Geoscience Reference
In-Depth Information
Fig. 2.22
Map of the Orlova
Chuka Cave (North Bulgaria)
with rose-diagram of the
cumulative length (in %) of
the galleries, compared to the
directions of the open (tensile)
fractures (in gray on diagrams
A and B) generated by the
tectonic stress fields in the
Aptian limestones
at 1.20 m below the top of sedimentary sequence in an
excavation in the cave (Evlogiev et al.
1997
).
The reconstruction of the stress fields in the SE
Moesian Platform in Bulgaria has been made by means
of earthquake fault-plane solutions, tectonic fractures,
and fold patterns (Shanov
2005
) in rocks restricted
stratigraphically from the Early Cretaceous up to the
Late Pliocene. The philosophy of the study is that the
recognition of the older stress fields for a given area can
be successful if the characteristics of the youngest ones
are known. The contemporary stress field is discussed
using the fault-plane solutions from earthquakes and
the kinematics of the activated faults. The reconstruc-
tion of the Post-Pliocene paleo-stress field was made
by studies of conjugate shear joints systems in Upper
Pliocene limestones, covering the area east of the
location of the cave and containing traces from only
one tectonic deformation (Post-Pliocene).
Measurements of elements of tectonic fractures
were also performed in situ on more than 60 outcrops
of rocks of Aptian age. A more complete study of the
tectonic meso- and micro-structures has been per-
formed on the Sarmatian sediments in the SE Moesian
Platform. This study includes a description of the
discovered folds, brittle tectonic analysis, and recon-
struction of the Post-Sarmatian paleo-stress field. As a
result, it was deduced that the compression after the
Early Cretaceous period is NE-SW directed. The
direction of compression since Sarmatian to Early
Quaternary was NW-SE. A clockwise rotation of the
main stress axes was established for a number of sites.
The contemporary contraction is directed also
NW-SE, according to the fault-plane solutions deter-
mined for crustal earthquakes in the region. This
result is tested using the data from the GPS mea-
surement recently performed in this part of the Balkan
Peninsula (Shanov
2005
).
Having in mind this regional analysis, it is easier to
understand the results from the reconstruction of the
tectonic stress fields using measurements of joints
(106 joints) from an outcrop at the entrance of the
Orlova Chuka Cave (Fig.
2.22
).
At the first stage of its formation, the Post-
Cretaceous fracturing (the age is not evaluated but the
deformation happened before the end of Sarmatian
time) probably had some role to predestine the NE-
SW trend of the open fractures, able to form karstic
galleries (solution A in Fig.
2.22
). In any case the
longest gallery in the cave is parallel to the regional
fault striking NE-SW.
The second impact B was due to the more recent
deformations, probably related to the Post-Sarmatian
processes. The Post-Sarmatian (Late Neogene?)
deformations marked a total change of the preferential
direction of cave formation due to the interchange of
the orientations of r
1
and r
3
tectonic stress axes.
Since this time, because the tectonic regime as gen-
eral tendencies has not changed till present, the main
