The research within the STRAIN project was conducted along two main lines of research: laboratory experiments & geological fieldwork, both complemented by extensive microscopy studies.
I designed state-of-the-art friction experiments to study the role of mechanical strength evolution, i.e. the resistance to sliding in faults smulated in the laboratory. From these experiments we retrieved the sheared materials that were analyzed under optical and electron microscopes to study the evolution of the microstructures in response to the deformation (e.g. deformation/breaking of crystals, particle size evolution, extent and geometrical arrangement of deformed zones in the samples etc.).
We documented the (low) friction of important rocks commonly found in faults of the Earth's Crust, such as serpentines and foliated clays, which control the strength if fault interfaces a many levels of faults at plate boundarires. In addition, in the laboratory we simulated two fundamental modes of fault slip: i.e. sliding within rock gouges (powders) and sliding along solid bare surfaces, which are commonly observed in nature. With sliding experiments on bare rocks, we discovered that macroscopic frictional strength is extremely sensitive to surface roughness: we observed that low roughness surfaces have extremely low strength and the slip behaviour is unstable giving rise to laboratory earthquakes. This and other peculiar processes of the sliding surfaces may be strongly akin to processes of slip in shallow natural faults and open new possibilities to study the stability and regularity of slip in the rocks. On the other hand, experiments made sliding rock powders documented how friction evolves in rocks when we have progressive comminution of the grain size in the powders. We found that when powders are extremely fine, frictional behaviour becomes unstable or irregularly unstable, depending on the conditions surrounding the fault (e.g. elasticity, pressure). This allowed us to study what happens at the end of the natural processes of grinding and comminution in natural faults. We demonstrated, with the aid of microstructures, that the stable or unstable behaviour is due to the trade-off between the efficiency of brittle and viscous processes. Brittle and viscous processes are both active at the comminution limit of the rocks, even if the low ambient temperature of the fault would result, in principle, into sole brittle deformation.
In the field, we investigated several faults exhumed to the Earth's surface, including shallow faults in the NE Apennines of Italy, intermediate depth faults in the W Apennines, and deep faults in the Caledonides of NW Scotland. We described in detail the architecture of fault zones, showing the extent of damage imposed by the sliding and flowing of rocks. We documented also the deformation processes in the fault zones and the evidence of fluid pressure and migration ad depth. In particular, we recognize that dissolution-precipitation processes have a paramount role in controlling the strength of faults, the cycle of fluid pressure and the overall structure of the fault zone, at all crustal depths.
The integration of mechanical data and field observation led us to formulate a model to put forward the genesis of slow slip and arrest in tectonic faults, one of the promising avenues to study the genesis of earthquakes in the last decades. In our model, slow slip is generated by brittle shear in conjunction with dissolution-precipitation forming a network of mechanically weak platy minerals and the slowness and arrest of the process is due to the geometrical complexity of the fault zone we observe in the field.
