Opposing upper-mantle reflectivity beneath the northern Fennoscandian Shield: A possible record of multiple Paleoproterozoic subduction systems

The historical offshore BABEL seismic lines from the Bothnian Bay have played a key role in shaping interpretations of Paleoproterozoic tectonic evolution within the Fennoscandian Shield. However, the legacy data from BABEL lines 2–4 have remained underutilized since their original acquisition and interpretation more than 35 years ago. Here, we reprocess these lines using modern seismic workflows, resulting in substantially improved images of crustal and upper-mantle structure. The reprocessed data reveal two distinct bands of oppositely dipping upper-mantle reflectivity: the previously recognized northeast-dipping mantle reflectivity, re-imaged with greater structural detail and comprising a 10−15 km-thick band of reflectivity, and a newly resolved southwest-dipping reflectivity that extends laterally for 50 km and penetrates to depths of at least 30 km beneath the Moho. We interpret the northeast-dipping reflectivity as possibly recording a relict Paleoproterozoic subduction system, while the southwest-dipping reflectivity may represent a subduction-related suture spatially associated with the Pajala Deformation Belt and the Savo Belt. Together, these oppositely dipping domains may record a more complex Paleoproterozoic collisional architecture than previously recognized, potentially involving subduction-polarity reversal or tectonic switching during the Svecofennian accretion. The mantle reflectivity is also spatially associated with major mineralized belts, including volcanogenic massive sulfide deposits in northern Sweden and Finland, suggesting that lithospheric-scale seismic features may fingerprint metallogenic processes. Our results demonstrate the continued scientific value of legacy crustal-scale seismic datasets and provide new geophysical constraints on Paleoproterozoic tectonic evolution and mineral systems in northern Fennoscandia.