GNGTS 2017 - 36° Convegno Nazionale

170 GNGTS 2017 S essione 1.2 The upper crust in Friuli: a new seismotectonic model. Matching interpretation of seismic profiles of the Friuli prealpine area and the outward data carried out by means the new detailed geological survey (CARG – FVG project: 066 – Udine and 049-Gemona del Friuli sheets: Zanferrari et al. , 2008a, 2013), we propose and discuss a new geological section crossing the upper crust of the eastern SouthernAlps from the Fella valley to the piedmont plain south of Udine (Fig. 2). We can underline two main features: 1. the Paleogene External Dinarides thrust-belt represents a pervasive framework inside the eastern Southalpine chain in Friuli. On the bases of the geological and structural surveys we propose that the upper crust in Friuli is arranged in a stack of Paleogene tectonic units (among which the Gemona-Kobarid, the Susans-Tricesimo, and the Palmanova one, progressively incorporated and merged in the eastern Southalpine chain by means the polyphase Neogene-Quaternary compressive tectonics. The Paleogene Dinaric thrusts frequently show evidence of S-verging Neogene-Quaternary reactivation; 2. the identification of a wide, steeply N-dipping, right lateral strike slip fault-system along the Resia and Tagliamento valleys. It deals with a broad and anastomosed right lateral N 110°-115° strike slip fault system, coupled with synthetic (N135° striking) and antithetic (N30° striking) faults, NW- or SE- verging reverse faults, en echelon fold systems, strike slip duplexes, contractional and releasing bends, positive or negative flower structures (Zanferrari et al. , 2013). This strike slip system is the prosecution in the Friuli region of the Idrija system in the W-Slovenia and therefore it was named Idrjia-Ampezzo strike- slip system (IA in Figs. 1 and 2). Neotectonic activity of the Idrija-Ampezzo strike slip system is demonstrated by: 1) widespread deformation of Pliocene - Middle Pleistocene continental successions (Monegato and Stefani, 2011); 2) medium to high historical and instrumental seismicity that hit the Tolmezzo area (see for example the Verzegnis earthquake (1928/3/26; Mw: 6.0) that shows strike-slip FPS congruent with the average striking of the Idrija-Ampezzo strike slip system; 3) the coseismic deformations recorded by the topographic high precision survey performed after the M=6.45 and M=5.95, 1976 Friuli earthquakes (Talamo et al. , 1978) , which now may be linked to the present activity of the Idrija fault system in the Tolmezzo-Venzone area. Discussion. On the bases of the discussed data, we propose a relationship between the set of low angle SSW-verging thrust of the Julian Prealps (i.e. Gemona-Kobarid, Susans-Tricesimo and Buia thrusts) and the Idrija-Ampezzo strike-slip fault system. On the bases of the evidence of active deformation along the Idrjia-Ampezzo strike slip fault and the available knowledge on the kinematics of the region, we suggest that the transpressive slip is probably splits between the Idrjia-Ampezzo strike slip fault-system and the compressive thrust-belt developing along the Julian prealpine area (Fig. 1). Slip partitioning on splays of oblique structures has been observed in many cases from across the world, both as for the coseismic and long-term displacements (e.g., Bemis et al. , 2015; Henyey et al. , 1999; Walker et al. , 2003). This new seismotectonic model may have crucial consequences for the assessment of the seismic hazard in Friuli. References Bressan G. and Bragato P.L.; 2009: Seismic deformation pattern in the Friuli-Venezia Giulia region (north-eastern Italy) and western Slovenia. Boll. Geof. Teor. Appl. 50 /3 255-275. Bemis S. P., Weldon R. J., and Carver G. A.; 2015: Slip partitioning along a ontinuously curved fault:Quaternary geologic controls on Denali fault systemslip partitioning, growth of the Alaska Range, and thetectonics of south- central Alaska . Lithosfere, 7/3, 235-246. Galadini F., Poli M.E. and Zanferrari A.; 2005: Seismogenic sources potentially responsible for earthquakes with M≥6 in the eastern Southern Alps (Thiene - Udine sector, NE Italy). Geophys. J. Int., 161 , 739-762. Henyey T. L., Fuis G. S., Benthien M.L., Burdette T.R., Christofferson S. A., Clayton R. W., Davis P.M., Hendley J. W., Kohler M. D., Lutter W. J., McRaney J.K., Murphy J. M., Okaya D.A., Ryberg T., Similia G. W. and Stauffer