Svecofennian Crustal Domain as a Target Crystalline Basement and Sedimentary Cover

The Svecofennian Crustal Domain is divided into 2 sub-regions (see Fig. 1): (a) SCD within the Fennoscandian Shield and (b) SCD within the Russian Platform in the Baltic States and adjacent areas of Belarus and north-western Russia. The Baltic Sea together with the Gulf of Finland determines the NE-SW-trending boundary between the two sub-regions. Restricted areas with remnants of Meso- to Neoproterozoic (Gulf of Bothnia seabottom with grabens extending into the mainland, Lake Ladoga district) and Lower Palaeozoic (Bothnian submarine district, Lake Siljan area and others) sequences are considered to belong to the shield area (Fig. 1).

Inactive crustal discordances, i.e., transform movements or deformations that cut the primary continuation of the blocks, border the region. The Karelian-Svecofennian Transition Zone (KSTZ, Fig. 1), which was created during the accretion of the Svecofennian orogen to the Karelian palaeocontinent (Korja et al. 1993), forms the northeastern border zone of the region. The SCD is separated from the Sarmatia crustal segment to the southeast by the northeast trending Volhyn-mid-Russian aulacogen system. The Tornquist-Teisseyre Zone (TTZ, Fig. 1) forms the fundamental boundary to southwest. Western frontier of the SCD is represented by the late Paleoproterozoic Transscandinavian Igneous Belt (TIB, Fig. 1), which probably has inherited a transform fault (Gorbatschev and Bogdanova 1993). The Caledonide Orogenic Belt borders the area from northwest and north.

The age of the SCD, a uniform crustal segment containing a range of varied metamorphic and igneous lithologies, is ~1.9 Ga (Gorbatschev and Bogdanova 1993). Although presently the region unites geologically different shield and platform areas, with a submerged Baltic sea-bottom in between, the pre-Quaternary history (Table 3) of it was very much of the same kind. During 1.9 - 1.6 Ga, the Svecofennian orogen went through compression and crustal thickening coupled with folding, metamorphism, intrusion, migmatization, faulting, shearing, and uplift and erosion processes. In places, suites of granulite metamorphism, formed at depths of 10 - 15 km, were exposed at the surface (Koistinen et al. 1996). At 1.65 - 1.5 Ga, faulting and block mountain building and formation of rapakivi-anorthosite igneous centres with central and fault swarm- related volcanic apparatus featured the region (Puura and Floden 2000). The following erosion and peneplanation led to formation of flat and stable environments favourable for long-term survival of impact structures. From these periods, no impacts are known yet. Since about 1 Ga (during 1.5 - 0.6 Ga) lasting prevalence of continental conditions, eroded mountain environments of post-Svecofennia gradually transformed into a large shield-type area. However, the 1.5 - 1.9 Ga crystalline rocks presently exposed on the shield or sub-cropping under late Vendian-Phanerozoic sedimentary cover were temporarily largely covered by Mesoproterozoic molasse-type (Jotnian, ~1.4 Ga, in the Baltic Sea-Gulf of Bothnia area) or Neoproterozoic (Bothnian Bay-Lake Ladoga area and Volhyn-Orsha-Mid-Russian Aulacogen) siliciclastic deposits (Amantov et al. 1996, Nikishin et al. 1996, Puura and Floden 1997). Remnants of this cover are preserved in tectonic grabens or depressions (on the bottom of the Bothnian Sea, Bothnian Bay, Lake Ladoga, and Volhyn-Orsha-Mid-Russian Aulacogen).

Table 3. Generalized geological history of the Svecofennian Crustal Domain (SCD).

Age (Ga)

Geological history

1.9 - 1.6

The Svecofennian orogen, compression and crustal thickening

1.65 - 1.5

Faulting and block mountain building, formation of rapakivi-anorthosite

igneous centres

1.6 - 1.4

Rapid erosion of block and volcanic mountains, peneplanisation of the

territory

1.4 - 1.3

Early platform-type sedimentation (Jotnian)

1.3 - 0.6

Prolonged erosion and peneplanisation

0.6 - 0.3

Accumulation, formation of the sedimentary cover all over the SCD

0.3 - 0.0015

Erosion is dominating, destruction of the sedimentary cover in the western

part of the SCD, incision of the basin of the present Baltic Sea, alternating

accumulation and erosion processes within the SW marginal zone of the

SCD

0.0015 - present

Glacial erosion and accumulation

Late Neoproterozoic erosion and peneplanation led to formation of flat and stable environments on top of differently aged Precambrian structures and lithologies. These processes generated the pre-Late Vendian unconformity that, nowadays, spreads all over the East European Craton.

In the pre-Quaternary times, the boundary between exposed crystalline (paleoshield) and sedimentary (paleoplatform) areas differed from the present as well as varied in time. During the Phanerozoic sedimentation, depocenters were located either in marginal parts of the SCD (from Vendian to Early Cambrian in the east, from Middle Cambrian to Silurian in the NW and SW parts, from Mesozoic to Cenozoic in the SW part) or in its centre (from Devonian to Early Carboniferous). Differentiated erosion, caused by localized tectonic uplifts, repeatedly reshaped the distribution of platform sediments. After the dominant sedimentation periods during Cambrian, Ordovician, and Silurian, almost the whole SCD belonged to the platform.

