Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift

Abstract

Lithospheric thinning is a fundamental process associated with the transition from continental to oceanic regimes during continental rifting. Precisely how and when this lithospheric thinning proceeds are first order controls on rift basin evolution. The Main Ethiopian Rift, part of the ∼ 2000 km long East African Rift System, is the archetypical modern example of continental rifting, and a key location in which to study the evolution of the lithosphere during extension. This study explores lithospheric modification in the interface region between the Main Ethiopian rift and northward propagating rifting from Kenya through a major and trace element study of rift initiation and maturation using the 19 myr magmatic record preserved in this region. Initial rifting in southern Ethiopia is coincident with the eruption of basalts along the rift shoulders that are characterized by deep fractionation trends (0.5 GPa) and poorly developed magmatic pathways. The earliest of these basalts are derived from melting columns where the aluminum phase is garnet-dominated (Tb N/Yb N ∼ 1.8–2) and has geochemical characteristics interpreted as melting of the lithospheric mantle. The transition from initial rift shoulder magmatism to Quaternary magmatic–tectonic fault belts on the modern rift floor at Arba Minch (6°N) is coincident with a shallowing of the melting column (Tb N/Yb N ∼ 1.3–1.7), less significant contributions from the lithospheric mantle, and the establishment of a shallow fractionation regime (0.1 GPa). At Chencha (∼ 6.3°N) newly dated (12.32 ± 0.17 Ma) magmatism on the rift shoulder has similar fractionation paths to contemporaneous magmatism to the south (0.5 GPa), but is derived from a different, shallower mantle source (Tb N/Yb N ∼ 1.3–1.5) that we interpret results from lithospheric thinning associated with the now-inactive Chow Bahir rift. Between 6.5 and 8°N, significant surface faulting and shallow magmatic fractionation paths (0.1 GPa) in the dominant Quaternary structure of the Main Ethiopian Rift (the Wonji Fault Belt and Silti-Debre Zeyit Fault Zone), highlights the strong connection between magmatism and extensional tectonics in these structures. Along the eastern rift margin, Wonji Fault Belt magmas are derived from a dominantly shallow melting column (Tb N/Yb N ∼ 1.4–1.7) that is similar in composition to the older rift shoulder lavas at Chencha. Adjacent to the western rift margin, magmas erupted in the Silti-Debre Zeyit Fault Zone are interpreted to have erupted through a thicker lithosphere as these magmas are derived from a deeper melting column (Tb N/Yb N ∼ 1.7–2.1) that contains some minor apatite. The inferred variations in lithospheric thickness in southern Ethiopia outlined in this study illustrate the interaction between northward and southward rift propagation in addition to lateral variations across the rift floor as extension migrates into zones of focused magmatic intrusion. The results of this investigation show that geochemical techniques can be applied to probe the history of lithospheric modification during rifting and provide new constraints for models of rift development.