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 understand the manner in which plate convergence is accomodated. 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: (i) regional-scale folds; (ii) isoclinal folding, steep pervasive cleavage and vertical stretching, synchronous with peak metamorphism; (iii) and strain localization into ductile reverse shear zones. The persistence of a relative flat envelop for the Paleozoic sedimentary pile and Variscan isograds, and the absence of Alpine crustal-scale faults in the core of the Axial Zone, suggest 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 bears important implications for crustal evolution and inheritance in mountain belts more generally.

Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data

“…16A contains a number of assumptions, as key relationships have been eroded away. But it fits well the lack of Alpine thrusts of large displacement in the Axial Zone, as well as the relatively flat envelope of the folded Paleozoic sedimentary pile and the Variscan isograds (Soler et al, 1998;Laumonier, 2015;Cochelin et al, 2017). Note that the hanging wall of the NPF is constituted by post-Paleozoic rocks (the Aulus basin).…”

“…3B). This interpretation is discussed by Soler et al (1998), Laumonier (2015 and Cochelin et al (2017), who argue that the Gavarnie thrust has a much lower displacement in…”

“…16A follows the trace of the ECORS-Pyrenees profile across the eastcentral Pyrenees, crossing the North Pyrenean Zone in Ariège, the Axial Zone and the southern Pyrenees along the Noguera Pallaresa valley. Descriptions and references of the surface geology can be found in Baby et al (1988), Déramond et al (1988), Muñoz (1992), Berástegui et al (1993), Vergés (1993), Ford et al (2016) and Cochelin et al (2017). The deep structure was surveyed by the ECORS-Pyrenees and PYROPE E geophysical profiles Chevrot et al, 2015) (Figs.…”

Tectono-sedimentary evolution of a rift system controlled by Permian post-orogenic extension and metamorphic core complex formation (Bidarray Basin and Ursuya dome, Western Pyrenees)

“…This metamorphism occurred between 310 and 290 Ma (see Denèle et al, 2014;Lemirre et al, 2019;Poitrenaud et al, 2019) and was responsible for partial melting of the middle and lower crust, which then intruded into the base of the upper crust of the Axial Zone as gneiss domes in compressional settings ( Fig. 1; Denèle et al, 2007;2009;Cochelin et al, 2017Cochelin et al, , 2018aCochelin et al, , 2018b. Formed in an overall context of N-S horizontal shortening, these domes are elongated parallel to the regional direction of stretching in the middle and lower crust, like Journal Pre-proof J o u r n a l P r e -p r o o f 7 "a-type" MCCs (Cochelin et al, 2017(Cochelin et al, , 2018b.…”

Kinematics and strain distribution in an orogen-scale shear zone: Insights from structural analyses and magnetic fabrics in the Gavarnie thrust, Pyrenees

“…Variscan foliations were locally re-activated during the Cenozoic compression, thus contributing to the overall shortening of the Axial Zone (Mirousse and Barrère, 1993), considering the parallelism between Variscan and Alpine structures in some areas (Cochelin et al, 2017).…”