Magnetic anisotropy and paleomagnetism of precambrian rocks in the Fennoscandian shield

Sammanfattning: This thesis presents three applications of the anisotropy of magnetic susceptibility (AMS) technique, dealing with tectonic and magmatic flow fabrics of Proterozoic igneous rocks in the central and northern parts of the Fennoscandian Shield. Paleomagnetic measurements were performed in attempts to put the different rock units into a geological and plate tectonic context. The Storsjön-Edsbyn deformation zone (SEDZ) separates the Rätan granite of the Trans Scandinavian Igneous Belt (TIB) and the southern Svecofennian subprovince in the southwest, from the central Svecofennian subprovince in the northeast. The zone strikes NNW-SSE, it is c. 200 km long and 5 – 30 km wide. The study of the SEDZ was performed in order to define the tectonic character of the zone and to test a previously presented tectonic model. A close-up investigation of the Rätan granite was carried out in an attempt to test if the granite was emplaced in a tectonic regime governed by compression or extension. Large parts of the Rätan granite, and also the SEDZ, have been intruded by post-Jotnian dolerite sills belonging to the Central Scandinavian Dolerite Group (CSDG). The ancient magma flow directions in different parts of the CSDG sills were defined in attempts to locate the magma source. Paleomagnetism was used to look for eventual geographical differences in time of the intrusions and also to identify new areas in the shield intruded by the CSDG. On basis of AMS data the width of the Storsjön-Edsbyn deformation zone is estimated at 10 – 12km in its central-southern part and 20 – 25km in its northern part. The orientations of the susceptibility axes vary systematically across the zone, in a way that suggests that the SEDZ is a pure shear dominated transpression zone caused by a compressive stress from the present SSW. This supports a previously proposed tectonic model of the SEDZ. Paleomagnetic data from a post-Jotnian dolerite sill and from a dyke that cut the SEDZ indicate that no plastic deformation has occurred along the zone during the past 1.25 Ga, and probably not for the past 1.5 – 1.6 Ga. The compressive stress field from the present SSW indicated in this study could be related to an initial stage of plate collision between the southwestern Scandinavian subprovince and older parts of the Fennoscandian Shield. AMS data from the 1.70 Ga old Rätan granite indicate a primary magnetic/magmatic fabric. The Rätan granite seems to have been emplaced in a tectonic environment primarily governed by NW-SE directed extension and the pluton was most likely fed obliquely with a magma source situated to the present northwest. The CSDG dolerites were previously known in four complexes in central Sweden and Finland (Ulvö, Jämtland, Dalarna and Satakunta) and from the results of this paleomagnetic study another complex (the Västerbotten complex) has been identified in northern Sweden. These complexes cover an area of c. 150 000 km2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites. The measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform c. NW or SE directed magma flows. The dolerites of the CSDG are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings. In a paleomagnetic reconstruction of Baltica versus Laurentia at c. 1.27 Ga the two continents were joined with NE Greenland attached to NW Baltica. AMS and geological data from a few dolerites and a basalt in NE Greenland indicate magma flow directions that in the tectonic reconstruction is more or less parallel to the flow of the dolerites in Sweden. This may suggest a common magma source located in the reconstructed contact between Baltica and Laurentia. The tensional regime that is reflected by the huge sill complexes is in this interpretation related to the break up of Baltica from Laurentia at c. 1.27 Ga ago.