Structural inheritance in the Central Pyrenees: the Variscan to Alpine tectonometamorphic evolution of the Axial Zone

Abstract

Estimating structural inheritance in orogens is critical to understanding the manner in which plate convergence is accommodated. The Pyrenean belt, which developed in Late Cretaceous to Paleogene times, was affected by Cretaceous rifting and Variscan orogeny. Here we combine a structural and petrological study of the Axial Zone in the Central Pyrenees to discuss structural inheritance. Low-grade Paleozoic metasedimentary rocks were affected by a Variscan transpressional event that produced successively: (1) regional-scale folds; (2) isoclinal folding, steep pervasive cleavage and vertical stretching, synchronous with peak metamorphism; (3) strain localization into ductile reverse shear zones. The persistence of a relatively flat envelope for the Paleozoic sedimentary pile and Variscan isograds, and the absence of Alpine crustal-scale faults in the core of the Axial Zone, suggests that the Axial Zone constitutes a large Variscan structural unit preserved during Pyrenean orogeny. This configuration seems to be inherited from Cretaceous rifting, which led to the individualization of a large continental block (future Axial Zone) against a hyper-extended domain along the North Pyrenean Fault zone. This study places the currently prevailing model of Pyrenean belt deformation in a new perspective and has important implications for crustal evolution and inheritance in mountain belts more generally. Supplementary materials: Raman spectroscopy of carbonaceous materials data and a figure illustrating peak-fitting of the Raman spectrum of carbonaceous material and Raman spectra from the various samples of the Pallaresa cross-section are available at https://doi.org/10.6084/m9.figshare.c.3906247