Matteo Demurtas

Fracturing and force distribution during shearing in bimaterial gouges

Fault gouges are amongst the main products of strain accommodation in the brittle upper crust. During shear, fracturing has a direct effect on grain size (and shape) evolution and ultimately grain-grain interactions. Such changes are believed to influence how stress is accommodated within the gouge and therefore control the mechanical and petrophysical properties of granular materials. Recent observations of both natural and experimental strain localisation features have highlighted the potential for compositional anisotropy within a deforming rock to play a key role in control fracturing processes. As a result, the occurrence of a polyphasic mineral assemblage in natural gouges may strongly influence the rheology of a fault during deformation. Numerical simulations using the Discrete Element Method (DEM) approach have proved, in recent years, to be a powerful tool in revealing grain scale processes associated with frictional sliding of granular fault gouges. Such an approach affords us unusual access inside a deforming material to precisely track the active grain processes and how they interact and evolve. We performed DEM simulations designed to investigate the influence of a bimaterial mineral assemblage on fracturing, grain size distribution evolution, force  and stress distribution within a deforming gouge applied to the carbonate system, with a calcite-dolomite mixture. Our simulations show how even small amounts (c. 20%) of a more competent phase can results already in significant changes in the fracturing processes occurring during shearing, ultimately bearing the potential of controlling its slip behaviour.