Paleomagnetism of Gedemsa magmatic segment, Main Ethiopian Rift: Implication for clockwise rotation of the segment in the Early Pleistocene

Abstract

One-hundred-seventy oriented paleomagnetic core samples from 23 sites were collected from volcanic lava flows belonging to the Nazareth Group (1.7–1.9 Ma) and the Wonji group (≤0.3 Ma) along the Gedemsa magmatic segment (MS) in the, Main Ethiopian Rift (MER). Samples were subjected to progressive thermal (Th) and Alternating Field (AF) demagnetization for directional analyses, and representative samples were used for rock-magnetic experiments. The rock-magnetic study indicated that titano-magnetite, titano-hematite and magnetite are the primary carrier of remanent magnetization. Two components of Natural Remanent Magnetization (NRM) were revealed: the first, low stability component is removed by AF of 20-30mT or heating up to 200 °C. The second, high stability component isolated above these steps is directed toward the origin and considered Characteristic Remanent Magnetization (ChRM). All specimens we studied show normal geomagnetic polarity. The overall mean direction for the Wonji group is Decs = 0.60, Incs = 5.10 (α95 = 6.50, K = 35.8, N = 14) while for the Nazareth group is Decs = 18.90, Incs = 13.40 (α95 = 10.30, K = 35.6, N = 7). When these mean directions from the two groups are compared with their respective reference geomagnetic dipole fields (Besse and Courtillot, 1993, 2002), the former shows no significant block rotation and a clockwise rotation R = 170 ± 3.40 were detected in the latter. These results indicate the Gedemsa magmatic segment is characterized by complex deformation due to the opening of DebreZeit-Butajira (DzB) and Yerer – Tullu Wellel Volcano-tectonic Lineament (YTVL segments. Our study indicates that detailed paleomagnetic observations in the Rift are important to identify quantitative deformation history of the region.