Thematic Articles

The Late Paleozoic convergence and collision between Gondwana and Laurentia resulted in along-strike variations in the Alleghanian–Mauritanide–Variscan orogeny during the assembly of the greater part of Pangaea. A series of ca. 380–290 Ma events segmented the orogen into two principal geodynamic domains with contrasting tectonic evolutions. In the northeast, the European Variscan belt records multiple subduction–collisional tectonic events, including indentation by Laurussian and later Gondwanan promontories and by Gondwana-derived terranes. Late-stage events (330–290 Ma) produced strongly curved deformation belts (oroclines), and late- to post-orogenic extension. In contrast, the southern Appalachians formed southwest of the promontory collisions where subduction of Rheic Ocean remnants produced a continuous Andean-style orogenic arc that preceded ca. 290 Ma terminal collision. We explain Pangaea amalgamation using a global model of mantle convection like that of modern Earth.

The orogenic crust of the European Variscan belt is granite-rich and only locally has a mafic lower layer. The core of the belt originated by massive melting of fertile quartzo–feldspathic sources (felsic meta-sedimentary or meta-igneous) derived from an Ediacaran–Ordovician accretionary system. An unusually felsic lower crust formed either by relamination of previously subducted continental crust or by melting of crustal rocks to produce a granitic upper crust and a laminated, restitic lower crust. This is in strong contrast to conventional models, developed mainly for magmatic arcs, that find or infer mafic lower crustal compositions. Thus, global estimates on the nature and evolution of the continental crust should consider the heterogeneity of the deep crust produced in various types of orogenies.

Tectonically emplaced peridotites and mantle xenoliths present complementary aspects of the evolution of the Variscan lithospheric mantle. The former have diverse origins and document complex histories of melt–rock reactions, exhumation along various pressure–temperature–time (P–T–t) paths, and emplacement into the crust, unravelling plate boundary evolution during Variscan subduction and collision. Mantle xenoliths exhumed by Cenozoic volcanism reveal ancient partial melting and mostly post-Variscan metasomatism episodes. Yet, their coarse-grained textures potentially record Variscan deformation. Dominantly belt-parallel fast seismic directions of the in situ Variscan lithospheric mantle may record flow normal to the convergence direction, but parallel to the boundaries of the Baltica and Avalonia blocks in central Europe, and to the main strike-slip faults and late extension in the Massif Central and Iberia.

Thick and relatively cold cratonic lithosphere of Laurussia and Gondwana shaped the Variscan orogen as those continents collided diachronously to form Pangaea. Herein, we summarize and integrate geological and geophysical results that show how cratonic lithosphere of those composite continents created the Variscan geologic foundation of Europe and northwestern Africa. Our analysis focuses on the lithospheric architecture of Baltica, Avalonia, and Gondwana-derived terranes to distinguish preserved cratonic domains from reworked zones. Zircon provenance analysis further constrains terrane origins and accretion history. The European Variscan belt is distinguished by the large proportion of Gondwana-derived terranes compared with its orogenic core. Its tectonic system reflects inherited rift architecture and the influence of rigid lithospheric promontories, setting it apart from other collisional belts.

Numerical and analogue modeling provides insights into dynamic processes shaping convergent plate boundaries. In the case of the Variscan orogeny, efforts to explain observations using physics-based models started in the late 1990s with 2D numerical simulations and have evolved towards advanced 2D petrological–thermomechanical numerical simulations and limited analogue experiments. Here, we review and discuss advances in six key research directions: (1) pre-orogenic processes, (2) buoyancy-versus tectonics-driven exhumation of high-pressure–high-temperature rocks, (3) relamination and trans-lithospheric diapirism, (4) origin of complex pressure–temperature paths, (5) origin of crust–mantle rock associations, and (6) origin of ultra-potassic and alkaline magmatism. We conclude by outlining future research directions that require the continuation of joint cross-disciplinary efforts of “modelers” and “observers.”

Deformed Variscan rocks crop out across much of Europe and northwestern Africa and tell the story of the Paleozoic welding of Gondwana and Laurussia to form Earth’s last supercontinent, Pangaea. Although mainly preserved as continental products, this event was driven by the opening and closing of three oceans: first the Rheic Ocean’s Late Silurian subduction northward beneath Laurussia, then also southward beneath northeastern Gondwana in the Mid-Devonian. Devonian slab rollback along Laurussia’s southern margin then opened the Rhenohercynian Ocean while the Rheic Ocean continued subducting beneath Gondwana’s northeastern edge. Early Carboniferous retreat of that trench then rifted eastern Gondwana, opening the wedge-shaped Paleotethys Ocean. The Rheic and Rhenohercynian oceans then closed, melding the continents, contemporaneous with subduction along northern Paleotethys, widespread intracontinental magmatism, and then orogenic collapse.