Continental rifting: magnetotelluric fieldwork adventures in Africa
The 2005 Presidential Address, by Prof. Kathy Whaler
at the 2005 January Meeting of the Royal Astronomical Society
(Poster, 126k pdf)
Africa has experienced several major episodes of rifting. The best known is the current extension along the East African rift, from which we expect Africa eventually to split into two from the Afar triangle southwards along a new axis of sea floor spreading. But older rifting has also been important in shaping the landscape, and governing the location of resources. Many older rifts contain sedimentary basins with hydrocarbon potential (or are currently producing); others are modern-day analogues of basins that have eventually developed into hydrocarbon bearing basins elsewhere in the world.
Magnetotelluric (MT) studies use natural magnetic fields generated externally to the earth as a probe of the electrical structure of the interior. Changing external fields induce changing internal magnetic and electric fields (eddy currents), whose ratio depends on the electrical conductivity of the sub-surface. We can probe the depth dependence of conductivity by measuring the ratio as a function of frequency of the variation, based on the concept of the skin depth.
MT is a good way to investigate structures associated with rifting, since there is usually a good contrast between the electrical resistivity of the surrounding plateau and rocks within the rift. In particular, fractured structures containing fluids, sedimentary basins formed subsequently as weathered rock from the uplifted rift sides has been deposited on the rift floors, and magma and partially molten rock from volcanic activity associated with rifting, all have enhanced conductivity.
Examples of studies from several rift areas will be presented. The main emphasis will be on recent investigations of the northern Ethiopian rift, where continental rifting is evolving to sea floor spreading. In 2001-3, a number of geophysical experiments were carried out under the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) by scientists from institutions in Ethiopia, Europe and North America. Data from the different methods are currently being modelled and interpreted. Joint interpretation is giving new insights into processes taking place, and strengthening conclusions that would be more tenuous if based on a single physical parameter. MT is proving to be an important partner in this.