3D structure and physical properties of the Kuhmo Greenstone Belt (eastern Finland): Constraints from gravity modelling and seismic data and implications for the tectonic setting

In our paper we study 3D structure of the Kuhmo Greenstone Belt (2800-2760 Ma) located in the Finnish part of the Archean Karelian craton in the Fennoscandian Shield. Although the surface position of the belt is relatively well constrained by geological studies, its 3D structure is poorly known. Previously, two groups of competing geodynamic models have been proposed to explain formation and stabilization of the belt. The first group suggest that the belt may be formed in intracratonic, extensional rift environment with igneous rocks being derived from hot, upwelling mantle plume. In this case one would expect that the belt has a diapiric shape with deep root. The other suggests that the belt may be formed by collision and subduction processes, resulting in thin elongated shape and shallow root. Thus the knowledge of the 3D structure of the belt is essential to distinguish between two groups of models. The belt is marked by a regional-scale positive Bouguer anomaly, indicating strong density contrast between the belt and surrounding bedrock. In spite of that, previous seismic studies (SVEKA'81 wide-angle reflection and refraction profile and FIRE1 near-vertical reflection profile) revealed pronounced change in the crust-mantle boundary beneath the belt (from almost 60 to 40 km), but no any velocity or sharp reflectivity contrasts between the belt and its surrounding. In order to explain this apparent disagreement, we estimated seismic velocities and density of the main rock types of the belt from modal mineralogy. The result of the modelling suggests that the main rocks types of the belt have low seismic velocities and high densities due to high content of amphibole, biotite and muscovite. This explains why the belt is not seen in the seismic data, in spite of the gravity high. Thus we applied 3D gravity modelling and inversion in order to obtain the 3D density structure of the belt. Our result demonstrated that the Kuhmo Greenstone Belt is a surface structure that is 10-12 km wide with depth less than 7 km and average density of 2.84 × 103 kg/m3. The belt has no deep “root” in the middle crust and lower crust. The difference in Bouguer anomaly west and east of the belt is explained by the regional trend produced by the Moho uplift. Our model implies that the belt was formed as a result of plate-tectonics like processes rather than mantle diapirism, although these processes not directly comparable to the Phanerozoic ones due to the higher heat flux in Late Archean.