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

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description We have begun analysing our seismic data from the Ethiopia/Kenya area. We have assembled new constraints on the thickness of the crust below the Turkana Depression, and the variations in mantle wave speed below the region. We have discovered that the low-lying nature of the Turkana Depression has to do with crustal thinning, not a lack of dynamic support from the mantle below. We have also discovered precisely which areas of Turkana are seismically active, with important implications for seismic hazard and geothermal energy potential.
Exploitation Route Having discovered precisely which areas of Turkana are seismically active, with important implications for seismic hazard and geothermal energy potential. A geothermal energy company, Reykjavik Geothermal LTD, and the Ethiopian Government, are thus taking an active interest in our work.
Sectors Education,Energy,Environment

URL https://agu.confex.com/agu/fm20/prelim.cgi/Paper/695324
 
Description Through this award, we have interacted considerably with Ethiopian and Kenyan research groups, as well as school and clinic personnel in the countryside in both countries. They have learned a considerable amount about seismic hazard in their homeland.
First Year Of Impact 2019
Sector Environment
Impact Types Societal

 
Description Geothermal Energy Exploration in Ethiopia
Geographic Reach Africa 
Policy Influence Type Participation in a advisory committee
 
Description Underpinning food and water security through geothermal exploration in Ethiopia and Kenya
Amount £100,788 (GBP)
Organisation United Kingdom Research and Innovation 
Department Global Challenges Research Fund
Sector Public
Country United Kingdom
Start 11/2018 
End 06/2019
 
Title Turkana Rift Arrays to Investigate Lithospheric Strains - UK component (TRAILS-UK) 
Description TRAILS-UK comprises a network of broadband seismic (Guralp ESP and 3T) sensors that will operate concurrent with a geodetic in the Turkana Depression and its immediate surroundings, ultimately to differentiate between competing models for rifting and magmatism in the Turkana Depression, the topographic low between two broad zones of uplift. Straddling the Ethiopian/Kenyan border, this network complements the US-component of TRAILS (Ebinger), which will operate concurrently in northern Kenya. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? No  
Impact First broadband seismic dataset ever collected within the Turkana Depression. 
URL https://www.fdsn.org/networks/detail/9A_2019/
 
Description Research seminar at the Centre for Earth Evolution and Dynamics (CEED), Oslo, Norway 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Research seminar at the Centre for Earth Evolution and Dynamics (CEED), Oslo, Norway
Year(s) Of Engagement Activity 2018
 
Description Talk to the Imperial College London Geophysics Society 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Undergraduate students
Results and Impact I gave a talk to the Imperial College Geophysics Society about working in Africa on the TRAILS project.
Year(s) Of Engagement Activity 2019
 
Description Talks at numerous Ethiopian and Kenyan schools 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact While installing seismograph stations in Ethiopia and Kenya as part of project TRAILS in January/February 2019, we spent considerable time talking with teachers and pupils about seismology, volcanoes, earthquakes, and natural hazards.
Year(s) Of Engagement Activity 2019