Late orogenic structures and orogen-parallel compression in the Northern Apennines

The Northern Apennines is an orogenic belt characterized by a northeastward propagation of deformation during the main collisional event. Field mapping, together with structural analyses, pointed to the presence of several later transversal and parallel structures, with respect to the main trend of the chain. These are mainly represented by strike-slip faults and folds. Steeply dipping NW-SE trending folds are related to a continuing NE-SW direction of shortening. One of the most striking transversal faults is the dextral fault located in the northern side of the Apuan Alps, near the boundary with the Tuscan Nappe. It trends nearly ENE-WSW and it develops for about ten kilometers. Other transversal structures are represented by upright folds, recognized in the Tuscan Nappe and in the Massa Unit outcropping in several areas of the Northern Apennines. The direction of shortening related to these structures, both folds and faults, is oriented roughly NW-SE, i.e. parallel to the main trend of the Northern Apennines. This orogen-parallel compression is related to the development of the metamorphic domes. The two directions of horizontal shortening induced buckling and vertical growth in the metamorphic domes, so enhancing the process of exhumation of the metamorphic rocks. The domes are later affected by collapse folds and low- to high-angle normal faults related to extensional tectonics. Our structural data together with the available data on fluid inclusions in syntectonic veins in the Tuscan Nappe indicate a post-D1 decreasing pressure. This suggests that the Tuscan Nappe was exhumed in a compressive tectonic setting starting from the D1 deformation phase up to the later phases.