Compressional and extensional tectonics inside collisional belts: the case of the western portion of the Northern Apennines

Coeval extensional and compressional tectonics are a common feature in orogenic belts, especially in post-collisional 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. The deformation acquired by these units during their translation across collisional belts, thus, are very likely imprinted by the development of extensional and compressional 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 geological data (seismic lines, boreholes), paleothermal data (vitrinite reflectance, mixed layer illite-smectite) and thermochronological data (apatite fission tracks) in order to investigate the deformation history and, consequently, the record of the extension-compression relationships inside these far-travelled/allochthonous units. The integrated approach allowed us to: 1) build a 3D representation of the Ligurian and Subligurian units present-day geometry; 2) document an early tectonic thinning of the Ligurian and Subligurian units occurred in the late Miocene, followed by a later reshaping in the Pliocene to Recent (?) times; 3) identify the tectonic exhumation and uplift of the deepest foredeep units as one of the main causes which led to the activation of low-angle extensional faults responsible for the thinning of the far-travelled/allochthonous units since late Miocene; 4) relate the later Pliocene to Recent (?) reshaping phase to both high-angle extensional faults and increased erosion rates; 5) recognize lateral variations in the extent of uplift, exhumation and thinning inside the western Northern Apennines. These results allowed us to strengthen the hypothesis that in the Northern Apennines extensional and compressional tectonics were coeval and tightly connected by a cause/effect relationship, explained by an over-thickened orogenic wedge which decreased its surface slope’s angle and returned to its gravitational equilibrium state. This latter was achieved by means of tectonic and surface erosional processes, acting during two major evolutionary stages: 1 - late Miocene) low-angle extension of the shallowest portion (the uppermost 4-6 km) of the orogenic wedge causing the thinning of the far-travelled/allochthonous units; 2 - Pliocene to Recent?) high-angle extensional tectonics coupled with increased surface erosion. The study of the late orogenic deformation and reshaping of the far-travelled/allochthonous Ligurian and Subligurian units, thus, revealed that the western Northern Apennines were dominated by an overall compressional regime which induced episodes of gravitational instability in the shallower portion of the Apennines orogenic wedge. Then, this study provides elements indicating the possibility that the processes characterizing the second stage were active till Recent times, because of the relationships highlighted between deep Apenninic compressional structures and Holocene surface gravitational processes. The importance of surface erosion processes during the second stage led to the attempt to quantify these processes through the use of numerical modelling, which gave an average erosion/exhumation rate of 0.64 km/My over the last 5.35 My). The new data and results presented in this work have important implications on the geodynamic framework characterizing the Northern Apennines. They reveal that, from middle Miocene to Recent, the western Northern Apennines were affected by underplating processes spaced out by episodes of frontal accretion possibly related to the subducting slab retreat.