GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 65 footwall are often conjugated and range in dip from 80 o to 45 o , thus possibly associated to higher confining pressures during formation and slip with respect to the Alto di Cacchia DGSD end- member case (Fig. 2). Sant’Erasmo (minor antithetic fault at the footwall of a main seismogenic fault and reactivated as DGSD) . The about 400 m long scarp is steeply dipping (~70 o ) and includes secondary scarps at the hangingwall forming small trenches. The Sant’Erasmo DGSD is hosted in the footwall of the Rocca Preturo 10 km long normal fault and includes opened subvertical joints but also lower angle conjugate fractures cutting the main scarp. This fracture pattern could be related to gravitational reactivation of pre-existing secondary faults and fractures of the damage zone of the Rocca Preturo seismogenic fault. Serrone DGSD (minor antithetic fault at the footwall of the Gioia dei Marsi Fault) . The scarp is about 500 m long along stirke, discontinuous and with sinistral en-èchelon step-overs. The dip ranges from over 70 o at the scarp tips to about 45 o in the central section, where the scarp appears more polished and less karstified. As for the Sant’Erasmo DGSD, this structure is located at the footwall of a main seismogenic fault (the Venere-Gioia dei Marsi Fault) and opened sub-vertical joints (with spacing larger than 20 cm) and conjugate fractures were found. Microstructural investigations. Optical microscope observations of the PSZs of end-member cases (i.e., shallow and small DGSD cutting Quaternary deposits like the Alto di Cacchia DGSD) vs. large NF producing > Mw 6.5 earthquakes like the Venere Fault) and intermediate cases (i.e., NF reactivated as DGSD: Valle Force Fault ) have shown systematic differences in the deformation style and microstructural maturity (Fig. 2). The slipping zones associated to the NF are dominated by cataclastic to ultracataclastic fabric ( sensu Sibson, 1977), including well- defined and cm-thick PSZs. Instead, the slipping zones associated to the DGSDs consist mainly of protocataclasites or crush breccias and cataclasites are extremely thin or absent. Instead, Scanning Electron Microscope observations of the matrix of the cataclasites of the PSZs both NF and DGSD reveal that the matrix is formed by micrometer to nanometer in size highly-packed calcite grains, sometimes with foam-like fabric, and cut by stylolites (Fig. 2). These microstructures could be the result of cataclastic processes plus diffusive mass transfer (pressure solution and precipitation) and sintering processes. Sintering is a thermally-activated Fig. 2 - Comparison between the macro- and micro-structural observations of the Alto di Cacchia (AC) DGSD (the DGSD end-member case) and the Valle Force (VF) minor fault. The AC is characterized by opened joints dipping at high angle (>75 o ) and by a thin (~3 mm thick) cataclastic PSZ. Instead, the VF is characterized by a higher dispersion of the attitude of the joints in the footwall (i.e., they dip both at low and high angles), and by a thick cataclastic PSZ. At the micro- to nano-metric scale (SEM and FESEM backscatter images), the matrix of the PSZs of both the AC and the VF consist of highly packed calcite nanograins possibly resulting from sintering and solution creep processes.