592 
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5C 5D 
Figure 5c. Schematical localisation of geochemical 
anomalies (black squares) at the intersection of brec- 
ciated lineaments defining closures of synforms. 
Figure 5d. Stereographic projection of minor fold axes 
in the emerald bearing Eastern Cordillera, Colombia. 
2.3.Economic aspects 
The deviatoric part of the horizontal stress field 
seems to split the crust in the direction of maximum 
horizontal compression before it fails by shear faul 
ting. Fractures with this appropriate direction and 
relation to shear fractures appear filled up by epi 
genetic solutions of possible economic interest. If 
the Zambian quartz-dykes are examples of a non-minera- 
lized fill, kimberlite dykes and pipes outcropping un 
der the same favourable conditions with respect to 
strike and attitude in the Lofoi area (Shaba, Zaire), 
are more interesting targets. 
Geochemical exploration in the emerald bearing part 
of the Eastern Cordillera, Colombia, illustrates the 
effects of the stretching and thorough fracturing in 
the lowermost beds of the passively folded and faulted 
synforms. All anomalies indicating environments fa 
vourable for emerald mineralization are situated at 
the intersection of brecciated lineaments bordering 
some of the synforms (fig. 5c). 
3 THE CONTINENTAL STRESS FIELD 
It is currently accepted that, except for uplifted and 
isostatically compensated regions where the stress 
field would be tensional (Artyushkov 1973), most of 
the continental plates are slightly compressional (i.g 
Fleitout & Froidevaux 1983). The lithosphere act as a 
guide for this deviatoric field and may be treated as 
a continuum. The repetitive pattern of trends of line 
aments and their spatial relationship with geological 
structures indeed suggest that the former are the 
effects of a uniform strain emanating from continuous 
and homogeneous processes. As far as the distribution 
of the stress field within the Earth's lithosphere and 
crust is concerned, the classical theory of a conti 
nuous elastic plate may be applied. Much of the obser 
ved strain in the crust however is of ductile nature 
and results from plastic processes. The applicability 
of elastic theory to geological processes in nature 
is thus restricted to the time period between the on 
set of loading of the planar continuum to the moment 
the elastic material fails by brittle fracturing or 
deforms in a plactic way. 
3.1 The homogeneous field 
The stress field in the plate interior depends largely 
on the strength of the applied forces, on the geometric 
outline of the loaded plate and on the boundary condi 
tions. Pure shear does not rotate the strain axes and 
stress and strain remain co-axial. A study implemented 
on a triangular body (Dehandschutter in prep.) shows 
a good gross correlation of the obtained stress tra 
jectories and lines of elastic slip with the observed 
strain in the triangular corner of the South America 
Plate, the northern Andes. 
Nevertheless is it very clear as well that factors 
of lower order do control stress distribution and 
strength of the deviatoric field. The elements of the 
analysis given in 2 above suggest that a prime factor 
in control of the stress field and the resulting fi 
nite strain, is the presence of intersecting groups 
of lineaments which apparently act as discontinuities 
in the uniform crust. The elastic constants of the 
solid in the lineaments may either be higher or lower 
than the corresponding values in the plate's interior. 
3.2 The discontinuous field 
In order to evaluate the influence weak discontinui 
ties may have on the stress field, a triangular plate 
made of photoelastic resin was cut in two perpendi 
cular directions (figs. 6,7) representing a possible 
combination of ENE and NNW lineaments. The lineaments 
or weak discontinuities in the model were filled with 
another resin the modulus of elasticity of which is 
5 times less stiff than the material in the interior 
of the plate. The photoelastic analysis was seconded 
by a numeric finite elements control (fig. 7). 
Some results of the experiments done on the trian 
gle are extendable to more universal conditions. It 
is found that the primary or undeviated field which 
is known from the investigation of the homogeneous 
triangle obtains in the interior of the blocks se 
parated by weak discontinuities. 
The weak lineaments create a field of influence 
around their strike (fig. 6a) in which the expected 
trajectories (fig. 6a) of maximum compression (pri 
mary field) are deflected towards parallelism with 
the lineaments and the edges of the relatively stiff 
blocks (fig. 6c). The adges of the stiff blocks have 
the tendency to act as stress guides. The degree to 
which trajectories inside the blocks are deflected 
depends on the elastic contrast between the block 
and the weak lineament. 
Figure 6. Elastic stress distribution in a triangle 
cut by weak discontinuities. A. Fields of influence 
of weak zones and sigma 1-trajectories in homogeneous 
triangle; B. Azimuth and magnitude of axes of maximum 
and minimum compression, thickness in lineaments = 
thickness in blocks; C. Refracted and reflected tra 
jectories; D. Axes of max. and min. compression, 
thickness lineaments = thickness blocks/2. 
Trajectories entering or leaving the weak linea 
ments are strongly refracted (fig.6c). The axes of 
minimum compression are re-oriented inside the zones 
where they strike parallel to the trend of the line 
aments. The axes of maximum compression correspon 
dingly turn towards perpendicularity with the res 
pective lineaments. The angle of incidence of the 
Figure 7. 
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