Paleomagnetic Study and Structural Mapping of Dofan Magmatic Segment of Northern Main Ethiopian Rift in Afar, Ne Ethiopia
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Date
2015-12
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Addis Ababa Universty
Abstract
The paleomagnetic investigation was carried out on outcropping Dofan volcanic rocks, southern Afar, Main Ethiopian Rift. The aim of this paleomagnetic study was to quantify vertical axis crustal block rotation and identifying the potential magnetic minerals carrier the remanence of rock sampled. Structurally the area is characterized by intense Quaternary faulting and fracturing. Two sets of faults are mapped in the study area namely; NNE–SSW and N-S trending youngest rift faults and few NNW–SSE striking rift margin normal faults.
A total of 26 paleomagnetic sites were sampled within the well exposed Dofan volcanic rocks (outcrop of basalt, ignimbrite and trachyte) for the first ever paleomagnetic investigations. After routine paleomagnetic sample collection and preparation procedures 170 specimens were subjected to stepwise progressive alternative field and thermal demagnetizations techniques. The careful directional analysis of individual specimens revealed one or two components of natural remanent magnetization (NRM) at the end of the demagnetization process as: 1) A pervasive overprint carried by low blocking temperature (<300°C) and low coercivity (<20mT) grains which complicate and contribute uncertainty to the interpretation of Characteristic remanent magnetization (ChRM) directions which is effectively, eliminated by alternative demagnetization 2) ChRMs are recognized above those steps. After removal of overprinted components most magnetization directions displaying straight lines directed toward the origin of the orthogonal plot. Paleomagnetic directional analysis derived from the 24 reliable sites in Dofan volcano are distributed in cluster about a normal polarity mean direction.The blocking temperature and coercivity ranges and isothermal remanent magnetization examination representative specimens indicates that the critical magnetic minerals that do have a significant response for paleomagnetism are pure magnetite as dominant remanence carrier mineralogy. The ChRM obtained at 500-580°C, showing an evidence of magnetite and titanomagnetite. However, the high temperature component was isolated between 600-640°C; indicating hematites are also present as subordinate constituents. The overall observed mean directions from 24 accepted sites calculated using high quality ChRM directions calculated by excluding the anomalous direction (DF7 and DF3 sites) (Ds = 351.8°, Is = 11.5°, N = 24, K = 21.4, α95 = 6.5°) compared with the expected mean geomagnetic dipole reference field directions of Africa for an average age of 1.5Ma. This result suggests that statistically significant rotation about the vertical axis has occurred at the Dofan locality which indicate∼9.2° counterclockwise crustal block rotations. This result is in prefect agreement with counterclockwise block rotation linked to transtensional deformation of the Fentale-magmatic segment reported by (Tesfaye Kidane et al., 2009).
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Paleomagnetic investigation