Fission track thermochronology applied to Phanerozoic thermotectonic events in central and southern Sweden

Central and southern Sweden is characterized by a Precambrian basement and a few Lower Palaeozoic sedimentary remnants, with a probable initial thickness of less than 500 m. Upper Palaeozoic deposits have not been documented in the Swedish part of the Fennoscandian Shield. Regional deposition of Upper Palaeozoic sediments on the platform appears possible though, since vast amounts of material must have been eroded from the Caledonian mountain range during this period. Fission track thermochronology has been used in this study in order to validate the former existence of a post-Caledonian sedimentary load, trace the variation in thickness and extent of the deposits and get an indication of the cooling history in the area. Samples from an east-west trending profile, from the Caledonian border zone in the west to the Aland Archipelago in the east, together with a few samples from surrounding areas, have been analysed. The obtained pooled apparent apatite fission track ages along the profile vary from similar to 250 Ma in the west to similar to 500 Ma in the east, while the mean track lengths decrease from 14 mu m in the west to 11 mu m in the east. In southern Sweden, ages between c. 170 and 320 Ma and a mean track length of 13.4 mu m were obtained. The dating results show that all samples have experienced raised temperatures since the pre-Cambrian peneplanation. Temperatures leading to total annealing of fission tracks during the Palaeozoic were reached in the western part of central Sweden and in southern Sweden, while partial annealing is traced further east to the Aland Archipelago. The obtained fission track results are interpreted by the former existence of a Devonian foreland basin covering most of central and southern Sweden. An estimation of the Upper Palaeozoic sedimentary thickness yields similar to 2.5 km of deposits in the Ludvika area and at least similar to 1 km in the Aland Archipelago. Inverse modelling of the fission track data resulted in several alternative modelled thermal histories. One of these suggests that central Sweden was covered by Palaeozoic and Mesozoic sediments until Tertiary time. In that case, the surface relief of western central Sweden has been chiselled out during the Tertiary. Another possible thermal history includes denudation due to surface level conditions during the Mesozoic in the west. In that case, reheating to similar to 40-80 degrees C during the Late Mesozoic-Cenozoic is required according to the present apatite annealing models. None of the proposed modelled thermal histories can be rejected so far. (C) 2000 Elsevier Science B.V. All rights reserved.