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

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The basic idea of plate tectonics is built on a simplified view that the lithosphere (the outermost layer of the solid earth) is broken into rigid pieces that drift around relative to each other without deforming internally, like puzzle pieces that are moved around to make a picture but don't themselves change shape or size. 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. This experiment is focused on quantifying the role of three different factors in influencing plate break-up: preexisting structures in the lithosphere; present topography; and the convecting mantle beneath. The Turkana Depression of northern Kenya and southern Ethiopia is an ideal place to investigate these issues: it is probably the location where anomalous mantle first interacted with the African lithosphere to produce magma, it has considerable inherited structure, and it has very little topography compared to adjacent parts of the rift. The scientific and broader impacts of this project are significant. An international team supported by the U.S. National Science Foundation and the U.K. Natural Environment Research Council and including scientists from the US, UK, Kenya, and Ethiopia will undertake both field observations and scientific computing. The work will support graduate and undergraduate students at the two collaborating US institutions. Knowing how new plate 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 East African Rift System (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. This project involves a multi-method geophysical 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. Systematic comparisons of the data products to one another, combined with 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. Such constraints on the dynamics of rifting will, in turn, enable a better understanding of 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 award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.