Deep structure of the southern Ural mountains as derived fromwide-angle seismic data

Abstract

P- and S-wave modelling of the data obtained during the seismic refraction–wide-angle reflection experiment of the URSEIS95 project demonstrate the presence of a 15–18 km thick crustal root beneath the Magnitogorsk-Tagil zone in the central part of the Urals orogen. However, the centre of this crustal root is displaced by 50–80 km to the east of the present-day maximum topography. Also beneath the Magnitogorsk-Tagil island arc zone, an upper crustal body with a high P-wave velocity of 6.3 km s−1 at 4–9 km depth can be interpreted as consisting of mafic and/or ultramafic rocks. This, in turn, would help to explain the positive Bouguer gravity anomaly and the surface heat-flow minimum associated with the zone, and would also be consistent with the known surface geology of the zone. Another major feature of the seismic model is the presence of high P- and S-wave velocities (7.5 and 4.2 km s−1, respectively) at the base of the crustal root. If the deeper parts of the thickened crust also have high densities (small density contrast of about −0.1 g cm−3 with respect to the uppermost mantle) then this helps to explain the absence of a pronounced gravity minimum associated with the root. These high velocities and densities can be most easily explained by mafic rocks or a mix of mafic and ultramafic rocks. Within the structural framework of Berzin et al. (1996) these rocks would belong to the lower Russian plate, which was being subducted beneath the Siberian plate during the Uralian orogeny. It is possible that the crustal root is formed from the remnants of oceanic crust or a mix of oceanic crust and mantle attached to the Russian plate. This would mean that little or no continental crust has been subducted or that subduction, and hence the Uralian orogeny, stopped when there was no more oceanic crust or when an attempt was made to subduct lighter continental crust.