Palaeointensities of Oligocene and Miocene volcanic sections from Ethiopia: Field behaviour during the Cainozoic

Abstract

© 2018 The Author(s). We conducted an absolute palaeointensity (API) survey on Ethiopian volcanics from Early Oligocene (Belessa section) and Middle Miocene (Debre Sina section). After a careful selection based on the reversibility of high-field thermomagnetic curves from samples yielding unambiguous palaeodirections, we restricted our analysis to five (resp. six) cooling units from Belessa (resp. Debre Sina), submitted to both Thellier and Wilson-style experiments. X-ray diffraction measurements and microscopical observations under reflected light gave us good confidence in the reliability of the remanence carriers of the Belessa basalts, with pseudobrookite exsolutions in titanomagnetite (TM) grains, synonymous of an advanced stage of higherature (i.e. deuteric) oxidation. In contrast, the Debre Sina trachybasalts may have been submitted to lowerature (i.e. post-consolidation) alteration featured by granulation of TM grains and leading to a higher dispersion of API estimates. Compared to the 'world palaeointensity database', the lower dipole moment found in Ethiopia for the Early Oligocene (30 ± 8 ZAm2) than the Middle Miocene (65 ± 20 ZAm2) is compatible with a global increase of dipole strength from 30 Ma to present, and also suggestive of a dipole low during the Oligocene. This observation must however be treated with caution because of the over-representation of Plio-Pleistocene determinations in the global database and the partial correlation between dipole strength and number of palaeointensity determinations.