Disputas: Mattias Lundmark

Prøveforelesning

Bedømmelseskomité

Jim Connelly, PhD, University of Texas at Austin, U.S.A.
Øystein Nordgulen, PhD, The Geological Survey of Norway (NGU)
Håkon Austrheim, PhD, Institutt for geofag, Universitetet i Oslo

Sammendrag

In this thesis high-precision uranium-lead geochronology, allowing the precise dating of rocks, is used to study the growth of the Baltic Shield during a time period that spans nearly 1.700,000,000 years.

The study focuses on rocks in the Jotunheimen in south-western Norway. These rocks make up thrust sheets that were pushed onto the Norwegian basement by the Caledonian collision with East Greenland, one of several continent-continent collisions that led to the formation of the supercontinent Pangea.

It is shown that most of the rock types in the Jotunheimen thrust sheets formed ca 1650 million years ago. This probably took place in a tectonic setting similar to the present-day Andes in South America, where oceanic crust colliding with the continent gives rise to volcanism and mountain building. The Jotunheimen rocks were then metamorphosed, partially melted and deformed ca 1000 million years ago during the Sveconorwegian collision with an unidentified continent (Greenland?). The collision formed an ancient “Himalaya” along the south-western edge of Norway and Sweden and was part of the assembling of the supercontinent Rodinia. The ages show that the Jotunheimen rocks themselves come from the lower part of the Baltic Shield, ripped off and thrust onto Norway during the later Caledonian collision.

Towards the end of the Sveconorwegian collision, ca 965 million years ago, a vast complex of anorthositic rocks south-west of the Jotunheimen National Park (centred around Gudvangen) was formed. These rocks form only under special circumstances, and it is therefore proposed that cold mantle below the Baltic Shield was peeled off from the bottom of the continent and sank towards the end of the Sveconorwegian. The cold mantle was replaced by hot, up-welling mantle, leading to magmatism and formation of the anorthositic rocks. These were subsequently metamorphosed twice more during the Sveconorwegian, illustrating the complexity of the evolution of the lower crust during the final stages of a continent-continent collision.

The youngest rocks dated are 427 ± 1 million year old granites that intrude the older rocks as sub-parallel, meter-thick dykes. It is demonstrated that the geometry of the granites was controlled by deformation of the Jotunheimen rocks during the Caledonian continent-continent collision. The age of the granites therefore date the thrusting of the Jotunheimen rocks onto the Norwegian basement.

Kontaktperson

For mer informasjon, kontakt Nils Roar Sælthun.