The structural pattern and geological evolution of the Stephano-Permian basins known under the pre-Mezozoic basement of SW Paris basin (France)
Résumé
Our presentation deals with the subsurface late Variscan basins identified in France under the Mesozoic cover of the Paris basin. Well-known late orogenic basins, Stephano-Permian in age, crop out in small places in and around the French Variscan basement. But very little is known about their occurrences, and therefore their geological evolution, under their Mesozoic cover. Our study is based on the modern reprocessing and the interpretation of seismic profiles acquired by the oil industry in the 80', representing about 1400 km of seismic lines. The recognition of these basins on the seismic profiles helped us to answer previously open questions regarding their internal geometry, sedimentary filling, structural control, extension, and thickness. We focus on three characteristic features of these hidden Stephano-Permian basins, giving a totally new picture of their geological evolution. (1) We depict for the first time the stepwise formation of such a basin, with the sedimentary and structural record of its progressive opening. (2) We also illustrate the differential erosion of these basins before the arrival of the first Triassic sediments. More than 1 km of sediments has been removed over distances of about 100 kms. (3) Finally we produce the first detailed structural map at the top of the Permian sediments in the Paris basin. This structural scheme is based on the faults interpreted on the seismic lines, with the help of cartographic faults at the 1/50000 scale, and aeromagnetic and gravimetric records. All these results may create new scientific opportunities, for instance regarding the transition between the late Variscan period and the initiation of the Mesozoic basins.