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 O. Averbuch a, ⁎, C. Piromallo b a Université Lille-Nord de France, Lab. Géosystèmes, UMR CNRS 8217, 59655 Villeneuve d'Ascq cédex, France b Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy abstract article info Article history: Received 2 December 2010 Received in revised form 31 May 2012 Accepted 24 June 2012 Available online 30 June 2012 Keywords: Lithospheric delamination Post-orogenic collapse Mantle tomography Variscan belt Paris basin The Paris basin (northern France) is a Late Paleozoic-Mesozoic intracratonic basin that settled upon the col- lapsed 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 sub- sidence 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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the question of the long-term evolution of litho- spheric roots of collisional mountain belts has gained a growing interest from the scientific community. One of the main points emphasized by first conceptual studies is that a thickened continental lithosphere com- monly “delaminates” at a certain degree of the orogenic development, the subducted lithospheric mantle being detached from the overlying crust and subsequently buried in the deeper mantle (e.g., Bird, 1979; Gvirtzman, 2002; Kay and Kay, 1993; Nelson, 1992; Sacks and Secor, 1990). The detailed processes by which such sub-crustal delamination can operate are still poorly known, but thermo-mechanical models argue for the possibility of the base of the lithospheric orogenic root to be removed either by diffuse convective processes (Houseman and Molnar, 1997; Houseman et al., 1981; Platt and England, 1993; Pysklyvec et al., 2000) and/or by localized decoupling along the Moho and break-off of the down-going slab due to its negative buoy- ancy (Bird and Baumgartner, 1981; Davies and von Blankenburg, 1995; Moore and Wiltschko, 2004; Morency and Doin, 2004; Nelson, 1992). The most recent models explore a variety of slab detachment modes and scenarios, ultimately controlled by variations of thermal and rhe- ological properties of the lithosphere involved, i.e. slab strength (Burkett and Billen, 2010; Duretz et al., 2011; van Hunen and Allen, 2011). Regardless of the mechanisms involved, the removal of the oro- genic root has been suggested to induce the rise of the asthenospheric mantle at some lithospheric level below the mountain belt, resulting in significant heating, uplift and extension of the previously thickened crust (Arnold et al., 2001; de Boorder et al., 1998; Duretz et al., 2011; Gögus and Pysklywec, 2008; Platt and England, 1993) as well as in intense magmatism produced by decompression-induced melting of the upwelling asthenospheric unit (e.g., Davies and von Blankenburg, 1995; Kay and Kay, 1993; Keskin, 2003; Macera et al., 2008). The net Tectonophysics 558–559 (2012) 70–83 ⁎ Corresponding author at: Université de Lille 1, Lab. Géosystèmes, UMR CNRS 8217, Cité scientifique, 59655 Villeneuve d'Ascq cédex, France. Tel.: +33 320 33 71 30; fax: +33 320 43 49 10. E-mail addresses: olivier.averbuch@univ-lille1.fr (O. Averbuch), claudia.piromallo@ingv.it (C. Piromallo). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.06.032 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto