Kinematics and geometry of Miocene low-angle detachments and exhumation of the metamorphic units in the hinterland of the Northern Apennines (Italy)

This paper deals with the lateral segmentation (crustal boudinage) and exhumation of the metamorphic units in the hinterland of the Northern Apennines during the Late Miocene extension. The study area (Mt. Leoni area) is part of the Middle Tuscan Ridge, a regional morpho-tectonic feature in which the deepest tectonic units of the Northern Apennines are broadly exposed. Crustal seismic profiles and deep boreholes indicate that the Middle Tuscan Ridge coincides with the outcropping part of a laterally discontinuous geological body, mainly made up of HP-LT and greenschist facies metamorphic units. Extensional detachments characterised by top-to-the-East sense of shear and staircase geometry produced the lateral segmentation of the metamorphic units. Their activity can be ascribed to Late Tortonian–Late Messinian times. Geometry and kinematics of the extensional detachments have been compared with the Miocene extensional structures developed at shallow depths in the overthickened crust of the Northern Apennines, allowing understanding of the extensional tectonic evolution which reorganised the hinterland Northern Apennines collisional belt.