Quaternary tectonic activity in the western Carnic Prealps (Eastern Southern Alps, Friuli)

In the framework of the DPC-INGV S1 project "Base-knowledge improvement for assessing the seismogenic potential of Italy", detailed geological, structural and morphotectonic investigations integrated with the analysis of topographic maps, high resolution-DEM and ENI - AGIP seismic profiles, was carried out in the western Carnic Prealps. The aim of the project was to characterise as many as possible geometric, kinematic, dynamic and chronological parameters of the major active faults located at the front of the Eastern Southern Alps (ESC) in western Friuli. Here the outer sector of the ESC is made of a series of arched SW-NE trending, SE-verging thin-skinned thrusts, stacking on the foreland and involving more external portions of the plain through the activation of new external splays. In the adjoining plain, thrusts are mostly blind but deformation affects the upper Pleistocene to Present alluvial and glacial deposits. In the wester Carnic Prealps the Quaternary front is made of: Arba-Ragogna, M. Jouf-Maniago and Polcenigo-Montereale thrust systems. The Mt. Jouf - Maniago tectonic unit shows evidence of Quaternary activity near Maniago locality, where the terraced Middle and Upper Pleistocene units (respectively Maniago conglomerates and Maniago gravels) are back-tilted and uplifted on the present piedmont plain. Moreover lacustrine deposits (9090 ± 90 years 14C BP), cropping out along the Colvera creek, a few kilometres north of Maniago, are gently folded and fractured. Moreover on the bases of stratigraphic and morphotectonic analyses carried out on Pleistocene terraces at the outlet of the Meduna valley, a post-LGM-Holocene vertical slip rate of about 0.6 mm/y was detected for the Maniago thrust. South of the Maniago-M. Jouf tectonic unit, the Miocene succession is thrust and folded by the Arba-Ragogna thrust-system that shows evidence of Quaternary activity, as testifies the angular unconformity between the lower Messinian (Montello Conglomerate) and the lower Quaternary (San Pietro di Ragogna Conglomerate). The recent tectonic activity of the Arba-Ragogna thrust system is also testified by drainage anomalies and gentle scarps connecting uplifted paleo-landscapes of Quaternary age. A vertical slip-rate of about 0.2 mm/y has been calculated for the Arba- Ragogna thrust during the last 21 kys. Evidence of Quaternary activity come also from the analysis of the ENI-AGIP reflection seismic lines. The Polcenigo-Montereale thrust- system runs at the base of the Cansiglio-Cavallo carbonate plateau giving rise to the major morphological element of the area. In particular it piles up the Upper Jurassic - Upper Cretaceous limestones of the Friuli Carbonate Platform not only on the Miocene Southalpine molassa but also on the pre-LGM conglomerate that are 10° back-tilted. Near Aviano the Holocene alluvial fan of Ossena river is gentle folded and its post-Roman age gravels are back-tilted. Near Polcenigo, the Budoia-Aviano splay deforms the LGM alluvial fan of the Artugna creek giving rise to considerable drainage anomalies. Near Montereale along the Malnisio thrust the deposits related to the LGM Cellina fan are displaced of about 10 m, suggesting also for this sector a recent vertical slip rate of about 0.6 mm/y.