Vertical strain partitioning in hot Variscan crust: Syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax

Abstract

-A new structural map of the Paleozoic crust of the Pyrenees based on an extensive compilation and new kinematic data allows for the evaluation of the mechanical coupling between the upper and lower crust of the abnormally hot foreland of the Variscan orogen of SW Europe. We document partitioning between coeval lower crustal lateral flow and upper crustal thickening between 310 and 290 Ma under an overall dextral transpressive regime. Partitioning also involved syn-convergence transtensional gneiss domes emplacement during this period. Late orogen-normal shortening of the domes and strain localization in steep crustal-scale transpressive shear zones reflects increasing coupling between the lower crust and the upper crust. The combination of dextral transpression and eastward flow in the Pyrenees results from the shortening and lateral escape of a hot buoyant crust along the inner northern limb of the closing Cantabrian orocline at the core of the Iberian-Armorican arc between ca. 305 and 295 Ma. Delamination or thermal erosion of the lithosphere enhanced orocline closure and explains (1) the switch from crust-to mantlederived magmatism in the Iberian-Armorican arc and (2) the abnormally hot and soft character of the Pyrenean crust that escaped the closing syntax.Keywords: Variscan / Pyrenees / syntax / orocline / gneiss dome / transpression / hot orogen Résumé -Partitionnement vertical de la déformation dans la croûte varisque chaude : échappement des Pyrénées de la syntaxe ibéro-armoricaine. Une nouvelle carte structurale harmonisée et une série de coupes de la croûte paléozoïque des Pyrénées sont présentées sur la base d'une compilation et de nouvelles observations de terrain. Cette carte permet d'évaluer les modalités du couplage mécanique entre la croûte supérieure et la croûte inférieure de l'avant-pays anormalement chaud de l'orogène varisque. La croûte inférieure a flué latéralement pendant l'épaississement de la croûte supérieure dans un contexte transpressif dextre entre 310 et 290 Ma. Dans ce contexte convergent et grâce à un couplage crustal partiel, la croûte inférieure a pu être partiellement exhumée dans des dômes gneissiques transtensifs. Le serrage tardif des dômes et la localisation progressive de la déformation dans des zones de cisaillement transpressives d'ampleur régionale attestent de l'augmentation du degré de couplage entre croûtes supérieure et inférieure pendant le refroidissement régional. La combinaison de la transpression dextre et du fluage latéral de la croûte des Pyrénées résulte du raccourcissement et de l'échappement du matériau piégé dans la syntaxe ibéro-armoricaine lors de son amplification et la fermeture de l'orocline cantabrique en son coeur entre 305 et 295 Ma. L'érosion thermique ou la délamination du manteau lithosphérique a permis (1) la transition entre un magmatisme crustal et un magmatisme mantellique ou hybride dans la syntaxe et (2) la fermeture de l'orocline et l'échappement contemporain du matériel pyrénéen chaud qu'elle contenait.

Wrench‐Related Dome Formation and Subsequent Orogenic Syntax Bending in a Hot Orogen (Variscan Ibero‐Armorican Arc, the Ouessant Island, France)

“…Gutiérrez‐Alonso et al () suggested that buckling involves asthenosphere‐lithosphere interactions producing delamination or thermal erosion. These processes imply the soft character of the surrounding lithosphere that is able to escape from the closing structure by lateral flow along the branches of the arc, for example, along the North Pyrenean shear zone or the SASZ (Augier et al, ; Cochelin et al, ; Gapais et al, ; Llana‐Fúnez & Marco, ). From that point of view, the SASZ could have acted as a transfer zone between the cold Central Armorican domain and the hot South Armorican domain (Gapais et al, ), in a manner similar to other arcuate belts like the Aegean, the Apennine‐Calabria, and the Rif‐Gibraltar‐Betics regions (Brun & Faccenna, ; Faccenna et al, ; Palano et al, ; Rutter & Valetti, ).…”

“…The West European Variscan belt exposes many large strike‐slip shear zones, revealing wrenching that largely contributed to orogenic construction (e.g., Arthaud & Matte, ; Gapais & Le Corre, ). These shear zones are interpreted as the result of (1) the accommodation of the lateral component of oblique convergence between Gondwana and Laurasia plates (Braid et al, ; Martínez Catalán, ; Pastor‐Galán et al, ; Shelley & Bossière, ), (2) the accommodation of indentation of an irregular pre‐Variscan Gondwanan margin (e.g., Braid et al, ; Brun & Burg, ; Casas & Murphy, ; Dias & Ribeiro, ; Matte & Ribeiro, ; Matte, ; Kroner & Romer, ; Llana‐Fúnez & Marco, ; Murphy et al, ; Perroud & Bonhommet, ; Quesada, ; Ribeiro et al, , ), and/or (3) the accommodation of oroclinal buckling leading to the Cantabrian orocline/Ibero‐Armorican Arc (Casas & Murphy, ; Cochelin et al, ; Edel et al, ; Gutiérrez‐Alonso et al, ; Llana‐Fúnez & Marco, ; Martínez Catalán, ; Murphy et al, ; Pastor‐Galán et al, ; Ribeiro et al, ; Shaw et al, ; Figure a). Many of these strike‐slip shear zones bound gneiss domes and were active during exhumation of their cores with different tectonic scenarios proposed: transfer zones (Augier et al, ; Gapais et al, ), pull‐apart bounding structures (Echtler & Malavieille, ; Roger et al, ), or transpressional zone boundaries (Cochelin et al, ; Denèle et al, , ; Rabin et al, ).…”

Two‐stage Variscan metamorphism in the Canigou massif: Evidence for crustal thickening in the Pyrenees

“…Gibson, 1991;Vissers, 1992;Wickham & Oxburgh, 1986); 3) regional transpression (e.g. Carreras & Capella, 1994;Cochelin et al, 2017;Denèle, Olivier, Gleizes, & Barbey, 2009;Druguet, 2001;Gleizes, Leblanc, & Bouchez, 1998;Mezger, 2009). The latter have progressively evolved from models invoking pure transpression (e.g.…”

“…Carreras & Capella, 1994) to those admitting a significant contribution of localised extension (e.g. Cochelin et al, 2017;Mezger & Passchier, 2003).…”

“…This metamorphism develops in gneissic to migmatitic dome-shaped structures that refold an initial flat-lying foliation, whereas a steeply dipping cleavage affects the overlying low-grade metamorphic rocks (De Sitter & Zwart, 1960;Zwart, 1979). This vertical metamorphic and structural zonation led to contrasting interpretations ranging from transpressional to transtensional tectonics, including deformation partitioning between the lower and the upper crust (see Carreras & Capella, 1994;Cochelin, Chardon, Denèle, Gumiaux, & Le Bayon, 2017). Whatever their differences, most of these tectonic models imply an early crustal thickening.…”