Tectonics of Gedemsa Magmatic Segment: Insight from Paleomagnetic Investigation, Central Main Ethiopian Rift

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Addis Ababa University


The paleomagnetic and tectonic investigation was conducted on Gedemsa Magmatic Segment, Central Main Ethiopian Rift. The main objective of this study was to investigate crustal block rotation of the Gedemsa magmatic segment and to identify the magnetization carrier minerals in the sampled rock units. To address the set scientific objectives, a total of 170 core samples (from 23 sites) were sampled and analysed from the outcrop of basalts, ignimbrites, volcanic tuffs and rhyolite units. From these samples a total of 243 specimens were prepared. The specimens were then subjected to progressive demagnetization to isolate the ChRM from the secondary overprint magnetization, and impulse magnetization (23 specimens) to identify magnetization carrier minerals. Rock magnetic experiments show that the dominant magnetic minerals are titanomagnetite, magnetite, titano-hematite and small amount of hematite. The demagnetization analyses reveal two components (low stability and high stability components) of Natural Remanent Magnetization (NRM). Demagnetization process removes the low stability components by applying up to 30mT or by heating up to 520oC. The high stability ChRM components were isolated above those steps and the ChRM vector direction characterized to be directed towards the origin with a straight line segment. All the selected sites show normal polarity. An overall mean direction calculated from individual accepted site mean directions were D=359.10, I=9.60, N=20, K =20, α95=7.50. When these observed mean directions are compared with the mean expected reference geomagnetic field direction for stable Africa (D=10, I=13.50, N=32, K=105.6, and α95 =2.5), from the apparent polar wander path reference curve for Africa, found to be statistically different in small amount. This suggests that a small amount of counterclockwise crustal block rotation (~30) about the vertical axis has occurred at the Gedemsa Magmatic Segment relative to stable Africa. The result is in agreement with most of other paleomagnetic investigations which advocates an increasing magnitude of counter-clockwise vertical axis rotation towards the north which characterised the heterogeneous deformation of the Main Ethiopian Rift. The study area is characterised by normal faults with NNE-SSW, NE-SW and some N-S orientation which are part of the WFB.



Characteristic Remanent Magnetization, Crustal Block Rotation, Magmatic Segment, Paleomagnetism, Tectonics