Active faulting in the Alps as seen by GNSS: comparative case-studies from the Belledonne and High-Durance fault systems

In the W-Alpine context, where active deformation is slow and seismicity is moderate, the question of determining and characterizing active faulting remains a scientific challenge. However, previous studies have shown that reliable geodetic strain rates can be assigned to the High-Durance fault (HDF) system, in accordance with the regional seismicity. In this paper, we propose a comparative analysis of two major faults recognized in the W-Alps: the HDF in the Briançon vicinity and the Belledonne fault (BDF) close to Grenoble. The aim is to investigate the constrains that can be brought by GNSS, in slow deformation ranges of 0.1-1 mm/yr, to decipher active faulting, both in terms of localization at the regional scale and quantitative strain rates. We also aim to compare the seismotectonic framework with the geodetic results. Last but not least, these two major faults bear witness of the main kinematics found within the realm of the W-Alps, that is to say extensional mechanism in the internal zone (along the HDF) and strike-slip in the external zone (along the BDF). They can thus be considered as two major fault systems representative of the overall W-Alpine current deformation. From a quantitative viewpoint, the BDF extends over about 50 km along the western side of the Belledonne External Crystalline Massif. Using up to 20 years of data from 22 permanent GNSS stations, approaches exploiting the redundancy between the individual station velocity estimates provide dextral strike-slip kinematics with a rate of 0.2 ± 0.2 mm/yr. This rate is coherent with a unique strain tensor calculated over the 50 km wide local network, evaluating a NNE-SSW extensional axe of 2.0 ± 0.8 nanostrain/yr, with a WNW-ESE shortening axe of 4.7 ± 1.2 nanostrain/yr. Strain calculations by alternative methods on regular grids evaluate a lower total amplitude of strain rate close to the BDF trace, in particular less compression. Comparatively, the HDF is investigated thanks to a dense network of 30 GPS stations covering the Briançon area, which was surveyed during 5 temporary campaigns in 1996, 2006, 2011, 2016, and 2021. The redundancy of the dense network and the long observation interval after the addition of the fifth campaign in 2021 allow to increase the accuracy of the velocity fields. The average horizontal strain rate over the entire network located in the center of the Briançon Seismic Arc has been evaluated at 20 ± 2 nanostrain/yr of E-W extension across the 50 km network, yielding about 0.5 mm/yr of extension across the NS trending HDF. From a seismotectonic viewpoint, the comparison with seismicity highlight the coherency between seismotectonic and geodetic deformation fields, both for the HDF and BDF systems, in terms of style, direction, and amplitude of deformation.