PROTEROZOIC RIFTING IN THE BALTIC SHIELD REVISITED: LADOGA IS NOT A RIFT

The Lake Ladoga region in the southern part of the Baltic Shield hosts a series of mafic dykes, sills and plateau-basalts of Mesoproterozoic ages (ca. 1.53-1.46 Ga). Based on chiefly geochemical data, the region is conventionally interpreted as an intracratonic rift. We question the validity of this geodynamic interpretation by analyzing regional geophysical data from the Lake Ladoga region. We also compare the geophysical data with a similar in age Midcontinent rift and Valday rift, and provide alternative explanations for Mesoproterozoic geodynamic evolution of the southern Baltic Shield.

We present a complete list of tectonic, magmatic, and geophysical characteristics typical of continental rifts in general and demonstrate that, except for magmatic and, perhaps, some gravity signature, the Lake Ladoga region lacks any other rift features. We propose that Mesoproterozoic mafic intrusions in southern Fennoscandia may be associated with a complex deformation pattern during reconfiguration of (a part of) Nuna (Columbia) supercontinent, which led to magma intrusions as a series of mafic dykes along lithosphere weakness zones and ponding of small magma pockets within the cratonic lithosphere. Consequent magma cooling and its partial transition to eclogite facies could have led to the formation of a series of basement depressions, similar to intracratonic basins of North America, while spatially heterogeneous thermo-chemical subsidence, with phase transitions locally speeded by the presence of (subduction-related) fluids, could have produced a series of faults bounding graben-like structures.