The French Stephano-Permian basins as witnesses of the Variscan-Mesozoic transition - compilation map and open scientific questions
Résumé
Late orogenic Stephano-Permian Basins (SPB) are widespread in present-day western Europe; they are known at outcrop in the internal and external zones of the Variscan belt (e.g. around the French Massif Central, or the Sarre-Nahe basin in Germany) as well as in its northern foreland (e.g. Great Britain and North Sea-Poland basins). Their subsurface prolongation beneath the Mesozoic and Cenozoic sedimentary cover is mostly attested by well data and seismic industrial surveys (oil and gas exploration-production). Indeed, apart from scientific research, SPB were extensively studied for their resources, such as oil, gas, coal, uranium or other.
SPB were extensively studied by the French geologists during the 80' and 90', and were mostly considered as intramontane post-orogenic basins, developed in relation with the extension and stretching of the continental crust/lithosphere at the end of the Variscan orogeny. SPB outcrop in several limited locations in and around the present-day French Variscan basement (Massif Central, , Alps, Vosges…). Their subsurface occurrences are little known under the post-depositional sedimentary cover, with only scarce studies based on few available subsurface data (well, seismic, gravimetric and aeromagnetic data).
We first propose a compilation geological map of the SPB in France, resulting from the merge of previously existing outcrop and subsurface maps and newly interpreted subsurface data. We then point out crucial geoscientific questions regarding their extension (initial and present-day), thickness, sedimentary filling, internal geometry and opening mechanism (local structural control vs regional subsidence), which have hardly been addressed and therefore still remain open.
Answering these questions may help to solve geoscientific problem from both the Variscan and Mesozoic point of view; for example estimating volumes of the SPB from their extension and thickness might represent a proxy of the amount of sediment eroded at the end of the Variscan orogeny; in the same way, a better knowledge of their opening mechanism may give a new starting point to discuss the initiation of the subsidence of the Paris basin.