Multi fluid-flow record during episodic mode I opening: A microstructural and SIMS study (Cotiella Thrust Fault, Pyrenees)

Syntectonic veins commonly have been used to assess the composition and source of fluids involved in fault zone activity. Such veins also provide information on the ambient stress conditions during deformation and mineralization. Based on bulk sampling and bulk O- and C-isotope analysis, combined with fluid inclusion microthermometry, many studies have demonstrated that syntectonic veins provide snapshots of fluid composition and stress conditions over the course of fault history. This is widely acknowledged for mode I extension veins that develop in the damage zones of faults. However, an important and unanswered question is the extent to which such veins record a more detailed fault history at the micron scale. In this study, we present new detailed in-situ micron-scale δ18O data, measured using Secondary Ion Mass Spectrometry (SIMS), combined with detailed fluid inclusion microthermometry and Δ47 clumped isotope thermometry to document the fluid and temperature conditions during mode I vein growth related to deformation along the Cotiella thrust fault (Pyrenees). All the studied veins show three distinct episodes of vein opening, recording a complex history of varying fluid composition and temperature. Results show that the studied fault portion passed from a hydrological rock-buffered system, in which formation waters were in isotopic equilibrium with the host sediments, to a fluid-buffered system involving meteoric water. However, such information is only achievable at the micron scale using Secondary Ion Mass Spectrometry. This study demonstrates the potential of these new micro-beam techniques for investigating fault behavior in more detail than previously available, particularly with respect to the nature of the fluids involved and the P-T conditions extant during fault activity.