THE PERCHED PALEOSURFACES OF THE PYRENEES -CANTABRIAN BELT

International audiencePerched paleosurfaces of the Central and Eastern Pyrenees have been first described bygeomorphologists for a long time (Birot, 1937; Lagasquie, 1982; Calvet, 1996) and more recentlyby geologists (Bosch et al., 2015; Monod et al., 2015; Bosch, in progress). These studiesare of peculiar interest since they provide robust constraints for the timing and mechanisms ofrelief creation in a collision belt.Pyrenean paleosurfaces have been recognized over large areas in the Axial Zone betweenthe Aure valley to the West and the Cerdagne and Capcir grabens to the East. The actual elevationof theses surfaces varies between 1000 and 2500 m above sea level. In Cerdagne, the grabenfilling has been dated of Vallesian age (Middle to Late Miocene). Based on this constraint,the entire Pyrenean perched paleosurfaces have been considered of Miocene age. Since thepalynofacies of the Vallesian sediments is interpreted as a near-sea level deposit, the presentelevation of the paleosurfaces should have been acquired after the Vallesian assuming thatall paleosurfaces have nearly a Miocene age. In the vicinity of Cerdagne and Capcir grabens,the central Pyrenees paleosurface appears connected to the uppermost terraced paleosurface,suggesting an age prior to the Vallesian times.A common feature of the Pyrenean paleosurfaces is the preservation of thick lateriticweathering profiles. In the Eastern Pyrenees, the paleosurfaces display a terraced pattern withthree or more levels, the highest being likely the oldest. In the Central Pyrenees (from the AstonMassif to the East to the Neouvielle Massif to the West), The single paleosurface is apparentlyoffset along numerous faults. Field work will be conducted in order to collect structural dataallowing a detailed tectonic evolution to be reconstructed.In the Louron valley (close to the Aure valley), the central Pyrenean paleosurface is apparentlydownthrown along N160 trending faults facing the valley that define a graben-like featurewith an overall NS trend. Field work will be conducted to check the structural relationshipsof such possible features with the reverse faults and thrusts linked to the Pyrenean compression.Preliminary investigations show that fault planes here bear shallow dipping slickenslidesindicating a probable transtensional mechanism. A second set of normal faults trending N100to N130, parallel to the belt, seems to offset the central Pyrenean paleosurface. These faults areobserved in the Massif of Aston (SW of Ax les Thermes) where a detailed tectonic analysis willbe conducted.Paleosurfaces will be followed from the Aure valley (central Pyrenees) to the East in orderto decipher their origin and evolution: are they terraced surfaces or a single surface offsetalong recent faults having a general N160 and N100-130 orientation? Special attention willbe payed to the occurrence of the preserved lateritic weathering profiles that allows reconstructingthe lateral correlation of the paleosurface remnants. Finally, we plan to map thesepaleosurface remnants from the central Pyrenees to the Cantabric belt with a special attentionto possible faults that might offset some portions. An attempt to date the weathering profilesusing syn-weathering minerals (adulaires) will be conducted in the meantime. These data willcollectively help better understanding the significance of paleosurfaces in collision belts.This work is part of the project “Source to Sink”, we thank Total and BRG