Paleogene phreatomagmatic volcanism on the western main fault of the northern Upper Rhine Graben (Kisselwörth diatreme and Nierstein–Astheim Volcanic System, Germany)

At the western margin of the northern Upper Rhine Graben, Germany, six deeply eroded vents make up the Nierstein–Astheim Volcanic System. Among these vents the phreatomagmatic Kisselwörth maar-diatreme volcano erupted directly on, or at least very close to, the western main fault of the Upper Rhine Graben, which is still active today. This sheds new light on the early formation of one of the most conspicuous grabens of the European Cenozoic Rift System: While the oldest sediments within the northern end of the graben were biostratigraphically dated to Late Eocene/Early Oligocene (36–33 Ma), the Kisselwörth maar-diatreme was 40Ar/39Ar dated to 55.8 ± 0.2 Ma. Thus, c. 20 million years before subsidence started the western main graben fault not only provided a path for rising magma but was also hydraulically active to allow phreatomagmatic eruptions.