EUROPE/The Urals 91 Figure Schematic crustal cross section along the URSEIS profile (see Figure 2), showing the bivergence of the Uralides and the crustal root beneath the Magnitogorsk arc The geometry of the western foreland thrust and fold belt is constrained by surface geology and reflection seismic and borehole data, and has been balanced and restored Figure Automatic line drawings of the ESRU and URSEIS reflection seismic profiles, with the major crustal boundaries and the Moho indicated These profiles clearly show the bivergent collision architecture of the Uralide orogen and the thickening of the crust towards its central axis lower crust reflectivity is diffuse in the case of the Magnitogorsk Arc, but quite strong in Tagil The crust-to-mantle transition, or Moho, is not imaged beneath the Magnitogorsk zone, but is a fairly sharp transition beneath the Tagil zone East of the arc complexes, the upper and middle crust region is imaged as clouds of diffuse reflectivity interspersed with, or cut by, sharp, predominantly west-dipping reflections that extend from the middle part of the crust into the lower crust, where it appears to merge with the Moho In the Middle Urals (ESRU), it is characterized by abundant lower crustal reflectivity In both the URSEIS and ESRU data, the Trans-Uralian zone dips westward beneath the East Uralian zone The velocity structure of the Uralide crust is best characterized along the URSEIS transect (Figure 6A and B) The upper crustal pressure wave velocities (Vp) reach up to 6.3 km s 1, and the shear wave velocities (Vs) reach up to 3.9 km s 1, with the higher values being in the Magnitogorsk Arc In the middle and lower crust, Vp ranges from 6.5 to 6.8 km s 1, reaching 7.1 km s above the Moho in the central and eastern part of the transect; Vs ranges from 3.7 to 3.9 km s 1, increasing to between 3.9 and 4.0 km s at the Moho The crust–mantle boundary is marked by an increase in Vp to >8.0 km s and in Vs to >4.6 km s Using less resolved data, the Middle Urals appears to have a Vp structure similar to that of the URSEIS transect, although higher values (7.6–7.8 km s 1) are reached near the Moho where the crust is thickest The Uralide heat flow density (HFD) is characterized by a strong minimum along the central part of the orogen (with values as low as 10 mW m 2), reaching typical continental crustal values (up to