Slip and stress in block-in-matrix shear zones: 1. microstructure and mineralogy of a serpentine-filled dilational jog

We document the structural setting, microstructure, and mineralogy of a crack-seal-type dilational jog that developed in the stepover between two faults cutting through a phacoid of massive serpentinite, itself embedded within a serpentinite shear zone at the base of the Dun Mountain Ophiolite, New Zealand. Our outcrop and microstructural measurements allow us to constrain the boundary conditions for a numerical model (part 2) that quantitatively explores the relationships between stress, fault slip, and incremental cracking in block-in-matrix shear zones. The dilational jog is c. 3 cm wide and contains hundreds of crack-seal bands, each c. 20–30 μm wide. Internally, the jog comprises two mineralogically distinct crack-seal domains: serpentine-only domains and serpentine-andradite garnet domains. Additionally, individual crack-seal bands have a double-layer structure: in serpentine-only domains each band comprises a thin (<2 μm) layer of chrysotile and a thicker layer (c. 25 μm) of polygonal serpentine/lizardite, whereas each band in serpentine + andradite domains comprises a thinner (c. 5 μm) layer of microcrystalline andradite and a thicker layer (c. 15 μm) of polygonal serpentine/lizardite. Micro-CT analysis shows that the serpentine + andradite domains have conic or ellipsoidal shapes with long axes subparallel to the inferred jog opening direction, and that andradite is smeared along micro-transform surfaces inside the jog. Our conceptual microstructural model invokes jog formation during progressive serpentinization of the host rock. Incremental crack opening along the jog-wall rock interface promotes relatively rapid initial precipitation of chrysotile or andradite at high fluid:rock ratios. As cracks fill and pressure re-equilibrates, relatively slow growth of polygonal serpentine/lizardite is favoured until the cracks are sealed and the cycle repeats. Our observations suggest that the precipitation of andradite (instead of chrysotile) was controlled both by structural boundaries within the jog (e.g., micro-transform surfaces) and by local element transport (e.g., Ca from serpentinizing clinopyroxene grains in the host rocks) to patches of the crack wall.