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 synmetamorp hic veins exposed in greenschist facies rocks on NW Sardinia was investigat ed with the aim of gaining insight into the processes controlling the fract ure spacing. The vein spacing distribution measured along linear profiles i s non-uniform and approximately log-normal. Although the vein abundance and absolute vein spacing is strongly influenced by rock type, the vein spacin g distribution is independent of rock type. Comparison of the observed data with synthetically generated fracture distributions indicates that the obs erved distribution can be reproduced by Kolmogorov fragmentation, but canno t be reproduced by a random: even-bisection or a fractal (Cantor Dust) proc ess. These results are interpreted to indicate that the positions of fractu res that become veins are interdependent. In an attempt to explain the inte rdependency of fracture positions. the perturbations of stress and fluid pr essure associated with an isolated underpressured axial planar fracture wer e investigated. Simple calculations show that the fluid pressure drop insid e a fracture coinciding with failure locally perturbs the stress and fluid pressure fields such that the surrounding rock, extending over a lateral ex tent equivalent to approximately two fracture lengths, is instantaneously d riven into a critical stress state that satisfies the failure condition. Nu merical simulations demonstrate that this process tends to localise the nuc leation of subsequent fractures close to existing fractures. This mechanism can explain the existence of non-random or clustered fracture or vein spac ing distributions forming by hydrofracture. (C) 2000 Elsevier Science Ltd. All rights reserved.