The CIFALPS seismic experiment: first high-resolution data on the crust and upper mantle structures of the southwestern Alps

The Alpine belt, being a most studied mountain belt by geologists, is an ideal natural laboratory for understanding the processes and mechanism of orogeny. A number of questions on the dynamics of the Alps, however, remain open due to the lack of detailed data on its lithospheric and sublithospheric structures. This is particular true for the very arcuate southwestern part of the belt. In order to improve images of the crust and upper mantle beneath the southwestern Alps, we have installed a temporary broadband seismic array across the belt from the Rhone valley (France) to the Po plain (Italy). The main sub-array of the CIFALPS (China-Italy-France Alps seismic survey) project is a 350-km long roughly linear profile of 46 stations trending WSW-ENE from Bollène (France) to the north of Alessandria (Italy). Its average station spacing is smaller than 10 km, with a densification to 5 km in the internal Alps. Nine additional temporary stations located ~40 km to the north and south of the main profile complement the adjacent permanent broadband networks to improve the 3-D constraints on the deep structures. Most stations are equipped with three-component broadband sensors (50Hz -120s). The array was installed in the summer of 2012 and will be operated till to September 2013. We used available dataset to compute receiver functions and analyze shear-wave splitting from SKS phases. A common-conversion point migrated receiver function section displays a strong Moho P-to-S conversion beneath the western end of the profile, which fluctuates in amplitude and depth from beneath the Vocontian basin to the Penninic front, and becomes hardly distinguishable beneath the internal zones. This image is consistent with the results of the controlled-source ECORS-CROP profile in the northwestern Alps. The lateral change in Moho signature on the ECORS-CROP line was interpreted as the result of signal attenuation across the very heterogeneous upper crust of the internal zones. We however favor the hypothesis of a change in Moho reflectivity from the European crust in the west to a continent-ocean transitional crust beneath the internal zones. Using SKS shear wave splitting measurement, we confirm that the major fast polarization directions from the studied stations are roughly NW-SE oriented, supporting the interpretation of mantle deformation around the deep high-velocity body interpreted as continental or oceanic lithosphere plunging beneath the Po plain.