The existence of a crustal root beneath the Urals which would deflect
the position of the Moho by some 20 km is still largely controversial.
A French-Russian project carried out a wide-angle-reflection seismic
experiment across the Middle Urals to image the Moho topography along
a 175-km profile running approximately east-west north of Ekaterinburg
. New data show a 6-km Moho deflection beneath the central part of the
The Moho reflectivity is variable along the orogen. section, with ver
y sharp reflections beneath the Russian platform (45-km depth), and fa
inter attenuated signals in the root zone (51-km depth). Even if this
crustal root is not as thick as indicated by some previous speculation
s, it makes the Urals the only Palaeozoic orogen in the world to show
such a peculiarity. A major ultramafic overload in the upper crust wou
ld partly balance the crustal root, in accordance with isostatic equil
ibrium. We finally postulate that sharp wide-angle reflections from th
e Moho can be considered indicative of a layered lower crust. This wou
ld apply to the Russian platform which maybe gained this structuring d
uring the Ordovician extensional regime.