Annakaisa Korja

The Svecofennian orogen: a collage of microcontinents and island arcs

Abstract Based on an integrated study of geological and geophysical data, a tectonic model for the Palaeoproterozoic evolution of the Svecofennian orogen within the Fennoscandian Shield at the northwestern corner of the East European Craton is proposed. The Svecofennian orogen is suggested to have formed during five, partly overlapping, orogenies: Lapland-Savo, Lapland-Kola, Fennian, Nordic and Svecobaltic. The Svecofennian orogen evolved in four major stages, involving microcontinent accretion (1.92-1.88 Ga), large-scale extension of the accreted crust (1.87-1.84 Ga), continent-continent collision (1.87-1.79 Ga) and finally gravitational collapse (1.79 and 1.77 Ga). The stages partly overlapped in time and space, as different processes operated simultaneously in different parts of the plates. In the Lapland-Savo and Fennian orogenies, microcontinents (suspect terranes) and island arcs were accreted to the Karelian microcontinent, which itself was accreting to Laurentia in the Lapland-Kola orogeny. The formation of the Svecofennian orogen was finalized in two continental collisions producing the Nordic orogen in the west (Fennoscandia-Amazonia) and Svecobaltic orogen in the SSW (Fennoscandia- Sarmatia). The collisions were immediately followed by gravitational collapse.

Palaeoproterozoic accretionary processes in Fennoscandia

Abstract Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c. 1×106 km2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c. 1.89 and c. 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

Focal mechanisms of three earthquakes in Finland and their relation to surface faults

Abstract Focal mechanisms for three recent earthquakes in Finland are determined using P-wave polarities together with SV/P and SH/P phase amplitude ratios. The events occurred on May 11, 2000 in Toivakka, Central Finland (ML=2.4), on September 15, 2000 in Kuusamo, northeastern Finland (ML=3.5), and on May 2, 2001 in Kolari, western Finnish Lapland (ML=2.9). In order to obtain reliable estimates of the source parameters, one-dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep-seismic sounding results. The starting models are modified by one-dimensional ray tracing using the earthquake observations. The events are relocated by employing P- and S-phase arrival times from the nearest seismic stations and the final velocity models. Synthetic waveforms, calculated with the reflectivity method, are used to further constrain and verify the source and structural parameters. The Toivakka earthquake indicates thrust- or reverse-faulting mechanism at a depth of 5 km. After comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane (358°/42°) was the fault plane. The best-fitting fault plane solution of the Kolari earthquake suggests pure thrust-faulting at a depth of 5 km. The nodal plane striking 035°/30° correlates well with surface observations of the postglacial, possibly listric fault systems in the source area. The Kuusamo earthquake (focal depth 14 km) has a normal-faulting mechanism with the nodal planes trending 133°/47° or 284°/47°. Preference is given to the SW-dipping nodal plane, as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age. The three earthquakes have occurred in old Precambrian faults and shear zones, which have been reactivated. The reactivated faults are favourably oriented in the local stress field.

A case study of lateral spreading: the Precambrian Svecofennian Orogen

Abstract We have studied the crustal structures of the Palaeoproterozoic Svecofennian (c. 1.9 Ga) Orogeny with the help of large scale seismic reflection surveys (FIRE 1–3), preliminary structural field work and geological and geophysical databases. The central part of the orogen is occupied by the Central Finland Granitoid Complex, which comprises two suites of granitoid rocks and associated mafic and volcanic rocks. The complex and the surrounding supracrustal belts are cut and deformed by numerous shear zones and faults; here divided into six groups. The most prominent reflections are usually shear zones or faults on outcrops. The granitoid complex is interpreted as a deep, lower-level section of an old core complex, where the younger granitoid intrusions form the basins and older granitoid intrusions and associated volcanic rocks form the horsts. The upper–middle crust detachment zone is exposed at the northeastern edge of the complex and middle crust is exposed in the migmatitic domes at northern and western margins. The seismic reflection sections display a frozen image of orogenic thickening and lateral spreading. The decoupling of the upper, middle and lower crust during spreading resulted in the formation of layered superstructure–infrastructure of the crust.

Examining Three‐Dimensional Crustal Heterogeneity in Finland

The marriage of several high-quality seismic experiments in Finland over the past 30 years has shown that the saying “something old, something new, something borrowed” can result in the cost-efficient analysis of large-scale, three-dimensional (3-D) seismic structures. Standing alone, each data set gives a partial view of complex 3-D structures. When combined, they reveal a 3-D block structure embedded in a layered crust and enable the analysis of dynamics involved in forming stable cratonic crust. Efforts to collect large 3-D data sets around the globe include EarthScope (funded by the U.S. National Science Foundation (NSF)), the European Science Foundations (ESF) 4-D Topography Evolution in Europe: Uplift, Subsidence and Sea Level Change (TOPO-EUROPE), and the European Space Agencys Gravity Field and Steady-State Ocean Circulation Explorer (GOCE). Such endeavors are fundamental to modern crustal research. Huge emphasis is placed on collecting and archiving these data, but often only a fraction of data are used in initial studies. Fortunately, new data sets can be complemented with vintage ones (e.g., the NSF-funded Consortium for Continental Reflection Profiling (COCORP) and ESFs European GeoTraverse (EGT), as well as continent-wide science programs on continental evolution in Canada (LITHOPROBE), Europe (ESF-funded EUROPROBE), and the Himalayas (NSF-funded International Deep Profiling of Tibet and the Himalaya (INDEPTH)). Because fieldwork and archiving have already been completed for these vintage projects, new information can be extracted by new methods, with considerably less effort and funding.