Monitoring paleo-fluid pressure through vein microstructures

A comparative study carried out on the Northern Apennines of Italy has revealed that syndiagenetic extensional and shear veins developed in mud-rich sediments because of fluid overpressure. They are characterized by a crystallization sequence that includes key steps, represented by different crystal morphologies, such as network, breccia, fibres and pressuresolution. The vein network is visible at both the microscale and the mesoscale. Evidence of two opening directions implies variation of the main stress regime. The other steps provide insights in the fluid pressure evolution, so that the breccia and the fibres indicate different opening rate in overpressured sediments, while pressure-solution marks the end of each episode. Vein evolution shows that stress regime and fluid pressure are intimately related and that they drive the occurrence of fracture. Moreover syn-diagenetic veins reflect the transient nature of stress and fluid flow offering the possibility to investigate the very early stages of deformation and compaction of sediments.