Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 31

an intense tectonic extensional regime that mainly affected the eastern Baltic Sea
and neighbouring areas.
The tectonic activity ceased during the Late Ediacaran and Cambrian with the
onset of wide marine transgression. The tectonic strain accumulation shifted to the
Teisseyre-Tornquist zone in the west. This is reflected, for example, in the for-
mation of funnel grabens with extensions between 1 and up to 200 m and depth
ranges between 1 and over 50 m. They formed mainly during the Early Cambrian
and partly also during Middle and Late Cambrian and are documented from south
Sweden/Scania (Lindström 1967 , Scholz et al. 2009 ) . In general, no evidences of
significant major faulting are recognized during the Cambrian, Ordovician and Early
Silurian times, suggesting low tectonic stresses affecting the Baltic basin. Fractures
filled with Lower Cambrian sandstones are mapped in the northern Baltic Sea
region. According to Drake et al. ( 2009 ) , they might have formed in relation to far-
field extensional effects of the opening of the Iapetus Ocean. Cambrian sandstone
fractures in the coastal region around Simpevarp generally follow the orientation
of the basement fracture sets with dominant NNE-ENE directions (Drake 2008 ) .
In Bornholm, a well-exposed sandstone dyke swarm strikes NW-SE. The opening
and filling of the fissures were caused by normal extension movements in NNE-
SSW direction in several steps, probably during the Early Cambrian (Katzung and
Obst 1997 ) . Funnel structures and clastic dykes are also reported from the coast of
the Baltic Sea south of Vik (Scania). Their formation is also related to extensional
tectonics (Scholz et al. 2009 ) .
In some seismic profiles, evidences of Late Ordovician faulting were reported
from the Lithuanian and the Latvian offshore areas. Although fault amplitudes reach
only a few dozens of metres, some of them controlled the growth of Ordovician reefs
(Kanev and Peregudov 2000 ) . They mainly show reverse kinematic features imply-
ing compressional tectonic activity during Late Ordovician when the lithosphere
flexuring was initiated due to East Avalonia docking.
2.4.2 Late Silurian-Early Devonian Phase
The main structuring phase of the Baltic Sea basin took place during the time period
between the latest Silurian and the earliest Devonian. A detailed structural analysis
revealed that the region was exposed to NW-SE-directed horizontal compression
in relation to the collision of Laurentia and Baltica (Šliaupa 1999 ) . Two dominating
groups of E-W (ENE-WSW) and NE-SW (NNE-SSW) striking reverse faults have
been formed. Typical for the first group are transpressional geometries, while the
second fault group belongs mainly to a compressional type.
The faulting was focussed on special areas (Fig. 2.8 ) . Main tectonic strain accu-
mulated in the Liepaja-Saldus ridge zone and the Telsiai fault zone in the central
part of the Baltic basin. Such a selective faulting can be explained in terms of struc-
tural inheritance. The Liepaja-Saldus ridge is an ancient fault zone that is marked
by sharp changes in the Moho depths, thus pointing to a first-order structure. The
Telsiai fault zone is also a reactivated Palaeoproterozoic zone (Šliaupa et al. 2002b ) .