Late Vendian-Early Palaeozoic tectonic evolution of the Baltic Basin: regional tectonic implications from subsidence analysis

摘要

Subsidence analysis was performed on 43 boreholes penetrating the Upper Vendian-Lower Palaeozoic sedimentary succession of the Baltic Basin. The results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development. Tectonic subsidence patterns are consistent throughout the basin for the time period studied. An extensional tectonic subsidence event, possibly of two phases, is indicated from the Late Vendian to the beginning of the Middle Cambrian. This event is seen in the southwestern part of the Baltic Basin (Peri-Tornquist zone) until the earliest Cambrian after which it is also observed in the SW-NE-trending Baltic Depression part of the basin. Basin development during this time is interpreted as recording the latest stages of break-up of the Precambrian super-continent Rodinia and ultimately the formation of the Tornquist Sea. The late Middle Cambrian to Middle Ordovician tectonic subsidence pattern of the Baltic Basin is characteristic of post-rift thermal subsidence of the newly formed passive continental margin of Baltica, developed along its southwestern edge. A gradual increase in subsidence rate is observed from the (Middle?) Late Ordovician and throughout the Silurian (particularly for Ludlow and Pridoli times) creating subsidence curves with convex shapes typical of foreland basin development. The rate of Late Silurian tectonic subsidence increases significantly towards the southwest margin of the Baltic basin, adjacent to the present location of the North German-Polish Caledonides. The Baltic Basin therefore appears to have developed primarily as a flexural foreland basin during Silurian oblique collision of Baltica and Eastern Avalonia. A foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the Late Silurian. Compressional deformation structures of Early Devonian (Lochkovian) age are seen in seismic sections in the central part of the Baltic Basin (Lithuania). These, together with a change in subsidence pattern, mark the end of the Caledonian stage of basin development of the Baltic Basin.