Low-temperature thermochronology of the flanks of the southern Upper Rhine GrabenAbstract The Upper Rhine Graben (URG) is the most perceptible part of the European Cenozoic Rift System. Uplifted Variscan basement of the Black Forest and the Vosges forms the flanks of the southern part of the graben. Apatite and zircon fission-track (FT) analyses indicate a complex low-temperature thermal history of the basement that was deciphered by inverse modelling of FT parameters. The models were tested against the observed data and independent geological constraints. The zircon FT ages of 28 outcrop samples taken along an E-W trending transect across the Black Forest and the Vosges range from 136 to 312 Ma, the apatite FT ages from 20 to 83 Ma. The frequency distributions of confined track lengths are broad and often bimodal in shape indicating a complex thermal history. Cooling below 120°C in the Early Cretaceous to Palaeogene was followed by a discrete heating episode during the late Eocene and subsequent cooling to surface temperature. The modelled time-temperature (t-T) paths point to a total denudation of the flanks of URG in the range of 1.0-1.7 km for a paleogeothermal gradient of 60°C/km, and 1.3-2.2 km for a paleogeothermal gradient of 45°C/ km since the late Eocene.

Low-temperature thermal history of N-Switzerland 255 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. ZUSAMMENFASSUNGNeue Zirkon-und Apatit-Spaltspurdaten werden aus vier Bohrungen, welche die mesozoische und pre-mesozoische Sedimentbedeckung, sowie das oberste kristalline Grundgebirge der Nordschweiz durchteufen, vorgelegt. Inverse thermische Modellierung der gemessenen Apatit-Spaltspurparameter erlaubt, die Niedrigtemperaturgeschichte (unter ~120 °C) des kristallinen Grundgebirges der Nordschweiz zu enträtseln. Die Zirkon-Spaltspuralter (Zentralalter) und die Einzelkornalter zeigen eine Häufung um 250 Ma. Daraus folgt, dass die maximalen Paläotemperaturen nach der spätvariszischen Konsolidierung des Kristallins ~330 °C nicht mehr überschritten haben. Apatit-Spaltspuralter (Zentralalter) variieren zwischen 25 und 87 Ma. Die mittleren Spaltspurlängen reichen von 9,3 µm bis 11,6 µm und weisen auf eine substantielle Ausheilung in den Apatiten hin. Die modellierten Zeit-Temperatur-Pfade bieten ein klares Bild der Niedrigtemperaturgeschichte des kristallinen Grundgebirges in der Nordschweiz: Einer kretazischen Abkühlung folgt ein eozänes Wärmee-reignis und anschliessend eine Abkühlung zur heutigen Temperaturverteilung. Der eozäne Wärmepuls ist zeitgleich mit der initialen Riftingphase des nahegelegenen Oberrheingrabens und der damit verbundenen erhöhten vulkanischen Aktivität. Zum Nordschweizer Permokarbontrog-System gehörige Stö-rungen im Krustenmassstab könnten als Fliesswege für zirkulierende Fluide gedient haben, die zum beobachteten mittel-bis späteozänen thermischen Ereignis führten.

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The thermal history of the south-westernmost Black Forest (Germany) and the adjacent Upper Rhine Graben were constrained by a combination of apatite and zircon fission-track (FT) and microstructural analyses. After intrusion of Palaeozoic granitic plutons in the Black Forest, the thermal regime of the studied area re-equilibrated during the Late Permian and the Mesozoic, interrupted by enhanced hydrothermal activity during the Jurassic. At the eastern flank of the Upper Rhine Graben along the Main Border Fault the analysed samples show microstructural characteristics related to repeated tectonic and hydrothermal activities. The integration of microstructural observations of the cataclastic fault gouge with the FT data identifies the existence of repeated tectonic-related fluid flow events characterised by different thermal conditions. The older took place during the Variscan and/or Mesozoic time at temperatures lower than 280°C, whereas the younger was probably contemporary with the Cenozoic rifting of the Upper Rhine Graben at temperatures not higher than 150°C.

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