Paleomagnetism and Geochronology in Ethiopian Afar: Constraints on Reversal Ages, Inclination Shallowing in Lava and Long Term Geometry of the Geomagnetic Field, and Determination of a 2Ma Reference Pole for Stable Africa

Joint French-Ethiopian field trips in 1995-1996 focused on the stratoid (flood-like) basalts in Ethiopian Afar, yielding new geochronologic and paleomagnetic data which significantly expand our knowledge of the recent magmatic and tectonic history of the Afar depression, where the East African rift meets the terminations of the Red Sea and Gulf of Aden ridges. Twenty four new K-Ar ages range from 0.6 to 3.3 Ma. There is quite good agreement between magnetic polarities and the Geomagnetic Reference Time Scale (GPTS) of Cande and Kent (1995). Eight age determinations with uncertainty less than 50 kyr could be used in future reassessments of the GPTS (notably for the upper and lower Olduvai/Matuyama reversals, and the Reunion and Mammoth subchrons). Paleomagnetic analysis of 865 cores from 133 sites confirms that low-Ti magnetites are the main carrier of the Characteristic Remanent Magnetization (ChRM). A positive tilt test (based on two subgroups with respectively 63 and 23 sites) confirms that this ChRM is most likely the primary magnetization. The main paleomagnetic results include 2 Ma reference pole for stable Africa (l__ = 87.2 ^oN, l__ = 217.1 ^oE ;A95 = 4 ^o), based on 26 sites located on either side of the northern termination of the East African rift. This pole is in agreement with the synthetic African pole of Besse and Courtillot (2001). A 4.6 ^o+/-1.8 ^o (2 sigma) inclination anomaly (shallowing) is identified within a population of 231 stratoid lava flows, consistent with a global axial quadrupole of 6%+/-2% of the axial dipole, and consistent with recent 0-5 Ma global field models (e.g. Carlut and Courtillot, 1998). Tectonic implications of these observations are reported elsewhere.