The Montello Thrust and the Active Mountain Front of the Eastern Southern Alps (Northeast Italy)

New field and subsurface data are combined to define the geometry and evolution of the mountain front of the eastern Southern Alps. The Montello thrust (MT), the southernmost frontal structure, is reconstructed by recently published well-located microseismicity and is connected at depth with the larger Bassano-Valdobbiadene (BV) thrust. The latter started at 10-9 Ma, whereas the MT evolved at 8-6 Ma, as documented by growing unconformities in the foreland deposits and the nature of the natural gas in the anticline culmination. The latter, named Montello anticline, is growing from the upper Miocene to the Quaternary, as shown by surfaces of abrasion and deposition, including the Biadene windgap. The folded bottom of the windgap is a fan unit recently dated to the MIS 3. The MT is presently creeping and its deformation rates are much lower than previous estimates. Most of the late Miocene to Pleistocene deformation at the front of the eastern Southern Alps is accounted for the BV thrust. The reconstructed average shortening rates are in agreement with the geodetic velocity field from literature. The BV and MT are a unique active thrust system with variable slipping and locked patches, whose interactions should be further studied. Plain Language Summary The definition of the hazard in a seismically active area, such as the front of a mountain chain, requires a precise reconstruction of the geometry of the tectonic structures. In the densely populated foothills of the eastern Southern Alps of Italy, these structures do not reach the surface and are difficult to study. Combining surface and subsurface data with precisely located microseismicity, we provide a synthetic view on the geometry and evolution of the active mountain front. We find that the frontal structure is larger and deeper than previously thought, and it reaches the outskirts of one of the main cities of the area, Treviso. The tectonic stratigraphy of the area, and the geomorphology of a gentle fold (Montello anticline), reveal that the structure (Montello thrust [MT]) is much older, therefore its rates of deformation much lower, than previously thought. Most of the active deformation is focused in a northern structure, named Bassano-Valdobbiadene thrust. Together, these structures account for the Adria-Europe convergence, as defined by geodetic velocity field. Their behavior as thrust system is complex, with locked patches and freely slipping portions, such as the MT and would require a future modeling of their interactions.