Thermal history of Triassic sandstones from the Vosges Mountains-Rhine Graben rifting area, NE France, based on K-Ar illite dating
The thermal history of the Buntsandstein (Lower Triassic) sedimentary rocks that crop out in the Vosges Mountains, or are deeply buried in the Rhine Graben, was evaluated on the basis of a combined mineralogical, morphological, oxygen isotope and K-Ar-dating study of illite separates from five locations over a limited 120 km 2 area. The results indicate a complex pre-rift evolution characterized by the following features: (1) in the northern Vosges, no record of diagenetic illite formation could be detected; (2) in the nearby Saverne rifting fracture-field, tectonic activity favoured the formation of faults in the crystalline basement ~210 Ma ago. This induced fluid migrations into the porous sandstone cover with illite precipitation occurring either repetitively over a period of ~25 Ma or as two separate events, the latter at ~185 Ma, affecting rocks to the east in the deeper part of the post-Eocene Upper Rhine Graben system; this latter stage of tectonic activity and fluid migration continued until 160 Ma, unless overprinted by another tectonic pulse at that time; (3) afterwards, the main tectonic activity moved to the Soultz-sous-Forets area where two episodes could be identified at ~95 and 70 Ma by dating mineral crystallization; the latter illite age was also detected in more deeply buried Buntsandstein lithologies of the rift basin, west of Strasbourg, reflecting diverse crystallization under different physical and/or chemical conditions; (4) further illitization which at present is poorly constrained may have restarted ~55 My ago in the same deep part of the rift graben. The youngest detected illitization age is clearly older than the main Miocene rifting episode of the Rhine Graben, suggesting that no illite crystallization occurred in the Buntsandstein sandstones during this tectonic event. This is explained by the fact that these rocks, as well as most of the underlying crystalline basement, were already sealed by fluid migration and mineralization during late Mesozoic rifting, and were therefore almost impermeable to the circulation of Tertiary fluids. The overall evolution of the region between the Vosges Mountains and the central Rhine Graben appears to be more complex than that of equivalent rock lithologies elsewhere in Western Europe. Several episodes identified here have been detected in other regions, but no single location exists which was affected by all events.