Incrementally Developed `Dilational Hydro-Shears' Forming at High Angles to σ1 in Foliated Mélange Matrix

The Chrystalls Beach accretionary mélange, New Zealand, contains an extensive fault-fracture mesh, in which mutually cross-cutting, incrementally developed, subhorizontal slickenfibre shear veins and subvertical extension veins intersect at an oblique angle of ~80° (Fig. 1). Slickenfibre shear veins commonly have multiple internal stylolitic slip surfaces subparallel to ubiquitous cleavage in pelitic mélange matrix, macroscopic `crack-seal' textures, and dissolution selvages along the vein margins. Stylolites, internal slip surfaces, and the long axes of elongate crystals within slickenfibre shear veins are inclined at a low angle (<15°) to the vein margin. Where shear and extension veins intersect, crystal fibres within the extension vein are parallel to elongate crystals within slickenfibres. The two kinematic vein types therefore had the same crystal-growth direction and are inferred to have formed contemporaneously in the same stress field. Thus field and microstructural observations suggest that slickenfibre-coated shear surfaces in the Chrystalls Beach mélange were active at ~80° to σ1, despite the constraint from Coulomb mechanics that new-forming faults, in cohesive, low porosity rocks, should lie at an angle θi = 45° - φ/2 to σ1. The slickenfibres appear to have formed by reactivation of subhorizontal weak planes (e.g. stylolites parallel to cleavage) which act as micro-transforms linking subvertical extension fractures opening parallel to σ3. This requires Pf > σ3 and low differential stress depending on the frictional resistance on the stylolites, and allows for shear failure at high angle to σ1 in fluid overpressured heterogeneous shear zones. The reshear conditions are similar to the extensional hydrofracture criterion, but low tensile strength, low intrinsic cohesion, and localised elevated shear strain controlled by material heterogeneities and anisotropy, create a situation where shear occurs essentially by extension under local hydrofracture conditions between weak planes. The term `dilational hydro-shears' is suggested for these slickenfibre surfaces to reflect their dilational component and the requirement of the hydrofracture criterion to be locally achieved for shear to occur. Schematic diagram (not to scale) of the geometrical relationships between shear- and extension veins, and inferred compressional stress trajectories in the Chrystalls Beach mélange.