Fragmentation of the Adriatic Promontory: New Chronological Constraints From Neogene Shortening Rates Across the Southern Alps (NE Italy)

Abstract

The eastern Southern Alps are located at the northern tip of the Adria microplate, which imposes 2.0–2.5 mm/year of N-S-convergence relative to stable Eurasia. We map surface evidence of recent folding/faulting in this area from a 5-m Digital Elevation Model (DEM). In the eastern part of the belt, observations reveal a 30-km-wide zone of active folding composed of at least five growing anticlines. The most recent ones warped the postglacial alluvial surface by <10 m. Combining these findings with published geological and geophysical data allows us to infer that active thrusting occurs along a single deeply rooted thrust, which accommodates the indentation of the Adriatic crust. Resolving the observed pattern of uplift on the inferred fault geometry indicates that NNE-SSW shortening across the eastern Southern Alps has occurred at a rate of about 1.5 mm/year over the postglacial period. On the other hand, a balanced cross section for the eastern Southern Alps at the scale of the upper crust constrains a minimum of 43 km of finite shortening over the last 14 Ma, yielding a shortening rate of about 3 mm/year, which is 2 times higher than the postglacial shortening rate. This decrease in the shortening rate is associated to the Pleistocene activation of new thrusts that is compatible with a change in the direction of compression. The inferred local change in the kinematics of thrusting during the Pleistocene is consistent with a change from Nubia-imposed to Adria-imposed convergence indicating that the fragmentation of the Adriatic promontory could have occurred 1–2 Ma ago.