Jurassic palaeogeography and subsidence of the Poitou high (western France), influence of the variscan orogene structural inheritance on the onset of a Mesozoic sedimentary basin.
Résumé
The Poitou high of Western France is situated at the junction between the Paris Basin, the Aquitaine Basin, and two major Variscan metamorphic massifs (Armorican and Central massifs). This area steps over the southern Variscan suture, separating allochtonous units in the south from autochtonous units to the north. We performed joint studies on the metamorphic basement and the Mesozoic sedimentary cover, in the aim to understand the influence of the structural inheritance of the Variscan orogene on the onset of the Mesozoic basin subsidence. The basement's structure below relatively thin sedimentary cover was reconstructed from geological knowledge and geophysical methods (ground gravimetry and airborne magnetism, BRGM dataset). Palaeogeography and sedimentary geometries were recomposed from field studies and well logs issued from water wells. Mesozoic sedimentation on the Poitou high started during Lower Jurassic (Lias). Our data demonstrate that Liassic subsidence pattern is closely linked to the Variscan orogenic heritage. The autochtonous, poorly deformed thick crust on the northern Poitou high did not subside during most of Lias. On the opposite, in the allochtonous southern part, onset of subsidence took place since the early Liassic times. The first deposits are observed along narrow areas, elongated along N160 oriented geophysical anomalies. This structural direction corresponds to Late Carboniferous to Permian tardi-orogenic extension. On the opposite, N110 syn-orogenic structures do not influence the distribution of Lias sediments. Onset of sedimentation, fault plays and subsidence patterns during Lias extension, are thus closely related to the variscan heritage, and particularly with the extensive tardi-orogenic history.