Variscan structures in the Kent Coalfield, southeast England, and their regional significance

Abstract

The Upper Carboniferous Kent Coalfield lies concealed beneath various Mesozoic formations, its southern areas lying about 20 km north of the commonly accepted position of the main Variscan Deformation Front. However, despite intense intra-coal deformation, the existing literature is ambivalent about compressional Variscan features in Kent, the general view being that coal deformation is largely the product of the depositional environment. The main deformation is interpreted here as the result of Variscan compression, the structural style being imposed by the sandstone-dominated lithology. This conclusion is necessitated by the regularity of deformational structures revealed by mine workings, and supported by coal sequence irregularities suggestive of thrusting, especially in the lower Westphalian strata, all of which is paralleled in parts of the South Wales Coalfield. The Kent data indicate that, as in South Wales, a zone of thrusting many tens of kilometres wide lies in advance of the main deformation front. Structural trends are consistent with an overall swing in the front from east–west across much of central-southern England, to more northwest–southeast across northeastern France. This swing may represent a transpressional transfer zone, within which stress deflection and block rotation produced thrust vergence oblique to the overall direction of maximum compression.

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

“…The origin of such steeper dip of the main frontal thrust in the BoulonnaisArtois still requires to be explained. Some large-scale oblique convergence active along the Artois-Boulonnais-SE England Variscan segment [Rippon et al, 1997;Averbuch et al, 2004] as well as some possible foreland basement culmination due to the reactivation of Caledonian thrusts [Minguely et al, 2008] could be possible driving factors for such increased dip of the thrust front. New constraints on the structure at depth and analogue modelling studies are thus still needed to provide further insight into the main 9. -Carte temps-profondeur du chevauchement frontal principal varisque (faille du Midi) établi à partir de l'interprétation de profils sismiques ré-partis le long du front de déformation varisque du Nord de la France (d'après Minguely [2007]).…”

Geometry and kinematics of the Boulonnais fold-andthrust belt (N France): implications for the dynamics of the Northern Variscan thrust front

“…11). As previously stated by Rippon et al [40], the SE England-Boulonnais thrust belt would thus represent an oblique transfer zone between the Ardenno-Rhenish and SW England frontal belts. Therefore, the Northern Variscan thrust front appears as a highly segmented belt developed in response to an important Early Devonian basin pre-structuration.…”

“…The style of folding and thrusting within the remaining deposits of these basins suggests that they are situated few kilometres north of the major Variscan deformation front i.e. the Midi thrust zone in northern France and the Dorking-Biddenden structure in England [40].…”

La Faille du Midi, mythe et réalités

“…Mais similitude ne signifie pas nécessairement continuité, comme le montrent les Calcaires de la Viosne, dans le Massif Armoricain (Pelhâte et al, 1991), également semblables aux dépôts ardennais de même âge. Toutefois, des dépôts westphaliens sont connus dans le Boulonnais (Bonte, 1969) comme dans le Kent (Rippon et al, 1997). Les caractères minéralogiques des sédiments qui leur sont sous-jacents conduisent à estimer à environ 2 000 m l'enfouissement subi par ces minéraux (Averbuch et al, 2004).…”

“…Cette couverture est plissée et écaillée, montrant clairement une cinématique de transport vers le nord-est (Averbuch & Mansy, 1998). Quoique moins sévères, des déformations témoignant d'une structuration tangentielle à vergence nord-est, ont aussi été observées dans le Kent (Rippon et al, 1997).…”