A deformed wedge-top basin inverted during the collapse of the Variscan belt: the Permo-Carboniferous Lorraine Basin (NE France)

A new structural model is presented for the Permo-Carboniferous Lorraine Basin (NE France), a major intramountain basin that developed during the latest stages of the Variscan orogeny (ca 315–270 Ma). This basin is buried in NE France below the Paris Basin but outcrops in southern Germany (Saar-Nahe Basin). Digitalized well logs and reprocessed seismic data were used to decipher the kinematic evolution of this basin located along the Rhenohercynian orogenic suture zone. The basin initiated during the late collision stage (Late Namurian-Westphalian) in a wedge- top position upon the Saxothüringian retrowedge. The syn-orogenic sequences are delimited to the north by the major SE-directed Metz Thrust, which is part of the backthrust system that propagated during Late Westphalian times. Seismic data provide evidence of negative tectonic inversion, allowing the formation of syn-rift depocenters (Stephanian-Early Permian) above the former anticlines. Erosion of these anticlines results in a major unconformity (base of Stephanian) marking the onset of post-orogenic collapse stage. The late Early Permian shortening (Saalian phase) reactivated former thrusts and normal faults, thus generating uplift of the basin. The post- orogenic phase is complex and diachronous at basin scale, and both compression and extension can be recorded in the same area over a short period (<10 Ma). The Late Carboniferous negative tectonic inversion along the Rhenohercynian suture zone is proposed to result from the lithospheric delamination of the Variscan orogenic roots. The associated upwelling of asthenospheric material is recorded by intense magmatic activity, and can be, in turn, considered as the main trigger for the subsequent thermal subsidence of the Paris basin.