652 MOHO DISCONTINUITY Figure (Upper part) Structure of the transitional zone between the Baltic Shield and the Variscan crust adjacent to the south, which has undergone Permo Mesozoic extension STZ, Sorgenfrei Tornquist zone; RFH, Ringkbing Fyn High; TEF, Trans European Fault (Lower part) North south section from the northern margin of the Rhenohercynian zone to the northern margin of the Alps Although the internal crustal structure is very heterogeneous and the section cuts prominent suture zones, the crustal thickness is rather constant (28 30 km) Modified from Ansorge et al (1992) North German Basin and the northern margin of the Alps Along this profile, the recent Moho-discontinuity lies at a depth between 28 and 30 km The depth increases to 34 km at the northern margin of the Alps The present-day morphology of the Moho-discontinuity does not show any significant relicts of Variscan roots Velocities in the uppermost mantle change significantly from a minimum value of 7.8 km s in the north to as much as 8.4 km s under the southern margin of the Mid-German Crystalline High; from there it decreases again to a more normal value of 8.1 km s The Variscan orogenic systems were formed during Late Palaeozoic times by the collision of numerous continental fragments or terranes of different size with the Baltic Shield and the East European Platform Because seismic reflection profiles provide evidence of important crustal stacking, compressional deformation must have formed thick and deep crustal roots, comparable with the mountain roots of the recent Mediterranean orogens These roots have not survived Destruction of the Variscan roots is usually referred to as ‘collapse’ of the orogen This comprehensive term summarizes a variety of processes, including erosion and isostatic uplift; gravitational spreading of hot, low-viscosity materials; extension by active rifting; and delamination of dense lower crust and mantle lithosphere Spectacular Crustal Root of the Uralides The Ural Mountains are the product of collision of the East European Craton with the Siberian Shield (see Europe: The Urals) Figure shows the results of a seismic profile across the southern Uralides The Moho-discontinuity, well defined by seismic reflection data, dips from the west and east under the central part of the belt, down to a depth of 50 km Wide-angle reflections in the central part of this belt reveal a still existing crustal root, with the Mohodiscontinuity situated at a depth of 50–60 km This unusual preservation of an orogenic root may be explained by the small density contrast (only 100 kg m 3) between the root and the surrounding