Geoscience Reference
In-Depth Information
Fig. 2.32
Expressive
tectonic control on the karst
along the southern boundary
of Vratsa Block near Ivan
Pousti Monastery, Bulgaria
We suppose that this tectonic stress field is related
to the Sub-Hercinian Tectonic Phase (at the end of the
Turonian). On these three sites, a second group of
clearly expressed joint systems exists, assumed to be a
result of the Pyrenean Tectonic Phase (end of the
Eocene). The importance of the deformations from
the Sub-Hercinian Tectonic Phase is reflected also as
well expressed orientation of the ellipse of anisotropy
of the Lower Cretaceous limestones (Vertical Elec-
trical Sounding at the site E1). Less expressed is the
anisotropy ellipse for the upper part of the electrical
profile. The long axis of the ellipse could be accepted
as an indication of younger deformations, but arbi-
trary attributed to the Pyrenean Tectonic Phase.
Only one of the sites of the measurements (No 6)
has shown deformations that could be related, with
some level of probability, to the Neotectonic stage.
The argument for this is the similarity of the recon-
structed tectonic stress axes directions to the result of
the fault-plane solution from the earthquake with
magnitude M = 3.6 northwards of the studied area.
But, all analyzed data do not show evidence for car-
dinal changes of the principal directions of tectonic
strain after the Pyrenean Tectonic Phase.
