Inter-array and intra-array kinematics of en échelon sigmoidal veins in cross-bedded sandstone, Merimbula, southeastern Australia

Abstract

The kinematics of en échelon arrays of quartz veins hosted by sandstone of the Upper Devonian Worange Point Formation are strongly influenced by bedding and cross-bedding. Bulk deformation recorded by the vein arrays (inter-array kinematics) was influenced by the anisotropy of the host rock, such that, three types of arrays are observed: (1) east-verging arrays parallel to bedding; (2) east-verging arrays oblique to bedding with veins parallel to cross-bedding; and (3) west-verging arrays parallel to cross-bedding with veins parallel to bedding. These three array types correspond to flats, ramps and backthrusts, respectively and represent the earliest stages of thrust fault formation. Flat and ramp arrays form a staircase geometry and conjugate ramp and backthrust arrays form pop-up structures. The interpretation of the morphology of veins and wall-rock (intra-array kinematics) indicates rotation of bridges was the primary kinematic process in the arrays. The opening of vein space by rotation of bridges which maintain constant thickness is predicted from measurements of the orientations of vein–wall-rock interfaces at vein tips and at the array centre. Comparison of the predictions with direct measurement of the proportion of vein to wall-rock along the centreline of arrays demonstrates the applicability of the bridge-rotation model which has been generalised to include the thickening or thinning of bridges during rotation.