Dyke swarms of the Ethio-Sudanese lowlands
In construction... the problem is that there
are two many things to say. For the moment, here is a synthesis of some
results obtained on the trap-feeding dykes observed on the Abyssinian
plateau, which consists of the lava pile, and the Ethio-Sudanese lowlands,
where the lava pile has been eroded over 700-1000 m vertically down to
its base (locally to the basement).
See the Publications
page for more details (especially the two papers by Mège and Korme,
2004, and Mège, 2009). The poster
presented at IDC-6
gives a visual summary.
It includes all the reliable dyke ages obtained to date
by Nicolas Arnaud at Gésciences Montpellier. We will publish these
data in a peer-reviewed paper as soon as possible.
In Abyssinia, many dykes are observed
despite the lava cover, but can usually not be followed over long
distances. Most dykes are intensely altered by hydrothermalism. Nevertheless,
in central Abyssinian, reliable ages of 30.97±0.41 and 30.7±0.36
Ma have been obtained for a dyke from a swarm in the Lalibela region,
and another one north of the Angareb ring complex. Along the Afar
margin, slightly younger Oligocene ages have also been found, as well
as lower and upper Miocene ages, respectively prior to and during
the birth of the Afar depression. Remarkably, no dyke has been reported
to cross the Blue Nile river valley, which incises the Abyssinian
lava pile over a distance of 400 km and a depth of 1000 m, emphasizing
uneven feeder dyke distribution in the volcanic province.
The Ethio-Sudanese plain West of Lake Tana
displays thousands of dykes. The largest swarm, called the
Serpent-God Dyke Swarm, trends NE-SW and includes thick (5-20 m) silicic
dykes and thinner (< 5 m) mafic dykes outcropping over distances
up to 30 km that could be dated 31.3±0.36 to 29.3±0.98
Ma. The Serpent-God Dyke Swarm dyke swarm seems to follow the Precambrian
Tulu Dimtu shear zone, suggesting that the lithospheric fabric and
basal topography may have played a role in trapping the melts. Analysis
of dyke population length distribution suggests that the mafic dykes
less than 10 km long in the present topography may not have erupted.
The distribution law for mafic and silicic dyke populations does not
have the same coefficient and exponent, perhaps due to a different
mode of emplacement. The silicic dykes could originate from crustal
melting whereas the mafic dykes originate from the mantle, a duality
also noted in the geochemistry of the erupted lavas. Dyke thickness
is related to length following a square-root power law, demonstrating
that elastic fracture mechanics can account for the geometrical aspects
of their emplacement, and that the emplacement depth was shallow enough
for fracture toughness-based analysis to make sense. Field evidence,
anisotropy of magnetic susceptibility measurements, and high fracture
toughness, indicate a syn-emplacement component of shearing, perhaps
in response to the slight orientation difference between the orientation
of the principal stress field during emplacement and the orientation
of the Precambrian crustal fabric. The Ethio-Sudanese plain is intruded
by many other mafic dykes displaying a complex geometry, denoting
local-scales variations in stress field orientation, and perhaps proximity
of buried magma chambers. Those dykes are younger, probably upper
Oligocene or Miocene.
Finally, an undated N-S-trending swarm of at least
a dozen of long (tens of km) and thick (up to 20 m) dykes, mainly
mafic, remains poorly characterized due to difficult field access.
Existence of many other dykes North of a Gonder-Metema line
is evident but for the same reason they remain unexplored.
This work has been carried out with Nicolas Arnaud (Univ. Montpellier
II), Hervé Diot (Univ. La Rochelle), Tesfaye Korme (RCMRD), and
Tewodros Rango (Univ. Ferrara).