Starting from the Middle Palaeozoic, late-Caledonian compression and uplift destroyed the previously accumulated sediments and the uppermost part of the Precambrian complexes. In Middle Palaeozoic, local fault-related uplifts were eroded in the internal and distal part of the Baltic Early Palaeozoic sedimentary basin (Baltic synecline in Baltic countries and mid-Baltic seabottom). However, in the NE part of the SCD, from NE Estonia to the Lake Ladoga area, pre-Devonian erosion removed layers of 100 - 400 m thick. In the Lake Ladoga area, pre-Devonian erosion cut off Early Palaeozoic deposits and reached Vendian deposits (Puura et al. 1996).

Devonian marine, partly lagoonal, sediments accumulated and survived in the basin covering most of the Baltic area and the eastern part of the

SCD. During the Devonian, probably, the Caledonides-derived continental sedimentation took place all over the north-western SCD including southern and central Sweden as well as southern and central Finland, whereas shallow evaporate marine basins were characteristic to the present territory of Baltic countries. The area of sedimentation decreased already in the Late Devonian as Early Carboniferous sedimentation continued in the remnant basins in the southern Baltic area, and in the eastern SCD. Large-scale Devonian sedimentation buried the Silurian and older impact structures under up to 1 - 2 km thick cover. At the same time, Devonian became a thick sedimentary target for cratering through Late Palaeozoic and, where survived, Meso- and Cenozoic.

In the SW and W areas of the SCD, Late Carboniferous - Early Permian tectonics (rifting along the Tornquist-Teysseire Lineament and in Oslo Graben) caused uplift of a large area in NE Poland (Mazury Massif) and the southern part of Sweden (Puura et al. 2000), where deposits of Cambrian to Devonian age were eroded. At the same time, central Sweden was unroofed (as dated from Siljan impact structure; see Grieve 1987). In Mazury, up to 2 km thick Late Permian, Meso- and Cenozoic sediments covered the unroofed basement again. Southern Sweden was temporarily covered by Cretaceous, but then unroofed again (Lidmar-Bergstrom 1996).

The Tertiary uplift of Fennoscandia was another epoch of destruction of the sedimentary cover, including the surroundings of Lake Ladoga, the Gulfs of Finland and Bothnia, the central and northern Baltic Sea, and the NW part of the East Baltic mainland. The final reshaping of the shield-platform distribution took place during the Cenozoic in connection with regional tectonic/palaeogeographic events, such as the beginning of ocean floor spreading in the Norwegian Sea, general uplift and erosion of Fennoscandia, and reshaping of the river systems and watersheds within the neighbouring Russian Platform (Puura and Floden 1997). The area from NW Poland to North Sea was loaded under clastic sediments whose provenance area was Fennoscandia, the present depression of the Baltic Sea and its surroundings with valley-type landforms. Pleistocene glacial erosion and accumulation diversified and smoothed the valley-shaped landscape. Pleistocene active glacial excavation removed the remnants of sedimentary cover from the present shield area, in many areas even the topmost parts of the basement (as evidenced by large number of erratic material in the SE and S surroundings of the Fennoscandian Shield) and the topmost parts of the pre-Quaternary sedimentary cover in present platform area. On top of the Lauhavuori hills in southern Finland, basal layers of probable Early Vendian have survived (Puura et al. 1996) suggesting the pre-glacial existence of sedimentary cover in southern Finland. Therefore, in large areas, especially in surroundings of the present

Baltic Sea (west coast) and Bothnian Gulf, Tertiary and Quaternary erosion exhumed the Vendian unconformity level.

Generally, duration of continental erosion dominated over the marine accumulation periods. The Devonian continental sedimentation period was considerably short and restricted to the hinterland of Caledonian mountains, NW part of the SCD. The Pleistocene glacial and Holocene accumulation was unevenly distributed forming a blanket of variable thickness.

Although the geological settings and palaeogeographical conditions at the SCD were generally uniform, there were large differences in details. As a result, scenarios of crater formation were variable (see Fig. 2). Two types of impact targets have been possible: (a) crystalline shield and (b) sedimentary platform. The last could have been covered by sea or not. Also, depending on the thickness of sedimentary cover and magnitudes of impacts, the structures may have been penetrated through the sediments (and water) into the crystalline basement.

Scenarios of crater survival and reshaping were related to post-impact evolution of areas hosting impact sites. Often, the scenarios included recurrent erosion and burial. Depending on the vertical extent of impact structures, the presence of covering sediments, and/or vertical extent of erosion, the preservation level (Dence 1972) may vary between 1 and 7.

Formation Svecofennian

Fig. 2. Typical syn-impact geological settings of meteorite impact structures within the Svecofennian Crustal Domain. 1-3 represent structures on mainland, 4-5 at sea; 1 occurs at the crystalline basement; 2 and 4 have impacted into sedimentary rocks only, 3 and 5 into both, sedimentary and crystalline rocks.

Fig. 2. Typical syn-impact geological settings of meteorite impact structures within the Svecofennian Crustal Domain. 1-3 represent structures on mainland, 4-5 at sea; 1 occurs at the crystalline basement; 2 and 4 have impacted into sedimentary rocks only, 3 and 5 into both, sedimentary and crystalline rocks.

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