Aretusini, Stefano (2013) Architecture and seismic markers of an exhumed fault zone in dolostones (Foiana line, Italian Southern Alps). [Magistrali biennali]
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The earthquake sequences of Bovec 1998 (strike-slip focal mechanism, MD 5.6), L'Aquila 2009 (normal, Mw 6.1) and Emilia 2012 (thrust, Mw 6.0) had the main shocks ruptures propagating and several foreshocks and aftershock nucleating within sedimentary successions mostly made of carbonatic rocks. These earthquake sequences are often long lasting (several months to years) and follow a complex spatiotemporal evolution, which is probably the result of the geometry of the fault-fracture network, fault rock distribution, ingression of uids, etc. Seismic waves inversion analysis has limited spatial resolution (in most cases > 1 km), and does not allow to reconstruct the geometrical complexity of fault zones and the distribution of fault rocks. Here the need of dedicated studies, from metric to kilometric scale ("multiscalar") of the architecture of faults hosted in carbonatic sedimentary rocks. This information, associated with mechanical data obtained from friction and rupture experiments, could be used in physically-based earthquake forecasting models and in simulation of seismic ruptures for earthquake hazard studies. In this thesis, I used a multidisciplinary approach which includes remote sensing analysis, detailed geological eld survey and microstructural studies to quantify the architecture of the 30 km long north-south striking Foiana Fault Zone (FFZ, Linea della Foiana auctores: Val di Non, Italian Southern Alps). The FFZ activity lasted from the Permian (mainly normal faulting) to the Miocene (mainly strike-slip faulting). In the Val di Non area, the FFZ is mainly hosted in Triassic dolostones (Sciliar Fm.), was exhumed from 1.5 km (southern segment) to 2.5 km (central segment) depth and, in his southern part, curves to the south-west into a restraining bend producing a fault-propagation anticline. As discussed here, the FFZ underwent seismic activity during the Miocene as attested by eld and microstructural evidences. The remote sensing analysis, thanks to the high resolution (2.5 to 1 m) 7 of the LIDAR-based digital terrain models images, allowed a detailed reconstruction of the structural lineaments of the FFZ and neighbor areas. The lineaments were organized in groups based on their strike. The Riedel-shear model was used to interpret their geometrical arrangement, which was consistent with the left-lateral kinematics of the FFZ. Detailed structural eld geology survey (which included systematic rock sampling) was performed in three localities (from south to the north: Carnalez, Salobbi and Doss de la Ceora) covering a ca. 7 km long segment of the southern part of the FFZ. I traced sections parallel and orthogonal to fault strike to determine the attitude and spacing of the main structural features (faults, joints, bedding, etc.) and the distribution and characteristics of the fault zone rocks. An intriguing aspect was the presence of large volumes (up to 300 m in thickness) of in-situ shattered (fracture density from 3 cm to few mm) dolostones with negligible or null shear strain, cut by ultracataclastic slipping zones with mirror-like surfaces. The presence of clasts truncated by the mirror-surfaces and of clasts with radial fractures ("exploded" grains) immersed in the ne matrix of the ultracataclasites, is indicative of the seismic origin of these fault rocks, consistently with recent experiments reproducing seismic slip deformation conditions. In general, the attitude of the minor faults of the FFZ evolves from scattered with dominant reverse dip-slip reverse component in the southern exposures (Carnalez) to sub-parallel faults with dominant strike-slip towards the northern exposures (Doss de la Ceora). This major change in the architecture of the FFZ is concomitant with a decrease in the thickness of the damage zone (in-situ shattered dolostones) from 300 m in Carnalez to 200 m in Doss de la Ceora. These architectural along strike variations are here interpreted as the result of the presence of southern fault bend or of dynamic coseismic rupture processes. In the latter case, the larger scatter in the attitude (and sense of shear) of the fault-fracture network with respect to the northern exposures may reect rupture directivity eects and the dierent depth of seismic faulting (larger the pressure connement, larger the scatter of the attitude of the faults), consistently with recently published earthquake simulations models. Lastly, the pre-existing bedding surfaces appeared to be primary factor in controlling the nucleation of slip surfaces and fault zone thickening and growth during progressive deformation.
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