Geoscience Reference
In-Depth Information
Middle Oligocene—Lower Miocene age. Probably,
the Jurassic and Triassic sediments were overthrusted
over Vitosha Pluton during Post-Oligocene time.
Conditions were created for typical allogenous karst
formation in a complex geologic-tectonic setting.
During the Pliocene-Pleistocene, as a result of the
activation of orogenic processes, the morphological
units around Bosnek Karst Region were shaped
(Matova and Angelova
1994
). The initial tectonic
impulse was not so strong and for that reason the
Struma River keeps its outflow direction toward west.
Both the relicts of this old river valley and the
accompanying them alluvial deposits could be seen
SE of Bosnek Village.
The Quaternary stage begins with a change in the
intensity of tectonic processes. As a result of repeated
movements and subsequent vertical tectonic defor-
mations, the rock complexes affected by karstification
were exposed on the surface and the formation of an
underground karst complex started. The valley and
karst complex development are considered as related
to the changes of the character of the tectonic
movements, phases of the glacial epoch, lithostruc-
tural features and Quaternary tectonic activity. During
the Upper Pleistocene, segments of Pernik Fault Zone
were activated. The NW-SE trending faults of Bosnek
Karst region belong to this fault zone. As a result of
this process, two separate areas are formed in Bosnek
Karst Region. The tendency of regional uplift and
river incision continued after the Middle Pleistocene.
The karst processes from the Pleistocene up to now
are determined by Struma River, being simulta-
neously a rivershed and a drainage system (Benderev
and Angelova
1999
). The faster block uplifting of
Vitosha Mountain led to the formation of a steep
slope of Struma River valley and consequently the
river water begun to flow underground in the zones of
carbonate rocks.
The alternation of active uplift and relatively low
tectonic activity led to formation of different levels of
galleries, some of them even under the contemporary
river valley. As a fact proving the presence of dif-
ferent stages in the karst development, mainly the
structure of Duhlata Cave system should be consid-
ered where the cave galleries are much longer and
where seven horizontal levels could be clearly
delimited. These levels and their number correlate
closely with river terrace fragments found on the
valley slopes (Benderev and Angelova
1999
). The
oldest and hypsometrically highest galleries could be
considered as the most upper and oldest way of
underground water flow that corresponds to the pla-
nation during the Pliocene-Pleistocene time. Thus, the
age of that process could be accepted as a beginning
of a more intensive karstification of the whole Bosnek
Karst Region. As a result, a complex multi-leveled
labyrinth cave with a basic underground stream and a
few small tributaries has been created. The process of
the cave morphogenesis continues up to present days.
Tectonic Stress Fields Reconstructions
The reconstructions of the tectonic stress fields were
made using the statistical approach for analyzing the
conjugated shear joint systems, the kinematics of the
deformations along faults discovered in Duhlata Cave
(well-preserved striations on slickensides), and the
fault-plane solutions for some local earthquakes. The
studied sites are outcrops of limestones, as well as
inside the Duhlata Cave (Fig.
2.44
). Initially, two
expressed
tectonic
stress
fields
have
been
recon-
structed (Table
2.6
).
The older stress field is determined to be from Post-
Oligocene time (Benderev and Shanov
1997
). This
stress field reflects the tectonic phase of trusting of the
Mesozoic rock complexes over the volcanic rocks of
Cretaceous age of Vitosha Pluton. The studies in the
cave discovered one older tectonic stress field. The
orientation of the minimum tectonic stress axis r
3
is
subvertical whereas the maximum tectonic stress axis
is sub-horizontal, and this is a clear indication for
trusting regime of the deformations.
The inferred second tectonic stress field is of Pli-
ocene-Quaternary age. The tectonic compression was
directed NW-SE and the active sub-horizontal NE-
SW tectonic extension created possibilities for open-
ing of the NW-SE fractures and their karstification.
The logical consequence was that the karst galleries
and the way of the underground waters have the same
general direction (Fig.
2.43
). Namely this deforma-
tion is reflected in the movements along fault No. 2 in
Fig.
2.44
and the formation of slickensides on the
fault surface.
During the works in the cave Duhlata we discov-
ered also the second generation of tectonic striations.
They are well preserved in the cave and are probably
very young because the clay is remaining on the
striations (Fig.
2.45
). The solution practically con-
firms
the
Pliocene-Quaternary
orientation
of
the
