Structural and geochronological implications of the Fentale Volcanics at a nascent passive margin of the Main Ethiopian Rift: Constraints from magnetostratigraphy study at the Kereyou Lodge, Ethiopia

We sampled twenty-four cooling units for magnetostratigraphic study from a 110 m thick Early Pleistocene volcanic section near Kereyou Lodge in the Main Ethiopian Rift. Seven samples were used from each site for analysis of remanent magnetization using thermal and alternating field demagnetization techniques. The Natural Remanent Magnetization (NRM) direction comprises two vector components in most samples. The first, low-stability component is isolated generally by heating to 100 °C–250 °C or by demagnetization using an alternating field intensity of 10–15 mT. After these steps most magnetization directions define straight lines directed towards the origin, and record the primary NRM or ChRM (Characteristic Remanent Magnetization). Magnetization decay curve analyses show the possible magnetic minerals to be titanomagnetite, magnetite, titanohematite and hematite. Paleomagnetic site mean directional analyses revealed that upper 18 flows are of reversed polarity, and the bottom 4 flows are normal. Using available age information the normal interval is correlated to the Olduvai Subchron on the Geomagnetic Polarity Time Scale (GPTS) of Cande & Kent (1995) and the flows at the polarity transition are dated to be 1.77 Ma. The overall declination and inclination differences calculated ΔD = −4.8° ± 4.4°, ΔI = 0.8° ± 4.2°, are consistent with the 7° counterclockwise reported (Kidane et al., 2009). Site-mean inclinations show a systematic change with flow position. Inclinations get progressively shallower with depth for the top 17 volcanic flows and progressively steeper for the bottom 4 flows consistent with progressive backward and rift ward block tilting mechanisms respectively.

Research Highlights
► Identification of Olduvai Subchron and its geochronologic implications for the studied rock sequences.
► Inclination shallowing and steepening resulting from fault geometries.
► Vertical axis rotation due to oblique deformation.