Crustal section across the Main Ethiopian Rift showing the P-wave (compressional body wave) velocity structure in colors, highlighting the inferred locations of gabbroic intrusions in the lower and middle crust below the main magmatic segment. Credit: Ebinger et al., 2017, Figure 10b, from Keranen et al., 2004
Source: Tectonics

The Cenozoic East African rift, Cameroon Volcanic Line and Atlas Mountains all formed on the slow-moving African continent. Ebinger et al. [2017] evaluate a robust compilation of geophysical data to evaluate the role of magmatism in shaping these crustal features in Africa. Within the southernmost Eastern rift, the authors find that crust comprises about 20 per cent of new magmatic material that has ponded in the lower crust sills, and intruded as sills and dikes at shallower depths. In the Main Ethiopian rift, intrusions comprise 30 per cent of the crust below axial zones of dike-dominated extension. Meanwhile, in the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges.

This work suggests that transitional crust, including seaward-dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15 to 20 kilometer thick crust. The mid-Cenozoic to recent Cameroon Volcanic Line lacks a hotspot age-progression and is associated with small tensile stresses, which inhibit the creation and rise of magma into the crust. The authors interpret their new results to indicate that magma and volatiles are migrating from the asthenosphere through the African lithosphere, modifying rheology and contributing significantly to global carbon and water fluxes.

Citation: Ebinger, C. J., Keir, D., Bastow, I. D., Whaler, K., Hammond, J. O. S., Ayele, A., Miller, M. S., Tiberi, C. & Hautot, S. [2017]. Crustal structure of active deformation zones in Africa: Implications for global crustal processes. Tectonics, 36.

—John Geissman, Editor-in-Chief, Tectonics

Correction, 10 January 2018: The figure caption has been updated to credit the original source of the figure.

Text © 2018. The authors. CC BY-NC-ND 3.0
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