Exhumation and reshaping of far-travelled/allochthonous tectonic units in mountain belts. New insights for the relationships between shortening and coeval extension in the western Northern Apennines (Italy)

Coeval extensional and contractional tectonics are a common feature in orogenic belts, especially in postcollisional phases. The attempt to constrain their relative timing and the responsible mechanisms led to several models taking into account different geodynamic contexts of various mountain chains. In peri-Mediterranean collisional orogens, extension and compression are commonly associated and coeval to the emplacement of far-travelled/allochthonous units. Therefore their internal deformation acquired during the translation across collisional belts is likely imprinted by the extensional and contractional tectonics affecting the orogen. In the western portion of the Northern Apennines of Italy, one of the northernmost peri-Mediterranean Tertiary orogens, the far-travelled/allochthonous units, i.e., the Ligurian and Subligurian units, are widely exposed. The present study integrates published and newly acquired surface geological data (geological maps, cross-sections), subsurface boreholes data, and paleothermal (vitrinite reflectance, mixed layer illite–smectite and Kübler index) and thermochronological data (apatite fission-tracks) in order to investigate the deformation history and, consequently, the record of the extension–compression relationships within these far-travelled/allochthonous units. The study of modes and timing of reshaping of the far-travelled/allochthonous units allowed us to strengthen the hypothesis that extensional and contractional tectonics were coeval in the Northern Apennines and tightly connected by a cause/effect relationship. In particular, the late orogenic low- and high-angle extensional tectonics represent the response to an overall contractional regime which induced episodes of gravitational instability in the orogenic wedge.