Investigation of the Rheology of Gabbros: Example from the Queyras Ophiolites (France)

Abstract

The rheology of the lower oceanic crust remains poorly constrained despite its central role in plate tectonics and strain localization at slow-spreading ridges. This study investigates the petrology, deformation mechanisms, and tectonic and metamorphic evolution of gabbros from three Alpine ophiolite localities in the Queyras region (Western Alps, France), interpreted as remnants of a slow-spreading oceanic core complex. Petrographic observations have been integrated with scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and X-ray computed microtomography (µXRCT) to characterize mineral assemblages, microstructures, and the spatial distribution of weak phases. The gabbros are dominated by clinopyroxene, Ca-rich plagioclase, and Fe-Ti oxides, and record a complex history of deformation and metamorphism. Clinopyroxene porphyroclasts exhibit both ductile (undulose extinction, bending) and brittle (fracturing, bookshelf sliding) deformation, indicating a transition across the brittle–ductile regime. Strain localization is associated with Fe-Ti oxides, which occur preferentially in pressure shadows and along foliation, suggesting they act as mechanically weak phases that facilitate deformation. Plagioclase forms a fine-grained recrystallized matrix, while metamorphic assemblages including glaucophane, chlorite, epidote, and actinolite record fluid-assisted alteration under blueschist- to greenschist-facies conditions. Microstructural and chemical data indicate that deformation initiated near a slow-spreading ridge under high-temperature conditions, followed by hydrothermal alteration and progressive deformation during subduction. Subsequent exhumation was accompanied by fluid infiltration and retrogression under greenschist facies conditions. The absence of eclogite-facies minerals suggests limited subduction depths and relatively rapid exhumation. These results provide new constraints on deformation processes in the lower oceanic crust and highlights the importance of multiphase interactions and fluid-assisted weakening in governing lithospheric strength.