U-Pb calcite geochronology attests Late Cretaceous S-verging thrusting in the central Southern Alps, Italy

The central Southern Alps (cSA) form a complex south-verging polyphase fold-and-thrust belt developed from the Late Cretaceous onward due to the Alpine convergence (Schönborn, 1992; Zanchetta et al., 2015). The oldest ages related to thrust stacking have been obtained from pseudotachylythes along the Orobic Thrust exposed in the northern part of the belt (Zanchetta et al., 2011), stacking the Variscan basement onto the Permian-Triassic cover.

Aim of this contribution is to present new U-Pb radiometric ages of calcite tectonites obtained in the central and southern portion of the belt, where no precise time constrains of thrust stacking are available. E-W trending Eocene dike swarms crosscutting thrust planes are the only indirect evidence of a pre-Eocene age of the oldest stages of the Alpine contraction (D’Adda et al., 2011).

The central part of the cSA shows a thick pile of thrust sheets deforming the Lower to Middle Triassic carbonate successions. Our new U-Pb calcite ages obtained on growth fibers along fault planes, veins and calc-mylonites sampled along some of the most important regional thrust planes mainly result in Late Cretaceous ages. We obtained similar ages also within the southern portion of the belt, where the Norian “Dolomia Principale” thrust sheets, override the Rhaetian Riva di Solto Shale immediately to the north of the frontal portion of the belt. Younger ages resulted from the Paleogene units which are involved in the exposed frontal part of the belt, which is mostly buried under the recent infilling of the Po Plain forming the Milan Belt.

These data confirm previous interpretations of the central Southern Alps as part of a Late Cretaceous doubly vergent pre-collisional belt (Zanchetta et al., 2012). Research supported by the FAST PRIN Project (2021-NAZ-0299, Italian MUR).

D'Adda, P., Zanchi A., Bergomi M. A., Berra F., Malusà M. G., Tunesi A., & Zanchetta S. (2011) - Polyphase thrusting and dyke emplacement in the central Southern Alps (Northern Italy), International Journal of Earth Sciences, 100, 1095–1113.

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Zanchetta S., D'Adda P., Zanchi A., Barberini V. & Villa I.M. (2011) - Cretaceous-Eocene compressions in the central Southern Alps (N Italy) inferred from 40Ar/39Ar dating of pseudotachylytes along regional thrust faults. Journal of Geodynamics, 51, 245-263.

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Zanchetta S., Malusà M. & Zanchi A. (2015) - Precollisional development and Cenozoic evolution of the Southalpine retrobelt (European Alps). Lithosphere, 7, 662-681.