The low-temperature thermal history of northern Switzerland as revealed by fission track analysis and inverse thermal modelling

Abstract

New zircon and apatite fission track (FT) data from four boreholes, which penetrate the Mesozoic and pre-Mesozoic sediments and crystalline basement of northern Switzerland, are presented. Inverse thermal modelling of the measured apatite FT parameters unravels the low-temperature (below ~120°C) thermal history of the crystalline basement of northern Switzerland. Zircon FT central and single-grain ages cluster around 250 Ma, thus maximum palaeotemperatures did not exceed ∼ 330°C after late-Variscan consolidation of the crystalline basement. Apatite FT central ages vary between 25 and 87 Ma. Confined mean track lengths range between 9.3 μm and 11.6 μm, suggesting substantial track annealing within all apatite samples. Modelled time-temperature paths offer a clear picture about the low-temperature thermal history of the crystalline basement of northern Switzerland: Cretaceous cooling is followed by an Eocene heating event and subsequent cooling to present-day temperatures. The Eocene heating episode is contemporaneous with the initial rifting stage of the nearby Upper Rhine Graben and the associated increasing volcanic activity. Crustal-scale faults of the Permo-Carboniferous Trough of northern Switzerland could have acted as major pathways for circulating hydrothermal fluids giving rise to the observed Middle to Late Eocene thermal event.