The Cretaceous retro-belt of the Alps and the early indentation of Adria

Most paleotectonic reconstructions assume the indentation of Adria subsequent to the Periadriatic magmatism, after 32-26 Ma. Some consider an even younger (post 14-10 Ma) retrobelt of the Alps. These reconstructions contrast with evidence of a late Cretaceous to Eocene retro-belt in the western Southern Alps, intruded by the Adamello pluton and associated magmatic bodies. Recent work suggest this retro-belt continued eastwards into a relief extending from the Texelgroup towards the Transdanubian Range, allowing detritus to feed the retroforeland basin. In the eastern Southern Alps, remnants of this basin occur in the northernmost sectors, and recent work documented the Late Cretaceous northward flexuring of the Adria foreland.

Collectively, these observables confirm the occurrence of a Late Cretaceous retrobelt, subsequently cut in the Oligocene by the Periadriatic Line: the western part of the retro-belt remained in the Southern Alps, whereas, to the east, the Cretaceous double vergent belt was left north of the Periadriatic Line, only leaving the tip of the retro-foreland basin in the Southern Alps. This Eastern Alps Cretaceous belt is well recognized, following the so-called eclogite belt.

The Cretaceous retro-belt was sinistrally reworking the Jurassic Giudicarie fault system, finally defining it as first-order transverse range pre-existing the Periadriatic Line. This latter reworked the indented Adria plate in the west, where the crustal doubling prevented any possible deeper source for the Periadriatic magmatism. The lower plate break-off, therefore, seems a very unsuitable hypothesis.