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Josselin BERTHELON

Resolving the Pyrenean vertical movements from InSAR images and coupling with the crustal structure, seismicity and H2 fluid migration

Supervisors:
Charlotte Fillon and Sylvain Calassou, Total, CSTJF, Pau
Bertrand Nivière, Université de Pau et des Pays de l’Adour

Major Results

Using high-density INterferometric Synthetic-Aperture Radar (InSAR) measurements, we map and quantify for the first time the slow vertical motions of the Central-Western Pyrenees. The distribution of the substantial post-orogenic uplift rates stresses that inherited crustal density contrasts determine the post-orogenic vertical motions and drives the current seismicity.

Abstract

The post-orogenic decay of mountain ranges should reflect the transition towards increasing control by isostasy on orogen evolution. In that setting, slow (<0.5 mm.yr-1) vertical motions and quiet seismic activity is expected. Yet, in the Pyrenees, substantial uplift and erosion are coeval with important seismicity, despite the negligible horizontal motions measured by GPS. We resolve the range’s vertical motion using high-density InSAR measurements for the 2015-2018 temporal interval, using the Sentinel-1 missions and SqueeSAR® processing technology. >

The accuracy of these measurements reveals present-day uplift rates much larger than the background horizontal displacement. Replaced in the long-term geodynamic history, we show that the inherited density contrasts arising from the occurrence of mantle rocks at depth drive the localization of isostasy-driven vertical motions. Consequently, the differential accommodation will favor seismogenic activity on normal structures. This study emphasizes that intraplate earthquakes in regions of slow convergence may reflect original density contrast rather than fault weakness.

Relationships between the Pyrenean lithosphere structure, seismicity and the present-day vertical motions.
(a) Map of the Airy isostasy, Moho depth, neotectonic activity and InSAR vertical velocity in the Pyrenees.
(b) Schematic bloc diagram of the Pyrenean neo-tectonics at crustal scale.