Contrasting Palaeozoic basements in the Western Alps: inferences for the geometry and kinematics of the eastern boundary of the Variscan belt.
Abstract
The boundary between the Helvetic and the Penninic (= Briançonnais) Zones has long been recognized as a major
fault (“Penninic Front”) in the Western Alps. A narrow oceanic domain has been postulated at least along part
of this boundary (the Valaisan ocean). However, the information provided by the pre-Triassic basement has not
been fully exploited, and will be discussed here in detail. The igneous and metamorphic history of the pre-Triassic
basement shows significant differences between the External Massifs from the Helvetic Zone, with abundant late
Carboniferous granites, and the basement of the Briançonnais Zone, including the Internal Massifs (Dora-Maira,
Gran Paradiso, Monte Rosa), devoid of Carboniferous granites. A major coal-bearing basin, the “Zone Houillère”,
opened along this boundary. This limnic intramontane basin has never been properly investigated. The Zone
Houillère is not comparable with the external, paralic, flexural, basins on both sides of the Variscan belt, but shows
similarities with the Saar-Saale basin. Like the latter, we interpret the Zone Houillère as a transtensional basin
opened along a major, crustal-scale, fault zone, namely the East Variscan Shear Zone. The Permian magmatism
and sedimentation displays contrasting distributions, being absent or very localized in the Helvetic Zone, and
widespread in the Penninic Zone. The above data indicate that the structural inheritance from the Variscan belt
plays a major role in defining the future location of the Valaisan basin, i.e. the boundary between the European
palaeomargin and the Briançonnais microcontinent. In addition, these data allow a better definition of the geometry
and kinematics of the southern margin of the European Variscan belt.