Combined transient electromagnetic and magnetotelluric study of the southern Kenya Rift Valley

The transient electromagnetic (TEM) method and the magnetotelluric (MT) technique have been applied to determine the electrical resistivity structure across the southern Kenya Rift Valley. The main profile extends from the shores of Lake Victoria, west of the Rift Valley, to the north of the Chyulu Hills volcanic chain, 150 km SE of the rift. A second profile runs parallel to the Chyulu Hills volcanic trend. Data from 19 stations along the two profiles have been processed using classical techniques and in the case of MT, analysed with modern tensor decomposition methods.;The TEM data have facilitated the removal of static shift effects from the MT data and recovery of the near-surface (<300 m) geoelectric structure. One-dimensional joint inversion of TEM and MT data yielded an approximate geoelectric structure for the region. Subsequent two-dimensional modelling has revealed a more realistic resistivity distribution for the complex environment of the Kenya Rift.;A resistive (>2000 .m) Archaean crust 30 km thick, with a 10-12 km mid-crustal conductive (100 .m) zone, resting on a moderately resistive (100 .m) mantle appears at the west end of the main profile. A conductive fault-like zone extending to mantle depths in the area of the Oloololo Escarpment coincides with the exposed boundary between the Archaean Nyanza Craton and the Proterozoic Mozambique Belt. A poorly constrained highly resistive (>10000 .m) (Proterozoic ?) crust is found at the western flank of the rift. Low resistivities (<50 .m) are found down to the base of the crust in the rift zone and are possibly due to the presence of sedimentary fill deposits at shallow depths, and the presence of magmatism and partial melt at deeper levels. East of the rift a less sharply defined geoelectric margin, offset from the accepted topographic and geologic boundary of the rift, marks the transition to a more resistive (1000 .m.) Proterozoic crust. Significantly enhanced conductivities (<100 .m) are implied in the complex 3-D region of the Chyulu Hills.