Some structural features of the Alpine lithospheric root

Abstract

In order to investigate the structure of a postulated lithospheric root below the Alps in some detail, P-wave travel time residuals from eleven seismograph stations on a N-S profile crossing the Swiss Alps were evaluated. Ninetyfive teleseismic events were analyzed. Relative travel time residuals ( RRes) were calculated using the J.-B. travel time table and the most northern station on the profile (near Schaffhausen) as a reference station. The azimuth-independent station mean values broadly reflect the varying travel time delay in the Alpine crust. Relative to the Gräfenberg Observatory (GRF) 200 km north of the Alpine rim, however, the upper mantle below the Swiss Alps seems to be faster. The relative residuals can be interpreted as a 3% velocity increase in a depth interval of 110 km. This type of isotropic lithospheric root is in general agreement with other investigations but is not the key problem in the present study. The most conspicuous feature of the observed RRes is a sharp minimum in the direction of the strike of the Alps for all the Alpine stations but not for those outside the Alps. This has also been confirmed by taking the GRF as a reference. The interpretation concentrates on this particular feature. Model calculations show that neither an isotropic lithospheric root nor transverse crystal anisotropy could account for the observations. However, the high-frequency approximation for wave propagation in a subvertial stack of higher and lower velocity rock sheets along the strike of the Alps appears to be at least one possible interpretation. The thickness of pairs of rock sheets is estimated to be 30–80 km. The sheeted lithospheric root reaches its maximum depth of the order of 200 km (depending on velocities assumed) below the southern Alps and appears to consist of between three and eight pairs of sheets. A geological interpretation in terms of this model is discussed based on subsequent subduction episodes of lithosphere by plate convergence during the closure of the Tethyan basins.