Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 44

Fig. 2.20 Effective elastic
thickness of the lithosphere
(Ershov and Šliaupa 2000 )
and Moho depths (Rapakivi
granites and related rocks are
indicated by the dashed lines )

from the Mid-Atlantic ridge as indicated by the general NW-SE-orientated maxi-
mum horizontal stress (Olsson 2002 ) . However, this model alone cannot explain the
recent seismic activity in the region. Isostatic glacial rebound movements strongly
influence the tectonic stresses of the shield (Muir Wood 1993 ) . GPS measurements
indicate a doming of the crust centred in the Bothnian Bay. The eccentric shift of the
GPS sites is coherent with a vertical doming (Scherneck et al. 2001 ) . Examination
of the strain field of Fennoscandia by means of a glacial isostatic adjustment model
suggests that elastic extension is the dominant style of deformation, controlled by
horizontal displacement (Scherneck et al. 2003 , Marrota and Sabadini 2004 ) .
The Baltic Sea is located on the western flank of this Fennoscandian dome and
thus may be a part of this geodynamic system. This suggestion is supported by
recent GPS measurements in the Baltic countries (Pan and Sjöberg 1999 ) . The mod-
elling of the stress field distribution from those GPS data revealed two major stress
provinces. For the western parts of Lithuania and Latvia and most of the Estonian
area, uniaxial and diaxial tectonic extensions are shown, while the eastern part of the
Baltic region is exposed to compression; the strain rate is in the order of 10 -8 -10 -9
year -1 (Zakarevicius et al. 2008 ) . Therefore, it is hypothesized that the western part
of the Baltic region and the Baltic Sea area are affected by the same geodynamic
mechanism as the Fennoscandian dome.
It is established that the Fennoscandian doming preceded the Quaternary glacia-
tion and it is thus obvious that the Baltic Sea area was exposed to an extensional
regime before Quaternary time. Moreover, comparison of data of GPS sites around
the Baltic Sea (BIFROST) carried some authors to the conclusion of the existence
of a dextral strike-slip fault with a relative velocity of about 1.5 ± 0.5 mm/year along

a N-S direction in the middle of the Baltic Sea.