The relationship between brittle tectonics and bedrock morphology of Central Fennoscandia

The bedrock of Central Fennoscandia has been shaped by a long and complex geological history involving ductile deformation and metamorphism which dates back at least to the 1.9–1.8 Ga Svecofennian orogeny. Subsequent geological processes including Precambrian brittle faulting and fracturing, younger fault reactivations and several stages of hydrothermal activity and alteration processes, provided further contribution to defining the present-day bedrock structure and mechanical properties. Eventually, extensive glaciation affected the exposed upper part of the bedrock through structurally selective erosion, which is largely responsible for the morphology of the bedrock erosion surface. As such, the brittle tectonic history, involving faulting and fracturing near the Earth’s surface, has played a significant role in shaping the current bedrock topography. However, also the preceding ductile structures played a role as they caused the localisation of the brittle deformation through the process of structural inheritance.

Based on previously published results the brittle tectonic development within Central Fennoscandia initiated in response to N–S compression at around 1.75 Ga. Based on our new datasets, consisting of isotopically dated fault gouge samples and brittle structural observations (Nordbäck et al., 2022), N–S extension at around 1.65 Ga and a E–W extension at around 1.6 Ga were associated with 1) reactivations of previously formed major structures of the bedrock, 2) rapakivi magmatism and 3) the development of a (failed) continental rift between Finland and Sweden. Our structural data from within the 1.58 Ga rapakivi granites indicate that strike-slip tectonics prevailed during Mesoproterozoic times. According to isotopic and structural data from Olkiluoto in southwestern Finland, thrust faults were generated in response to E–W compression during the Sveconorwegian orogeny between 1.1–1.0 Ga. The younger stress changes that induced faulting activity, have been found to cause merely reactivations of the fault systems that were formed already by late Mesoproterozoic times. Based on our structural datasets from the 1.58 Ga rapakivi granites, paleostress analysis and observed relative age relationships between faults and joints, Neoproterozoic exhumation of the bedrock appears to have resulted in extensional bedrock stresses and the development of Precambrian bedrock joints.        

Erosional processes during the Quaternay glaciations interacted strongly with the existing brittle structures which were preferably eroded during the glacial advances and retreats. Especially the intensely fractured major fault zones greatly impacted the current bedrock morphology while smaller structures, such as individual joints or shear fractures, only have a local impact (Skyttä et al., 2023).

References:

Nordbäck, N., Mattila, J., Zwingmann, H., Viola, G., 2022. Precambrian fault reactivation revealed by structural and K-Ar geochronological data from the spent nuclear fuel repository in Olkiluoto, southwestern Finland. Tectonophysics 824, 229208. https://doi.org/10.1016/j.tecto.2022.229208

Skyttä, P., Nordbäck, N., Ojala, A., Putkinen, N., Aaltonen, I., Engström, J., Mattila, J., Ovaskainen, N., 2023. The interplay of bedrock fractures and glacial erosion in defining the present-day land surface topography in mesoscopically isotropic crystalline rocks. Earth Surface Processes and Landforms. https://doi.org/10.1002/esp.5596