Uncovering fossil subduction in a mineral-endowed Paleoproterozoic terrain: Reprocessing legacy BABEL seismic lines from Bothnian Bay, Sweden

In 1989, as part of a collaborative effort involving 12 research institutions and known as the BABEL Project (Baltic and Bothnian Echoes from the Lithosphere), 2,268 km of crustal-scale seismic lines were acquired in the Gulf of Bothnia and the Baltic Sea. The lines were acquired using near-vertical reflection, and wide-angle refraction methods, providing insights into the evolution of plate tectonic processes during the Paleoproterozoic era.

The offshore, near-vertical seismic data were collected using a 3 km-long cable comprising 60 groups of 64 hydrophones, positioned at a depth of 15 m. An airgun array, consisting of six identical subarrays, was used as the seismic source and towed at a depth of 7.5 m. The group spacing, shot interval, and record length varied between the lines. Specifically, in this study, the record lengths for lines 3 and 4 were 25 s, with a group spacing of 50 m and a shot interval of 75 m, while for line 2, the record length was 23 s with a group spacing of 25 m and a shot interval of 62.5 m, respectively. A sampling rate of 4 ms was used for all three lines.

Lines 2, 3, and 4 in the Bothnian Bay are located between the volcanic-hosted massive sulphide belt of Skellefte in Sweden and Vihanti-Pyhäsalmi in Finland. Given the historic value of the data and within the scope of a mineral systems workflow, we have recovered these data digitally to take advantage of modern processing and imaging solutions. Original processing showed divergent reflectivity reaching the lower crust of a Precambrian crystalline basement in the Baltic Shield. In addition, a prominent dipping reflector extending into the upper mantle was imaged, offsetting the Moho by 5-10 km. These findings led to the suggestion, for the first time, of active plate tectonic processes during the Paleoproterozoic time.

The reprocessing work reveals reflections as shallow as 1 s and shows a series of individual reflections and diffraction signals. The Moho boundary is significantly improved in terms of both its signature and trackability and, as in previous investigations, we show a set of sub-Moho reflections dipping down to 23-25 s. Not only have we brought the data to life, but we have also turned them into compelling narratives, providing an enhanced understanding of lithospheric structures in this mineral-endowed region of the world.

Acknowledgments: This work is supported by the Smart Exploration Research Center. The center has received funding from the Swedish Foundation for Strategic Research (SSF) under grant agreement no. CMM22-0003. This is publication SE25-003.