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Paleomagnetism and geochronology of Oligocene and Miocene volcanic sections from Ethiopia: geomagnetic variability in the Afro-Arabian region over the past 30 Myr
  • Florian Lhuillier,
  • Stuart Gilder
Florian Lhuillier
Ludwig Maximilians University of Munich

Corresponding Author:[email protected]

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Stuart Gilder
Ludwig Maximilian University of Munich
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We report palaeomagnetic and K-Ar geochronologic results of two volcanic sections from Ethiopia. One section, dated around 29-30 Ma and spanning ~1 km in thickness, is related to the Oligocene Afro-Arabian traps. The second ~700-m-thick section was emplaced during the Miocene in two pulses around 10-11 and 14-15 Ma. We sampled 67 flows (550 cores) of predominantly basaltic rocks at the Oligocene section and 59 rhyolitic to trachybasaltic flows (500 cores) at the Miocene section. The Oligocene section was correlated to subchrons C11r to C11n.1n, with an average emplacement rate of 1m/kyr and instantaneous rates increasing with time from ~0.5 m/kyr near the base to ~1.36 m/kyr towards the top. We combined our results to the available paleomagnetic studies for the Early Oligocene (N = 4; 167 sites), Middle Miocene (N = 2; 125 sites) and Plio-Pleistocene (N = 8; 249 sites) to better understand how geomagnetic secular variation changed through time in the Afro-Arabian region. Recentred directional distributions for all three periods are elongated in the meridian plane (e = 2.7 ± 0.4, 1.8 ± 0.6, 2.3 ± 0.4, respectively), in coherence with field models for a dipole-dominated field. The angular dispersion S of the virtual geomagnetic poles, representative of the vigour of the palaeosecular variation, was higher during the Early Oligocene (S=14.2°|13.2°15.4°) and the Middle Miocene (S=15.0°|13.8°16.5°) than during the Plio-Pleistocene (S=9.7°|9.0°10.5°). As the reversal frequency f during the Early Oligocene is half that for the Plio-Pleistocene, it appears that S and f are uncorrelated in this near-equatorial region.