Paleomagnetism and geochronology of Oligocene and Miocene volcanic
sections from Ethiopia: geomagnetic variability in the Afro-Arabian
region over the past 30 Myr
Abstract
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.