Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 33

In the southern part of the Baltic basin, a set of large E-W striking faults were
established. This faulting is associated with the intense doming of the lithosphere
that also leads to the erosion of Devonian and older sediments. The largest fault
of this group is the Kaliningrad fault striking across the Gdansk Bay and further
onshore to the east (Fig. 2.8 ) . The amplitudes of those faults are in the range of
30-50 m.
The Leba ridge is built of a wide set of N-S striking faults, the activity of which
led to the truncation of more than 1 km of Devonian and uppermost Silurian sedi-
ments (Domzalski et al. 2004 ) . The seismic profiles reveal the compressional nature
of the Leba faults. The other Caledonian faults were also reactivated in a compres-
sional regime during the Permocarboniferous. The most intense fault reactivation is
reported from the Liepaja-Saldus ridge (Figs. 2.13 and 2.14 ) .
A peculiar feature of the Permocarboniferous phase is the activation of igneous
processes in the southern part of the Baltic Sea and in northern Poland (Fig. 2.12 ) .
The intrusions were dated to 340-355 Ma (Birkis and Kanev 1991 , Šliaupa et al.
2002c ) which is contemporaneous to the Chmielno volcanic formation of the
Pomeranian basin in Poland. So far 21 intrusions have been identified by character-
istic magnetic anomalies (Šliaupa et al. 2004 ) . They are connected mainly to N-S
and E-W trending faults (Fig. 2.12 ) . Well C8-1 drilled, for example, a 6-m-thick
intrusion hosted by Silurian shales, which is connected to the Kaliningrad fault.
Well D1-1 penetrated a 25-m-thick sill also hosted by Silurian shales and connected
to an E-W striking fault. This fault also hosts another intrusion located close to
the well D5-1 (Fig. 2.12 ) . This fault is very well traced by 30 m offset of Upper
Permian layers in the onshore area. It is noticeable that this fault shows inverse rela-
tionship offset of Devonian sediments pointing to tectonic inversion. The chemical
composition of D1-1 diabases is close to the continental rift basalts (Motuza et al.
1994 ) . Diabases are of sub-alkaline composition with modal olivine and nepheline,
of porphyritic texture (3-5% of plagioclase phenocrysts). D1-1 sill intruded in two
phases - the early phase is represented by fine-grained diabase, while very fine-
grained diabase intruded in the second phase. The chemical composition suggests
a formation of the magma chamber at 150-120 km depth. The magnetic source
depth modelling of the magnetic field data indicates that diabase sills are mainly
hosted by Cambrian and Silurian sediments and only partly by the crystalline
basement.
Contemporaneous igneous activities are documented from northern Poland, the
region that experienced the most intense uplift during the Permocarboniferous. Rb-
Sr ages of the Elk massif are around 355 Ma and thus similar to K-Ar ages of the
Pish gabbro intrusion (Depciuch et al. 1975 ) . Several smaller ultramafic and mafic
intrusions were identified in the area and show age dates between 347 and 344 Ma
(Depciuch et al. 1975 ) . A second phase of igneous activity took place in northern
Poland between 295 and 265 Ma. This phase corresponds to the phase of intense
magmatism in the North German basin (Benek et al. 1996 ) .
Furthermore, a 355-Ma Rb-Sr age of Ordovician K-bentonites was identified in
Estonia (Kirsimae et al. 2002 ) . All these data point to basin-scale thermal processes
during the Permocarboniferous.