The Nappe des Marbres Unit of the Basque-Cantabrian Basin: The Tectono-thermal Evolution of a Fossil Hyperextended Rift Basin

The architecture of the Pyrenean-Cantabrian belt results from the inversion of a series of former Cretaceous rift basins. A HT-LP metamorphic event dated at 105 to 85 Ma ago is commonly associated with an Albo-Cenomanian episode of hyperextension of the continental crust. This metamorphism is well known in the eastern Basque-Cantabrian Basin within the Nappe des Marbres Unit (NMU) that is preserved from intense compressional deformation during the Pyrenean orogeny. Based on a structural study at the scale of the eastern Basque-Cantabrian Basin and on a dense sampling for T_RSCM estimates with the Raman spectroscopy of carbonaceous material (RSCM) method (Raman Spectrometry on Carbonaceous Material), we show following results: (1) the NMU has recorded two major phases of deformation, related to a localized extensional ductile foliation (S₁) during the Cretaceous rifting and later by the regional N-S shortening, recorded by a regional cleavage (S₂) observed only in shale sediments. (2) The NMU is affected by early salt tectonics related to several diapirs and salt walls fed by the Upper Triassic evaporitic layer. (3) The NMU recorded maximum temperatures exceeding 550°C, which represent some of the highest temperatures along the Pyrenees. These new data demonstrate that there is no E-W lateral metamorphic gradient across the belt and that hyperextension rifting occurred consistently within a high-thermal regime; (4) metamorphic isograds are oblique in respect of the main structures, suggesting that the metamorphic event postdates most of the early salt-related deformations prior to or coeval with early stages of rifting. As it represents one of the best preserved example of preorogenic hyperextended basin, the eastern Basque-Cantabrian Basin record may be used regionally to better understand the rift to orogen evolution of the Pyrenean Internal Metamorphic Zone but also more generally as an analog of salt-bearing hyperextended rift record.