Jean-Louis Mansy

Dynamics and inversion of the Mesozoic Basin of the Weald-Boulonnais area: role of basement reactivation

Abstract The geometry and dynamics of the Mesozoic basins of the Weald–Boulonnais area have been controlled by the distribution of preexisting Variscan structures. The emergent Variscan frontal thrust faults are predominantly E–W oriented in southern England while in northern France they have a largely NW–SE orientation. Extension related to Tethyan and Atlantic opening has reactivated these faults and generated new faults that, together, have conditioned the resultant Mesozoic basin geometries. Jurassic to Cretaceous N–S extension gave the Weald–Boulonnais basin an asymmetric geometry with the greatest subsidence located along its NW margin. Late Cretaceous–Palaeogene N–S oriented Alpine (s.l.) compression inverted the basin and produced an E–W symmetrical anticline associated with many subsidiary anticlines or monoclines and reverse faults. In the Boulonnais extensional and contractional faults that controlled sedimentation and inversion of the Mesozoic basin are examined in the light of new field and reprocessed gravity data to establish possible controls exerted by preexisting Variscan structures.

Structural analysis of the adjacent Acadian and Variscan fold belts in Belgium and northern France from geophysical and geological evidence

Abstract Two Palaeozoic fold belts are juxtaposed in Belgium and northern France. The northern belt of the Brabant Massif, traditionally considered a part of the European Caledonides, was folded during the Acadian orogeny in the early Devonian. Although it is largely concealed, gravity and magnetic maps give a general picture of the existing structures. A curved gravity trend is interpreted as an arc-shaped succession of granitic batholiths. Structures north and south of this arc are completely different. The northern subdomain, containing magnetic lower Cambrian metasediments, shows multiple thrust faults towards the south over the granites, whereas the southern subdomain, without magnetic rocks, is moulded around the granitic core. The southern Variscan belt is characterized in general by thrusting with a north, NW and NE vergence. During the Variscan orogeny the Brabant Massif acted as an obstacle to the general transport of the Ardennes Massif to the N–NW. The oblique convergence with the Brabant Massif caused differential displacement towards the northwest, the western areas having travelled farther, as seen from a pattern of WNW-oriented strike-slip faults.

Influence of heterogeneous lithostructural layering on orogenic deformation in the Variscan Front Zone (eastern Belgium)

The ramp-flat thrust model and other classical models based on Rocky ountains or Appalachians structures allow a better understanding of ault and fold interaction in a sequence dominated by competent rocks, n which, in most cases, thrusts propagate forward in-sequence. This attern, however, fails to explain the deformation history of rock uccessions with low proportions of competent rocks. In such ithologies, out-of-sequence faults are more common features. The ariscan Front Zone in eastern Belgium serves as a case study for this on-classical deformation model. Cross-sections are well-constrained by urface data covering a large area. The Devono-Carboniferous sequence, esting unconformably on Caledonian basement, has shortened drastically, nd the tectonic history can best be explained by a composite structural odel. Continuous and discontinuous deformations took place in uccessive steps. A lame anticline developed first. It evolved ubsequently into either break-thrust or translated fault-propagation olds. Out-of-sequence thrusting by the Theux-Tunnel fault formed the esdre Nappe. The latter is the northern part of the Ardenne Allochthon. he main discrepancies with the ramp-flat thrust model are emphasized. articularly, the Vesdre Nappe, which is the highest thrust sheet, is ot the youngest event of deformation. Common elements of the Variscan ront Zone between northern France and western Germany are discussed. he Theux-Tunnel fault has the same kinematics significance as the idi-Aachen fault. The footwall of the Vesdre Nappe is the eastward rolongation of the Brabant foreland. The complex structure of the Theux indow reflects imbrications under the allochthon and can be compared ith other wedges and thrust sheets

Calcaires à Archéocyathes du Cambrien inférieur du Nord de la Colombie britannique (Canada). Implications paléogéographiques et précisions sur l'extension du continent Américano-Koryakien

