Late Alpine brittle faulting in the Rotondo granite (Switzerland): deformation mechanisms and fault evolution

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.

Hydraulic subsurface measurements and hydrodynamic modelling as indicators for groundwater flow systems in the Rotondo granite, Central Alps (Switzerland)

“…These are the Gamsboden/Cacciola‐Granite (C) and the Fibbia‐Granite (F). Throughout the area of the Gotthard‐Massif and Aar‐Massif, alpine faulting and foliation can be observed (Labhart, ; Zangerl et al ., ; Luetzenkirchen and Loew, ). The former are often following pre‐existing structures such as lithological boundaries, intrusive contacts or dykes and are found today mainly with steep to subvertical inclinations.…”

Transient surface deformations caused by the Gotthard Base Tunnel

“…This is because highly conductive (highly diffusive) preferential groundwater pathways have limited extents due to tectonic heterogeneity [16,30] and quickly develop hydraulic interaction (leakage) with with small fracture networks belonging to the matrix. Leakage in the fractured rock mass is also clearly supported from distributed small magnitude dripping tunnel inflows [16,42].…”

“…Most faults in the study area show a complex Alpine deformation history with a ductile phase overprinted by strong brittle deformations [29,30]. All faults with significant surface expressions have been mapped, classified and projected to the tunnel elevation prior to the construction of the base (Fig.…”

“…Brittle faulting during Miocene mainly occurred along pre-existing ductile shear zones and is characterized by strike slip motion [28][29][30]. Based on Lützenkirchen and Loew [30] [31]: Its northern part (1145 m length at the elevation of the GBT) is composed of mica-gneisses and schists with an extremely strong Alpine tectonic overprint which occurred both under ductile and brittle conditions. The southern part of the TZM (1382 m length) consists of rocks with only localized brittle shear deformation (faults with a few meter width make up 5% of the entire section).…”

Dynamic modelling of a mining-induced fault slip

“…It is found out from the studies that fault zones generally consist of fault damage zones and fault cores, 34 to which shear displacements are localized (Bradbury, Barton, Solum, Draper, & Evans, 2007;Chester & Chester, 1998;Chester & Logan, 1986;Lutzenkirchen & Loew, 2011;Noda & Shimamoto, 2009). The fault cores are composed of fault gouge, clay-like materials or cataclasite layers, and significantly thin compared to the damage zones.…”