Middle Proterozoic-early Palaeozoic evolution of central Baltoscandian intracratonic basins: evidence for asthenospheric diapirs

The three intracratonic sedimentary basins located in central Baltoscandina via, namely the Bothnian Gulf basin, the Bothnian Sea basin and the Baltic basin, developed in response to Middle Proterozoic and Late Proterozoic tec tonic events, separated in time by about 800 Ma. Only the Baltic basin was subsequently affected by Caledonian orogenesis and Mesozoic rifting. Crusta l extension was minor or did not take place during the Proterozoic basin ev olution phases. However, according to the Moho topography, crustal thinning did take place. This was probably a result of subcrustal magmatism. On a c raton-wide scale, the ages of granitoids, which intruded during the Middle Proterozoic basin formation, generally decrease from east to west. This fac t, combined with the evidence provided by mantle-derived flood basalt magma tism, points to a moving asthenospheric diapir as the cause for basin devel opment. Asthenospheric upwelling was probably also responsible for the seco nd, Late Proterozoic, basin evolution phase, as evidenced by the lack of cr ustal thinning and extension, and the occurrence of tholeiitic intrusions. In addition, a Late Proterozoic thermally induced palaeo-high, located at a bout the position of the intracratonic basins, is compatible with indicatio ns from glaciations. As the ages of Late Proterozoic intracratonic basins a lso decrease from east to west across the craton, the location of asthenosp heric diapirism during this time interval was also moving. For the Fennosca ndian lithosphere, the presence of fundamental lithospheric weakness zones (e.g. terrane boundaries) might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about th e same location in the Fennoscandian Shield. The spacing and size of the Pr oterozoic intracratonic basins suggest that the asthenospheric diapirism wa s not deep seated. Therefore, sublithospheric convective processes might be the cause for the asthenospheric upwellings. Such processes are related to Rayleigh-Taylor instabilities in the sublithospheric mantle. Emplacement o f an asthenospheric diapir causes a thermal bulge at the surface of the lit hosphere. Modelling results demonstrate that erosion of the surficial high, succeeded by cooling of the lithosphere, can explain the accumulation of e arly Palaeozoic sediments in the Bothnian Sea basin, taking into account po st-Ordovician vertical and lateral erosion of the basin fill. (C) 1998 Else vier Science B.V. All rights reserved.