Resume Les coupes du Precambrien superieur et du Cambrien du Nord de la Colombie Britannique (Canada) (chaine de Cassiar et Montagnes Rocheuses) offrent un bon materiel permettant a la fois de reconstituer la paleogeographie de la region et detudier les archeocyathes du Cambrien inferieur. Letude de la repartition des differences lithologiques, des epaisseurs, des facies et des associations darcheocyathes et de coralomorphes des deux cotes du Fosse des Montagnes Rocheuses (FMR) implique un jeu dextre de 400–450 kilometres le long de la faille. A linterieur de chacun des blocs, on observe des successions de facies qui permettent detablir la paleogeographie du Nord de la Colombie Britannique et des territoires adjacents a la fin du Precambrien. Les roches rouges a caractere proximal predominaient; elles ont ete recouvertes par un facies de plage dorthoquartzites ubiquiste. A la base de ces orthoquartzites, ou pres de celle-ci, se place la periode de transition entre le Precambrien et le Cambrien. Au tout debut du Cambrien la situation etait similaire, sauf au Sud et a lOuest ou se trouve un facies deau plus profonde (avec la formation de gres et de schistes pelitiques de couleur sombre). Pres du FMR, une tendance a lemersion est alors marquee. Lapparition de roches rouges hematitiques et de conglomerats grossiers tout le long de la chaine de Cassiar jusqua celle de Cariboo est le signe dune regression. La fin de la zone a Fallotaspis et le debut de la zone a Nevadella , ou, dapres les archeocyathes, lintervalle qui setond depuis les zones les plus anciennes jusqua celles a Claruscoscinus fritzi/Metacyathellus caribouensis , correspond, en partie, a letablissement de trois ceintures caracteristiques (dEst en Ouest: facies detritique externe, facies carbonate median et facies detritique interne). Une transgression rapide et de peu damplitude eut lieu au debut de la zone a Bonnia-Olenellus (zone a archeocyathes Claruscoscinus fritzi/Metacyathellus caribouensis et Pycnoidocoscinus? serratus/Tabulaconus kordeae ); elle sest accompagnee dapports terrigenes considerables. Dans la deuxieme partie de cette zone de trilobites correspondant aux zones darcheocyathes Archaeocyathus atlanticus et Tegerocyathus greenlandensis/Pycnoidocyathus pearylandicus , on observe une migration des divers facies vers lOuest. Il semble que les assemblages darcheocyathes dun meme âge se soient legerement modifies perpendiculairement a la ceinture carbonatee mediane, cest-a-dire vers la mer ouverte; cependant des etudes complementaires seraient necessaires pour resoudre ce probleme. On peut seulement remarquer que lassemblage darcheocyathes contenu dans les petites bioconstructions de la partie proximale de la ceinture detritique externe pres de la riviere Gataga est proche des assemblages des Mts Mackenzie et de lAlaska oriental, tandis que lassemblage darcheocyathes contemporain de la ceinture mediane carbonatee contient des elements que lon rencontre au Sud de la Colombie Britannique et de lEtat de Washington. Dans leur ensemble, les assemblages darcheocyathes dAmerique du Nord sont caracterises par leur uniformite sur toute la peripherie du craton depuis lAlaska et les Mts Mackenzie jusquau Grand Bassin et Sonora (Mexique) et depuis les Appalaches jusquau Labrador et lOuest de Terre-Neuve. Ceci permet de considerer toute cette region, ainsi que certains terrains de Koryakie (Extreme Est du Nord siberien), comme une unique province a archeocyathes, americano-koryakienne, et de lui associer une echelle biostratigraphique unifiee, dont les zones ont ete enumerees plus haut, de la plus ancienne a la plus recente.

Déformations cassantes et plicatives dans le Jurassique du Boulonnais (France), influence lithostructurale et héritage paléozoïque

Abstract The Jurassic sedimentary rocks (Bajocian to Tithonian) in the Boulonnais area (NW France) unconformably overlie a structured Palaeozoic basement. The Jurassic rocks are composed of three litho-structural units: 1) the Bajocian and Bathonian carbonates form a thin brittle unit; 2) the Callovian and Oxfordian clays form a thick, weakly folded unit; 3) the Kimmeridgian and Titho- nian layers of alternately competent and incompetent rocks form both a faulted and a folded unit. Each unit deforms specifically. The mapping of the Jurassic rocks in the Boulonnais area was completed in the submarine western prolongation from the interpretation of seismic profiles. The structural pattern of the Boulonnais is marked by faults striking N110-120, N090 and N030, and by folds with axes trending N090 and N120. The submarine structure is marked by numerous E-W faults and folds. The analysis in the field and the paleostress computation reveal both brittle deformation and folding, from which we interpret a relative chronology of tectonic events. The comparison between the Jurassic and Palaeozoic deformations makes the similarity of their major directions obvious. A structural inheritance of the Palaeozoic basement is thus noteworthy when the Jurassic cover deforms, shown by various types of folding. Nevertheless, not all of the paleozoic faults were reactivated during Mesozoic and Cenozoic tectonics.

Le Pas de Calais, un détroit cénozoïque à ouvertures multiples

The Dover Strait is open during the Paleogene, closed from the Upper Eocene and reopens from the middle Quaternary; but Pliocene foramifers assemblages are identical on both sides of the strait. The re-analysis of the Messinian shore line of the Monts de Flandres (Diestian at the Noires Mottes) in the prolongation of the Landrethun Flexural zone, of the Slack Formation, and the discovery of a new formation (Wimille Formation, inside the Boulonnais), all scealled by a goethitic pan, attest of an opening of the Dover Strait, controlled by the Messinian tectonical crisis and by the evolution of the Western Channel and its paleovalley system. Tectonical, geomorphological and climatic implications of this opening are discussed within the Western European context.