Slipping zones from exhumed faults in dolostones (Borcola Pass Fault, Italian Southern Alps)

Fault zones in limestones and dolostones represent significant seismogenic sources in many areas worldwide, including central Italy and the Italian Fore-Alps (e.g. Val di Noto 1693, estimated Mw = 6.9; Avezzano 1915, Mw = 6.7; Friuli 1976, Mw = 6.4; Irpinia 1980, Mw = 6.9; L’Aquila 2009, Mw = 6.3). Field and microstructural investigations of exhumed seismogenic fault zones and related fault rocks in carbonates are therefore important to document fault structure and the range of deformation processes active during the seismic cycle. The Borcola Pass Fault is a ca. N-S strike-slip branch of the Schio-Vicenza Line (a main lineament of the Italian Southern Alps) and is well exposed within a series of large dolostone quarries (Borcola Pass, Trento). Estimated depth and temperature conditions during faulting are ca. 1.6-1.7 km and 50°C. The fault zone consists of a > 80 m thick damage zone surrounding a 2-3 m thick fault core containing dolomitic fault rock lenses bounded by principal slip zones up to 10 cm thick. The damage zone is cut by three systems of secondary faults striking N-S, E-W and NW-SE. N-S and E-W striking faults reactivated inherited (Jurassic to Paleogene) regional-scale joints and have an average spacing between 0.2 to 0.5 m, whereas NW-SE striking faults were newly formed during post-Paleogene slip activity along the Borcola Pass Fault and the Schio-Vicenza Line. Both principal and secondary slip zones consist of cement-supported dolomitic cataclasites and dolomite-filled veins. Some slip zones contain a sub-centimetre thick vein-like cataclastic layer (Layer A) located immediately beneath the principal slip surface and above a cement-supported cataclasite (Layer B). Layer A is white in colour and consists of sub-rounded dolostone grains ranging between 300 µm and 2.5 mm in size, suspended in a dolomitic cement. Layer B is grey in colour and consists of sub-angular dolostone grains ranging between 5 µm and 1 cm in size within a dolomitic cement. According to image analysis investigations, Layer A has a lower 2-dimensional fractal dimension (D < 1) and better grain sorting than Layer B (D≈ 1.6). The boundary of Layer A towards the principal slip surface is cuspate-lobate and includes a 2-5 mm thick zoned ultracataclasite whereas towards Layer B the boundary is sharp and truncates clasts. Several injection veins depart from Layer A into Layer B. These microstructural data suggest that while grain fragmentation models (e.g. constrained comminution) can account for the clast size distribution found in Layer B (D ≈ 1.6), other physico-chemical (maybe coseismic) processes such as localized layer fluidization and grain sorting may result in the unusual textural characteristics, including the presence of injection veins, found in Layer A.