591 
E lineaments 
graben 
eaments 
lénts Magdalena 
nterpretation 
' S-structure, 
isation). The 
en major ENE 
he structure 
ynsedimentary 
om one slip- 
f is composed 
d is formed and 
E lineaments, 
nt crustal dis- 
d the extension, 
phology than the 
as, in the pre- 
extral slip on 
btained over 
he same funda- 
synforms and 
Oligo-Pliocene 
the southern 
rn couple of 
ke to the NNW, 
NW-SE trans- 
y interprete 
in the same 
an counter- 
invoked in ex- 
. Another con- 
d in fig. 5b 
rms comparable 
sn be argued 
f compressive 
nents after 
ety of possi- 
nses on the 
aservation 
ain, cannot 
concern is 
Figure 3. Photogeological interpretation of the 
Chimbwe-synform S-structure, Zambia. Depocentre cre 
ated by sinistral slip on left-stepping oblique-slip 
faults. 
Figure 4. Photogeological interpretation of the Rio 
Chicamocha area, Boyaca, Colombia. The axes of syn- 
forms are strongly reminiscent of pull-apart basin 
geometries (S- and Z-basins). 
limited to the east by the arm-pit of the Cordillera 
where recent uplift attained its highest levels (inset 
to fig. 4). 
2.2 Structures related to horizontal compression 
Figure 5a shows a combination of Landsat interpreta 
tion and published geological mapping (Raasveldt 1956) 
of the area around Girardot, Colombia. The rather 
chaotic image is caused by coeval shortening around 
two almost mutually perpendicular axes. Shortening is 
not equally distributed over the area. It is limited 
to well determined linear zones appearing as linea 
ments in which uncompetent strata are folded en-éche 
lon around horizontal or plunging axes. More competent 
beds are merely uplifted or drape folded (Stearns 1978) 
over the linear zones of thickening. Figure 5b sche 
matically represents the contours of these intersecting 
linear uplifts. In the field, they indeed appear as 
narrow and faulted anticlines. During the uplift of 
the anticlines, rocks inside the rhombic area outlined 
by them, is stretched and passively faulted. These 
Figure 5a. Landsat interpretation of published geolo 
gical map Girardot area. 
Figure 5b. Interpretation Girardot area. 
Structural level 1 : active shortening and uplift in 
lineaments bordering a central stretched area; 
Structural level 2 : passive drape folding and forma 
tion of synforms, syntectonic sedimentation. 
areas appear as large-wavelength synforms. At the 
apogee of the border uplift, a central graben is 
created which links two corners of the rhomb. The 
graben is filled with molassic sediments. 
This interpretation is corrobated by partial obser 
vations in profiles where shortening is evident and 
expressed in minor fold axes accompanied by axial 
plane and shear cleavage. Here, in the emerald bearing 
part of the same Eastern Cordillera, detail field map 
ping reveals that synforms are contained within a 
framework of crossing ENE and SE sub-latitudinal line 
aments (fig. 5c). The lineaments are the topographic 
expression of lines of brecciated and metasomatically 
altered country rock. Some measured axes of minor 
folds group inside a statistical plane which is paral 
lel to each of the sub-latitudinal lineaments (Ea and 
Wa in fig. 5d). More fold axes project within a plane 
comprising the sub-meridional lineament direction 
which is conjugate to the first sub-latitudinal mem 
ber of respectively the eastern and the western pair 
(Wb and Eb in fig. 5d). On the megascopic outcrop 
scale, one sees two groups of narrow anticlines, each 
group with different axial strike and adjacent to the 
corresponding transverse brecciated lineaments. The 
interposed synform is terminated by the intersecting 
lineaments. The divergent strike of the axes of shor 
tening or anticlines imposes important thinning in the 
lowermost portion of the rhombic synform (fig. 5b).