Michele Fondriest 1 , Steven A. Smith 2 , Dario Zampieri 1 and Giulio Di Toro 1,2

1. Field and microstructuralinvestigationofanexhumed fault zone in dolostones (SouthernAlps, Italy) Michele Fondriest1, Steven A. Smith2, Dario Zampieri1 and Giulio Di Toro1,2 (1) Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 00143, Rome, Italy • Motivations • Fault zones within carbonate-bearing rocks are common seismic sources in the Earth crust. Here we describe the architecture and rocks of a branch (the “Main Fault”, exposed in a dolostone quarry) of the Schio-Vicenza Fault zone (Italian Southern Alps, Fig.1A -B)ref.1. • Main fault architecture • Sinistral N-S vertical strike-slip fault with a sub-cm core cutting dolostones and dolomitic marbles; slip < 100 m. • >80 m wide damage zone cut by three minor fault systems striking N-S, E-W and NW-SE (Fig.2A-C). • Microstructuresofdolomiticcataclasites • Ambient conditions during faulting: P ≈ 40 MPa (1.6-1.7 km of depth), T ≈ 50°C. • Cement supported cataclasites (Fig.3) with vein filling and clasts substitution by dolomitic precipitation (Fig.4A-C). • Slipping zone with two cataclastic layers (A & B: Fig.4A). • Possible coseismic indicators • Injection veins (hydrofracturing) and truncated clasts within slipping zones suggest the occurence of pressurized fluids and high slip rates, consistent with those achieved during earthquake ruptures (1-3 m/s: Fig.5A-C). • Fractal dimension (D≈1.7-1.8) of most cataclasites is consistent with the the “constrained comminution” fracturing modelref.2 but in the slipping zones (layer A) fractal dimensions D > 3 suggest other deformation mechanisms(clast selection due to fluidization?), consistent with coseismic slip (Fig.6A-B). 3. Fault rocks Fig. 3 1cm LAYER A 4. Microstructures LAYER B 1cm Fig. 4A Monomineralicclast Polymictclast Fig. 4C 850 µm Fig. 4B 1. Geologicalsetting 5. Truncatedclasts and injectionveins Fig. 1B Fig. 1A Fig. 5C 100 µm Fig. 5A Main Fault Fig. 5B slip surface 1 slip surface 1 Schio-VicenzaLine Trento slip surface 2 slip surface 2 V1 Padova 900 µm Vicenza Borcola Pass V2 300 µm 2. Main Fault architecture ImageJ 6. CSD Methodref..3 Results Fig. 2B Basalticdykes Equivalent diameter d = 2 (A/π)0.5 Main Fault D=1.8 Log N = -D Log d Fig. 6A d ( ) • “constrained comm.” in layer B. • clast selection due to fluidization in layer A? Fig. 2A 5 m S N Layer A D=3.7 Fig. 2C E Layer B D=1.8 W N-S NW-SE 4.3 m d ( ) E-W Fig. 6B References: (1) Zampieri and Massironi, 2007, Geol. Soc., London, Spec. Pub., 290, 351-366.(2) Sammis et al., 1986, Pure and Appl. Geoph., 124, 53-78; (3) Di Toro and Pennacchioni, 2005, Tectonophysics, 402, 55-80.