Imaging deep crustal magmatic processes in the Central Main Ethiopian
Rift zone using 3-D Magnetotellurics
In active continental rifts, asthenospheric upwelling and crustal
thinning result in the ascent of melt through the crust to the surface.
In the Main Ethiopian Rift (MER) most volcanic activity is located in
magmatic segments in the rift centre, but there are also areas of
significant off-axis magmatism. Imaging the deeper parts of magmatic
plumbing systems is possible with several geophysical techniques
including magnetotellurics (MT). We collected MT data at 67 sites and
derived a three-dimensional inversion model of the electrical
conductivity in the Central Main Ethiopian Rift, testing inversion
parameters and model feature robustness. High conductivity indicating
the presence of melt and potential pathways in the upper crust (above 5
km depth) is found in only a few places. In contrast at mid crustal
level below 15 km depth, higher conductivity values associated with
partial melt is pervasive along the north-western part of the rift.
Using mixing models and geochemical estimates of melt conductivities we
derive melt content estimates for the middle to lower crust. We compare
the conductivity model with regional shear wave tomography results. In
the lower crust there are lower shear wave velocities coinciding with
higher conductivities, indicating the presence of partial melt.
Furthermore, there is a high velocity anomaly in the upper crust (5 km)
under Aluto volcano, where MT images a resistive body. Both observations
are consistent with an older cooled magma body.