Alpine structure and deformation chronology at the Southern Alps - Po plain border in Lombardy

A review of the structural grain of Southern Alps in Lombardy is here integrated with new subsurface data, in order to contribute to detailed reconstruction of the chronology of Alpine deformation of the South Alpine chain and foreland. According to the recent structural schemes, we firstly describe the northern structural zones of the mountain belt. From N to S these are: the Orobian Thrust, involving Variscan basement and Permian sedimentary cover; the Orobian Anticlines, which include basement to lower Mesozoic units (DE SITTER & DE SITTER KOOMANS, 1949); the Carbonate Allochthonous Units, forming a stack of imbricated thrusts of Mesozoic formations (GAETANI & JADOUL, 1979); the Flessura Pedemontana (DESIO, 1929), which involves the South Alpine succession up to the Cretaceous formations in an out of sequence, thrust-related fold belt of late Neoalpine age; the Structures of the South Alpine border (BERSEZIO et alii, 2001), which represent the southernmost marginal belt of faulted folds involving mostly the Cretaceous Lombardian Flysch. Secondly we describe three structural belts, which can be recognised at the buried front of Southern Alps and in the Po Plain foreland area: the Imbricate Tertiary Units BERSEZIO et alii, 2001), which involve the Mesozoic and Tertiary successions, the former underlying at great depth the stack of imbricated Tertiary clastic wedges; the Structures of the Central Lombardy plain (BERSEZIO et alii, 2001), a belt of regional folds due to propagation of deep fault planes ramping up from the metamorphic basement of the Po Plain; the Southernmost Po Plain Structures, which are represented by gentle folds affecting the Tertiary clastic wedges at the southern tip of the South Alpine thrusts. Analysis of deformation chronology has allowed us to identify the structures predating the Adamello magmatic intrusion (older than 42 Ma), both in the mountain chain (Orobic thrust) and in the foreland (inversion structures of the Lacchiarella and Villafortuna- Trecate areas), where Pre-Adamello foreland deformation is represented by gentle inversion of the pre-Cretaceous extensional basins. Post-Adamello (Neoalpine) deformation is responsible for foreland propagation of the thrust belt, with formation of the Imbricate Tertiary Units, which are kinematically linked with the Structures of the South Alpine border. Their deformation lasted until the Tortonian, as documented by preservation of Tortonian sediments in the footwall of the most external South Alpine thrusts. Neoalpine strong inversion of the Mesozoic basins occurred in the southernmost Po plain foreland. In the most external foreland area of the western Southern Alps (Piedmont-Lombardy area), Neoalpine compression came to an end after deposition of the Lower Messinian units and reasonably before the Pliocene. However, east of the Giudicarie line, South Alpine deformation lasted until the Pleistocene.