Deep Earth Structure in Fennoscandia: New Insights From Passive Seismic Imaging

Abstract

Fennoscandia represents a geologically diverse and tectonically complex region that offers a unique window into Earth's lithospheric evolution. Its crustal structure, shaped by the assembly of microcontinents during the Archean and Proterozoic, has been further modified by subsequent tectonic events such as orogenies, continental breakup, and rifting. Despite decades of research, significant uncertainties remain, primarily due to limited seismic data coverage and the complex tectonic history of the region.

One of the most intriguing aspects of the deep structure of Fennoscandia, and a primary focus of this thesis, is the origin of the Scandinavian mountains - Scandes. Unlike traditional orogenic belts, these mountains are not associated with active plate boundaries and their origin remains debated. Understanding the geodynamic mechanisms behind the formation of these enigmatic mountains requires detailed knowledge of the crustal and mantle structure beneath Fennoscandia.

To image the deep structure of Fennoscandia, this thesis integrates data from the ScanArray experiment (2012-2017) with existing seismic networks. This enhanced network allows unprecedented resolution in imaging crustal and mantle structures across Fennoscandia. By applying P- and S-wave receiver function (RF) techniques and common conversion point (CCP) stacking, the study maps the Moho depth, investigates the mantle transition zone, and identifies key structural features across the crust and subcrustal regions.

The results of this study highlight the laterally consistent geometry of the discontinuities of the mantle transition zone at depths of 410 and 660 km, with no mantle thermal anomalies detected that could account for the enigmatic Scandes. In contrast, RF reveal significant crustal heterogeneity. A network of RF stacked in CCP profiles allows delineation of the boundaries of major tectonic blocks, such as the Archean Norrbotten craton and Proterozoic Bothnia and Bergslagen regions, and maps both known and previously unseen structures. The latter include large-scale subcrustal dipping reflectors, such as paleosubduction features in southern Bothnia, which may represent some of the oldest seismic evidence for plate tectonics, and a possible eclogitic crustal root beneath a large portion of the Archean Karelian craton.

A key discovery of this thesis is the southwest-dipping extension of the Norrbotten craton beneath the Proterozoic Svecofennian domain, forming a 60-km-thick crustal stacking structure. This block extends offshore, forming a wide continental margin. The stacked structure likely acted as a mechanical barrier, controlling the formation of transform fault systems and influencing the geometry of the North Atlantic Ocean during continental breakup.

This stacking structure also proved crucial for understanding the origin of the Scandes. The low topography along the Norwegian coast aligns with a gradual lithospheric thinning where the stacking structure is located, whereas the highest elevations of the Southern and Northern Scandes coincide with sharp lithospheric steps around it. These steps, combined with mantle flow parallel to ocean spreading, create conditions for edge-driven convection, a dynamic process that generates localized mantle flow, adding buoyancy to the crust and supporting mountain uplift. This mechanism explains the distribution of both low- and high-topography areas across the entire Scandinavian margin and suggests a broader applicability to other passive margin settings.

Overall, this thesis provides a new structural framework for understanding the tectonic evolution of Fennoscandia. It resolves long-standing questions about the origin of the Scandes and offers broader insights into upper mantle processes, continental dynamics, and the Precambrian terrains of Fennoscandia, providing a robust structural basis for future geodynamic studies in the region. Show more