THE ROOT OF THE URALS - EVIDENCE FROM WIDE-ANGLE REFLECTION SEISMICS

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 very sharp reflections beneath the Russian platform (45-km depth), and fainter attenuated signals in the root zone (51-km depth). Even if this crustal root is not as thick as indicated by some previous speculations, it makes the Urals the only Palaeozoic orogen in the world to show such a peculiarity. A major ultramafic overload in the upper crust would partly balance the crustal root, in accordance with isostatic equilibrium. We finally postulate that sharp wide-angle reflections from the Moho can be considered indicative of a layered lower crust. This would apply to the Russian platform which maybe gained this structuring during the Ordovician extensional regime.