The geological evolution of East Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 16

The breakup of Gondwanaland
Fragmentaion of the superconinent took place
over a protracted period and there is controversy
over the exact dates at which separaion of the
consituent coninents took place. Sowerbutts
(1972) proposes that Antarcica and Africa
separated during the Karroo, while South
America is believed to have moved away from
Africa in the Lower Cretaceous, 125 myr BP
(Sclater & Tapscott, 1979) or 120 myr BP

(McConnell, 1977).
On the basis of diverging pole posiions for
India, Antarcica, and Africa for the mid-Creta-
ceous, McElhinny (1970) shows that the Indian-
Madagascar-Antarcic block broke away from
Africa between the � id-Jurassic and mid-Creta-
ceous, opening up the Indian Ocean for the first
ime. This fits with the geological evidence from
the East African coast already cited. Sowerbutts
(1972) views the Jurassic-Cretaceous rifing in
East Africa as being associated with the breakup
of Madagascar and Africa. Rabinowitz, Coffin &
Falvey (1983) are more precise and state that the
Africa-Madagascar separaion began 165 myr BP
and ended 121 myr BP. They note that the Africa-
Madagascar separation thus began at about the
same ime as the iniial breakup of Gondwana-
land, with the separaion of North America from
Africa (Sclater & Tapscott, 1979). Subsequently,
Antarcica parted from the Indian-Madagascar
block 100 myr BP and the Madagascar-Indian
subconinents separated from each other about
the ime of the Mesozoic-Teriary boundary 65
myr BP (McElhinny, 1970).
The palaeoposiion of Madagascar in the
Gondwana superconinent has been a subject of
intense debate over the last few decades, with
three possibiliies proposed (Figure 2.7).

Northern position

Southern .
position

position

Figure 2.7. Three altenaive palaeoposiions of Madagascar.

3. Its present posiion with respect to Africa
(Kent, 1972; Kuina, 1975; Kamen-Kaye,
1982).

The geological evidence supporing all of these
posiions is equivocal; however, physiographic,
palaeomagneic, and other geophysical evidence
avours the northerly posiion (Rabinowitz 1971;
Embleton & McElhinny, 1975, 1982; Rabinowitz
et al., 1983).

Plainlands of East Africa
In easten Africa a series of plains is found, vary-
ing from the remnant bevelled summits of the
Livingstone and Uluguru mountains to the exten-
sive areas of the Central Plateau in Tanzania, and
the smooth uplifted surfaces bordering on the Rift
Valley in Kenya. Down-fauling and down-warp-
ing of the coastal region accompanied the inland
uplifts. Marine transgression (an extension ofthe
seawater over a former land area) marked the
breakup of Gondwanaland, establishing a stable
coninental margin facing the developing Indian
Ocean (Kent, 1974) (Figure 2.8).
Because the verical movements characterisic

1. A northerly posiion adjacent to Somalia,
Kenya and Tanzania (du Toit, 1937;
Smith & Hallam, 1970; Dietz & Holden,
1970; Embleton & McElhinny, 1975;
Rabinowitz et al., 1983).
2. A southerly posiion adjacent to
Mozambique (Wegener, 1929; Wellington,
1954; Flores, 1970, 1984).