Stress variability and brittle tectonic structures.

The problem of stress variability, both in space and time, during brittle deformation of rocks is faced in this note. Several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed. A genetic partitioning of the stress tensor is proposed, showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific dgeneticT components. The genetic components discussed in this note are the gravitational, the tectonic, the fluid, the thermal and the diagenetic one. Each genetic component is a function of several parameters, but for all of them the possible important role played by the variable time is emphasised. According to numerous researches, based on theoretical and experimental grounds, brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure. Field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small- and large-scale spatial variations as well as about short- and long-term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event. The relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches.