The geological evolution of East Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 15

kilomeres. Rising above the plains are occasional
isolated hills called inselbergs. Eastern Africa has
its share of such monotonous landscapes,
although the Neogene-Recent rift tectonics and
volcanism have so profoundly changed its face
that some spectacular scenery is found in
counries such as Burundi, Rwanda, Uganda,
Ethiopia, Kenya, Tanzania and Malawi. Struc-
tural upheavals of this order (Figure 2.6) have not
affected Central and West Africa as recently or to
the same extent, so that the plainlands predomin-
ate there.

Kent (1972) asserts that the greatest crustal
adjusments of the East African mainland coast
took place during the Karroo when the moden
sructure ofthe fault-controlled coast was blocked
out. No later fauling has approached its
magnitude.
This period of rifing in East Africa occurred at
the same ime as the separation ofAfrica from the
Antarcic (Sowerbutts, 1972), and the huge out-
pourings of basalt in southern Africa.

Age of block fauling

Opinions regarding the age of the various block-
fault mountains in easten Tanzania vary. Qu en-
nel et al. (1956) regard the faulting in easten and
central Tanzania which produced the fault blocks
of the Pare, Uluguru and Usambara mountains as
being of Neogene age. However, east of the Ulu-
guru Mountains, the faulted edge of the Karroo
sedimentary basin coincides with the bounday
fault of the Ulugurus, and the mineralogical com-
posiion of the coarse-grained sediments indicates
that the source area lay to the west (Sampson &
Wright, 1964). This implies that the Uluguru
Mountains fault block had started to form a dis-
incive unit as far back as the Karroo. It seems
reasonable to suggest that the other fault blocks
and the Southern Highlands developed over tens
and even hundreds of millions of years. Probably
short periods of intense uplift associated with
major periods of verical movement on a con-
tinental scale along ancient Precambrian
faultlines were separated by long periods of
quiescence. The blocks were probably iniiated in
the Karroo period (290-180 myr BP), con-
temporaneous with the iniial breakup of Gond-
wanaland. Then, very long erosive phases,
interrupted by widespread but less significant
verical movements, planed off the landscape to
form the African and younger erosion surfaces
(King, 1967). Finally, during the development of
the East African Rift System the faults were again
reacivated (7-0 myr BP) and the moden moun-
tain blocks were the result.

Karoo ifing
During Karroo times rit auling prevailed and
produced horsts and grabens over much of the
country. Sediments accumulated in elongated
basins largely under terresrial condiions,
although openings to the sea took place to the
northeast in the Ruvu Valley basin area and to the
southeast in the Mandawa area (Kent et al. , 1971;
Kajato, 1982).
The orientaion ofthe rift fauling is often con-
cordant with pre-eising Precambrian sructural
directions, a coincidence widely recognised in
East Africa (King, 1970). For example, in westen
Tanzania, Karroo basins followed the trend ofthe
Precambrian northwest-southeast Ubendian oro-
genic belt and in tun were the forerunners of the
Westen Rift troughs of Lakes Tanganyika and
Rukwa. In conrast, in easten Tanzania Karroo
basins are generally elongated in a meridional
direcion concordant with the dominant trend of
the Mozambique Belt. For example, Karroo sedi-
ments flank the Uluguru Mountains to the east
and west.
The magnitude of the Karroo rifing is
highlighted by several authors. Dixey (1956)
shows that the Lake Nyasa basin started as a rift
n Karroo imes and coninued developing during
the Mesozoic. Kent et al. (1971) ideniied 3000
m of Karroo evaporites which accumulated in a
graben largely controlled by reacivated basement
auling (Kent, 1972). At the end ofKarroo imes
there was urther down-fauling which accounts
or the preservaion of Karroo sediments in many
places (Quennel, McKinlay & Aitken, 1956).