Analog models of the lateral spreading of a thick three-layer crust : implications for the Svecofennian orogen in Finland

Our knowledge of the crustal structure of the Precambrian Svecofennian orogen has been enhanced during the last decade. Much of the new knowledge is due to the deep seismic reflection studies (Finnish Reflection Experiments; FIRE), which transect the main lithological units and tectonic boundaries on the Finnish side of the Fennoscandian Shield, showing a frozen image of the crustal structure. Understanding of the crustal structures improves interpretations of crustal evolution. However, the tectonic events and processes causing the anomalies/structures cannot be directly defined from the profiles. The processes can be studied by analog modeling, which is a viable tool for filling in absent components of a puzzle of reconstruction of the crustal evolution. The analog modeling can yield information on the generation and the development of crustal scale structures and crustal components, for example, after mountain building process. In this thesis, post-accretion crustal evolution of thickened orogenic crust is studied via analog modeling. Geochronological, geochemical and geophysical data from the central part of the Svecofennian orogen are used to compare analog models to nature. The thesis presents two sets of analog modeling experiments, thermomechanical and centrifuge, which are simulating the lateral spreading of a three-layer crust after thermal relaxation. In the modeling experiments, it is assumed that the spreading is caused by the differences in gravitational potential energy between the thickened orogenic crust and the adjacent thinner areas. In the experiments, the spreading is dictated towards free space. In the centrifuge modeling, the spreading is gravitationally induced and produces lateral flow and ductile uplift. In the thermomechanical modeling, the spreading is thermally induced and it only produces lateral flow. The analog modeling results show that a thick accretional orogenic crust with ductile middle layer will undergo lateral and gravitational spreading in all crustal layers. The spreading results in subsiding of upper and middle crusts, uplifting of lower and middle crusts; thinning, thickening and elongation of middle and lower crusts; and crustal scale block rotation. The lateral spreading can also result in reverse faults and shortening of layers and blocks, although the extensional structures are predominant. The lateral spreading rotates tectonic boundaries towards the spreading direction by elongating and shortening those. The deformation is most dominant in the ductile middle layer. In the models where tectonic boundaries are present, the spreading is accommodated on both sides of the boundary zones indicating that a crust, undergoing gravitational spreading, suffers from deformation across the orogenic belt. The geophysical data – aeromagnetic, seismic reflection and refraction data – from the central part of the Svecofennian orogen have shown that the orogenic belt is composed of fragmental crustal blocks separated by large scale shear zones. The comparison to analog models suggests that the crustal scale shear zones and the wide bands of high reflectivity in seismic profiles are reactivated terrane boundaries and thick-skin stacking surfaces, which have elongated and rotated westward. Geochronological, geochemical and structural data from the Paleoproterozoic Central Finland granitoid complex show that the plutons have been emplaced at a post-accretion stage during middle crustal exhumation and lateral spreading. The exhumation and the lateral spreading started at ca. 1884 Ma and continued until 1870 Ma. The rocks can be divided into three groups by their geochemical and geochronological characteristics. Group 1 rocks have formed from melts from the lower crust and have been emplaced >1887 Ma to the upper and the middle crusts. Group 2 rocks have formed from melts from the middle crust and have been emplaced at 1890-1886 Ma and at 1884-1883 Ma to (sub)surface and to the upper part of the middle crust, respectively. Group 3 rocks have formed from melts from the lower crust and they have been emplaced at 1881-1880 Ma in the upper parts of the middle crust. Each magma group represents an important event in the crustal evolution: the Group 1 magmas are the first post-accretion melts of the thickened crust, the Group 2 magmas are the result of widespread middle crustal melting due to thermal relaxation, and the Group 3 magmas are emplaced during extensional event. The ongoing exhumation between 1884 Ma and 1870 Ma changed the deformation style of the plutonic rocks from pervasive foliation to localized shear zones. The comparison between the CFGC and the adjacent areas shows that the coeval deformation and magmatism are present in the entire central part of the orogen. Furthermore, the analog models and the data from the central part of the Svecofennian orogen in Finland suggest that the coeval exhumation, the lateral spreading and changes in the magma type across the orogen can be explained by westward gravitational spreading.