Investigation of material from three core sections of the RWTH-1 drill-hole in the Wurm syncline of Aachen, Germany, shows mineralogical and structural evidence of intensive hydrothermal activity in the footwall of the Aachen thrust. Mineral and microstructural data indicate minimum temperatures of 200–250°C. CISillite 001 values of 0.45–0.61 (Δ °2𝛉) and insignificant amounts of smectite indicate a late diagenetic grade for illite pointing to temperatures <200°C. Chlorite, mainly formed in veins and cleavage planes, has CISchlorite 002 values between 0.35 and 0.26 (Δ °2𝛉) which only in part point to anchizonal grade. In contrast to these illite and chlorite data, maximum temperatures up to 370°C can be expected based on comparison with recently published fluid inclusion and mineral thermometric data. Illite is neither significantly affected by the hydrothermal event nor by deformation, and mirrors the burial history of the Wurm syncline.
Chlorite grew syntectonically as is shown by bent and predominantly stretched sheets which do not, however, have deformed structures. Syntectonic hydrothermal growth by incipient nucleation along crystal edges limited domain size and thus also the CISchlorite 002 values. The hydrothermal event did not last long enough to allow further crystal growth. The retarded CISillite and CISchlorite grades can be best explained by limited duration (probably <5000 y) of the hydrothermal event which for a short time reached epithermal temperatures. The hydrothermal fluid flow was caused by dewatering of sedimentary rocks during thrusting and tectonic thickening within the Variscan orogen and it was focused along the Aachen thrust which represents the frontal Variscan thrust.