Preorogenic Folds and Syn-Orogenic Basement Tilts in an Inverted Hyperextended Margin: The Northern Pyrenees Case Study

The Chainons Bearnais (CB, North Pyrenean Zone) resulted from the Cenozoic contractional reactivation of the salt tectonics-bearing, hyperextended margin that initiated at the Europe-Iberia transition during the Early Cretaceous. In this tectonic scenario, assessing the relative contribution of extension and contraction to the present-day structure is crucial to reconstruct the hyperextended margin geometry and to quantify the subsequent shortening. This study undertakes this issue by defining the relationship between folding and two bedding-independent references: peak temperature isotherms and paleomagnetic data. Isotherms were reconstructed from 76 new measurements of Raman spectroscopy on carbonaceous materials (RSCM) and indicate temperatures at the time of peak metamorphism in the CB (110-85 Ma, end of extension). They are shallowly to moderately northwards dipping and cut across most of the folds deforming the Mesozoic units. Paleomagnetic data from 29 sites evidence a widespread remagnetization carried by pyrrhotite that was probably blocked during the early Paleogene (before the onset of continental collision) and postdated folding in the CB. Abnormal inclinations in this remagnetization suggest syn-collision tilts up to 60 degrees to the north in the back limb of the Axial Zone. Based on the presented data set, we propose that the folding of the cover above the evaporitic decollement was almost fully completed by the end of the Cretaceous extension, with similar to 85-100% of the dip of fold limbs being acquired before the remagnetization time. Cenozoic contraction reactivated the extensional faults in the shallow basement as top-to-the-S thrusts, leading to the passive transport and northwards tilting of the folded cover.