Spatial Heterogeneity of Uplift Pattern in the Western European Alps Revealed by InSAR Time-Series Analysis

Within the low-deforming western European Alpine belt, GNSS measurements show that uplift is the main signal characterizing current surface deformation in the range, reaching up to 2 mm/yr, while no shortening is observed across the belt. Based on the huge amount of satellite data available today, it now appears possible to constrain new high resolution surface velocities in the western Alps, which is of primary importance to better understand the links between surface deformation and neotectonics processes in this region.

Relying on ~ 170 radar acquisitions from Sentinel-1 satellite over four years, we propose for the first time an InSAR-based mapping of the uplift pattern affecting the Western Alps on a ~350x175 km-wide area. Their processing is challenging due to the high noise level inherent to mountainous areas and the low expected deformation signal. We thus use in this study the NSBAS small baseline approach (Doin et al., 2011) for interferograms corrections, unwrapping, and time-series inversion. Atmospheric corrections are made using ERA5 reanalysis model (Hersbach et al., 2020). We estimate regional line-of-sight (LOS) velocities by correcting the resulting time-series from outliers and by separating seasonal and linear signals through different approaches which all yield similar results, thus highlighting the robustness of the obtained LOS velocity field. Based on several assumptions, we finally convert LOS velocities to uplift rates using local incidence angles.

The corresponding InSAR-derived velocity field is validated by the comparison with GNSS solutions. They both show uplift in the core of the belt, with higher rates in its northern part, and subsidence at its periphery. Our approach however provides a denser spatial distribution of vertical motions compared with GNSS. Higher uplift rates are found within the external crystalline massifs compared with surrounding areas, in agreement with the variations expected from recent deglaciation and long-term exhumation data.

These results bring new insights into active tectonics in the Western Alps. While several distinct wavelength patterns can be identified within the uplift signal throughout the western Alps, we suggest that they may originate from common geodynamic processes, with differential surficial responses explaining their localization. These processes may involve glacial isostatic adjustment, erosion, and/or slab break-off.