Palaeomagnetism of Palaeozoic glacial sediments of Northern Ethiopia: a contribution towards African Permian palaeogeography

Abstract

One hundred fourteen oriented palaeomagnetic core samples were collected from 13 palaeomagnetic sites on subhorizontal to tilted glacial sediments at five localities of Northern Ethiopia. Combined alternating field (AF) and stepwise thermal demagnetization techniques were successfully applied to resolve the complete directional spectrum. A viscous remagnetization (VRM) and one stable component of magnetization were identified in most of the specimens. The VRM is removed between a temperature range of 120–350 °C and AF of up to 30 mT. Further heating until ∼650 °C results in smooth decay of the natural remanent magnetization (NRM) intensity to about 50 per cent and the rest of the NRM is efficiently removed by heating to 690 °C, while only 30–50 per cent of NRM is removed by the maximum AF available suggesting haematite as remanence carrier. Results of the magnetization decay curve plots and rock magnetic analyses using the variable field translation balance indicated the presence of magnetite with minor goethite, pyrrhotite as well. The high stability component defining a straightline segment, starting 350 °C and/or 30 mT is mostly directed towards the origin and interpreted as the characteristic remanent magnetizations (ChRMs). The direction of magnetization is determined both by best-fitting line using the least-square technique of Kirschvink and remagnetization circles of Halls for few unresolved overlapping components. The site mean directions of the sediments from two sites are normal polarity and are close to present-day field directions at the sample site. The site mean directions from 11 sites, on the other hand, are reversed in polarity with better grouping in the tilt-corrected coordinate and pass the McFadden fold test. This overall site mean direction is Dec = 143.4º, Inc = 58.8º (N = 11, α95 = 9.7º) with a corresponding mean pole position of Lat = 26.0º, Lon = 249.5º (N = 11, A95 = 13.1º). This geomagnetic pole position is later rotated into West Africa coordinates to allow for extensional rifting in the Benue Trough about an Euler pole position, at 19.2ºN, 352.6ºE through an angle −6.3º (clockwise). The resulting pole position is located at ϕs = 246.6ºE, λs = 31.8ºS (N = 11, A95 = 13.1º), this pole with its 95 per cent confidence circle intersects the 270–310 Ma, segment of the APW path for West Africa consistent with ages of between late Carboniferous and early Permian. The result also implies that the Late Carboniferous Dwyka land ice sheet had probably extended more than 1000 km further north to Ethiopia than previously known.