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

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