Investigation of Mantle Structure beneath Eastern Africa: Implications for the Origin Of the Cenozoic tectonism and propagation of the Rift System

Open Access
Author:
Mulibo, Gabriel Daudi
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 06, 2012
Committee Members:
  • Andrew Arnold Nyblade, Dissertation Advisor
  • Andrew Arnold Nyblade, Committee Chair
  • Charles James Ammon, Committee Member
  • Sridhar Anandakrishnan, Committee Member
  • Derek Elsworth, Special Member
Keywords:
  • Tomography
  • Receiver function
  • Superplume
  • East African Rift System
  • Cenozoic Tectonism
Abstract:
It has long been debated whether or not the geodynamic processes of Earth’s deep interior influence surface dynamics across the African continent. In this dissertation, I investigate the seismic structure of the earth beneath a portion of the African continent that has experienced Cenozoic tectonism, eastern Africa, to address the origin of the tectonism found there and determine to what extent the source of the tectonism lies deep in Earth’s mantle. The approach used in this dissertation involves combining proven seismic imaging methods with new, extensive seismic data sets from eastern Africa. In Chapter 2, I use body wave tomography to image seismic wave speed variations at upper and mid-mantle depths. Model results reveal a low wave speed anomaly (LWA) that dips to the SW beneath northern Zambia, extending to a depth of at least 900 km. The anomaly appears to be continuous across the transition zone, extending into the lower mantle, and can be attributed to a temperature anomaly of ~300 K. The depth extent of the anomaly is not easily explained by models for the origin of the Cenozoic East African tectonism that invoke a plume head or small scale convection either by edge flow or passive stretching of the lithosphere. However, the depth extent of the LWA is entirely consistent with a model invoking a through-going mantle anomaly beneath eastern Africa that links to the African Superplume anomaly in the lower mantle beneath southern Africa. Nonetheless, even though the Superplume model is favored, a 200-300 km separation of upper and lower mantle anomalies across the transition zone cannot be ruled out. To further investigate the depth extent of the LWA, in Chapter 3 I image the structure of the mantle transition zone discontinuities using receiver function stacks. I find that the average transition zone thickness (TZT) is 210±10 km across a wide region extending from central Zambia to the NE through Tanzania and into Kenya. This finding indicates that a thermal anomaly of ≥300 K beneath Zambia and the East African plateau extends through the transition zone. The regionally thinned TZT from Zambia, Tanzania into Kenya confirms that the upper mantle anomaly beneath eastern Africa connects to the lower mantle Superplume structure. Thus, results presented in this Chapter, when combined with the results from Chapter 2, make a very strong case for the Superplume providing the primary source of heat to drive Cenozoic rifting, volcanism and plateau uplift in eastern Africa. This conclusion suggests that geodynamic processes deep in the lower mantle are influencing surface dynamics across the Afro-Arabian rift system. Results from Chapter 2 don’t clearly show the direction of the rift propagation at the end of the Eastern branch of the East African rift system (EARS) in Tanzania, leaving open question of how the EARS will continue to develop. Therefore, in the third part of this dissertation, I have investigated the seismicity of southeastern Tanzania to better determine the southward propagation direction of the rift system. The results show that most of the seismicity (~75%) is concentrated in a SE trend extending from the eastern margin of the Tanzania Craton at ~5.25° latitude to Stiegler’s Gorge at ~8° latitude. A lesser amount of seismicity (~25%) is found along the SE corner of the Craton extending southward towards Lake Nyasa (Malawi). This finding can be more easily explained by a plate kinematic model invoking a new microplate (Ruvuma) than a competing model linking the Eastern and Western branches of the EARS via a zone of active rifting south of the Tanzania Craton. The seismicity pattern therefore suggests that the EARS will continue to develop in a southeasterly direction across southern Tanzania rather than in a southerly direction towards Lake Nyasa and the Western branch.