Shortening of the axial zone, pyrenees: Shortening sequence, upper crustal mylonites and crustal strength

Abstract

The strength of the lithosphere may be constrained qualitatively by field observations on 19 localized vs distributed modes of deformation and by the mineral assemblages formed during 20 deformation. The internal deformation of the Bielsa basement unit of the Pyrenean Axial zone 21 is investigated through structural, microstructural and thermometric data. In this area, 22 shortening is widely distributed as attested by the folded attitude of the interface between the 23 basement and its sedimentary Triassic cover. Shortening is estimated around 1.7 km (13%) 24 from a regional balanced cross-section and should be considered in pre-Pyrenean 25 reconstructions. Shortening probably occurred before strain localization on crustal ramps as 26 suggested by zircon fission-track analysis. Distributed shortening is characterized at small-27 scale by low-temperature mylonites and cataclasites. In thin-section, feldspar originally 28 present in magmatic rocks is partially to totally sericitized. This transformation led to 29 significant weakening of the rock and took place in the 250-350°C temperature range. 30 Sericitization is ubiquitous, even in un-deformed granodiorites. This shows that the 31 weakening effect of sericitization not only occurs in ultra-mylonites, ultra-cataclasites and 32 2 phyllonites but also more generally in the upper crust early during the shortening history, with 33 implications for the shortening style. Estimates of the geothermal gradient suggest that 34 inherited thermicity may also have influenced the style of shortening. We propose that the 35 upper crust was very weak before or at the onset of its shortening due to high-thermal gradient 36 and fluid circulation that induced large-scale sericitization in greenschist facies conditions. 37 This has strong implications on the rheological evolution of the upper crust submitted to 38 metamorphic alteration in the greenschist facies and below.

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

“…Fluid inclusion trapping pressures in the range 2–5 kbar are reported by Henderson and McCaig (), consistent with pressure estimates from white mica in mylonite veins (Wayne & McCaig, ). The lowest pressure values of ~2 kbar are consistent with the maximum burial depth estimated for the Bielsa (Bellahsen et al, ; Jolivet et al, ).…”

“…Petro‐chronological results and microstructural analyses show that late to post‐Variscan lower greenschist facies metamorphism in the Bielsa massif transformed an initially strong assemblage made of plagioclase, K‐feldspar (forming together close to half of the primary modal composition) and amphibole into weaker, hydrated minerals such as white mica and chlorite, without associated deformation (as suggested by Bellashen et al, ). The breakdown of feldspar to white mica decreases by up to two orders of magnitude the sustained shear stress (at a constant strain rate) at 400°C, lowering the depth of the brittle–ductile transition by at least a third (Gueydan et al, ; Wintsch et al, ).…”

“…Close to Lake Urdiceto (Figure c,d), granitic to granodioritic rocks (Román‐Berdiel et al, ) are unconformably overlain by Triassic continental red sandstones and pelites. The basement and its sedimentary cover were deformed (folded) during the Alpine compressional phase (Figure d; Bellashen et al, ; Casas, Oliva, Román‐Berdiel, & Pueyo, ; Román‐Berdiel et al, ) with a wavelength of ~1 km. Several sub‐vertical S‐ or N‐dipping normal or reverse steep faults bound the antiforms and synforms (Figure d).…”

“…They exhibit a sub‐vertical to N‐dipping cleavage (e.g. Figures b and c), with nord side up sense of shear kinematic indicators above the sediment‐cover interface (Bellahsen et al, ). The granitic basement is heterogeneously deformed.…”

“…Maladeta, Néouvielle; Wayne & McCaig, ). In addition, structural and microstructural studies show significant distributed shortening at the scale of the massif (Bellahsen et al, ; Román‐Berdiel, Casas, Oliva‐Urcia, Pueyo, & Rillo, ), raising questions about the link between low‐grade metamorphism and strain accommodation mechanisms. Early feldspar destabilization has been proposed to have a major control on subsequent mechanical softening in Bielsa (Bellahsen et al, ), but the conditions and the age of such a reaction still remains unconstrained.…”