Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 32

The amplitude of the Liepaja-Saldus ridge reaches 600 m. Bounding faults dip at
angles between 50 and 80 ◦ . They show very complex geometries. Flower structures
are typical implying strike-slip-type faulting.
To the south of the Liepaja-Saldus-Telsiai zone, the prevailing direction of
Caledonian faults is NE-SW. The amplitudes are in the range of 50-200 m. These
faults are rather regularly spaced at a distance of about 30 km and show quite sim-
ple compressional geometries. They dip to the west at high angles of 70-80 ◦ .The
Leba ridge faults were also probably established during the Caledonian stage, but
their main activity happened during the Permocarboniferous. The onset of this large-
scale feature during the Late Silurian is supported by the presence of associating gas
fields in the Polish offshore area. As it is shown below, the gas was generated dur-
ing Silurian, while source rocks were already overmature by the beginning of the
Devonian.
The faulting north of Liepaja-Saldus ridge is only of minor intensity, which is
somehow surprising as the stress source is located in the northwest (Scandinavian
Caledonides). Several faults trending NE-SW are reported from Estonia. The ampli-
tudes are in the range of 10-30 m only. A network of smaller faults striking NW-SE
is mapped in northeast Estonia (Sokman et al. 2008 ) . Here too, amplitudes reach
only a few metres. The faults are dipping mainly to the northwest at predominating
angles of 60-70 ◦ and show a compressional style.
Detailed seismo-acoustic surveys of the northern Baltic Sea area revealed a clus-
ter of linear disturbance zones with 1-4-km-wide spacing. These zones strike several
tens of kilometres north-south and show offsets of several tens of metres. The seis-
mic profiles revealed a weak flexure-like bending of the layers in the zones; locally
they are intersected by small-scale faults (Tuuling and Flodén, 2001 ) . There is so
far no stratigraphic control to estimate the time of this faulting.
Small-scale faulting associated with the migration of hydrothermal fluids is
known from the Early Devonian in the northern part of the Baltic basin. This
hydrothermal activity is about 10-15 Ma younger than the corresponding ones
in Sweden and Finland (Alm et al. 2005 ) . Fluid inclusion investigations of the
fluorite-calcite-galena veins in the Baltic basin indicate depositional temperatures
of 100-150 ◦ C (Alm and Sundblad 2002 ) .
2.4.3 Permocarboniferous Phase
During the Permocarboniferous, tectonic processes were reactivated. Most intense
tectonic deformation took place in the southwesternmost Baltic Sea area along the
Bornholm-Darlowo fault zone which is a part of the Teisseyre-Tornquist zone.
This NW-SE striking fault system forms an up to 100 km broad zone of horsts
and grabens (Vejbæk 1985 , Krzywiec et al. 2003 ) . These structures were mainly
formed in Late Carboniferous-Early Permian (Wikman 1986 ) and are related to
post-orogenic destruction of the Variscan foreland initiated by wrenching and
strike-slip movements (Brochwicz-Lewinski et al. 1984 , Ziegler 1990 ) .