Correlations between fluid composition and deep-seated structural style inthe footwall of the Simplon low-angle normal fault, Switzerland

Structural and fluid-inclusion analyses reveal a correlated change in struc tural style and fluid composition along strike in footwall mylonites of the Simplon low-angle normal fault of Switzerland. In the south, early postmyl onitic structures are semibrittle shears and brittle faults; synkinematic f luid inclusions within the faults contain a carbonic component, whereas tho se in the shears lack CO2. To the north, early postmylonitic structures are extension veins and kinkbands and synkinematic fluid inclusions are aqueou s. Entrapment conditions for all fluid inclusions are estimated to be >500 degrees 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 ind uced brittle failure, resulting in development of abundant, closely spaced brittle faults. In areas lacking carbonic fluids, ductile deformation conti nued to lower pressures and temperatures.