Present-day uplift of the western

Collisional mountain belts grow as a consequence of continental plate convergence and eventually disappear under the combined effects of gravitational collapse and erosion. Using a decade of GPS data, we show that the western Alps are currently characterized by zero horizontal velocity boundary conditions, offering the opportunity to investigate orogen evolution at the time of cessation of plate convergence. We find no significant horizontal motion within the belt, but GPS and levelling measurements independently show a regional pattern of uplift reaching ~2.5 mm/yr in the northwestern Alps. Unless a low viscosity crustal root under the northwestern Alps locally enhances the vertical response to surface unloading, the summed effects of isostatic responses to erosion and glaciation explain at most 60% of the observed uplift rates. Rock-uplift rates corrected from transient glacial isostatic adjustment contributions likely exceed erosion rates in the northwestern Alps. In the absence of active convergence, the observed surface uplift must result from deep-seated processes. The western Alps, the highest topography of Europe, formed during Oligocene-Miocene times, as a consequence of the convergence and indentation of the Adriatic microplate toward Europe 1 . As much as 280 km of shortening was accommodated by underthrusting of the European margin beneath the Adriatic microplate 1–3 . This process resulted in crustal thickening, nappe stacking, and exhumation of crystalline basement, contributing to the building of the present-day topography. During the last million year, the end of active subduction in the Apennines 4 led to a change of the regional plate-kinematics, with the progressive development of extension in the core of the Apennines and counter-clockwise rotation of the Adriatic micro-plate 5 . A decade of continuous GPS measurements show that, with respect to the Alpine foreland, the GPS sites in the western Po plain have an averaged residual motion of 0.1 mm/yr and provide an upper bound of 0.3 mm/yr (95% confidence level) for possible right-lateral strike slip motion across the western Alps (Supplementary Information). Similarly, south of the western Alps, sites in Corsica and Sardinia show less than 0.4 mm/yr of shortening with respect to the southern western Alps 6 . Therefore, the western Alps is presently a mountain range with virtually zero horizontal velocity boundary conditions, offering a unique opportunity to evaluate the contribution of processes unrelated to horizontal tectonics to the evolution of orogens. In addition to continuous GPS data, we use levelling data spanning a century in order to study the present-day vertical motion in the western Alps and its surroundings. We show in the Supplementary Information that both data sets have an average internal precision of 0.2 mm/yr and are mutually consistent at a level of 0.3 mm/yr over the studied area. GPS sites located west of the Alpine foreland all show zero vertical rates (± 0.2 mm/yr) and therefore define a stable reference frame that we use to express regional vertical rates.