Late Alpine brittle faulting in the Rotondo granite (Switzerland): deformation mechanisms and fault evolution |
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Authors: | Volker Lützenkirchen Simon Loew |
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Institution: | 1.Matousek, Baumann & Niggli AG,Baden,Switzerland;2.Swiss Federal Institute of Technology (ETH Zurich),Zurich,Switzerland |
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Abstract: | The unlined Bedretto tunnel in the Central Swiss Alps has been used to investigate in detail the fault architecture and late
Alpine brittle faulting processes in the Rotondo granite on macroscopic and microscopic scales. Brittle faults in the late
Variscan Rotondo granite preferentially are situated within the extent of preexisting ductile shear zones. Only in relatively
few cases the damage zone extends into or develops in the previously undeformed granite. Slickensides suggest a predominant
(dextral) strike-slip movement along these steeply dipping and NE–SW-striking faults. Microstructures of these fault rocks
illustrate a multi-stage retrograde deformation history from ductile to brittle conditions up to the cessation of fault activity.
In addition these fabrics allow identifying cataclastic flow, fluid-assisted brecciation and chemical corrosive wear as important
deformation mechanisms during this retrogressive deformation path. Based on the analysis of zeolite microfabrics (laumontite
and stilbite; hydrated Ca–Al- and Na–Ca–Al–silicate, respectively) in fault breccias, cataclasites and open fractures we conclude,
that the main phase of active brittle faulting started below 280°C and ceased ca. 14 Ma ago at temperatures slightly above
200°C. This corresponds to a depth of approx. 7 km. |
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