Abstract. An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40 Ar-39 Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

The prerift Mesozoic sequences of the Cretaceous passive margins fossilized in the North Pyrenean Zone (NPZ) are characterized by high-temperature deformation in relation with thinning of the continental basement. Our compilation of chronological and geological data confirms a clear correlation between the distribution of the highest paleotemperatures in the prerift sedimentary cover and the loci of extreme crustal stretching. Geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic prerift sediments indicate an early attenuation of the rifted continental crust. This leads us to propose a mechanism of rifting involving boudinage of the continental crust. The lateral extraction of the Paleozoic basement occurred under the prerift cover that is decoupled on the Triassic clays and evaporites. The thermal conditions allowing coeval ductile deformation of the crust and of the prerift sediments leaded to the widening of basins devoid of large faulted blocks. We discuss the implications on the origin and significance of the granulites and the relations between flysch deposition and high-temperature metamorphism of the prerift sediments. In the NPZ, Albian-Cenomanian flysch sequences were deposited synchronously with the synmetamorphic ductile deformation of the prerift sequences. Since the base of the flysch deposits also recorded locally the high-temperature tectonic event, we propose an original mechanism for the evolution of the basins involving continuous deformation of the prerift metamorphic sediments. At the scale of the Pyrenean domain, our results suggest a strong lateral variability in the tectonic style of passive margins, in direct link with their thermic pattern.