Abstract
The Main Ethiopian Rift is accompanied by extensive volcanism and the formation of geothermal systems, both having a direct impact on the lives of millions of inhabitants. Although previous studies in the region found evidence that asthenospheric upwelling and associated decompression melting provide melt to magmatic systems that feed the tectono-magmatic segments in the rift valley, there is a lack of geophysical models imaging these regional and local scale transcrustal structures. To address this challenge, we use the magnetotelluric method and image subsurface electrical conductivity to examine the magmatic roots of Aluto volcano, quantify and interpret the melt distribution in the crust considering established concepts of continental rifting processes and constrain the formed geothermal system. Specifically, we combined regional (maximum 30 × 120 km2) and local (15 × 15 km2) magnetotelluric data sets and obtained the first multi-scale 3-D electrical conductivity model of a segment of the central Main Ethiopian Rift. The model unravels a magma ponding zone with up to 7 vol. % melt at the base of the crust (30-35 km b.s.l.) in the western part of the rift and its connection to Aluto volcano via a fault-aligned transcrustal magma system. Melt accumulates at shallow crustal depths ( 4 km b.s.l.), thereby providing heat for Aluto's geothermal system. Our model suggests that different volcano-tectonic lineaments in the rift valley share a common melt source. The presented model provides new constraints on the melt distribution below a segment of the rift which is important for future geothermal developments and volcanic hazard assessments in the region.
Key Points
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3-D multi-scale magnetotelluric model imaging the entire transcrustal tectono-magmatic system across a segment of the Main Ethiopian Rift
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A lower crustal magma ponding zone feeds a fault-aligned magmatic mush column supplying heat for the geothermal system of Aluto volcano
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Eastern and western volcano-tectonic lineaments likely share a lower crustal magma source with an estimated melt fraction of 7 percent
Plain Language Summary
Continental rifting is a fundamental process of plate tectonics that breaks continents apart to ultimately form new oceans. The landscape of the Main Ethiopian Rift is characterized by abundant volcanism and hot springs, which indicate the presence of geothermal resources formed by magmatic heating of subsurface fluids. Here we present a new 3-D subsurface electrical conductivity image of the magmatic system and geothermal reservoir beneath the Aluto volcano in the Main Ethiopian Rift. The model allows us to estimate the amount and distribution of magmatic melt. This is the first model that provides a high-resolution image of the entire magmatic system below a central part (maximum 30 × 120 km2) of the Main Ethiopian Rift from the deep magmatic melt source up to the surface. The new model shows that the geothermal reservoir under Aluto has been formed as a consequence of rifting-related volcanic activity thereby providing a geophysical illustration of fundamental geological processes. These results also have a high societal relevance by providing a basis for volcanic risk assessment and contributing to a better understanding of how the sustainable green geothermal energy resources form.
