NSFGEO-NERC: Crust and mantle structure and the expression of extension in the Turkana Depression of Kenya and Ethiopia

The theory of plate tectonics is built on a simplified view that the lithosphere (the outermost layer of the solid Earth) is broken into infinitely rigid pieces that drift relative to each other without deforming internally. This works well to capture the motions of pieces once their boundaries are well-developed, but doesn't explain how the pieces are made in the first place, such as how the African piece breaks into several parts along the East African Rift System (EARS). This experiment is focused on quantifying the role of three different factors in influencing plate break-up: preexisting structures and density contrasts in the lithosphere inherited from long past tectonic processes, present topography that may supply potential energy for breakup, and pushes and pulls on plates from motions of the convecting mantle beneath. The Turkana Depression of northern Kenya and southern Ethiopia is an ideal place to investigate the contribution of these factors because it is probably the location where anomalous mantle first interacted with the African lithosphere to produce magma, it has a lot of inherited structure, and it has very little topography compared to adjacent parts of the rift. By measuring the detailed properties of the lithosphere and crust in Turkana using seismic techniques and simultaneously measuring the rate and location of stretching using geodetic techniques, we can compare the importance of each of the three factors in influencing the initiation and evolution of a new plate boundary. Knowing how new boundaries form in space and time allows us to better understand the tectonic evolution of the planet over its long history, to identify past, current, and future plate boundaries, and to understand the natural hazards associated with tectonic boundaries, such as earthquakes and volcanos.

Nonlinear interactions among mechanical competence, gravitational potential, mantle dynamics, and magmatism determine how continental plate boundaries evolve over time. The EARS, is an ideal natural laboratory for rifting processes. For example, because the far-field boundary conditions on the whole EARS are the same, systematic comparisons of strain accommodation in melt-rich and melt-poor sectors have illuminated the role of heating and composition. Comparing sectors with and without large lateral material heterogeneities has revealed the role of pre-existing lithospheric architecture; comparing sectors with different total finite strain can be used as proxies for evolution. What remains to be considered, however, is the role of gravitational potential energy (GPE) through a comparison of a rift sector in high topography to one in low topography. Although the seismically and volcanically active Turkana Depression appears to represent the end member conditions of very low topography, very high material heterogeneity, and elevated mantle geotherms, it has yet to be investigated in detail with modern geophysical methods. We propose a multi-method investigation of the Turkana Depression, combining seismic and geodetic data collection for seismic imaging, earthquake source mechanisms, surface kinematics, crustal strain rates, and structural architecture. Analysis of these data, and inverse models of geodetic, structural, and earthquake data and limited forward numerical simulations of rift topography and strain patterns will test basic hypotheses about the role of GPE and crustal architecture in continental rifting. Doing so will help to resolve the longer-term rift evolution, especially the role of one or two mantle plumes, inherited continental structure from Mesozoic rifting, and topographic feedbacks in contributing to and shaping continental breakup. We aim to better understand the exchange of mass and heat between the lithosphere and mantle, long timescale continental tectonic plate and boundary behavior, and the spatial and temporal distribution of hazards and resources associated with magmatic rifting.

This project necessitates a close collaboration among six partner institutions, two in the U.S. (Tulane University and the University of Montana), one in the UK (Imperial College), and three in Africa: the University of Addis Ababa, the University of Nairobi, and the Turkana Basin Institute (TBI). The proposed work will train at least three graduate students, provide an international REU experience for one US student, and several students from African institutions. The seismicity and seismic imaging data sets will enable, for the first time, assessment of earthquake and volcanic hazards in N. Kenya and SW Ethiopia, and serve as a foundation for expansion of geothermal exploration. Deployments at secondary schools will bring introductory geophysics curriculum to East Africa. More advanced geophysics instructional materials will be incorporated into the Turkana Basin Institute's field school for undergraduate students, including the use of data collected at the TBI campuses. We will deliver scientific lectures highlighting our new findings at each of the partner institutions in Africa, in conjunction with our field operations. Finally, by making data from three of our seismograph stations and all of our continuous GPS stations publicly available from day one, global and continent-scale tomographers and geodesists will have the opportunity to fill a critical gap in their models of East Africa.