Syn-diagenetic evolution of shear structures in superficial nappes: an example from the Northern Apennines (NW Italy)

Abstract

In the superficial nappes of the Northern Apennines, meso- and microstructures in sheared mudstones and fine-grained turbidites consist mainly of networks of striated faults and calcite vein arrays. The striated faults correspond to the first generation of structures. They form two sets, respectively, synthetic and antithetic to the general shearing. Locally, a sigmoidal microfabric is associated with the synthetic faults in scaly deformation bands up to a few metres thick. Late laterally extensive calcite veins are superposed on these structures. They result from the opening of multiple releasing oversteps between shear surfaces. The ‘crack-seal’ mechanism of deformation played a major role in their formation. Structures of the first generation result from heterogeneous ductile deformation of initially poorly lithified and water-rich sediments. The late formation of the calcite veins corresponds to a transitional ductile-brittle behaviour in sufficiently lithified sediment. Evolution of structures probably depended on a strain-hardening process and was dependent on the progressive compaction and dewatering of the sediment. The gently dipping faults and veins probably formed the main pathway for the interstitial fluids, possibly overpressured, expelled from the sediment. We emphasize that the geometry of the structures described provides important shear-sense criteria for the study of superficial nappe tectonics.