Résumé
We presents the velocity field of nearly equal 160 continuous GNSS sites in France. No significant horizontal motion can be detected to the level of + or -0.5 mm/yr (95% confidence), but significant uplift rates up to 1-2 mm/yr are observed for sites in the Western Alps, an area underlined by moderate, but frequent, seismicity. Focal mechanisms in the upper crust show extension normal to the range axis in the axial zone, and compression in the foothills. This deformation has usually been interpreted as the result of gravitational collapse of the range under its own weight. Classical models of gravitational collapse suggest horizontal flow of the range, and imply horizontal strain rates higher than the vertical ones. The present day velocity field does not support this idea since the vertical rates are at least 5 times larger that the horizontal ones. Based on 2D finite element modeling taking into account ad hoc geotherms and rheology laws for the crust and the upper mantle, we show that erosion-induced uplift displacements of the mountain range can trigger extension below the high topography region and compression in the foothills. We further suggest that deciphering vertical motion in low convergence mountain ranges is key to better understand their geodynamics. Combining vertical rates together with seismology and gravimetry in numerical models will take us a step further in the comprehension of the earth dynamics.