PALEOMAGNETIC EVIDENCE OF COUNTERCLOCKWISE ROTATION OF THE DOFAN MAGMATIC SEGMENT: IMPLICATIONS FOR THE TECTONIC HISTORY OF MAIN ETHIOPIAN RIFT

The movements of the Nubian, Arabian and Somalian plates over the past 30 million years have created a series of different tectonic regimes with their own distinct structural style. The Main Ethiopian Rift (MER) is characterized by left-lateral transtension due to an oblique divergence between Somalia and Nubia plates. The aim of this study was to obtain paleomagnetic directions of the Dofan magmatic segment (DMS) volcanic rocks to determine the tectonic deformational process along the MER. Twenty-six paleomagnetic sites from basalt and trachyte flows and ignimbrite deposits were sampled in the DMS, MER. The oriented core samples were cut into 200 standard specimens and their Natural Remanent Magnetization (NRM) directions were measured using a JR6A spinner magnetometer. Most specimens were subjected to the stepwise alternating field (AF) and at least one specimen per site to thermal (TH) demagnetization and isothermal remanent magnetization (IRM). After magnetic rock measurements and stepwise Th and AF demagnetization, most of the samples taken were characterized by one magnetization component after viscous magnetization removal. A characteristic remanent magnetization was obtained at 500–580 °C, showing evidence of magnetite and titanomagnetite with a small amount of Ti. However, in some cases, a high-temperature component was isolated between 600 and 640 °C, indicating a titanohematite composition. The mean direction obtained from the DMS (Dec=354.1°, Inc.=+11.6° (N=23, K=35.1, α95=5.2°)) indicated counterclockwise rotation compared with the 1.5 Ma expected mean geomagnetic dipole reference field directions of the African Apparent Polar Wander Path Curve (Dec=1.0°, Inc.=+16.4° (N=32, K=105.6, α95=2.3°)), obtained a difference in declination ΔD=−6.9° ± 4.7° and inclination ΔI=+4.8° ± 5.5° are determined. The declination difference is interpreted as a very slight counterclockwise crustal rotation with respect to vertical axis and the result is consistent with previous paleomagnetic reports and with the recent analogue models of reorientation of extension directions and pure extensional faulting at the oblique rift margins of the Main Ethiopian Rift (MER).