Kinematic constraints on mantle-lithosphere interactions in East Africa
Rifting of continental lithosphere is a fundamental process that controls the growth and evolution of continents and the birth of ocean basins. It plays a crucial economical role by controlling the formation of hydrocarbon and thermal resources in rift basins and their successfully rifted counterparts, passive continental margins. Continental rifting involves the entire mantle-lithosphere system through heat transfer and magmatism, and possibly through the mechanical effect of mantle flow on lithospheric deformation. Continental rifts are therefore a prime setting for the study of coupling and exchanges between the deeper and shallower parts of our planet. The East African Rift (EAR), commonly cited as a modern archetype for rifting and continental breakup and the only rift on Earth that is currently splitting a continent over its entire length, provides a prime setting to such studies.
Sparse GPS and earthquake skip vector data in the East African Rift (EAR), coupled with recent tomographic images of sub-lithospheric structures and seismic anisotropy data, suggest that mantle flow, possibly associated with the African Superplume, interacts with the 200~km-thick lithospheric keel of the Tanzanian craton, driving lithospheric motions and continental rifting in eastern Africa. This hypothesis is being tested by (1) establishing the kinematics of the EAR using GPS measurements at new and existing sites in Tanzania, (2) assessing asthenospheric flow from an inversion of GPS velocities and seismic anisotropy data and from plume-lithosphere interactions models, (3) using the modeled and ``estimated'' flow as boundary conditions to a 3D finite element model of asthenosphere-lithosphere interactions.
This work is addressing some of the fundamental questions of continental plate tectonics: Are extensional strains broadly distributed or initially localized to narrow zones? Do interactions between mantle flow fields and cratonic keels influence the localization and orientation of strain within continental plates? In addition to these global tectonic issues, our work will answer the following questions specific to East Africa: What is the origin of the magma-poor Western rift system? What is the kinematics of linkage between the Western and Eastern rift systems?
The break up of continental lithosphere is a fundamental process that controls the growth and evolution of continents and the birth of oceans. It plays a crucial economical role by controlling the formation of hydrocarbon and thermal resources in rift basins and their successfully rifted counterparts, passive continental margins. Continental rifting involves the entire mantle-lithosphere system through heat transfer and magmatism, and possibly through the mechanical effect of mantle flow on lithospheric deformation. Continental rifts are therefore a prime setting for the study of coupling and exchanges between the deeper and shallower parts of our planet.This projects contributes to the understanding of the mechanics that controls the break up of continents. East Africa is a prime setting to study continental break up, as it offers direct exposure of the several stages of this process. In this project, we used Global Positioning System (GPS) data to determine, for the first time, the direction and speed of continental break up across eastern Africa. We showed that the process involves several microblocks, embedded into the boundary between the two major Nubian and Somalian tectonic plates. Although our working hypothesis was that breakup was facilitated by the diverging motion of mantle material beneath eastern Africa, as derived from seismological data, we find that continental break up in east Africa is best explained by the collapse of the high east African plateaus under their own weight, while lateral motions of mantle material play a minor role. A calculation of the forces involved shows that the initiation of the break up requires an additional process that weakens the continental crust.
