Geophysical Characterization of the Menengai Volcano, Central Kenya Rift from the analysis of magnetotelluric and gravity data

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ABSTRACT In this study, we qualitatively analyze detailed gravity and broadband magnetotelluric data in and surrounding the Menengai Volcano of the East African rift in Kenya to assess geothermal potential of the region. Three-dimensional gravity models obtained by inverting residual gravity anomalies and 2D resistivity models obtained by inverting the TE and TM magnetotelluric modes show several common features. Our models show that a low-resistivity zone above a higher resistivity zone correlates with a low-density region located 1–4 km beneath the volcano. These zones may be related to a high temperature gradient or hydrothermally altered, fractured rocks. Additionally, a low-resistivity (>20 ohm-m) and a low-density region located approximately 4–6 km below the volcano may be related to molten material that is the source of heat for the geothermal system. The low-resistivity (< 20 ohm-m) regions that correlated with a denser (2.8–2.9 g∕cc) region within the caldera are bounded by high-resistivity (>250 ohm-m), high density (>2.9) volcanic units implying that the dense and electrically resistive volcanic material is relatively cool and lacks significant fluid content that can lower resistivity. At shallow depths, 0.5–1.5 km below the caldera, a low-resistivity and low-to-moderate density region is interpreted as a zone with high fracture density that consists of clay minerals resulting from hydrothermal alteration. These results agree well with the results from previous seismic studies on the depth of the suggested molten rocks.

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