Abstract
The northern Ethiopian rift forms the third arm of the Red Sea, Gulf
of Aden triple junction, and marks the transition from continental
rifting in the East African rift to incipient oceanic spreading in
Afar. We determine the P- and S-wave velocity structure beneath the
northern Ethiopian rift using independent tomographic inversion of
P- and S-wave relative arrival-time residuals from teleseismic earthquakes
recorded by the Ethiopia Afar Geoscientific Lithospheric Experiment
(EAGLE) passive experiment using the regularised non-linear least-squares
inversion method of VanDecar. Our 79 broad-band instruments covered
an area 250 x 350 km centred on the Boset magmatic segment \~70
km SE of Addis Ababa in the centre of the northern Ethiopian rift.
The study area encompasses several rift segments showing increasing
degrees of extension and magmatic intrusion moving from south to
north into the Afar depression. Analysis of relative arrival-time
residuals shows that the rift flanks are asymmetric with arrivals
associated with the southeastern Somalian Plate faster (\~0.65
s for the P waves; \~2 s for the S waves) than the northwestern
Nubian Plate. Our tomographic inversions image a 75 km wide tabular
low-velocity zone (dVP=1.5 per cent, dVS=4 per cent) beneath the
less-evolved southern part of the rift in the uppermost 200-250 km
of the mantle. At depths of >100 km, north of 8.5N, this low-velocity
anomaly broadens laterally and appears to be connected to deeper
low-velocity structures under the Afar depression. An off-rift low-velocity
structure extending perpendicular to the rift axis correlates with
the eastern limit of the E-W trending reactivated Precambrian Ambo-Guder
fault zone that is delineated by Quaternary eruptive centres. Along
axis, the low-velocity upwelling beneath the rift is segmented, with
low-velocity material in the uppermost 100 km often offset to the
side of the rift with the highest rift flank topography. Our observations
from this magmatic rift zone, which is transitional between continental
and oceanic rifting, do not support detachment fault models of lithospheric
extension but instead point to strain accommodation via magma assisted
rifting.
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