Neogene-Quaternary Tectonic Stratigraphy of the Eastern Southern Alps, NE Italy

Based on the fact that the available chronology and stratigraphy of the Neogene-Quaternary sedimentary succession within the Eastern Southern Alps is very detailed, we investigated the external sector of the Eastern Southern Alps and performed a detailed structural and stratigraphic mapping of the Tortonian-Quaternary sedimentary units outcropping within a 120 km-long zone along the foothills of the mountain chain in Veneto and Friuli regions. The Eastern Southern Alps is a major strucural subdivision of the broader Alpine Chain and it is conventionally delimited to the north by the Periadriatic Lineament. From a tectonic point of view, the Eastern Southern Alps correspond to a distinct late Oligocene-Quaternary orogene. This south-verging fold-and-thrust belt was generated during the complex crustal collision and indentation of the Adria promontory underneath the Alpine chain. Due to the relatively large distribution of conglomeratic bodies within the considered stratigraphic interval, we focused our attention on numerous deformed pebbles characterised by pitted surfaces. These structural features are classically related to small-scale pressure-solution processes mainly occurring along those particle contacts that are roughly oriented perpendicular to the direction of maximum compression. Based on the careful analysis of the shape and orientation of the indented features on the pebbles' surface and following a statistical approach based on as many as possible measurements for each site, the mean orientation of the maximum compressive stress axis (σ1) is obtained by contouring the data on a stereonet and calculating the density peak. Our reconstruction of the late Tortonian-Quaternary Tectonic Stratigraphy in the Eastern Southern Alps has emphasized the occurrence of four distinct deformational events. It is noteworthy that these events do not represent 'simple' local variations of the stress field because for all datasets the corresponding sites of measurements are spread along the entire 120 km-long investigated area and in many cases, the same localities and sedimentary units are affected by more than one dataset (viz. stress field). In particular, the four events could be recognized and distinguished based on i) a different mean direction of compression and ii) their timing of activity. Accordingly during Late Neogene-Quaternary the stress trajectories of the Eastern Southern Alps were affected by repeated rotations with rapid flipping of the principal horizontal stresses. We refer to this behaviour as Twist Tectonics. The tectonic evolution of the Eastern Southern Alps has been compared with Neogene-Quaternary the convergence direction between Adria-Africa and Europe plates showing a good fitting with both timing and directions of compression of the major deformational events. However, since late Messinian, the Northern Apennines were close enough to the Eastern Southern Alps in order to perturb the stress field and generate short-lived (1-2 Ma) rotations of the stress trajectories (Twist Tectonics) possibly due to accomodation processes of the Adria indenter and overlapping effects of the two 'remote' engines (Africa and Apennines).