Evolution of anastomosing crack–seal vein networks in limestones: Insight from an exhumed high-pressure cell, Jabal Shams, Oman Mountains

We studied a special type of zebra carbonate in limestones of an overpressure cell exhumed from at 5 km depth, in outcrops on Jabal Shams, Oman Mountains. The rocks show anastomosing patterns of regularly spaced calcite veins in dark gray, fine-grained carbonate; microscopic observations reveal these as dense bundles of much finer veinlets, typically 10–50 μm thick. The vein bundles are up to 5 mm thick, they contain multiple sub-parallel arrays of host rock fragments embedded in the coarse-grained vein calcite. We interpret these structures as the result of numerous mechanically effective crack and reseal events together with strong growth competition or crystallization from sparse nucleation sites. Cementation produced mechanically strong veins so that new fractures were localized along the vein/rock interface or within the matrix itself. We present simple conceptional models relating the mechanical strength of the vein and the morphology of the resulting vein network.