GEOMETRY AND KINEMATICS OF CONVERGENT CONJUGATE VEIN ARRAY SYSTEMS

Conjugate arrays of quartz veins in the Neranleigh-Fernvale turbidite beds at Norries Head, eastern Australia occur in configurations in whi ch the trends of the veins, in a principal section, converge towards t he acute bisector of the conjugate arrays (a convergent configuration) . Such a configuration is common in vein arrays, and has been attribut ed to initiation of the veins as antithetic shear fractures based on g eometric arguments. The morphology of the Veins in this study indicate s that the veins are hosted by extension fractures which formed by en Echelon breakdown of faults. Previous models of extension fracture arr ays emphasise that all fractures lie parallel to the bisector of conju gate arrays and that arrays develop after initiation of extension frac tures and concurrent with fracture propagation. Sigmoidal vein shapes have been attributed to concurrent shear strain and fracture propagati on. An alternative model is proposed for the geometry of conjugate arr ays formed by en echelon breakdown of faults. The conjugate angle betw een parent faults is established before the en Echelon extension fract ures are formed. The fracture-array angle depends on the local displac ement of the parent fault, thus, there is no necessity for the fractur es in different arrays to be parallel. If the fracture-array angle is greater than half the conjugate angle between parent faults, a converg ent configuration of fractures is produced. The kinematics of opening of veins in this study involved bending of rock bridges between fractu res producing a gradation from planar to sigmoidal shapes in serial se ctions, without evidence of concurrent fracture propagation. Copyright (C) 1996 Elsevier Science Ltd