Phanerozoic evolution of Fennoscandia: Evidence from apatite fission track, fluid flow and geodynamic data in Finland and Estonia

Abstract

New apatite fission track (AFT) data from Estonia together with previously published data from Finland, all from crystalline basement drill core, constrain the Phanerozoic thermal history. Models show a consistent pattern of heating from Cambrian to end-Carboniferous, with peak paleotemperatures of 60 ± 10 °C when sediment cover was ∼ 0.5 km thicker in the Baltic Paleobasin and ∼ 1.5 km over Precambrian basement in Finland. Prior to the Phanerozoic, the Fennoscandia basement was exhumed to a peneplain by late Neoproterozoic. The evolution of Phanerozoic sedimentary cover can be linked to (1) the Scandinavian Silurian–Devonian Caledonian orogeny, (2) foreland basin and foreland uplift bulge development, (3) pre-drift extension and lithospheric uplift from Permo-Triassic to Cretaceous, and (4) Cenozoic rift-phase uplift following North Atlantic opening at ∼ 54 Ma. In the Devonian, Caledonian mountains attained elevations of ∼ 7–8 km, causing significant lithospheric elastic flexure resulting in a 6–7 km deep foreland basin and ∼ 500 m of foreland bulge uplift affecting Fennoscandia and the Baltic Paleobasin. The flexure relaxed and the foredeep basin was partly inverted as the mountain range exhumed and eroded during late orogenic collapse (∼400 Ma). The present shield area in Finland may have outcropped for a short time in the Devonian during the Caledonian forebulge phase but was covered shortly thereafter with sediments. The cover persisted until the final exhumation of Finland in the Mesozoic – Cenozoic. Some AFT data indicate hydrothermal disturbance by expulsion of hot fluids from the Caledonian thrust belt into the unconsolidated basal Cambrian sediments and crystalline basement. This craton-wide flow resulted in the precipitation of Devonian calcite-fluorite-Pb-Zn veins, Mississippi Valley type Pb-Zn deposits in the Swedish Caledonian front and dolomitic alteration of Ordovician-Silurian limestones in Estonia. Moreover, our model contributes to the interpretation of the evolution of topography in Norway and distribution of deep biosphere in Fennoscandia.