Synmetamorphic vein spacing distributions: characterisation and origin of a distribution of veins from NW Sardinia, Italy

The spatial distribution of a single generation of axial planar synmetamorphic veins exposed in greenschist facies rocks on NW Sardinia was investigated with the aim of gaining insight into the processes controlling the fracture spacing. The vein spacing distribution measured along linear profiles is non-uniform and approximately log-normal. Although the vein abundance and absolute vein spacing is strongly influenced by rock type, the vein spacing distribution is independent of rock type. Comparison of the observed data with synthetically generated fracture distributions indicates that the observed distribution can be reproduced by Kolmogorov fragmentation, but cannot be reproduced by a random, even-bisection or a fractal (Cantor Dust) process. These results are interpreted to indicate that the positions of fractures that become veins are interdependent. In an attempt to explain the interdependency of fracture positions, the perturbations of stress and fluid pressure associated with an isolated underpressured axial planar fracture were investigated. Simple calculations show that the fluid pressure drop inside a fracture coinciding with failure locally perturbs the stress and fluid pressure fields such that the surrounding rock, extending over a lateral extent equivalent to approximately two fracture lengths, is instantaneously driven into a critical stress state that satisfies the failure condition. Numerical simulations demonstrate that this process tends to localise the nucleation of subsequent fractures close to existing fractures. This mechanism can explain the existence of non-random or clustered fracture or vein spacing distributions forming by hydrofracture.