GEOPHYSICAL CHARACTERIZATION OF THE MENENGAI VOLCANO, CENTRAL KENYA RIFT FROM THE ANALYSIS OF MAGNETOTELLURIC AND GRAVITY DATA

Geophysical inverse problems are usually nonunique, and hence the interpretation of the resulting models in terms of geological units and structures is not straightforward. However, a integrated interpretation of models determined from different geophysical data may yield a more realistic geological model. In regions subjected to intense faulting and volcanism such as Menengai in the central section of the Kenya rift system, the integreation of multiple geophysical data is vital. The Quaternary Menengai volcano is located at the intersection the Molo tectono-volcanic axis and the Solai tectono-volcanic axis and is currently the site of geothermal energy exploration.

In this study, we qualitatively analyze detailed gravity and broadband magnetotelluric data in and surrounding the Menegai Volcano region. 3D gravity models obtained by inverting residual gravity anomalies and 2D resistivity models obtained by inverting the TE and TM magnetotelluric modes indicated that are several common features in both models despite that there is no unique relationship to link electrical resistivity with density. Our models show that a lower resistivity zone correlates with a less dense region located about 4 km in depth beneath the volcano and may be related to high temperature or molten rocks. Additionally, a low resistivity (> 20 ohm-m) and a less dense region located approximately 6-7 km in depth may be caused by molten material. The low resistivity regions are bounded by high resistivity volcanic units but these regions are associated with large low dense areas implying that the volcanic material is relatively cool and lacks anytype of fluids. At shallow depths, 0.5 km to 1.5 km below the caldera, a low resistivity and density region is interpreted to consist of clay minerals resulting from hydrothermal alteration. This result agrees well with the results from previous seismic studies.