Pre-collisional Late Cretaceous-Paleocene development of the Alps retrobelt in the hangingwall of the Alpine Tethys subduction: U-Pb carbonate dating of major tectonic structures in the Southern Alps (N Italy)

The Alps formed as a consequence of the collision between Europe and the Adria-Africa plate starting from the middle-late Eocene. Despite most of metamorphism, deformation and nappe-stacking were localized in the N-vergent part of the Alps (i.e. N of the Periadriatic Fault), significant crustal shortening affected also the S-vergent retrobelt, with the development of a fold-and-thrust belt that extends from the Canavese zone in the W to the Dolomites to the E.

Late Cretaceous high-pressure metamorphism in the Africa-derived Austroalpine units and fault activity along major tectonic structures in the Southalpine domain (i.e. the Orobic Thrust), already posed a question on the occurrence of pre-collisional deformation and metamorphism in the upper plate of the alpine Thetys subduction.

New U-Pb dating of calcite tectonites, obtained on growth fibers, calc-mylonites and shear veins along major thrusts of the central Southern Alps, mainly result in Late Cretaceous to Paleocene ages, pointing out that N-S to NW-SE directed compression already affected the Southalpine domain at those times. Younger ages resulted from the Paleogene units which are involved in the exposed frontal part of the belt, mostly buried under the recent infilling of the Po Plain forming the Milan Belt. The resulting ages do not follow an in-sequence pattern, but instead reveal that several structures, from the inner to the external part of the belt, were episodically formed and re-activated in the Late Cretaceous - early Eocene time interval. All together, U-Pb ages confirm that S(SE)-directed thrusting and folding affected the central Southern Alps since the Late Cretaceous, well before the onset of the Alpine collision.