Late orogenic deformation of the shallowest portion of an orogenic wedge: coeval activity of extensional and compressional tectonics in the western Northern Apennines (Italy).

In the Northern Apennines of Italy the Ligurian and Subligurian Units (LSU) were involved, since the Late Cretaceous, in the accretionary and orogenic processes related to the convergence between Europe and Adria. Since early Miocene the Ligurian Units completely overthrusted the Subligurian Units (REMITTI et alii, 2011) and both units stacked together (i.e. the LSU) translated towards NE, above the Tuscan-Umbrian foredeep deposits, until they reached the present-day topographic front in the NW portion of the Northern Apennines. In the NE-facing side of the western Northern Apennines the present-day geometry of the LSU does not reflect neither the shape of the late Cretaceous-middle Eocene oceanic accretionary wedge (MOLLI, 2008 and references therein) nor the shape acquired during the late Oligocene, when the early orogenic collisional phases produced a wedge whose geometry tapered out towards the foreland area, i.e. to the E-NE. In fact, at present, geologic evidences indicate that the LSU have a shape whose thickness ranges from less than 1 km at the main ridge zone (SW) up to more than 3,5 km along the NE slope of the chain and then tips out again few km N of the Northern Apennines topographic front, underneath the Pleistocene-to Recent sediments of the Po Plain foredeep. Therefore, the LSU thickness increases towards its NE external tip and tapers out again in the subsurface of the Po Plain. These data imply the activation of thinning processes in the inner portion of LSU and thickening processes in the central portion of LSU; these thinning and thickening processes have to be related to the post-early Miocene progressive emplacement of the LSU over the foredeep units and to the late orogenic extensional and compressional tectonics affecting the Northern Apennines (ARGNANI et alii, 2003, BOCCALETTI et alii, 2011). In particular, the thinning processes have been identified as being of tectonic nature in the innermost portion (mainly low- and high-angle normal faults: ARTONI et alii, 2006, BETTELLI et alii, 2002, VANNUCCHI et alii - 2008) and of sedimentary nature (mass-wasting deposits) (ARTONI et alii, 2010, PAPANI et alii, 1987, REMITTI et alii, 2011) in the outermost portion of the study area. The observed thickening in the central portion of the LSU body has been interpreted as a tectonic doubling caused by the late Miocene-Recent (?) thrusting which affected the whole Apennine orogenic wedge (CAMURRI et alii, 2001; CARLINI et alii, submitted) or, alternatively, as an accumulation zone of large scale and deep-seated gravitational processes (ARTONI et alii, 2006). In order to put new constraints on the timing and modes of thinning and thickening of the LSU we focussed on the western portion of the Northern Apennines, comprised between the main ridge, the Ceno and the Secchia rivers and the Apenninic topographic front. Here we adopted a multidisciplinary approach taking into account field evidences, low temperature thermal and thermochronological data (i.e. vitrinite reflectance, clay mineral analyses, apatite fission-tracks), numerical modelling of the cooling ages through the use of Pecube finite element code (BRAUN, 2003), results from recently published works on the evolution of the external slope of the chain, and a new interpretation of seismic lines and boreholes data. This multidisciplinary approach allowed us to: 1) build a 3D representation of the LSU present-day geometry; 2) constrain to the late Miocene-early Pliocene (in the innermost portion) and to the late Miocene-Pleistocene (?) (at the Apenninic front) the thinning of the LSU; 3) identify the tectonic exhumation and consequent denudation of the foredeep units as one of the main causes which triggered the thinning processes since late Miocene; 4) define first estimates of exhumation rates affecting the LSU and the underlying foredeep units. These results allow us to give new insights, temporal and spatial constraints on the interplay between shallow (< 4 km) and deep (< 20 km), compressional and extensional tectonics which appear to be acting at the same time and at different depths within the study area since late Miocene to Recent.