Abstract
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.
Highlights
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]
Unlike horizontal motion that is predominantly controlled by tectonic processes, vertical rates may result from highly diverse mechanisms
In the western Alps, most Glacial Isostatic Adjustment (GIA) models predict less than ~0.3 mm/yr of present-day uplift of the Alpine range with respect to its foreland[8,9], but values reaching 1.8 mm/yr have recently been proposed for a 2D model with a low viscosity (1021 Pa.s) crustal root beneath the northwestern Alps[10]
Summary
Since the surface uplift rates are several times larger than horizontal velocities, they must result from upward tractions applied at the base of the Alpine lithosphere This hypothesis is consistent with the analysis of gravimetric anomalies in the western Alps, which significantly depart from those predicted by the isostatic equilibrium or flexural elastic support of the plate[23]. Centennial-millennial timescale transient contributions of GIA cannot be ruled out as the dominant forcing to the present-day observed uplift, the good spatial correlation between geodetic results and exhumation rates[25] advocates for processes persistent at the million year time scale. Our geodetic results suggest that orogens can still be growing in the aftermath of the collision stage
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