Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 35

Caledonides on the East European craton were dated to 420-430 Ma, reflecting
volcanic activity in the adjacent orogen (Hansen 1995 , Obstetal. 2002 ) . Giese
et al. ( 1997 ) , Torsvik et al. ( 1996 ) and the MONA LISA Working Group ( 1997 )
assume a closure of the Tornquist Ocean between Avalonia and Baltica for the Late
Ordovician. Intense erosion of the uplifted orogen occurred afterwards. Within the
Rügen area the erosion period lasted until the Middle to Upper Devonian.
The Caledonian deformation front represents the northern boundary of the thrust
and fold belt. Its southern extension is not known because no single well has reached
the Lower Palaeozoic in the central part of the North German basin and in the basin
part south of Rügen Island. Both areas are characterized by a very thick overlying
sedimentary succession of Carboniferous to Cenozoic age (Fig. 2.6 , Hoth 1997 ) .
According to Hoffmann et al. ( 2001 ) the gently south-dipping Cambro-Ordovician
alum shales form the basal decollement on which the orogen wedge was thrusted
onto Baltica. This horizon is assumed to be located deeper than 10 km in the
mainland south of Darß and in the area south of the wells Greifswald 1, Loissin
1 and Grimmen 6 (Gd1, Loss1, Gm6 in Figs. 2.6 and 2.7 ) . Fault blocks down-
thrusted southwest were identified in offshore seismic profiles close to the Rügen
area (Schlueter et al., 1997 ) . They cut the basement, as well as Cambrian and the
lowermost Ordovician sediments, and thus document an intense rifting during the
earliest Palaeozoic.
For a long time, the boundary between Baltica and Eastern Avalonia was consid-
ered to be confined to the Caledonian deformation front (CDF) in the southwestern
Baltic Sea (Cocks and Fortey 1982 ) . However, since the EUGENO-S deep seismic
survey in the 1980s, it was realized that the major tectonic boundary between the
two plates is located further to the south and west. This was supported by deep
seismic sounding studies BABEL (Meissner et al. 1994 ) , DEKORP (Meissner and
Krawczyk 1999 ) and MONA LIZA (MONA LIZA Working Group 1998). Some
authors even concluded that the major suture is related to the Elbe zone (Abramovitz
et al. 1998 ) , which is located 200-300 km west and south of the CDF. A sporadically
strong sub-Moho structure dipping 20-30 ◦ to the NE was observed in the Bornholm
area. It is interpreted as the subducted slab of East Avalonia (McCann and Krawczyk
2001 , Krishna et al. 2007 ) . Furthermore, a series of profiles of the Baltic'96 exper-
iment show north-dipping reflections at Moho level in the southwestern Baltic Sea
area. Hence, northward subduction in the uppermost mantle is indicated by the avail-
able information (Thybo 2000 ) . During the Variscan stage, the southwestern Baltic
Sea area represented the northeastern margin of the foreland basin predominantly
controlled by flexure induced by the Variscan orogen to the south.
2.6 Present Morphology of the Baltic Sea Depression
The present Baltic Sea depression was formed during Cenozoic time. There is still
no consensus with respect to the role of erosional and tectonic processes for the
formation of the depression. A group of researchers suggest that the main forms