Indenters and Ribbons: Cratonic Lithosphere in the Variscan Belt

The Variscan orogen of Europe and northwestern Africa represents one of the most complex collisional systems on Earth, assembled during the diachronous convergence of Laurussia and Gondwana in the late Palaeozoic. Unlike classic continent–continent collisions dominated by the interaction of two large cratonic masses, the Variscan belt developed through the progressive accretion, reworking, and collision of numerous continental fragments derived mainly from Gondwana. Here, we synthesize geological, geophysical, and provenance data to evaluate how the inherited architecture of cratonic and transitional lithosphere controlled the construction, geometry, and internal differentiation of the Variscan orogen.

Our compilation integrates crustal thickness models, lithosphere–asthenosphere boundary (LAB) depth estimates, lithospheric mantle–to–crust thickness ratios, and detrital zircon provenance constraints across western and central Europe and adjacent Gondwanan domains. These datasets allow us to distinguish preserved cratonic lithosphere from zones that experienced partial or complete destruction of their cratonic character during rifting and collision-tectonic accretion. Particular emphasis is placed on the contrasting behaviour of Baltica, Brunia, Avalonia, Armorica, and Gondwana-derived terranes such as Saxo–Thuringia, Teplá–Barrandia, and the Variscan Internal Zone.

The results show that Baltica is the only cratonic block involved in the European Variscides that fully retained its thick, cold lithospheric mantle, with a LAB reaching depths of ~250 km. This cratonic lithosphere directly underthrust the Variscan orogen for distances of up to 100–150 km and acted as a rigid mechanical buttress, exerting a first-order control on the curvature and reorientation of the Variscan belt from a NE–SW trend in western Europe to a NW–SE trend in central Europe. In contrast, Gondwana-derived terranes are characterized by systematically thinned lithospheric mantle and shallow LAB depths, reflecting extensive pre-Variscan lithospheric modification during Ordovician rifting along the northern Gondwana margin. These terranes preserve widespread Gondwanan zircon age signatures, yet their lithospheric architecture indicates that they were already detached from the Gondwanan craton prior to collision.

Avalonia and Armorica occupy an intermediate position. Avalonia retained a relatively deep LAB inherited from its cratonic ancestry, but its moderately thin and reflective crust suggests significant pre-Variscan thinning. Armorica is the only Gondwana-derived terrane with a deep LAB comparable to cratonic domains, although its crustal structure resembles that of transitional lithosphere. The Variscan Internal Zone represents the most intensely reworked segment of the orogen, where Gondwana-derived lithosphere underwent profound crust–mantle decoupling, subduction, and syn- to post-collisional reworking.

We conclude that the European Variscan belt is fundamentally shaped by inherited lithospheric heterogeneity. Rigid cratonic blocks of Laurussian and peri-Gondwanan affinity acted as indenters, while mechanically weakened Gondwana-derived ribbons localized deformation, metamorphism, and magmatism. This dominance of reworked Gondwanan lithosphere distinguishes the Variscan system from other major collisional orogens and highlights the critical role of cratonic lithosphere and inherited rift architecture in the assembly of Pangaea.