Collaborative Research: Geochemical and geodynamic investigation of lithospheric drip viability beneath the East African Rift

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Geochemical and geodynamic investigation of lithospheric drip magmatism beneath the East African RiftContinental rifting is a fundamental part of the Wilson Cycle that defines the geological paradigm of plate tectonics. In this context, however, the relationships among magmatism, lithospheric thinning and crustal extension are poorly constrained. The PIs will evaluate the role of lithospheric drip magmatism as a trigger for successful continental rifting by studying alkaline mafic lavas from the Bufumbira and Toro Ankole volcanic fields in western Uganda. A growing body of evidence suggests that density instabilities in the lowermost lithosphere can undergo abrupt localized destabilization in response to regional thermal or mechanical stress. This physical thinning of the lithosphere, occurring on a scale of 20-50 km, enhances both magmatism and the likelihood of extensional failure, i.e., successful continental rifting, and thus controls the ultimate shape of continental margins. The work couples detailed geochemical analysis of proposed drip-related lavas and their accompanying mantle xenoliths with geophysical modeling of lithospheric drip and the ensuing crustal extension. The implications of this work to the understanding of continental extension represent a new approach to predicting the locus of new ocean basins and the forces required to produce them. At a regional scale, this work is of high importance to nations along the East African Rift: drip magmatism does not sustain the shallow long-lived magma chambers that are necessary for viable geothermal energy production. In terms of human resources, this project supports female researchers from a wide range of educational institutions as well as graduate and undergraduate students in geochemistry and geodynamics, and brings a short-course in geophysics to geology undergraduates who seek careers in this field. The lithospheric mantle is foundational - literally and conceptually - to the construction, destruction and division of tectonic plates. Modification of the lithospheric mantle by magma and fluids affects its composition and, by extension, buoyancy. If the bottom of the lithosphere is made denser, the base of the lithosphere can detach and sink, leading to small-volume basaltic volcanism, lithospheric thinning, and potentially rifting. Geochemical data on lavas suggest lithospheric drips drive volcanism in several parts of the East African Rift. Also, xenoliths from this area show significant metasomatism by melts and fluids, and lithospheric removal has been invoked to explain the timing and rates of recent uplift. In order to test this hypothesis, the role of lithospheric drips will be evaluated by: 1) obtaining detailed petrographic and mineral-scale compositional data on lavas and pyroxenite xenoliths, 2) calculating the physical conditions recorded by lavas and xenoliths, and 3) applying those calculations to 2D and 3D geodynamic models to whether lithospheric drip tectonics and melting explain the rocks themselves. Lithospheric drip melting is somewhat paradoxical, because cold descending lithosphere should not melt unless it is heated through conduction more rapidly than it becomes compressed during descent. It may be that additional complexity to symmetrical dripping is required, such as edge-driven convection during rifting or a nearby upwelling plume. This project will promote collaboration between 3 U.S. universities, including an REU institution, and provide opportunity for researchers from Uganda.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.