Ubaldo Cibin ¹, Andrea Di Giulio^* 2, Luca Martelli ¹

Nowadays, the innermost part of the Northern Apennines chain is constituted by a belt of high- pressure metamorphic units extending from the Ligurian Alps to the Alpine Corsica, across the Ligurian Sea and the Northern Tyrrhenian Sea where the same units have been detected by geophysical surveys (Bartole et al., 1991). The units forming this tectonic complex represent the southward extension of the subduction-related Pennine metamorphic units of the Western Alps; they were stacked during the Cretaceous-Paleogene Alpine convergence and later involved in the Oligocene-Neogene Apennine collisional system.

As a consequence the overall subduction-exhumation path of that units is linked to two different orogenic systems and, therefore the study of the timing of their exhumation is at the same time of interest for the Alpine and Northern Apennines geology.

With this respect, the provenance study of clastic sediments accumulated during the Oligocene- Early Miocene in the piggy-back basins formed on top of the Northern Apennines orogenic wedge gives the opportunity of collecting information about the age of exhumation and the onset of erosion of that high-pressure metamorphic units.

This kind of study, systematically coupled with field mapping and biostratigraphic dating by the study of nannoplancton associations, has been performed on all the main coarse-grained Oligocene- Miocene clastic bodies mapped into the piggy-back successions between the Genova-Milano and Firenze-Rimini transects, i.e. approximately 300 km along the chain, in the framework of the CARG project dealing with the mapping of the new Italian Geological Map scale 1:50.000. The main results of this study are as follows.

Field mapping and biostratigraphic dating allow to detect and date several Oligocene-early Miocene clastic bodies included in the piggy-back successions; they are interbedded in a wide turbidite complex until the early Rupelian (Ranzano Formation), while they are interbedded in marly hemipelagic-bearing formations since the late Rupelian to the Burdigalian (Rigoroso, Antognola and Contignaco Formations). Generally, they are strongly confined sandstone-conglomerate turbidite bodies mappable on quite short distances and mostly made by poorly evolute, often amalgamated turbidite beds, thought to represent the remains of very small sand-rich turbidite systems, which are expected to have been quite directly linked to their detrital sources; thus their composition reflects the evolution of the emerged lands backward.

The study of sandstone provenance highlights a change of the main clastic source from the Ligurian Nappes of the Apennines to the Pennine complex occurring all over the studied area with the only exception of the Bologna area, where, since the Eocene to the Early Miocene, the bulk of clastics continued to be fed by a quartzose-feldspatic source, possibly due to the recycling of older Ligurian sandstones. At the moment, this source substitution, which records the complete exhumation and onset of erosion of the Alpine HP/LT metamorphic units of the Ligurian Alps and Alpine Corsica, can be related to two not mutually exclusive causes: 1) it could reflect the progressive erosion of the Ligurian Nappes and the related unroofing of the underlying HP/LT Pennine Complex, as suggested by subsurface data on the Tuscan Apennines and Northern Tyrrhenian Sea which detected piggy- back sediments unconformably covering both the Pennine and the eroded remnants of the overlying Ligurian units. 2) Otherwise, it can record the uplift of the Pennine metamorphic complex due to its out of sequence overthrusting onto the Ligurian units of the Northern Apennines during the Apennines Oligocene-Miocene evolution.

Whatever its cause, the substitution of source from Ligurian units to Alpine HP/LT units occurred in different times along the different transects across the chain between the Late Rupelian and the Middle Miocene, quite regularly migrating from west to east. As a matter of fact, it is clearly recorded in the late Rupelian along the Genova-Milano transect (Val Curone area), in the Chattian along the La Spezia-Reggio Emilia transect (Val d’Enza-Val Taro area), in the Aquitanian along the Viareggio-Modena transect (Val Secchia area), didn’t occurred in the Bologna area, but is still recorded in the Aquitanian along the Firenze-Rimini transect (Val Savio-Val Marecchia area).

This space/time trend indicates an exhumation timing of the HP/LT metamorphic units which, as a whole, was younger and younger going from west to east along 200 km of the belt, at least up to the Bologna transect, reflecting the different evolution of different belt segments probably bounded by transversal tectonic releases in the framework of the oblique convergence between the Corsica- Sardinia and Adria microplates; as a consequence, the timing of exhumation of the HP/LT Alpine metamorphic units of the Ligurian Alps and Alpine Corsica seems basically guided by the Northern Apennines geodynamics other than by the Alpine one.