Tectonic evolution and global crustal architecture of the European Variscan belt constrained by geophysical data

Abstract

The European Variscan belt is a unique orogen that is covered by comprehensive sets of seismic and potential field data from the Iberian Peninsula in the west to the Polish Sudetes in the east. The combination of both allows for a new interpretation of the structure and evolution of the European Variscides at the continental scale. The European Variscan belt has been divided into three domains according to distinct geological and geophysical characteristics: 1) the North-eastern Variscan Domain (NVD) outcropping in the Bohemian Massif, Black Forest and Vosges Massifs, and the Rhenish Massif, 2) the Central Variscan Domain (CVD) represented by the French Massif Central, Armorican Massif and British Variscides, and 3) The South-western Variscan Domain (SVD) represented by the Iberian Massif. The gravity data show the presence of high amplitude, short-wavelength gravity anomalies that are mainly correlated with the outcrops of eclogites, ultramafic rocks and ophiolites. These anomalies locate the main body of the Mid-Variscan Allochthon in the SVD and CVD and the Devonian Mid-Variscan suture in the NVD. The gravity data also show medium amplitude elongated long-wavelength gravity highs aligned parallel to the structural grain of the Variscan belt, represented by the deformed Teplá-Barrandian-Kraichgau upper crustal rocks, a Devonian supra-subduction basin in the Mid-Variscan Allochthon, and the autochthonous rocks of the Central Iberian Zone in the SVD, the Armorican Massif in the CVD and the Saxothuringian Zone in the NVD. The short wavelength negative gravity anomalies are mainly developed in the central part of the belt and coincide with Carboniferous (330–310 Ma) per- to meta-aluminous magmatic bodies, pre-Variscan orthogneisses and Carboniferous felsic granulite bodies. Noticeably, Permian (300–290 Ma) granitoids do not reveal any gravity lows indicating that these bodies are not deeply rooted. The magnetic data show two belts correlated to Carboniferous Rhenohercynian and Devonian Mid-Variscan magmatic arc granitoids. The Rhenohercynian and Mid-Variscan subduction systems are well imaged by moderately SE dipping primary A-type reflectors in reflection seismic lines in the NVD and CVD, while in the SVD the reflectors related to the Rhenohercynian subduction are dipping to the NE and the seismic signature of the Mid-Variscan suture is weakly developed. In the NVD, a third belt of SE-dipping reflectors is attributed to the Carboniferous subduction of the Saxothuringian continental lithosphere beneath the Mid-Variscan Allochthon. Younger B-type moderately dipping reflectors in the upper-middle crust coincide with outcrops of Carboniferous detachments, sometimes limiting granite plutons and core complexes along-strike the core of the Variscan orogeny. C-type reflectors occur mainly in the deep crust and are considered as an expression of lower crustal flow resulting from extensional re-equilibration of the previously thickened Variscan crust. A synthesis of P-wave velocity logs at the scale of the whole Variscan belt shows the existence of three different continental crusts: (i) cratonic crust marked by a thick, high velocity lower crust, (ii) transitional crust characterised by a relatively thin high velocity lower crust and intermediate velocity middle crust, and (iii) a thin Variscan orogenic crust defined by low velocity lower and middle crust. The latter crustal type coincides with regional outcrops of 330–310 Ma per- to meta- aluminous granitoids and associated gravity lows along-strike the belt. It is argued that the specific “Variscan” orogenic crust originated by Carboniferous extensional thinning and extensive melting of previously thickened “Tibetan” type crust and not from Permian tectogenesis, which is restricted to marginal parts of the orogen.