Magnetostratigraphy and timing of the Oligocene Ethiopian traps

A combined paleomagnetic, 40Ar/39Ar and geochemical investigation has been conducted in two type sections (65 sites) of the NW Ethiopian plateau volcanic pile. The main 2 km-thick complete section of the flood basalts at Lima-Limo contains a succession of only three magnetic chrons. The central normal chron appears to correspond to Chron C11n, according to the 40Ar/39Ar ages, which cluster around 30 Ma. Two alternative magnetostratigraphic interpretations both indicate a duration on the order of or less than 1 Myr. A detailed major element study of the section shows remarkable magma homogeneity, with two cycles of differentiation. Using correlation with other sections, we propose that the emplacement of the bulk of the Ethiopian igneous province (with an estimated volume on the order of 106 km3) in about 1 Myr or less should be linked with the Oi2 global cooling event, which occurs in Chron C11r. Further independent support for this identification comes from the finding of prominent tephra horizons in central Indian Ocean sediments from leg 115, with a biostratigraphic age coinciding with the Oi2 event. The 30 Ma African pole produced by this study (77°N, 208°E, A95=3.7°, N=53) is far-sided by 6°±6° with respect to the reference synthetic pole for Africa and yields a large paleosecular variation.