Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 43

2.8 Discussion and Conclusions
The Baltic Sea is a young geomorphologic feature that was established in
Cenozoic time, most probably during the Quaternary period as evidenced by
Middle Pleistocene marine sediments. However, some authors present evidences
of Neogene ages of the N-S striking features, which dominate the Baltic Sea
depression morphology (Grigelis 1991 ) . Glacial erosional processes undoubtedly
contributed to the shaping and deepening of the depression. But, even assuming the
essential role of erosion, it is rather difficult to explain the exceptional ice sheet and
melt-water activity in the area without a pre-existing tectonic depression.
A strong evidence for the tectonic nature of the Baltic Sea depression is the
coincidence of the outline of the Cambrian marine basin and the recent Baltic Sea
(Fig. 2.8 ) . The Cambrian marks the onset stage of the Baltic basin that was initially
established in response to the continent break-up, thus implying a strong exten-
sional regime during the Cambrian, as supported by structural studies. Furthermore,
the Mesoproterozoic time was marked by voluminous intrusions of rapakivi grani-
toids and related igneous rocks, which all concentrated in the Baltic Sea area. Early
Ediacaran tectonic extension and Permocarboniferous magmatism also anomalously
affected the Baltic Sea area.
Rheological modelling of the lithosphere, based on a rather dense network of
deep seismic sounding profiles both onshore and offshore (e.g. Baltic Sea, Babel),
proved that the Baltic Sea depression is characterized by the weakest lithosphere in
the Baltic region (Ershov and Šliaupa 2000 ) . The effective elastic thickness (EET) of
the lithosphere is in general between 20.5 and 21.5 km in the Baltic sea area (28 km
in the Gulf of Finland), while it is in the range of 30-40 km in surrounding territories
and more than 40 km outside the Baltic basin (Fig. 2.20 ) . Variations in mechanical
properties are mainly due to different lithologies and temperatures. It is noticeable
that those variations are discordant to crustal thickness variations which are dom-
inated by E-W and NW-SE trends, most likely reflecting the Palaeoproterozoic
accretionary system (Fig. 2.20 ) . These lithosphere strength variations are mirrored
in the sub-Quaternary surface of the Baltic region, reflecting the general shape of
the Baltic basin, and the Baltic Sea depression in particular.
If in-plane tectonic extension is strong enough, it can result in subsidence of a
weak lithosphere (Artyushkov et al. 2000 ) . The 2D dynamic modelling of the Baltic
lithosphere indicates that extensional tectonic forces, typical for cratonic areas, may
have induced 150-200 m of subsidence. Taking into consideration the deepening
effects of glacial erosion, this is in good agreement with the sub-Quaternary subsur-
face in the Baltic Sea area. Thus, the presented model implies an extensional tectonic
regime affecting the Baltic Sea area during the Quaternary time. The extensional
nature of the Baltic Sea depression comprising smaller scale graben-like structures
was also suggested by some previous studies (e.g. Schwab et al. 1997 ) .
There are only a few breakout stress field measurements in deep wells in the
southernmost Baltic Sea (Jarosinski 1994 ) that indicate N-S maximum stress orien-
tation, which is possibly related to the impact of the Alpine chain in the south. On
the other hand, the crust of Fennoscandia is believed to be affected by the ridge push