Il Bacino Terziario Epimesoalpino e le sue implicazioni sui rapporti tra Alpi e Appennino

The Priabonian and Rupelian successions that crop out in the Tertiary Piedmont Basin (TPB, NW Italy) and in the epi-Ligurian Basin (Northern Apennines) are part of a large episutural basin formed along the inter­ nai margin of the Western Alps after the Ligurian or Mesoalpine collisional event. This basin, referred to herein as the Tertiary epi-Mesoalpine Basin (TEMB), is the counterpart of the classic "trilogie priabonienne" ofL. Moret (Cal­ caires Nummulitiques, Marnes Bleues, and Gres d'Annot) in the external margin of the Western Alps (Chaines sub­ alpines). The TPB is the marginai portion of the TEMB recording a very complex synsedimentary tectonic evolu­ tion which involved the reactivation of Mesoalpine thrusting along the Sestri-Voltaggio, the Villalvernia-Varzi-Ot­ tone and the Monferrato fronts. The TPB succession can be subdivided into five informai stratigraphic units (from base to top: Pizzo d'Oca, Rio Trebbio, Molare-Borbera, Valla-Persi and Monastero units) bounded below and above by regionally extensive upper Eocene (M. Piano Mari) and upper Oligocene (Rocchetta Formation and Rigoroso Mari) mudstone units. These mudstone units are thought to record periods of crustal inflection associateci with the Mesoalpine and the eariy Apenninic phases of thrusting respectively. The sedimentary and tectonic evolution of the TPB includes the following six main stages. (l) Bartonian and Priabonian transgression associateci with thrust-re­ lated subsidence and followed by the deposition of Priabonian turbidites (Pizzo d'Oca Unit); this stage is only re­ corded on top of internai Alpine units (Ligurides). The transgression is thought to be related to a regional or glo­ bal sea level rise that affected both the Alpine and the Pyrenean-Provençal domains. The turbidites of the Pizzo d'Oca Unit were probably fed by the Corsica and Sardinia massifs, suggesting that the Alpine system was stililarge­ ly submerged at the end of the Mesoalpine Phase of thrusting. (2) Thrusting of the Valtri Group onta internai Al­ pine units (Ligurides) and their eariy TEMB cover (Marne di M. Piano, Pizzo d'Oca and Rio Trebbio units). Dur­ ing this phase, the Sestri-Voltaggio line was apparently reactivated as a transpressive zone. From the end of this phase (Ligurian Phase II) onward, the Valtri Group acted as a relative "high" during sedimentation. (3) Extensional tec­ tonics related to the opening of the Balearic Basin concomitantly with (4) the depositian of thick successions of fiood­ dominated, alluvial and shallow-marine conglomerates and sandstones grading upward into bioturbated and fossi­ liferous mudstones. Contrary to previous interpretations, these transgressive strata (Molare-Borbera Unit) are char­ acterized by a depositional setting and palaeocurrent directions delineating a transgression from the south (Balear­ ic Basin) and Alpine source areas located in the north. These Alpine units undergoing subaerial erosion are related to thrusting of stage (2). (5) Tectonic inversion of the previous extensional setting resulting into the cannibalization of the transgressive deposits of stage (4) and their resedimentation into small, fault-bounded troughs through south­ eriy-derived gravity flows. This stage, recorded by the Valla-Persi Unit, leads to basin inversion and the ensuing for­ mation of rapidly subsiding basins in the north. (6) Thick accumulations of turbidites (Monastero Unit) followed by regional drowning and basin starvation (Rocchetta Formation and Rigoroso Mari). This final stage is thought to record the inflection of the Adria plate and the beginning of the Apenninic wedge. The external portion of the TEMB is the M. Piano-Ranzano Basin of the Northern Apennines, entirely lying on internai Alpine units (Ligurides). Part of an intermediate domain between the M. Piano-Ranzano Basin and the TPB is stili preserved in the eastern sec­ tor of the latter, indicating an area of substantial uplift ("Antola High") associateci with the reactivation of the Vil­ lalvernia-Varzi-Ottone front during the Ligurian Phase II. The originai relations between this intermediate sector and the M. Piano-Ranzano Basin were completely obliterated by subsequent thrusting and subaerial erosion along the Villalvernia-Varzi-Ottone front. Although deposited in a permanently deep-marine environment and therefore of more difficult interpretation, the turbidite deposits of the Ranzano Sandstone can be cleariy subdivided into un­ conformity-bounded units which are correlateci herein with the main evolutionary stages of the TPB. Spectacular chaotic units (Specchio Unit) record renewed thrusting associateci with the tectonic inversion of the TPB (stage 5; Ligurian Phase III). Detailed correlatian patterns of selected outcrop areas show facies and thickness changes of turbidite strata produced by virtually continuous structural deformation. The Priabonian and Rupelian strata of the TEMB pose a series of problems involving the boundary between the Alps and the Apennines. Since they predate the late Oligocene and Neogene foredeeps of the Apennines, these sediments are an epi-Mesoalpine succession and have no temporal and spatial relations with the epi-Ligurian Basin of the Northern Apennines, a younger basin de­ veloped on top of Ligurian thrust sheets already involved in an Apenninic wedge. The TEMB is actually a transi­ tional and stili pooriy understood feature between the Mesoalpine Phase of deformation and the beginning of the Apenninic wedge with its associateci foredeeps. Much of the story and significance of the TEMB is thus probablyhidden in the sub-Ligurian Canetolo Basin, i.e. an equally poorly understood palaeogeographic and tectonic domain, originally separating the Ligurian Basins from the Tuscan Realm.