Shear zone-mediated transfer and buffering of CO2-rich fluid during orogenic degassing

CO₂ degassing in orogenic settings, derived from either prograde metamorphic decarbonation or deep mantle sources, is commonly linked to deep and shallow seismicity along major deformation zones. However, the scarcity of natural fossil analogues of these deformation zones hosting CO₂-rich fluid flow limits our understanding of the causative mechanisms and hinders validation of proposed explanatory models and inferences from geophysical and geochemical observations.

We describe a set of shear zone-hosted, carbonate-bearing breccias from the basement units of the Gotthard nappe, Aar, and Mont Blanc massifs (Central and Western Alps), interpreted as evidence of regional-scale, fault zone-hosted flow of CO₂-rich metamorphic fluids during Alpine orogenesis. Field observations, microstructures and geochemical data suggest these breccias serve as fossil analogues of fault/shear zone networks controlling CO₂-rich fluid flow in active orogenic settings, providing new insights on crustal-scale transport of carbonic fluids and its relationship with tectonic deformation.

The breccias are localized on a pre-Alpine fault network within the crystalline basement rocks and are mainly composed of coarse-grained (mm-to-cm in crystal size) blocky calcite/dolomite forming a matrix that encloses angular host rock clasts. The large volumes of carbonates and the macro- and micro-textures indicates formation during transient, but repetitive (carbo-)hydraulic fracturing in the presence of CO₂-rich fluids, potentially at amphibolite/upper-greenschist facies conditions. C-O stable isotopes (-8.49‰ < d¹³C_VPDB < +0.73‰, +8.07‰ < d¹⁸O_VSMOW < +16.39‰) and the enrichment of (Heavy) REE elements, as well as the characteristic Y/Ho and La_N/Lu_N ratios, suggest the fluids potentially originated from high-grade metamorphic decarbonation during Alpine collision.

These breccias, together with previously reported H₂O-CO₂ flow examples in the Central-Western Alps (e.g., carbonate shear zones along the Glarus thrust; retrograde calcite-bearing Alpine clefts), point to orogen-scale flow of CO₂-rich fluids spanning prograde, peak, to retrograde metamorphism during Alpine collision. Although evidence for seismogenic deformation is limited, field and microscale structures show evidence for transient tectonic deformation, potentially aided by elevated pore fluid pressure. Tectonic stress drops associated with fluid pressure changes in these zones might have promoted (transient) H₂O/CO₂ phase immiscibility, leading to carbonate saturation and voluminous deposition. The resulting large carbonate volumes suggest that the shear zones intermittently acted as both conduits and reservoirs for CO₂ -rich fluids transported from depth toward the surface. This highlights their dual role in controlling orogenic CO₂ degassing and buffering emissions, with implications for understanding fluid-mediated tectonics and carbon cycling in collisional orogens.