Friday, 25 July 2014

The origin of Lake Vättern.

Lake Vättern is the second largest lake in Sweden. It lies in the south of the country, and is 135 km long and 31 km in width at its widest point, with a total surface area of 1893 km2, with a maximum depth of 117 m (check). The lake lies in a graben (depression formed by extensional rifting) on the Sveconorwegian Front, which marks the boundary between the 900 000 000 to 1 140 000 000 year old rocks of the Sveconorwegian Province to the west and older Svecokarelian rocks to the east. The Lake Vättern Graben is thought to have originally formed between 700 and 800 million years ago, however the lake itself is not likely to be this old, and is thought to have formed by reactivation along the fault at the end of the Pleistocene, a time when considerable faulting is thought to have occurred in northern Sweden, when the sudden retreat of the ice sheets triggered a series of major Earthquakes in the region.

In a paper published in the journal Geology on 17 March 2014, a team of geologists led by Martin Jakobsson of the Department of Geological Sciences at Stockholm University describe the results of an investigation into the origins of Lake Vättern.

(A) Map of southern part of Lake Vättern in south-central Sweden, showing coverage of multibeam mapping and singlechannel seismic profiling (blue lines) carried out in 2008 and 2013. Red dashed line outlines location of central fault system assuming that it follows mapped bathymetric depressions. (B) Overview map showing location of Lake Vättern in Sweden and inferred faults in Lansjärv area SF indicates the Sveconorwegian front. Jakobsson et al. (2014).

Jakobsson et al. carried out echo sounder and seismic profiling of the lake bottom in 2008 and 2013, and sediment drill-coring in 2012. They found that the deepest part of the lake formed a south-southwest to north-northeast trending trough, reaching a maximum depth of 117 m. the deepest part of this trough was characterised by bathymetric undulations, interpreted as collapse structures or subsidence zones, which reach as much as 100 m wide and more than 10 m in depth. Similar structures were found in the strait between the island of Visingsö and the town of Gränna, and on the eastern slopes, where the lake-floor has the appearance of a staircase. These structures are interpreted as the surface expression of slumps caused by movement of the underlying bedrock.

Multibeam imagery illustrating two areas of Lake Vättern graben, with collapse structures and slides. (A) Mass wasting and collapse structures in southern part of Lake Vättern, near drill site. (B) Location of sediment core, strategically placed to capture timing of major seismic event. S1–S3 are slide scars. (C) Structures similar to those in A, along southeast coast of island of Visingsö (C is a perspective plot). Locations of A and B are shown in the top figure. Jakobsson et al. (2014).

These collapse structures are also clearly visible in acoustic stratigraphic profiles built up of the lake bottom, indicating that sediment layers close to the surface closely follow these structures. However in the strait between Visingsö and Gränna these structures are overlain by more recent sediments. Signs of sediment deformation were also found close to the island of Jungfrun in the northern part of the study area. 

(A) Seismic reflection profile A-A′ across graben in southern part of Lake Vättern. (B) Subbottom profile B-B′ (TWT—two-way traveltime). C: Enlargement of B, with location of drilling site marked in profiles. Major stratigraphic boundaries are inserted (GC II—glacial clay unit II; GC—glacial clay unit I; PGC—postglacial clay; GC—gyttja clay). Locations of profiles are shown in the top figure. Jakobsson et al. (2014).

Drill-coring revealed a layer of sandstone 164 m bellow the lake floor and a layer of granite 189 m below this. Carbon dating of a bulk sediment sample from the lake gave a date of 19 000 years ago, but since the area is known to have been covered by a thick layer of ice at this time this date is considered unlikely, and is thought to result from reworked organic material (i.e. organic material from outside the area that has been carried here by glacial or fluvial processes), a phenomenon which is common in glacial sediments. The dominant pollen types within the core were Betula (Birch), Juniperus (Juniper) and Pinus (Pine), Salix (Willow), Artemisia (Sagebrush), Chenopodiaceae (Goosefoot) and Grasses also present. This strongly suggests that the sediments date from the Younger Dryas–Preboreal transition zone (latest Pleistocene or earliest Holocene), within a few hundred years of the deglaciation of the region, as later pollen profiles become totally dominated by Birch and Pine.

Subbottom profiles across Lake Vättern graben. A,B: Profile C-C′ east of island of Visingsö. C,D: Profile D-D′ near island of Jungfrun in northern part of study area. Locations of profiles are shown in the top figure. Estimation of vertical displacement is inferred in B and D. Jakobsson et al. (2014).

Jakobsson et al. conclude that all the sediment structures were caused by a single tectonic event, with a rupture length of 125 km and maximum vertical displacement of 13 m, between Visingsö and Gränna. They further estimate that such an event would require a Magnitude 7.5 Earthquake, triggered by a sudden release of pressure associated with the rapid withdrawal of the glacial ice sheets. This is consistent with the estimated Magnitude 7–7.8 event on the Lansjärv Fault and estimated Magnitude 8.2 on the Pärvie Fault, both in northern Sweden, which are thought to have been roughly contemporary and which also resulted in large sublacustrine landslides.

The pollen analysis of the sediments suggests that this event took place slightly after the drainage of the Baltic Ice Lake at the end of Younger Dryas. This occurred when the Scandinavian Ice Sheet retreated north of Mount Billingen; the ice sheet had acted as a dam, trapping the waters of the lake, but once this had passed Mount Billingen it failed, resulting in the release of about 7800 km³ of water with an average depth of 25 m, over a period of less than two years. This water drained into the North Sea, resulting in a huge drop in pressure on the underlying rocks, and a series of massive Earthquakes caused by glacial rebound (the rocks of the Scandinavian lithosphere were pushed down into the underlying mantle by the weight of the ice and water; when this went they were pushed back up).

See also…


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