Pre‐Alpine (Variscan) Inheritance: A Key for the Location of the Future Valaisan Basin (Western Alps)

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 Basin). 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, and 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, that is, the boundary between the European paleomargin and the Briançonnais microcontinent.

Kinematics and Age of Syn‐Intrusive Detachment Faulting in the Southern Alps: Evidence for Early Permian Crustal Extension and Implications for the Pangea A Versus B Controversy

“…This dextral shearing can obviously not be related to the Pangea B to A transformation because it predates the Early Permian rocks that yielded the paleomagnetic data on which Pangea B is based (Muttoni et al, ). Whereas dextral shearing during the Carboniferous at ~340–320 Ma is well documented (e.g., Ballèvre et al, ), we found no documentation of fault‐rock kinematics of Permian dextral strike‐slip faults in the Alps. The only exception is the Cossato‐Mergozzo‐Brissago in the westernmost Southern Alps, which is, however, controversial (dextral according to Boriani et al, ; sinistral according to Handy et al, , and Handy & Streit, ).…”

“…Also shown in Figure is the time span of dextral transcurrent shearing in the External Massives of the Helvetic Zone. In this part of the Alps, dextral shear zones have been dated by Genier et al () at ~320 Ma, by Ballèvre et al () at ~320–310 Ma, and by Simonetti et al () at ~340–320 Ma. Retrodeformation of Alpine deformation results in an original SW‐NE strike for these shear zones.…”

The Atlas-East Variscan -Elbe shear system and its role in the formation of the pull-apart Late Palaeozoic basins

“…The Variscan orogeny is widely interpreted as a result of the Carboniferous collision and accretion of Gondwanaderived microcontinents and continental masses with those of Laurussia (Gutiérrez-Alonso et al 2008;Padovano et al 2014). The irregular boundaries of the colliding plates generated coeval transpressional and transtensional tectonics, accompanied by a complex pattern of intracontinental shear zones at the scale of the southern European Variscides (Arthaud and Matte 1977;Neubauer and Handler 2000;Gutiérrez-Alonso et al 2004, 2008Martínez Catalán 2011;Padovano et al 2011Padovano et al , 2014Dias et al 2017;Ballèvre et al 2018; Fig. 1).…”

Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

“…These units are distinct for their Alpine metamorphic evolution and for their internal structure and rock assemblage and they represent the fossil oceanic lithosphere of the Jurassic Tethyan ocean [43,44]. This ocean basin, separating the continental Adriatic/African and European plates, consisted of a main branch known as the Piemonte-Ligurian (or South Penninic) basin, and minor northern branches including the Valaisan (or North Penninic; e.g., see [81][82][83][84]), and the Antrona basins [85][86][87]. These latter two sub-basins were separated by the Briançonnais peninsula preserved in the W Alps [84,88,89] but thinned to the east, so that only one Penninic ocean basin gave rise rise to the ophiolites exposed in the E Alps at/near the Tauern tectonic window [90] (see also the review in [91]).…”

“…This ocean basin, separating the continental Adriatic/African and European plates, consisted of a main branch known as the Piemonte-Ligurian (or South Penninic) basin, and minor northern branches including the Valaisan (or North Penninic; e.g., see [81][82][83][84]), and the Antrona basins [85][86][87]. These latter two sub-basins were separated by the Briançonnais peninsula preserved in the W Alps [84,88,89] but thinned to the east, so that only one Penninic ocean basin gave rise rise to the ophiolites exposed in the E Alps at/near the Tauern tectonic window [90] (see also the review in [91]). This setting is thought to have resulted from an intense fragmentation of the oceanic spreading ridge promoted by regional-scale transform faults, as envisaged in several paleogeographic reconstructions [83,89,92].…”