Phanerozoic, pre-Cretaceous thermotectonic events in southern Sweden revealed by fission track thermochronology

Abstract

In shelf areas surrounding southern Sweden, offshore settings contain evidence of repeated subsidence and uplift during the Phanerozoic. Earlier published fission track (FT) studies in adjacent areas suggest extensive exhumation since the Late Paleozoic. The aim of this study is to reveal pre-Cretaceous thermotectonic events that affected the south-central part of the Fennoscandian Shield. Twenty-four apatite samples from the Precambrian basement in southern Sweden were analyzed by the FT dating method. The samples were collected at the present-day crustal surface, which more or less coincides with a sub-Cambrian paleosurface. Apatite FT ages range between ∼150 and ∼315 Ma, and the mean track lengths range between ∼12.3 and ∼13.6 μm. Together, the FT results confirm that all samples from southern Sweden were totally annealed, i.e. heated above at least 100°C, during the Phanerozoic. Furthermore, three areas with different trends among the FT results have been discerned. FT ages younger than 200 Ma are characteristic for samples from the southeast. A large discrepancy among the FT results is recorded in the northwest. In between, a belt with 230–280 Ma ages is distinguished. A tentative reconstruction of the pre-Cretaceous thermotectonic history in the southern Fennoscandian Shield suggests that Caledonian foreland basin deposits were responsible for the Palaeozoic heating event that affected southern Sweden. Tectonic vertical movements in the basement were probably triggered by the load, resulting in a depth differentiation in the basin. Cooling of southern Sweden took place during the Carboniferous–Middle Triassic due to uplift and exhumation of the southwestern part of the shield. Subsequent Jurassic exhumation is recorded in southeastern and parts of northwestern south Sweden.