594 
Figure 9. Thickening of the edges of a block by over- 
(C) or underthrusting (B) due to compression from 
within the weak zones (A). 
a synform (fig. 9). This scenario offers an alterna 
tive to the first one although both may concur and 
joinedly affect the block interior. 
The re-orientation of the axes of maximum compres 
sion towards perpendicularity with the trend of the 
weak lineaments impedes strike-slip faulting within 
the lineaments if the edge of the blocks are shear 
free and the principal axes of stress respectively 
parallel and perpendicular to the edge (fig. 10a). If 
however the axes are not refracted into a normal po 
sition, they are inclined to the edges of the block. 
The lineaments, filled with material of low shear 
strength, then offer ideal pathways for strike-slip 
faulting (fig. 10b). This fully confirms the obser 
vations in 2.2 which testified of oblique-slip strain 
inside the lineaments. 
SUMMARY 
Geological structures with precise tectonic signifi 
cances and indicative to various degrees of the orien 
tation of the paleo-stress field in which they were 
created, can be circumscribed within a framework of 
transverse lineaments visible on satellite imagery. 
Structures of this kind are tensional joints, verti 
cal faults like rift boundary faults, quartz dykes, 
sedimentary basins of pull-apart origin. Even struc 
tures of compressive nature seem to relate to the same 
framework. The latter though might be the sequel of 
the presence of discontinuities like synsedimentary 
faults which were later reactivated during compression. 
During the opening phase the basin marginal faults 
were seemingly related to the framework. The are res 
ponsible for two directions of shortening (B normal 
to B) operative during the same orogeny. 
The good positive correlation between the theoreti 
cal stress field within some homogeneous elastic 
triangle with observed large-scale tectonic features 
in the northern Andes confirm the validity of the 
approach. A second order factor in control of the 
stress field in the framework of intersecting weak 
discontinuities. The weak zones reflect stress tra 
jectories in a way which is compatible with the ob 
served variability of rift directions. This effect de 
pends on the one hand on the physical conditions in 
the weak zones and in the blocks and on the boundary 
loading conditions on the other hand. The refraction 
of the trajectories inside the lineaments explains 
how several azimuthal groups of lineaments can give 
way in coeval strike-slip faulting under the same 
stress field. 
REFERENCES 
Artyushkov, E.V. 1973. Stresses in the lithosphere 
caused by crustal thickness inhomogeneities. Journ. 
Geophys. Res. 78 : 7675-7708. 
principal axes of stress 
trajectories max. compression 
Figure 10. Possibilities of strike-slip faulting in 
lineaments. A : impossible; B : favoured. 
Crowell, J.C. 1974. Origin of late Cenozoic basins in 
southern California. In Dott, R.H. & Shaver, R.H. 
(eds.), Modern and ancient geosynclinal sedimentation. 
Spec. Bull. Soc. Econ. Paleont. Miner. 19 : 292-303. 
Dehandschutter, J. in prep. Lineaments in the northern 
Andes and their bearing on the geodynamic evolution 
in the leading corner of the South America Plate. 
Dehandschutter, J. & Lavreau, J. 1985. Lineaments and 
extensional tectonics : examples from Shaba (Zaire) 
and NE Zambia. Bull. Soc. Belg. Gdol. 94 : 209-221. 
De Swardt, A.M.J., Garrard, P. & Simpson, J.G. 1965. 
Major zones of transcurrent dislocation and super 
position of orogenic belts in part of Central 
Africa.Geol. Soc. Am. Bull. 76 : 89-102. 
Fleitout, L. & Froidevaux, C. 1983. The state of stress 
in the lithosphere. Tectonics 2 : 315-324. 
Hobbs, W.H. 1911. Repeating patterns in the relief 
and in the structure of the land. Geol. Soc. Am. 
Bull. 22 : 123-176. 
Jaeger, J.C. & Cook, N.G.W. 1976. Fundamentals of 
rock mechanics. London : Chapman & Hall. 
Mann, P., Hempton, M.R., Bradley, D.C. & Burke, K. 
1983. Development of pull-apart basins. Journ. Geol. 
91 : 529-554. 
Raasveldt, H.C. 1956. Mapa geologico de la Rep. de 
Colombia, Plancha L9, Girardot. Bogota : Ingeominas. 
Stearns, D.W. 1978. Faulting and forced folding in the 
Rocky Mountains foreland. In Matthews III, V. (ed.), 
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Am. Memoir 151 : 1-37. 
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