Strong clockwise block rotation of the Ali-Sabieh/Aïsha Block: evidence for opening of the Afar Depression by a ‘saloon-door’ mechanism
Tesfaye Kidane, 2016. "Strong clockwise block rotation of the Ali-Sabieh/Aïsha Block: evidence for opening of the Afar Depression by a ‘saloon-door’ mechanism", Magmatic Rifting and Active Volcanism, T. J. Wright, A. Ayele, D. J. Ferguson, T. Kidane, C. Vye-Brown
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Palaeomagnetic investigations were carried out on tilted Jessoma sandstones in the Aïsha block. The average natural remanent magnetization intensity was 0.026A m−1 with good behaviour on demagnetization both by thermal and alternating fields. The normalized magnetization intensity decay curves indicated that fine-grained, Ti-poor titanomagnetites were the carriers of magnetization. A well-defined single-component magnetization with a poorly defined secondary component was found. The latter was removed at temperatures below 300°C or at 10 mT. After these steps, a straight-line segment directed towards the origin was identified, which was interpreted as the characteristic remanent magnetization. Site mean direction coordinates of Decg=227.8°, Incg=–33.9° (n=7, α95=2.8°, K=478) (in situ) and Decs=225.6°, Incs=0.4° (n=7, α95=2.8°, K=478) (tilt-corrected) were obtained. Corresponding mean virtual geomagnetic pole coordinates of Long=117.5° E, Latg=43.3° N (A95=3.2°, n=7) (in situ) and Lons=143.5° E, Lats=43.3° N (A95=2.8°, n=7) (tilt-corrected) were obtained. The corrected in situ mean pole position was compared with the African apparent polar wander path curve and a deposition age for the sandstone of 60±5 Ma was determined, which is consistent with previous reports. A vertical axis rotation (R±ΔR) of the Aïsha block of 29.1±4.06° was obtained, which is equivalent to an inferred 30° counterclockwise rotation of the Danakil block, in agreement with a ‘saloon-door’ mechanism for the opening of the Afar Depression.
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A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a ten-day period, c. 2.5 km3 of magma were intruded along a 60 km-long dyke separating the Arabian and Nubian plates. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. This activity led to a renewed international focus on the role of magmatism in rifting, with major international collaborative projects working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This work has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.