0 s were collected during >24 h of installations at 21 sites, with an average station separation of 2.5 km along a profile perpendicular to the rift from the western margin of the rift (near Butajira) to AdamiTulu (near the rift axis). This work describes the use of phase tensor analysis to determine the dimensionality and geoelectrical strike direction, as well as the estimation of geoelectric strike parameters and the 2D inversion model of MT data. The results of dimensionality analysis of the majority of our MT data show a small beta value (|β|<30) at low periods (nearly below 10 s), which indicates 1D or 2D structures, while at long periods (almost above 10 s), the data show 3D structures with a large value of |β| >30. However, from the results of the dimensionality analysis of MT data, the 2D inversion model is suitable to image the resistivity of subsurface structures. The dominant geo-electrical strike is estimated to be N150E using a Z-invariant and phase tensor azimuth. The 2D inversion model was performed after the MT data were rotated to the proper geoelectric strike direction of N150E. The desired 2D inversion of the resistivity model was obtained for the MT profile using joint inversion of TE (transverse electric) and TM (transverse magnetic) mode data with an RMS misfit of 1.11. The results of the 2D inversion model reveal partial melt and upper crustal fracture zone (fault). They are attributed to a large mass of batholithic or magmatic intrusion found at a depth of 12–22 km that is characterised by a resistivity of <5 Ωm. The partial melt extends beneath stations B07 to B11 along SDFZ with a horizontal width of approximately 10 km. It could be related to the source of heat for the Ashute and Aluto geothermal fields.