Progressive accretion of antitaxial crystal fibres: Implications for the kinematics and dynamics of vein dilation

We made field and petrographic observations of antitaxial crystal fibres in the gypsum veins of the Mercia Mudstone (SW England) to examine the kinematic and dynamic implications of these fibres. The gypsum fibres are predominantly oriented oblique to vein walls, indicating that the veins were formed in a shear-extension mode rather than pure tension mode. Fibres in single veins are parallel aligned, regardless of the vein-wall curvatures, except those in the tapering tip zones. Field data demonstrate that there is no clear correlation between fibre plunges and vein dips. Fibres in many vein steps exhibit the same orientations as those in the main vein segments, suggesting that these steps were formed as reactivated pre-existing faults/fractures instead of dilatational jogs. Vein tips that propagated progressively, commonly curved towards the planes of neighbouring larger veins, or interacted with the tips of adjacent sub-parallel veins, suggesting that vein propagation may have been dominated by the local stress field rather than remote stresses. Fibres in vein branches may not share the same orientations as those in the parent veins. The host marls are commonly segmented into multiple parts by interconnected vein arrays that contain differently oriented fibres in each vein. Our study suggests that the gypsum veins lack evidence for crack-seal increments, and the crystal fibres in the veins had a continuous accretion towards vein walls. The fibres are parallel to vein opening directions as indicated by the correlative vein-wall irregularities, and are of great potential for kinematic analysis and deformation restoration of the host rocks. Vein dilation could be driven by force of crystallization of veins exerted on the enclosing rocks, which could have caused a growth-related compaction and fracturing rather than uplifting the overlying strata that was accompanied with a bulk extension.