Is there a remnant Variscan subducted slab in the mantle beneath the Paris basin? Implications for the late Variscan lithospheric delamination process and the Paris basin formation

The Paris basin (northern France) is a Late Paleozoic-Mesozoic intracratonic basin that settled upon the collapsed
Variscan collisional belt. The lithospheric roots of the Variscan orogenic system, below the Paris basin,
have been investigated using a European-scale P-wave velocity tomographic model. Tomography points out
the existence of a significant high velocity anomaly in the upper mantle below the western part of the basin.
At ~150–200 km depth, the anomaly extends with a NW–SE trend along the buried Northern France trace of
the Northern Variscan Suture Zone i.e. the Bray segment of the Upper Carboniferous Lizard–Rhenohercynian
(LRH) suture. Moreover, the high-velocity anomaly is spatially correlated with the prominent Paris Basin
Magnetic Anomaly. Its downdip extent reaches depths greater than 200 km below the southern margin of
the Paris basin. As suggested in previous tomographic studies below ancient suture zones, these data argue
for such anomaly being the remnant of a Variscan subducted slab that escaped the extensive late orogenic
delamination process affecting the lithospheric roots by Late Carboniferous-Early Permian times and that
was preserved stable over 300 Ma at the base of the lithosphere. On a general geodynamical perspective,
these results provide a new insight into the long-term evolution of subducted lithosphere into the mantle.
In the case of the Western European Variscan orogenic belt, they suggest that the subduction of the LRH
slab below the previously thickened Variscan crust, and its final detachment from the orogenic root, have
played an important role in the collapse of the belt, inducing thermal erosion and extension of the overriding
lithosphere. The spatial evolution of late orogenic extension across the belt and of subsequent thermal subsidence
in the Paris basin is suggested to result from the heterogeneous delamination of the lithospheric
roots along strike and from the resultant pattern of asthenospheric rise.