Paleomagnetism of the Fentale-magmatic segment, main Ethiopian Rift: New evidence for counterclockwise block rotation linked to transtensional deformation

We carried out the first ever paleomagnetic investigations in the area around the Fentale volcanoes, Main Ethiopian Rift. A total of 28 paleomagnetic sites were sampled around the two Fentale (Tinish and Tilik) volcanic centers. Lava and pyroclastic flows ranging in age from 2 Ma to 0.1 Ma was sampled for this study. After routine paleomagnetic collection and sample preparation procedures specimens were subjected to progressive alternating fields and thermal demagnetizations. The directional analysis shows either one or two components of Natural Remanent Magnetization to characterize the entire measurements. The low stability components are removed either by alternating fields up to 15 mT or by heating up to 250 degrees C. The high stability and primary components are isolated above those steps and vector is directed towards the origin, and it represents the Characteristic Remanent Magnetization. The principal component analysis and site means directions calculations yielded normal and reversed polarities together with two anomalous directions. When an overall mean direction is calculated from the individual site mean directions for the Fentale region, D-s = 354.3 degrees, I-s = 19.3 degrees, N = 27, K = 33, alpha(95) = 4.9 degrees are obtained. When this result is compared with the mean expected reference dipole geomagnetic field direction (D = 1.0 degrees, I = 16.4 degrees, alpha(95) = 2.3 degrees, K = 105.6, N = 32) for an average age of 1.5 Ma, the following differences of Delta D = -6.7 degrees +/- 4.3 degrees, Delta I = -2.9 degrees +/- 4.3 degrees can be obtained. These results from the region indicate similar to 7 degrees counterclockwise block rotations about vertical axis consistent with the transtentional deformations that characterize the Fentale magmatic segment. (C) 2009 Elsevier B.V. All rights reserved.