High-resolution imaging of the Kuusamo Belt, northern Finland, from ambient noise tomography

The Kuusamo Belt of northern Finland formed as an Early Proterozoic rift system within the Archean crust of the Fennoscandian Shield between ~2.5 and 2.0 Ga. Following its formation, it has recorded several major shortening episodes, resulting in a heavily folded medium-grade metamorphic belt. More recently, this region has garnered interest due to the presence of Au and Co deposits within the belt. As a part of the multidisciplinary project UNDERCOVER, we present a preliminary ambient noise tomography model of the region focused on the crustal architecture around these deposits, using 493 nodal seismometers that were deployed over a 35x35 km region from June to August 2025. This dense array was inset within a larger network of 35 broadband stations spanning 150x170 km. Combining these arrays, we extract path‐averaged Rayleigh wave phase‐velocity dispersion measurements for >130,000 interstation paths to constrain the shear wave velocity structure of the upper 40 km. Our preliminary model resolves the large-scale crustal structure with the bulk crust characterized by shear wave velocities >3.0 km/s up to very shallow depths, consistent with wavespeeds sampling Archean greenstones and Paleoproterozoic mafic rocks which outcrop within the study region. To first order, broad-scale velocity perturbations align well with the trend of large-scale folding in the region. Going forward, we hope to take advantage of the dense nodal array and incorporate high-frequency phase velocities in a single model to refine the shallow subsurface structure and better characterize the relationship between velocity anomalies, structural features, and mineralization.

This research has received funding from the European Union through the Horizon Europe project UNDERCOVER (Grant agreement No. 101177528).