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.

Crustal structure of the Middle Urals based on seismic reflection data

“…A key component in this model is that the East European lower crust does not extend very far to the east below the Main Uralian Fault. This is consistent with both recently proposed models based on interpretation of seismic refraction data (Stadtlander et al 1999) and older models based on gravity data and topography (Kruse & McNutt 1988). However, this model contrasts sharply with those suggesting that East European crust extends far to the east below the Magnitogorsk-Tagil arcs (Berzin et al 1996;Poupinet et al 1997;Knapp et al 1998;Drring & G6tze 1999;Diaconescu & Knapp 2002).…”

Crustal and uppermost mantle velocity structure along a profile across the Pamir and southern Tien Shan as derived from project TIPAGE wide-angle seismic data

“…Many other Palaeozoic orogens, for example the Variscides, Caledonides and Appalachians have no crustal root anymore, due to post‐orogenic extension and re‐equilibration of the lithosphere (Berzin et al 1996). The shape of the Moho presented in this study shows most similarity to that of the southern Urals (Carbonell et al 1996; Stadtlander et al 1999), although this picture may change somewhat if and when high‐quality, deep‐crustal seismic data from controlled‐source experiments become available.…”

Crustal structure of central Tibet as derived from project INDEPTH wide-angle seismic data

“…7) are within one standard deviation for average crust at 10 km depth worldwide (Christensen & Mooney 1995), the values of 5.6-5.7 km s x1 in the vicinity of the BNS between model km 170 and 270 are anomalously low. A search of a data bank containing velocity measurements for 416 rocks of many different types (Stadtlander et al 1999 and references therein) reveals that, amongst other rock types, serpentinite and spilite have such low velocities at 10-15 km depth. It is an attractive hypothesis to think that the low observed seismic velocities in the vicinity of the BNS could be explained as being due to a body of serpentinite and/or spilite.…”