Correlations between fluid composition and deep-seated structural style in the footwall of the Simplon low-angle normal fault, Switzerland
Correlations between fluid composition and deep-seated structural style in the footwall of the Simplon low-angle normal fault, Switzerland
Geology (Boulder) (August 1999) 27 (8): 715-718
- Alps
- brittle deformation
- carbon
- carbon dioxide
- Central Alps
- Central Europe
- composition
- controls
- deep-seated structures
- deformation
- ductile deformation
- Europe
- faults
- fluid inclusions
- fluid phase
- foot wall
- inclusions
- Lepontine Alps
- low-angle faults
- metamorphic rocks
- mylonites
- normal faults
- Simplon region
- Southern Swiss Alps
- structural analysis
- Swiss Alps
- Switzerland
- syntectonic processes
- Valais Switzerland
- Simplon fault zone
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 CO (sub 2) . 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 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 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.