Integrated Seismic Studies of the Baltic Shield Using Data In the Gulf of Bothnia Region: Babel Working Group*

In the autumn of 1989 a co‐operative experiment involving 12 research institutions in northwestern Europe collected 2268 km of deep seismic reflection profiles in the Gulf of Bothnia and the Baltic Sea. the 121 litre airgun array used for this profiling was also recorded by 62 muiticomponent land stations to provide coincident refraction surveys, fan‐spreads, and 3‐D seismic coverage of much of the Gulf of Bothnia. We thus have potentially both high‐resolution impedance contrast images as well as more regional 3‐D velocity models in both P‐ and S‐waves. In the Bothnian Bay a south‐dipping, non‐reflective zone coincides with the conductive Archaean‐Proterozoic boundary onshore in Finland. Between the Bothnian Bay and Bothnian Sea observed reflectivity geometries and velocity models at Moho depths suggest structures inherited from a 1.9Ga subduction zone; the upper crust here appears to have anomalously low velocity. Within the Bothnian Sea, reflectivity varies considerably beneath the metasedimentary/granitoid rocks of the Central Svecofennian Province (CSP) and the surrounding metavolcanic‐arc rocks. Numerous dipping reflectors appear throughout the metavolcanic crust, whereas the CSP has little reflectivity. Wide‐angle reflections indicate that the metasedimentary crust of the Bothnian Basin is 10 km thicker than the neighbouring Svecofennian subprovinces. Near the Åland archipelago Rapakivi granite plutons exhibit bright reflections, a contrast to the usual non‐reflective plutons elsewhere in western Europe. Additional dipping reflections deep in the crust of this area may support models of rifting and crustal thinning during emplacement of the 1.70‐1.54 Ga Rapakivi granites. Coeval gabbroic/anorthositic magmatism may explain the high reflectivity and high velocity of these plutons. the c. 1.25 Ga mafic sills and feeder dykes of the Central Scandinavian Dolerite Group also produce clear reflections on both near‐ and far‐offset seismic sections. Continued modelling will produce better velocity models of the crust and better constrained contour maps of crustal thickness in this part of the Baltic shield.