3-D flexural modelling of the Silurian Baltic Basin

The Baltic Basin (BB) is situated on the western part of the East European Craton (EEC). It was established initially as a passive margin basin in response to the breaking apart of the Rodinia megacontinent during the Latest Precambrian–Early Cambrian times. Thereafter, it has been suggested that the Late Ordovician–Silurian basin subsidence was generated by foreland bending of the western margin of the EEC as a result of the Caledonian convergence and collision of continental plates, namely Baltica, Laurentia, and Eastern Avalonia. This is based on the regional basin architecture, which indicates accelerating subsidence, rapid deepening of the basin, and significant thickening of the sedimentary succession towards the Tornquist–Teisseyre Zone (TTZ), along the southwestern margin of the EEC, during the Silurian. However, the very broad extent of the Silurian BB is not obviously explicable by a model of simple foreland flexure. This problem has been investigated by quantitative modelling of Caledonian foreland flexure, east of the TTZ, using a 3-D finite difference technique. The results are interpreted in terms of their implications for the rheology of the EEC and the geometry and temporal evolution of Caledonian orogenic loading. Assuming higher values of effective elastic thickness (EET) for the EEC than for Caledonian lithosphere, the model results demonstrate that the evolution of the BB cannot be explained by the effects of orogenic loads lying only on the west of the present Caledonian Deformation Front (CDF). Furthermore, the inferred shape of the BB deflection is not easily reproduced, even allowing lateral changes of EEC lithosphere EET. Rather, additional loads east of the CDF (and TTZ) were necessary, in order to replicate the shape and amplitude of the BB as a flexural basin. These are tentatively interpreted to be dynamic loads related to mantle flow above an eastward dipping Silurian subduction zone.