Structural and fluid-inclusion analyses reveal a correlated change in structural style and fluid composition along strike in footwall mylonites of the Simplon low-angle normal fault of Switzerland. In the south, early postmylonitic structures are semibrittle shears and brittle faults; synkinematic fluid inclusions within the faults contain a carbonic component, whereas those in the shears lack CO2. To the north, early postmylonitic structures are extension veins and kinkbands and synkinematic fluid inclusions are aqueous. Entrapment conditions for all fluid inclusions are estimated to be >500 °C and >5 kbar. We speculate that different wetting characteristics of carbonic versus aqueous fluids influenced the mechanical behavior of the rocks. Nonwetting carbonic fluids locally increased fluid pressure and induced brittle failure, resulting in development of abundant, closely spaced brittle faults. In areas lacking carbonic fluids, ductile deformation continued to lower pressures and temperatures.

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