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Pierre Lacan, UPPA (University of Pau, France) (France)
Bertrand Nivière, UPPA (University of Pau, France) (France)
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The ongoing tectonic activity of Western Pyrenees is characterized by a moderate and diffuse seismicity. This seismicity shows a general E-W trend and is clustered along the northern flank of the range. We focus on the Arudy area which recorded one of the major Pyrenean earthquakes (M=5,1) in 1980.
The late Cretaceous inversion of the Iberian margin, first through a left-lateral strike-slip motion, then through frontal convergence, resulted in a shallow pop-up structure near Arudy. This pop-up attests of the presence at depth of a crustal discontinuity interpreted herein as a normal fault of the former Iberian margin. Folding of Quaternary alluvial terraces above both thrusts delimiting the pop-up, and growth of alluvial depocenters at their footwall, testify to Pleistocene tectonic activity. Since the middle Pleistocene, this deformation is characterized by superficial folds of 1500 m wavelength and 5-10 m amplitude. Extrapolating this scheme, the whole Iberian margin could have been inverted using a similar mode and currently determines a crustal pop-up. This reactivation could proceed too in a transpressive mode during latest Cretaceous. It resulted in an ejection of the early Cretaceous infill.
Reactivation of normal faults is driven by obliquity of compression or by fault geometry (dip). After the initial transpressive stage, during a later more compressive episode (early Eocene to early Miocene), activity would be relayed by gently-dipping thrusts acting as short-cuts. The strong dip of the crustal faults of the margin prevented too from their reactivation and favoured such gently dipping short-cuts. They leaded behind the margin to stack of crustal units at the origin of reliefs in the High Chain.
The present-day geodynamic configuration may reactivate the former faults of the margin in a right-lateral mode. This reactivation would lead to a partitioning of strain between the deep crustal discontinuity that forced the lateral components of the displacement, and the shallow pop-up that accommodates the frontal component. Finally, the initial geometry of the Iberian margin makes the structural evolution of the Western Pyrenees very dependant of the stress regime.
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