TESTING PROTEROZOIC SUPERCONTINENTS: NEW PALEOMAGNETIC DATA FROM FINLAND

Two different Proterozoic supercontinents have been proposed of which Hudsonland (also known as Columbia or Nuna) prevailed from ca. 1800 Ma to ca. 1200 Ma and Rodinia from 1100 Ma to ca. 750 Ma. However, there are several questions to be answered. For example, a debate is ongoing of which continents were involved and what were the life spans of these supercontinents? Also, the exact times when each continent amalgamated to supercontinents are poorly defined. Finally, there are contrasting opinions of whether Baltica and Laurentia were joined throughout the ca. 600 Myrs life span of Hudsonland (e.g., Halls et al., 2007; Salminen and Pesonen, 2007; Pesonen et al., 2003). To shed further light into these question we have carried out paleomagnetic and rock magnetic studies of the ca. 1870 Ma old Keuruu dykes and host rocks in central Finland. In addition, we also sampled impactite and target rocks of the Keurusselkä meteorite impact structure, located slightly southwest from the Keuruu dyke swarm The age of the impact is poorly known but lies between 1.8-1.25 Ga. To better define the amalgamation of Baltica to Rodinia, we have studied paleomagnetic properties of the 1122 ± 5 Ma (U-Pb) Salla diabase dyke in northern Finland. This dyke, being 60-140 m wide and more than 130 km long, cuts all the older Paleoproterozoic rocks and belongs to the youngest rock formations in the area.

The new paleomagnetic data of Keuruu area and of the Salla dyke provide a refinement for the apparent polar wander path of Baltica during the Proterozoic. We will compare the apwps of Baltica and Laurentia in terms of the possible long joint history of the two continents and provide a new scenario of the onset of Rodinia.

References

Denyszyn, S., et al., 2007. Paleomagnetism and age of Proterozoic magmatic events in the Canadian high arctic and Greenland. In: The Pulse of the Earth & Planetary Evolution: Rates and Rhythms, Cycles and Cataclysms, W. Bleeker and P. Sylvester (eds.), GAC NUNA Conference, Sudbury, 13-17 June, 2007, Program & Abstract Volume, p.22

Pesonen, L. J., et al., 2003. Paleomagnetic configuration of continents during the Proterozoic. Tectonophys. 375: 289-324.

Salminen, J., and Pesonen, L. J., 2007. Paleomagnetic and rock magnetic study of the Mesoproterozoic sill, valaam island, Russian Karelia. Prec. Res. doi:10.1016/j.precamres.2007.06.009.