Structure, geometry and formation of brittle discontinuities in anisotropic crystalline rocks of the central Gotthard massif, Switzerland

Abstract

Spatial occurrence, structural architecture and formation of brittle fault zones and joints are investigated by outcrop observations, scanline mapping, and light- and scanning electron microscopy in an anisotropic crystalline rock mass (e.g. granites, para-gneisses and schists) of the central Gotthard massif in the Swiss Alps. The analysis presented illustrates that several pre-fault anisotropic features (i.e. dykes, ductile shear zones, foliation and presumably a pre-existing meso-scale fracture set) control the nucleation and propagation of brittle faults. Three sets of brittle fault zones striking NE-SW, NNE-SSW and WNW-ESE can be distinguished. They formed through cataclasis at temperatures below 300 °C, and were activated predominately in a strike-slip regime. Up to five joint sets were mapped and characterized according to orientation, frequency, spacing and formation. Finally a regional fan structure was established in the Gotthard Pass area, encompassing the main foliation, steeply dipping joints and brittle fault zones, each of which shows the same orientation and location of the symmetry plane (NE-SW orientated).