Late Pleistocene folding above the Mail Arrouy Thrust, North-Western Pyrenees (France)

The Western Pyrenees have experienced several major earthquakes in the last 400 years. Herein, we investigate the ongoing tectonic activity of the Arudy area affected by one of the largest earthquakes (M = 5.1; 1980) registered in the instrumental seismicity catalogue in the Western Pyrenees. Folding of alluvial terraces younger than 17 +/− 3 ky above the Mail Arrouy Thrust (MAT), Attest from a Late Pleistocene tectonic activity. This deformation resulted in a shallow fold with a wavelength of 2800 m and amplitude of 7 to 8 m. The MAT is rooted above a former normal fault of the Cretaceous Iberian margin, which is a potential seismogenic source.Such observations suggest that the northern flank of the Western Pyrenees is still subjected to shortening in response to a compressional stress regime. This compression is consistent with the African-Eurasian plate kinematics and the Arudy 1980 earthquake. Nonetheless, recent seismological data suggest the presence of an extensional stress regime in the range. This extension is indicated by some seismic events mostly observed in the Pyrenean high chain. Our hypothesis is that these extension-driven events could be due to a local stress-field induced by the elevation of the range. On the contrary, the compression, associated with the regional stress regime, could prevail in the outermost domain of the range.

► We performed a tectono-geomorphic study in the North Western Pyrenees. ► We analyzed deformation of alluvial terraces above the Mail Arrouy Thrust. ► We found pieces of geomorphological evidence of late Pleistocene tectonics. ► Pleistocene compressive deformation occurred in the North-Western Pyrenees. ► Results are integrated in the seismotectonic context of the Western Pyrenees.