Critical displacement for normal fault nucleation from en-échelon vein arrays in limestones: a case study from the southern Apennines (Italy)

The process of fault initiation by the coalescence of en-echelon arrays of tensile cracks has been discussed by several workers and is now well established. However, not many studies have tried to quantitatively describe this process in terms of displacement and shear strain that control the early stages of fault nucleation. In this study, a large number of conjugate en-echelon vein arrays showing extensional offsets have been analysed. These structures are exposed in well bedded micritic limestones deformed at very low-grade conditions in the Apennine mountain belt of southern Italy. Three different types of structures can be distinguished: (i) ‘brittle–ductile’ shear zones, characterised by arrays of en-echelon, calcite-filled tension gashes; (ii) shear zones showing incipient development of discontinuous shear-parallel fractures cutting through the vein array; and (iii) faulted shear zones, in which a continuous, discrete normal fault breaks through—and clearly developed from—an original en-echelon vein array. Critical values of displacement and shear strain have been determined for the onset of normal fault nucleation from ‘brittle–ductile’ shear zones, therefore suggesting that these parameters exert a major control on fault initiation. A diagram of shear zone population vs. displacement also shows that this parameter obeys a power law of the type generally followed by normal faults in the frictional regime. Our results suggest that, during the early stages of fault nucleation, displacement and shear strain control the switch from dominant ductile (viscous) deformation mechanisms—accompanied by brittle fracturing responsible for vein formation—to the frictional regime.