Deformation pattern in the underthrust carbonate-rich sequenze of the Sibillini Thrust (central Italy): insights for shear zone evolution in modern subduction complexes

Modern convergent zones at tropical latitudes are characterized by subduction of carbonate sediment. Although carbonate response to deformation is different from clay, they are commonly treated as having a similar rheology. This approximation, though, is inadequate since carbonate behavior is complicated by cementation and pressure solution. Our goal, here, is to focus on the deformation of carbonate sediment in the footwall of a major fossil thrust zone and compare it with a possible scenario in a modern environment. The focus area is the Monti Sibillini Thrust, in the Umbria-Marche sector of the Northern Apennines where the pelagic, Late Eocene-Oligocene carbonates of the Scaglia Cinerea Formation are underthrusted beneath the late Cretaceous-Middle Eocene Scaglia Rossa Formation. The data collected during this meso-structural study allowed for an interpretation of the temporal and spatial relationships between the observed deformation structures (S-C deformation bands, shear veins and stylolites). These results allow the development of a dynamic model for the deformation. The proposed model explains the observed features through cycles of fluid-pressure build up, hydraulic fracturing, and consecutive collapse, producing pressure-solution and stylolitization.