Subsurface Images of the Eastern Rift, Africa, from the Joint Inversion of Body Waves, Surface Waves, and Gravity: Investigating the Role of Fluids in Early-Stage Continental Rifting
Loading...
Date
2017-05-19
Journal Title
Journal ISSN
Volume Title
Publisher
Oxford
Abstract
The Eastern Rift System (ERS) of northern Tanzania and southern Kenya, where a cratonic lithosphere is in the early stages of rifting, offers an ideal venue for investigating the roles of magma and other fluids in such an environment. To illuminate these roles, we jointly invert arrival times of locally recorded P and S body waves, phase delays of ambient noise generated Rayleigh waves and Bouguer anomalies from gravity observations to generate a 3-D image of P and S wave speeds in the upper 25 km of the crust. While joint inversion of gravity and arrival times requires a relationship between density and wave speeds, the improvement in resolution obtained by the combination of these disparate data sets serves to further constrain models, and reduce uncertainties. The most significant features in the 3-D model are (1) P and S wave speeds that are 10–15 per cent lower beneath the rift zone than in the surrounding regions, (2) a relatively high wave speed tabular feature located along the western edge of the Natron and Manyara rifts, and (3) low (∼1.71) values of Vp/Vs throughout the upper crust, with the lowest ratios along the boundaries of the rift zones. The low P and S wave speeds at mid-crustal levels beneath the rift valley are an expected consequence of active volcanism, and the tabular, high-wave speed feature is interpreted to be an uplifted footwall at the western edge of the rift. Given the high levels of CO2 outgassing observed at the surface along border fault zones, and the sensitivity of Vp/Vs to pore-fluid compressibility, we infer that the low Vp/Vs values in and around the rift zone are caused by the volcanic plumbing in the upper crust being suffused by a gaseous CO2 froth on top of a deeper, crystalline mush. The repository for molten rock is likely located in the lower crust and upper mantle, where the Vp/Vs ratios are significantly higher.
Description
Keywords
Composition and structure of the continental crust, Africa, Joint inversion, Tomography, Crustal imaging, Continental tectonics: extensional
Citation
S. Roecker, C. Ebinger, C. Tiberi, G. Mulibo, R. Ferdinand-Wambura, K. Mtelela, G. Kianji, A. Muzuka, S. Gautier, J. Albaric, S. Peyrat, Subsurface images of the Eastern Rift, Africa, from the joint inversion of body waves, surface waves and gravity: investigating the role of fluids in early-stage continental rifting, Geophysical Journal International, Volume 210, Issue 2, August 2017, Pages 931–950, https://doi.org/10.1093/gji/ggx220