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

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 pseudo-brookite exsolutions in titanomagnetite (TM) grains, synonymous of an advanced stage of high-temperature (i.e. deuteric) oxidation. In contrast, the Debre Sina trachybasalts may have been submitted to low-temperature (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 +/- 8ZAm(2)) than the Middle Miocene (65 +/- 20ZAm(2)) 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.