Long-term tectonic evolution of the Eastern Southern Alps (Italy): a reappraisal from new structural and radiometric constraints

Abstract

Constraining the timing of tectonic events is of prime importance for the in-depth understanding of the complex evolution of orogenic deformation, particularly in the case of fold-and-thrust belts. In this work, we combined U-Pb dating of tectonic carbonates and K-Ar dating of fault gouges of selected key outcrops along two main thrusts of the Paleogene-Neogene Eastern Southern Alps (ESA), Italy. The ESA are the south-verging fold-and-thrust retrobelt of the Alpine orogen, offering spectacular exposures to study the details of past tectonic processes. However, despite a few published papers regarding the deformation mechanisms of a major thrust in the ESA (the Belluno Thrust), modern, multiscale structural and radiometric studies of fault zones in the ESA are missing, such that detailed reconstructions of the local and regional tectonic evolution through space and time remain only loosely constrained. We focused on the (i) Valsugana Thrust, which is a first-order thrust separating the Dolomites s.s. to the north from the Venetian Pre-Alps to the south, and (ii) the more external Belluno Thrust. We coupled U-Pb dating of tectonic carbonates and X-ray diffraction and K-Ar dating of clay minerals in fault gouges with structural analysis and microtextural characterization. We show that the Valsugana Thrust represents an inherited pre-Alpine structure that (i) registered far field deformation during the Early Cretaceous (K-Ar gouge age of 140 ± 32 Ma), (ii) strongly influenced the geometry and kinematics associated with deformation structures during the Alpine orogenesis and (iii) recorded multiple reactivations in Late Cretaceous (K-Ar age of 79.2 ± 8.4 Ma and 76.2 ± 1.4 Ma), late Miocene (U-Pb age of 9.1 ± 0.8 Ma), and Miocene-Pliocene (U-Pb age of 5.3 ± 1.6 Ma) times. Radiometric constraints from the Valsugana Thrust attest to remarkable out-of-sequence compressional movements in the inner ESA after the orogenic wave had progressed farther south to the more external Belluno Thrust, whose activity is constrained to the Oligocene by a 30.6 ± 5.8 Ma K-Ar gouge age and a 23 ± 14 Ma U-Pb syn-tectonic vein age.