Inversion tectonics at the northern margin of the Paris basin (northern France): new evidence from seismic profiles and boreholes interpolation in the Artois area

Abstract

A synthesis of existing borehole data and seismic profiles has been conducted in the Artois area (northern France), along the northern border of the Paris basin, in order to explore the possible control exerted at depth by the Upper Carboniferous Variscan thrust front on the distribution of Late Paleozoic-Mesozoic depositional centers and their subsequent uplift in Tertiary times. Such control was demonstrated recently in the Weald-Boulonnais basin (Eastern Channel area) that forms the western prolongation of the area under study but was so far poorly constrained in the Artois area. Presented data provide evidence for the topography of the Artois hills and the altitude of sedimentary layers to be controlled by the activity of a network of relaying WNW-ESE striking faults inducing the systematic uplift of the southern fault blocks. Those steeply S-dipping faults branch downward onto the ramp of the Variscan thrusts forming listric faults that locally limit to the north buried half-graben structures, filled with fan-shaped fluviatile Stephanian-Permian deposits. Such clear syn-rift geometry shows that the ramp of the main Variscan frontal thrust (the Midi thrust) has been reactivated as a normal fault in Stephanian-Permian times thus forming a very demonstrative example of a negative inversion process. The reverse offset of the transgressive Middle Cretaceous-Lower Eocene layers covering unconformably the Paleozoic substratum argue for a Tertiary (Middle Eocene-Late Oligocene?) contractional reactivation of the fault network thereby documenting a repeated inversion process along the Artois Variscan thrust front. The Variscan frontal thrust zone is thus shown here to represent a prominent crustal-scale mechanical discontinuity that localized deformation in the Artois-Boulonnais area since Upper Paleozoic times. Inversion tectonique en bordure septentrionale du bassin de Paris (nord de la France) : nouvelles contraintes à partir de profils sismiques et de l'interpolation de forages en ArtoisMots-clés. -Front de chevauchement varisque, Héritage structural, Inversion tectonique, Bassin de Paris, Géodynamique du Nord de la FranceRésumé. -Une étude synthétique des données de forage et des profils sismiques existant a été entreprise dans la région de l'Artois (Nord de la France) afin de caractériser le contrôle potentiel exercé, en profondeur, par le front chevauchant varisque du Carbonifère supérieur sur la distribution des aires de sédimentation au cours du Paléozoïque terminal et du Mésozoïque et leur soulèvement plus tardif au cours du Tertiaire. Un tel contrôle a pu être démontré récemment dans le bassin du Weald-Boulonnais (Manche orientale) qui forme le prolongement occidental de la zone d'étude mais reste, pour l'heure, encore assez méconnu dans la région de l'Artois. Les données présentées montrent que la topographie des collines de l'Artois ainsi que l'altitude des niveaux sédimentaires de couverture sont contrôlées par l'activité d'un ré-seau de failles en relais, orientées suivant une direction WNW-ESE, induis...

Fault activity in the epicentral area of the 1580 Dover Strait (Pas-de-Calais) earthquake (northwestern Europe)

“…Hence, their original normal offsets would have been much smaller and, so, overcompensated by the Tertiary tectonic inversion. This is consistent with the observations of Minguely et al (2010) and Underhill & Paterson (1998) in the North Artois shear zone (northeastern France) and the Wessex Basin (Southern England), respectively. Both studies showed several examples of faults that offset the upper and younger sedimentary strata reversely, while the offset in the lower and older strata remains normal.…”

“…However, considering the amount of deformation necessary to create such large basins, similar offsets should be observable in Quaternary deposits along the on-shore continuation of these structures, which is not the case (e.g. Minguely et al 2010). A tectonic origin of the fosse is thus very unlikely.…”

“…The present-day geometry of this fault zone results from several phases of post-Palaeozoic deformation that have induced different reactivations of structures inherited from the Variscan Orogeny (e.g. Chadwick et al 1993;Van Vliet-Lanoë et al 1998, 2002aMansy et al 2003;Minguely et al 2010). Since its formation, the North Artois shear zone has passed from the original compressional setting (Variscan Orogeny) through extension related to the opening of the Tethyan and Atlantic ocean basins during Jurassic and Cretaceous times, and again to compression during the tectonic inversion that started in the early Palaeogene epoch due to the Alpine Orogeny (Vandycke et al 1988;Bergerat & Vandycke 1994;Van Vliet-Lanoë et al 2002a,b, 2004Mansy et al 2003;Minguely et al 2010).…”

“…According to some authors, the Sangatte Fault may have been in active extension during the deposition of the Gault Clay and Lower Chalk formations in the Cretaceous (Warren & Harris 1996;Ottemöller et al 2009;Minguely et al 2010). On the 1:250 000 offshore geological maps published in 1988 and 1989 by the British Geological Survey (BGS) and the compilation of geological data performed by James et al (2002), the Sangatte Fault is represented as several WNW-ESE trending minor faults and folds traversing the Cretaceous bedrock (Fig.…”

Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

“…This margin, alike the adjacent North Sea and Wessex basin, was affected by a major Late Jurassic-Early Cretaceous rifting event (Minguely et al, 2010 and reference therein) coeval with the general propagation of the Atlantic rift zone, north of the Gibraltar-Newfoundland fracture zone. The related pattern of subsidence and uplift induced the preservation of shallow marine Upper Jurassic and continental Lower Cretaceous (Wealdian) deposits in a restricted fault-bounded Boulonnais embayment upon which the presentday exposure is molded ( Fig.…”