Geoscience Reference
In-Depth Information
Fig. 2.39
Situation at the
vicinity of Dushnika Cave
with a scheme of the cave and
the sites of performed studies
1—Limestones of Opletnia
Member (Lower-Upper
Anisian); 2—Quaternary
sediments (mainly alluvial);
3—Site of measurements of
shear joints; 4—Site of
performed electrical
anisotropy studies
similar solutions for all sites. The summarized dia-
grams using 110 measurements from the sites repre-
sent the statistically most stable solution. Two distinct
tectonic impacts have been inferred that created two
preferential directions of open fractures in the lime-
stones of Opletnia Member. These two directions
have the same orientations as shown for Vratsa and
Lakatnik karst regions—NW-SE and NE-SW
(Table
2.5
). The first one is controlling these galleries
of Dushnika Cave that are now active, the second is
more or less coinciding with the orientation of the
entrance part of the cave.
If so, the first tectonic deformation is the same that
impacted the Triassic carbonate rocks in Western
Balkan Mountain, and it is accepted to be a result of
the Late Cimmerian Tectonic Phase at the end of the
Jurassic
fractures
were
created.
These
fractures
obviously
control
the
NW-SE
branch
of
the
karst
galleries
(Table
2.5
).
The second reconstructed stress field favored the
opening of the northeastwards striking fissures. The
regional studies and the presented above interpreta-
tions for Vratsa and Lakatnik karst regions have
demonstrated that the resulting deformations, most
probably, reflect the impact of the Pyrenean Tectonic
Phase, and that the same regional tectonic stress
conditions had remained from the Oligocene to the
Pliocene. The NE-SW oriented galleries of Dushnika
Cave reflect one more time this situation (Table
2.5
).
The ellipse of the electrical anisotropy for the first
15 m from the surface (AB/2 = 30) has its long axis
oriented exactly in this direction. The fault-plane
solution for the nearest earthquake of 09.03.1980,
period.
Conditions
for
opening
of
NW
