590 
Mwembeshi dislocation zone, De Swardt et al. 1965, on 
the one hand imposes a NE azimuth to the rift axis, 
the Ubende chain, on the other, deturns the axis to 
wards the NW) and lots of downfaulted areas as Lake 
Mweru (eastern couple) and its southern extension in 
the Luapula River (western couple). 
Surprisingly similar pictures arise from the analy 
sis of the northern Andes (Colombia, fig. 2b). The 
Magdalena River flows from the south to the north 
linking a string of true grabens, half-grabens and 
faulted synclines, each segment of which in turn is 
limited by ENE and/or SE lineaments and faults. The 
Magdalena rifts have another particular characteristic 
in common : the individual segments are disposed in 
typical en-échelon fashion and some of the individual 
échelons are linked by tails protruding as either ENE 
or SE lineaments. 
The latter features are easily recognised in Central 
Africa too (fig. 2a) : the geometry and shape of Lake 
Mweru Wantipa is one of a sigmoid rhomb, the main 
body of which strikes NE. The northern and southern 
tails extend as ENE lineaments. This eastern couple 
of lineamental directions is repeated in downfaulted 
(?) swampy areas (Luwala and Bwela swamps). 
Quartz dykes, tens of kms long, have the same re 
lationships to the confining ENE and SE lineaments as 
the NNE and NNW Phanerozoic rifts have (fig. 2a). The 
absence of regional compression in that part of 
northern Zambia (Mporokoso village) rules out a com 
pressive stress field-related origin of the dykes. 
They are here interpreted as the products of dilatan- 
cy (Jaeger & Cook 1976) on the megascopic scale rela 
ted to the formation of tensional joints and linea 
ments parallel to the axis of maximum horizontal 
stress. 
11. Chimbwe 
Figure 2a. Central Africa : schematical representation 
of spatial relationship between basic rhomb and ver 
tical tectonic elements. 
The examples suggest that rifts and vertical faults 
are linked to transverse lineaments. They are there 
fore easily recognisable on satellite imagery. The 
similarities in the geometry of all mentioned struc 
tures chosen in geologically (cratonic against mobile 
belt) different terrains are interpreted as a hint 
helping to understand some genetic aspects they have 
in common. 
Pull-apart basins 
Crust that is subject to an overall compressive stress 
field is amenable to local stretching and extension in 
the zones between the overlapping strike-slip master 
faults (Crowell 1974). These processes lead towards 
S- and Z-shaped rhombic pull-apart basins (Mann et 
al. 1983). 
Figure 2b. En-échelon disposition segments Magdalena 
River, Colombia. 
Figure 3 shows the photogeological interpretation 
of the Pre-Cambrian Chimbwe synform or S-structure, 
northern Zambia (see fig. 2a for localisation). The 
S-structure is found in a corner between major ENE 
(Kibara) and SE (Ubende) lineaments. The structure 
is interpreted as the outcome of the synsedimentary 
process of transform of slip-motion from one slip 
line towards another. The southern half is composed 
of the synform or depocentre proper and is formed and 
deformed by left-lateral slip along ENE lineaments. 
The group of SE lineaments may represent crustal dis 
continuities or faults that accomodated the extension. 
The northern half, of less evident morphology than the 
southern one, has a weak Z-shape and was, in the pre 
sent interpretation, pulled apart by dextral slip on 
the SE lineaments. 
It is remarkable again that images obtained over 
the northern Andes, give evidence of the same funda 
mental geometric relationships between synforms and 
lineaments. Here the sediments are of Oligo-Pliocene 
age. It is obvious from figure 4 that the southern 
depocentres are controlled by an eastern couple of 
lineaments, the northern synforme strike to the NNW, 
their axes being influenced by strong NW-SE trans 
verse lineaments (western pair). We may interprete 
the origin of these sigmoid structures in the same 
way we did for their African Precambrian counter 
parts. Other processes may however be invoked in ex 
plaining the origin of the depocentres. Another con 
ceivable scenario, e.g., is represented in fig. 5b 
which illustrates the genesis of synforms comparable 
to the one under discussion. It can even be argued 
that the sigmoid shape is the result of compressive 
dextral simple -shear on the ENE lineaments after 
deposition of the sediments. This variety of possi 
bilities requires a variety of slip-senses on the 
ENE and SE lineamnets. The paramount observation 
however, i.e. manifest strike-slip strain, cannot 
seriously be challenged. The region of concern is 
N 
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