Deep structure of the Alps—what do we really know?

Abstract

In the last five decades the deep structure of the Alps has been probed by every geophysical method applicable, and the resulting amount of data is unmatched for any other orogen. In this study, an attempt is made to review the data and the proposed structural models with the aim of separating the certain from the probable and from the speculative information. This can be achieved by first reviewing the theoretical resolving power and ambiguity of the applied interpretation methods and then analysing the proposed models. The methods reviewed are inversion of surface wave data, teleseismic and local earthquake seismic tomography, near-vertical reflection seismology, wide-angle reflection and refraction seismology, and gravity modelling. All information about the Moho rated as certain is combined to give a Moho map of the Alpine area. The information rated as certain and probable, and additional qualitative arguments, are used to discuss a crustal model of the Western and Central Alps represented by two cross-sections. Major structural elements in this crustal model are a thick overthrust Penninic nappe system, wedging at mid- to lower-crustal levels, a discontinuous Moho and strong variations along the strike of the orogen. Whereas the structures of the European upper crust and of the Penninic nappe system are well constrained, only few and isolated lower-crustal structural elements are rated as certain. Finally, the shape of the lower lithosphere in the Alps is discussed by review and comparison of the results from surface-wave, teleseismic travel-time residual and tomographic studies. Qualitative arguments suggest the existence of a lithospheric root or slab beneath the Alps. Probable tomographic information suggests a south-vergent European lithospheric slab beneath the Southern Alps and the Po Plain. Despite the considerable number of studies aimed at resolving the deepest part of Alpine orogeny, the available quantitative information on the sub-Moho structure cannot be rated as certain.