Paleomagnetic evidence of early Pleistocene counterclockwise rotation in the Butajira volcanic zone, central Main Ethiopian rift

Abstract

A total of 208 paleomagnetic core samples were collected from twenty-seven sites within the Butajira volcano chains of the central Main Ethiopian rift (MER), providing insights into tectonic rotation. Core samples were taken from ignimbrite (∼1.7–2.54Ma), basalt, and trachyte rock units (<1.6Ma) along the western margin of the central MER. Each core sample underwent analysis using twin specimens subjected to stepwise alternate field (AF) and thermal (Th) demagnetization techniques. Furthermore, a single specimen from every site was employed for rock-magnetic experiments. Detailed rock magnetic experiments identified titano-magnetite, magnetite, and hematite as the remanence carrier magnetic minerals. The natural remanent magnetization (NRM) directional analysis revealed two components:1) a low-stability (LS) component removed by low-temperature (<300 °C) and low-field (<20 mT) and 2) a high-stability (HS) with heating >300 °C or 20 mT increments showed vectors pointing to the plot's origin, indicative of the characteristic remanent magnetization (ChRM). All sites, except one, displayed reverse polarity NRM. The tilt-corrected mean direction calculated from 19 sites, yielded D_o = 355.7°, I_o = 9.9° (N = 19, K = 22.9, α₉₅ = 7.3°), indicates a 6.2° ± 6.2° counterclockwise (CCW) tectonic rotation relative to the reference dipole geomagnetic field direction of 2Ma D_x = 1.9°, I_x = 15.5° (N = 32, K = 105.6, α₉₅ = 2.5°) for stable Africa (Besse and Courtillot, 2003). The oblique rifting and pure-dip slip kinematics that characterize the border faults in the study area could have caused this minor counterclockwise rotation and agrees with analogue model prediction of Corti et al.,2013.