Geometry and kinematics of convergent conjugate vein array systems

Abstract

Conjugate arrays of quartz veins in the Neranleigh-Fernvale turbidite beds at Norries Head, eastern Australia occur in configurations in which the trends of the veins, in a principal section, converge towards the acute bisector of the conjugate arrays (a convergent configuration). Such a configuration is common in vein arrays, and has been attributed to initiation of the veins as antithetic shear fractures based on geometric arguments. The morphology of the veins in this study indicates that the veins are hosted by extension fractures which formed by en échelon breakdown of faults. Previous models of extension fracture arrays emphasise that all fractures lie parallel to the bisector of conjugate arrays and that arrays develop after initiation of extension fractures and concurrent with fracture propagation. Sigmoidal vein shapes have been attributed to concurrent shear strain and fracture propagation. An alternative model is proposed for the geometry of conjugate arrays formed by en échelon breakdown of faults. The conjugate angle between parent faults is established before the en échelon extension fractures are formed. The fracture-array angle depends on the local displacement of the parent fault, thus, there is no necessity for the fractures in different arrays to be parallel. If the fracture-array angle is greater than half the conjugate angle between parent faults, a convergent configuration of fractures is produced. The kinematics of opening of veins in this study involved bending of rock bridges between fractures producing a gradation from planar to sigmoidal shapes in serial sections, without evidence of concurrent fracture propagation.