Shattering and structural inheritance at the onset of faulting in the Rotondo granite

Faults accommodate shear motion in the upper crust through brittle deformation such as cataclastic flow. Plenty of field evidence suggests that the width of the embrittled volume, the lateral extent of the fault plane, and the grain size distribution within the fault core scale with shear displacement, indicating fairly solid scaling laws. However, at the onset of brittle failure, faults commonly exploit pre-existing anisotropic structures that facilitate slip localization and affect the early fault geometry as well as the fabric of the cataclastic products. The Bedretto Underground Laboratory represents a unique chance to study the structure of immature faults, whose pristine brittle structures are hardly preserved elsewhere during exhumation and exposure to weathering. We present the case study of the Waterfault (WF), a brittle shear zone hosted in the Bedretto tunnel within the Rotondo granite, which exploits pre-existing Alpine mylonites.

On the tunnel wall, the brittle products of the WF are confined within a 50 cm thick volume, comprised between two boundaries defined by the local mylonitic foliation. The WF is characterized by a large water outflow, suggesting that the fault behaves as a major permeable conduit inside the granite host. Such high permeability is commonly related to the occurrence of intense but localised brittle damage. However, careful sampling across the fault and detailed microstructural investigations revealed that the brittle damage introduced by shear displacement is much less volumetrically widespread than expected. Cataclasis is in fact confined to a thin (5 mm) fine-grained gouge shear band developed at the boundary of the fault zone. The damaged rock surrounding this layer presents intense fracturing, dominated by oriented fractures at high angle to the shear plane, but no sensitive displacement down to the grain-scale. Damage distribution and geometry is further controlled by the anisotropic mylonitic fabric and its mineralogy, showing grain boundary cracking and shard-like fragmentation of quartz and feldspar, as well as micro-boudinage-like cracking of mica-grains. More than 50% in weight of the material recovered both within and in proximity of the gouge layer is finer than 125 µm. These observations are evidence of dynamic rock shattering, pointing to a seismic origin of the embrittlement. The seismic damage preferentially fractured along the grain boundaries of the mylonite, producing a fine-grained material that eased the onset of localised brittle shear.

Our microstructural observations suggest that the WF might represent an optimal model for the early onset of faulting in anisotropic rocks driven by initial seismic damage, which unlocked the cohesive shear zone to favour slip localization. The “shallow” estimated depths of this event (< 5 km) might also open an interpretative window for the small-magnitude natural and induced seismicity (M<2) recorded in the Rotondo massif.