Initiation of convergent extension fracture vein arrays by displacement of discontinuous fault segments

Abstract

The formation of extension fracture-hosted veins that converge toward the bisector of conjugate arrays is problematical. The orientation of the extension fractures suggests that they formed within zones of re-oriented stress axes, established prior to fracturing. However, the commonly observed acute conjugate angle indicates that brittle dilatancy was already occurring during the initiation of the zones. Previous work has shown evidence of convergent extension fracture vein arrays formed by en échelon breakdown of faults passing from arenitic to pelitic units in turbidites. The present work shows evidence of convergent extension fracture vein arrays in limestone formed by localised re-orientation of stress due to slip on discontinuous fault segments causing en échelon fracture. The calcite veins studied occur in the Siluro-Devonian Jack Formation exposed in the Jack Hills Gorge, 200 km west of Townsville, in the Broken River province of north Queensland, Australia. The vein array systems display a range of acute conjugate angles from 60 ° to 30 °. The veins of conjugate arrays are arranged mainly in convergent configurations (including cross-parallel or Type 1), but bisector-parallel (Type 2) configurations also occur. The shear strain recorded by markers crossing convergent arrays and the morphology of veins was found to be inconsistent with the possibility that convergence resulted from rotation of veins from an initial bisector-parallel configuration.