New evidence of the Vuoksi River origin by geodynamic cataclysm

Abstract

The territory of investigations is located in the SE periphery of the Fennoscandian Shield. It served as an arena of periodic significant restructuring of the hydrographic network associated with the filling and discharge of large late-glacial and Holocene basins during the degradation of the Scandinavian ice sheet and in postglacial time. One such restructuring is a sudden change of the Saimaa Lake direction of flow in the middle Holocene from the west to south to the Lake Ladoga basin via the drainage hollow, inherited by modern Vuoksi River valley. Origin of the Vuoksi River is associated with the catastrophic water breakthrough of the Saimaa Lake across the marginal ridge Salpausselkä I of about 5.7 cal. kyr BP. This event usually connects with water accumulation and overflow due to non-uniform post-glacial uplift according to modern concepts. The authors propose a great earthquake as the immediate cause of the break waters of Saimaa Lake. This suggestion is based on the study of specific deformations of the rocky riverbed in the area of breakthrough and of the loose deposits in the banks of the Vuoksi River valley downstream. Open cracks and horizontally displaced rock blocks were discovered in the area of the former rapids near town Imatra. Their systematic displacements on the both sides of the rocky gorge indicate the shear kinematics of fault zone. Different types of deformations had occurred in loose sediments of the low terraces (3–4 m) in the Vuoksi River valley and 20–30 km below the headwaters. In three studied stratigraphic sections the three cardinal different types of deformations were discovered: 1) normal fault with vertical displacements, 2) tectonic inclination, and 3) traces of catastrophic mudflow. The time diapason of the terrace forming (and of the corresponding deformations) is determined of 8.3 to 1.8 cal. kyr BP (by the ages of adjacent terrace levels), which corresponds to the origination time of the Vuoksi River. The earthquake, which presumably was a trigger for the formation of the Vuoksi River, was generated by the activation of ancient fault zone, manifested in the crystalline foundation. Periodic post-glacial tectonic activity of this zone is revealed in traces of strong seismic events both in the bedrock (initial emergence of the gorge, its renewal during the breakthrough), and in loose deposits (deformations in different levels of terraces).

Keywords

• Holocene
• Fennoscandian shield
• paleoearthquake
• seismic deformations
• Saimaa Lake
• catastrophic breakthrough