The Harz Mountains and the Flechtingen Hills are part of a nappe stack thrusted onto the southern margin of Avalonia or the Rhenohercynian Zone, respectively (Linnemann et al. in Geol Soc Lond Spec Publ 542:403–431, 2024; Zeh et al. in Int J Earth Sci 113:1197–1211, 2024). Both, the Harz Mountains and the Flechtingen Hills, record a geological history from the closure of the Rheic Ocean at c. 419 Ma via the opening and closure of the Rhenish Seaway to final Variscan collision. The composition and the observable plate tectonic history of the Harz Mountains and the Flechtingen Hills is very similar. Both areas contain rock units in the range from Ordovician to Serpukhovian. Further, Harz and Flechtingen show in the Northwestern part deposits of the Rhenohercynian (Avalonian) margin and in the Southeast rock units formed on the Saxothuringian (Cadomian) margin of the Rhenish Seaway. The central part is composed of flysch, melanges, and nappes thrusted onto the Avalonian (Rhenohercynian) margin. A plagiogranite in a harzburgite aged at 329 ± 2 Ma points to the existence of true oceanic crust in the Rhenish Seaway. Deformation and final emplacement of all rock units occurred in late Viséan and Serpukhovian times at or after c. 328 Ma reflecting Rhenish Seaway closure. 30 Ma later, around the Carboniferous-Permian boundary, major granitoid plutons intruded during a time window of c. 300–295 Ma caused by an extreme extensional regime, that marks the termination of the Variscan orogeny in the area.