Fluid-induced strain softening during the formation of the Pyrenean orogenic prism: In situ Rb/Sr dating from the Cap de Creus shear zones (Eastern Pyrenees, Spain)

The aims of this study is to understand how concurrent fluids and deformation influence the behaviour of the Rb/Sr geochronometer in micas. This study is conducted within a classical deformation framework, shear zones, key structures in the geodynamic evolution of rifts and orogens which can accommodate kilometer-scale displacements and facilitate significant mass transfer. While the last two decades have led to a better understanding of the processes of nucleation and widening of these structures, as illustrated by the world-famous case of Cap de Creus, the extreme localization of deformation they exhibit raises fundamental questions about the rheological evolution of the lithosphere.

By conducting a multiscale petro-tecto-geochronological study of the Cap de Creus shear zones, whose age is debated, we have underlined that these shear zones record a major event of fluid-induced strain softening during the formation of the Pyrenean prism. This event is characterized by concomitant muscovite blast neocrystallization and intense quartz dynamic recovery, whose contemporaneity can be uniquely demonstrated by evidence of Fe-rich surface-derived fluids that penetrated to depth. This event occurred between 60 and 50 Ma, as shown by in situ and in-context Rb/Sr dating of two shear zones. This study highlights the critical role of fluid-induced rheological softening in ductile reactivation of polycyclic basements and provides a context-dependent framework for interpreting the behaviour of the Rb/Sr geochronometer in muscovite during deformation.

The implications of these results for the three-dimensional formation and evolution of the Pyrenean orogenic prism, will be discussed, including the role of structural inheritance, fluid circulation, and their contribution to shear-zone reactivation processes.