Successive Fracturing and Fluid Flow Events During Thrust Sheet Emplacement: The Montsec Thrust Sheet (Southern Pyrenees)

Field data together with petrographic and geochemical analyses (δ¹³C, δ¹⁸O) of vein cements and host rocks are used to reconstruct the fluid flow evolution in a well-exposed carbonate thrust zone, the Montsec thrust sheet, Southern Pyrenees. The Montsec thrust places Upper Cretaceous marine limestones over Paleocene non-marine carbonates and clays. Within this thrust zone, four fracture sets including bed-parallel and bed-perpendicular fractures (F1 and F2, respectively), bed-parallel slip surfaces (F3) and strike slip faults (F4) have been identified. These fractures have been filled with four generations of calcite cements (Cc1-Cc4). Consequently, stable isotope analyses applied to these cements indicate three different geochemical trends: (i) similar δ¹³C values between all cements and their related host rocks (from 0 to + 2 ‰VPDB) that has been interpreted as buffering of the cement-forming fluids by the host carbonates; (ii) a progressive depletion in δ¹⁸O values from Cc1 to Cc3 (from − 7 to − 12 ‰VPDB) that is either attributed to a change in the fluid origin, a dilution of the fluid composition and/or an increase in the fluid temperature and (iii) an enrichment in the δ¹⁸O values from Cc3 to Cc4 (from − 12 to − 9 ‰VPDB), probably indicating the infiltration of a low-temperature, likely meteoric, fluid during thrust sheet exhumation and piggy-back transport of this thrust towards the south. The formation of successive fracture/vein sets as well as the geochemical variation of the vein cements through time indicates changes in the fluid origin, pathways and the extent of fluid-rock interaction during the emplacement of the Montsec thrust. Therefore, this study highlights how the evolution of carbonate veins could inform the fluid flow events during the geodynamic evolution of fold-and-thrust belt systems.