all that factors 
tribution and 
e elements of the 
at a prime factor 
ae resulting fi- 
rsecting groups 
3 discontinuities 
astants of the 
a higher or lower 
plate's interior. 
;ak discontinui- 
triangular plate 
a two perpendi- 
ating a possible 
3. The lineaments 
were filled with 
Lty of which is 
in the interior 
sis was seconded 
(fig. 7). 
ae on the trian- 
conditions. It 
:ed field which 
le homogeneous 
:he blocks se- 
of influence 
:h the expected 
apression (pri- 
rallelism with 
relatively stiff 
riff blocks have 
The degree to 
are deflected 
:en the block 
i in a triangle 
Is of influence 
:s in homogeneous 
: axes of maximum 
i lineaments = 
l; reflected tra- 
ompression, 
ks / 2. 
ie weak linea- 
. The axes of 
nside the zones 
:nd of the line- 
.on correspon- 
with the res- 
dence of the 
Figure 7. Elastic stress fields in a discontinuous body. 
A. Photoelastic analysis, only the axes of maximum 
compression are shown. 
trajectories entering the lineaments is determinative 
with respect to the consequences of the refraction : 
the axis of minimum compression of normally indicent 
trajectories is always parallel to the weak zone, in 
side as well as outside of it, and the trajectory may 
cross the weak zone freely. 
Diminishing the thickness of the lineaments as com 
pared to the fixed thickness of the blocks, greatly 
enhances the deviatoric field in the former (fig. 6d). 
Finally do isotropic points of zero deviatoric 
stress which are loci of sudden changes in the direc 
tions of the trajectories, appear in two out of the 
four corners of the blocks surrounded by weak discon 
tinuities. The exact location of the isotropic points 
within their corners depends on the boundary condi-' 
tions of the loaded plate (fig. 8). 
4 DISCUSSION 
Accepting the possibility that lineaments are somehow 
related to crossing discontinuities within the elastic 
crust which are weaker than the more homogeneous crus 
tal solids and which strongly influence the trajecto- 
rues of the primary stress field, provides possible, 
though not unique, clues in understanding the fixed 
spatial relationship to lineaments in which many geo 
logical structures appear. 
The deviatoric field in the weak zones is always 
higher than the field in the block interior. The 
latter, with high mean stress, is often close to litho 
static. 
Small increments of stress may bring the interior of 
the blocks to dilatation and vertical splitting in the 
direction of maximum horizontal compression. These 
directions are visualised by stress trajectories which 
appear to systematically contour the blocks. The 
direction of spreading of the trajectories over the 
block depends on the position of the isotropic points 
within the block. Figure 8 shows that, depending on 
the loading conditions, trajectories and resultant 
brittle tensional failure may be deviated from a dia 
gonal position in the centre, towards a less inclined 
Figure 7. Elastic stress fields in a discontinuous body. 
B. FEM-analysis, axes of maximum and minimum compres 
sion. 
position with respect to the weak zones in the vicini 
ty of the latter. This result of the investigation may 
explain the systematicity in the directions of geolo 
gical structures and their relation to confining line 
aments as weak zones which was discussed in 2.1. It is 
quite clear that the above discussion is limited in 
time to the first phases of rift formation. True ex- 
tensional rifting will be accomodated by only l.Out of 
all possible sets of transverse structures which even 
tually evolved into a transform fault. 
The weak zones proper, here modelled as an elastic 
medium, might in fact conform better with true elastic- 
plastic bodies or contain a visco-elastic element. A 
strong deviatoric field in thege zones, which is ab 
sent from the interior of the blocks, may eventually 
lead to permanent strain, shortening and uplift. The 
compression from within the lineaments onto the rhom 
bic blocks finally thickens the edges of the blocks. 
Stresses resulting from the consequential bending of 
the homogeneous rhombic block will then substitute the 
stresses emanating from the boundary forces. The block 
.interior fails by vertical rupturing or is bent into