Crustal structure variation from the Precambrian to Palaeozoic platforms in Europe imaged by the inversion of teleseismic receiver functions - Project TOR

Abstract

The TOR experiment (Teleseismic TOmography TORnquist) carried out in winter 1996/97 across the Trans-European Suture Zone (TESZ) in Germany, Denmark and Sweden has collected new data to investigate the transition zone between Precambrian and Palaeozoic Europe. In this study, seismograms of teleseismic earthquakes recorded by the broad-band TOR stations have been used to calculate the receiver functions. The time-domain inversion method has been applied to the receiver functions to compute S-wave velocities in the crust and uppermost mantle beneath each station. The results of inversion down to 60 km depth provide new, independent information about the distribution of S-wave velocity in this area. Beneath the Swedish stations on Baltica, the thickness of the crust varies from about 45 to 50 km with mostly gradually increasing S-wave velocity and no sharp discontinuities while for Danish and German stations the crust is thinner (29-38 km) with a sharp Moho discontinuity. A very distinct S-wave low velocity layer was found at depth of 8-16 km in the upper crust of Baltica, supported by the results of refraction/deep seismic sounding experiments using both P and S waves. The map of Vp/ Vs ratio beneath the c. 1000 km long TOR 'profile' was obtained using the Vp velocity model from previous investigations. The values of Vp/ Vs = 1.73 were found in the uppermost crust of Baltica and upper Avalonian crust. The low velocity layer in the upper crust of Baltica is characterized by high value of Vp/ Vs = 1.85. Relatively low S-wave velocities are observed in the lower crust of Variscides (Vp/ Vs = 1.79), Baltica (Vp/Vs = 1.83) and Avalonia (Vp/Vs = 1.91); in the uppermost mantle Vp/Vs values are 1.77, 1.79 and 1.82, respectively.