Tectonics of Gedemsa Magmatic Segment: Insight from Paleomagnetic Investigation, Central Main Ethiopian Rift
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Date
2020-09-29
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Addis Ababa University
Abstract
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.
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Keywords
Characteristic Remanent Magnetization, Crustal Block Rotation, Magmatic Segment, Paleomagnetism, Tectonics