The Opening of the North Atlantic Ocean and the Rise of the Scandinavian Mountains

Academic Background

The Scandinavian Mountains (Scandes) are a significant geomorphological feature in northern Europe, but their formation mechanism has long remained a mystery. Traditionally, mountain formation is associated with convergent plate boundaries. However, the Scandinavian Mountains are located far from any active tectonic setting, which has sparked widespread interest among scientists, particularly regarding the mechanisms behind their high-altitude topography. Although the last continental collision in this region occurred approximately 420 million years ago during the Caledonian Orogeny, and the last major geodynamic process was the opening of the North Atlantic Ocean around 55 million years ago, the connection between these events and the formation of the mountains remains unclear.

To explain this phenomenon, researchers have proposed various models, including dynamic support from the Icelandic Plume, underplating of the lower crust, delamination of the lower crust, and edge-driven convection. However, evidence for these models remains insufficient, particularly due to the lack of detailed data on the crust and mantle structure beneath the Scandinavian Mountains. Therefore, this study aims to reveal the deep structure of the Fennoscandia region through seismic receiver function analysis, providing new insights into the formation of the Scandinavian Mountains.

Source of the Paper

This paper was co-authored by Anna Makushkina, Benoit Tauzin, Meghan S. Miller, Hrvoje Tkalčić, and Hans Thybo. The authors are affiliated with the Research School of Earth Sciences at the Australian National University, Université Claude Bernard Lyon 1 in France, the Chinese Academy of Geological Sciences, Istanbul Technical University, and the China University of Geosciences (Wuhan). The paper was published online on October 15, 2024, in the journal Geology, with the DOI 10.1130/G52735.1.

Research Process and Results

1. Data Collection and Seismic Receiver Function Analysis

The research team first collected broadband seismic data from the Fennoscandia region, including the recently acquired SCANarray dataset. Using these data, the researchers employed common conversion point (CCP) stacking of receiver functions (RFs) to image the deep structure of the region. Receiver function analysis is a widely used seismological method that can reveal the structure of crust-mantle interfaces.

The team focused particularly on S-to-P (S-wave to P-wave) and P-to-S (P-wave to S-wave) converted receiver functions (SRF and PRF). Although the results from both methods were generally consistent, the SRF images provided clearer delineation of the deepest crustal boundaries, so the analysis primarily relied on SRF images.

2. Crustal Structure of Fennoscandia

Through SRF analysis, the researchers identified three distinct crustal blocks in the western Fennoscandia region: the southern block (south of approximately 63°N), the central block (between approximately 63°N and 67°N), and the northern block (north of approximately 67°N). Each block exhibits unique crustal characteristics:

• Southern Block: The crust is 30–45 km thick, with a relatively simple structure composed mainly of Proterozoic terranes and orogens.
• Northern Block: The crust is 40–45 km thick, with a pronounced mid-crustal discontinuity, primarily consisting of Archean terranes that may have been reworked during the Paleoproterozoic.
• Central Block: The structure is more complex, with crustal thickness ranging from 25 to 60 km, displaying two distinct discontinuities (M1 and M2). M1 is located at a depth of approximately 25–40 km, while M2 is found at 45–60 km. M2 is interpreted as a continuation of the crust from the northern block, likely representing a stacked crustal structure.

3. Formation Mechanism of the Stacked Crustal Structure

The researchers proposed that the stacked crustal structure in the central block may have formed during a late Proterozoic collision. This stacking could result from the underthrusting of the southern block’s crust beneath the northern block’s crust, preserving two original crust-mantle interfaces (M1 and M2). Alternatively, the top of the stacked crust may have undergone eclogitization, creating the strong impedance contrast observed at the M1 interface.

4. Relationship Between the Opening of the North Atlantic and Crustal Structure

The study also found that the crustal structure of Fennoscandia significantly influenced the opening of the North Atlantic Ocean. The thick crustal structure of the central block may have acted as a mechanical barrier during continental breakup, causing a jump in the rift axis and forming two major transform fault systems: the Jan Mayen Fracture Zone and the De Geer Megashear System. This structure controlled the geometry of the North Atlantic opening and created lithospheric steps at the continental margin.

5. Edge-Driven Convection and Mountain Formation

The researchers further proposed that the high topography of the southern and northern Scandinavian Mountains may be related to edge-driven convection at the lithospheric steps. This convection mechanism generates localized upwelling at steep lithospheric steps, providing additional dynamic buoyancy to the crust and supporting mountain uplift. In contrast, the wide continental shelf in the central block, where the lithosphere thins gradually, lacks this convection mechanism, resulting in relatively lower topography.

Conclusions and Significance

This study, through seismic receiver function analysis, revealed the deep crustal structure of the Fennoscandia region and proposed a novel model to explain the formation of the Scandinavian Mountains. The results indicate that the high topography of the Scandinavian Mountains is not supported by a crustal root but is instead driven by edge-driven convection. This finding not only provides a new explanation for the formation of the Scandinavian Mountains but also offers insights into the mechanisms behind high topography at other passive continental margins worldwide.

Research Highlights

1. Novel Crustal Stacking Structure: The study is the first to reveal the stacked crustal structure in the central block of Fennoscandia and propose two possible mechanisms for its formation.
2. Role of Edge-Driven Convection: The study is the first to link edge-driven convection to the formation of the Scandinavian Mountains, providing a new explanation for high topography at passive continental margins.
3. Control Factors of North Atlantic Opening: The study highlights the influence of Fennoscandia’s crustal structure on the opening of the North Atlantic, offering new perspectives on the geometry of continental breakup.

Additional Valuable Information

The study also validated the impact of thick crustal structures on continental breakup processes through analogue modeling, further supporting the research conclusions. Additionally, the research team provided detailed supplementary materials, including additional seismic profiles and data analysis methods, for reference by other researchers.

This study not only addresses the long-standing mystery of the formation of the Scandinavian Mountains but also provides important theoretical support for understanding the formation mechanisms of similar geomorphological features worldwide.