| Abstrakt: |
The East African Rift System provides a rare location in which to observe a wide scope of rifting states. Well‐defined active narrow rifting in the Main Ethiopian Rift (MER) transitions to incipient extension and eventually pre‐rifted lithosphere through the northwestern flank of the Ethiopian Plateau (EP). Although the MER is well studied, the off‐axis region has received less attention. We develop Rayleigh wave phase velocity maps, Ps receiver functions, and H‐κ stack surfaces, and jointly invert these data using a trans‐dimensional, hierarchical Bayesian inversion algorithm to create shear velocity profiles across the MER and EP. All shear velocities observed are slower than the PREM global average, a reflection of the elevated temperatures that persist from plume impingement. In the EP, we find a shallow mantle slow shear velocity lineament parallel to the MER axis, amidst otherwise faster shear velocities. The crust is shallow in the MER, and also in the northwestern‐most EP flank. Thicker crust found elsewhere throughout the plateau is caused by crustal underplating and flood basalt emplacement. Shear velocities more reduced than the already low regional average, in concert with surficial volcanic features, geodetic observations, and slow P‐ and S‐wave anomalies, support off‐axis extension in the Ethiopian plateau, requiring reevaluation of the localization of continental breakup in the narrow MER. Plain Language Summary: Ethiopia is a unique location where a continent is breaking apart, and will eventually evolve into a narrow ocean. Whether extension occurs only within narrow magmatic segments, or is broadly distributed elsewhere in Ethiopia has yet to be fully determined. To answer this question, we use information from earthquake‐generated seismic waves to image the lower crust and mantle outside the expected bounds of continental breakup. We find evidence of extension well into the flanks of Ethiopia, in a rift‐parallel linear band. This lineament shows more evidence of thermal effects associated with rifting in the north, suggesting it is more mature, or experiences a greater degree of rifting there. Elevated temperatures, volcanic features, small magnitude GPS measurements, and observations of slow material deep beneath the surface all indicate that extension is more complex, and more broadly distributed, than previously assumed, and that rifting processes are not localized to geographic areas where their surface effects are most visible. Key Points: The Ethiopian Plateau's shear velocity structure is investigated by jointly inverting Rayleigh wave phase velocities and receiver functionsSlow off‐rift shear velocities indicate elevated lithospheric temperatures in the Ethiopian Plateau that may require in situ meltA slow axis‐parallel lineament indicates off‐rift extension in the Ethiopian Plateau, with increasing maturity towards its northern extent [ABSTRACT FROM AUTHOR] |
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