Coseismic signature in poorly lithified sediments: field data and friction experimental results

The frictional properties at seismic and sub-seismic slip rates of poorly lithified sediments in forearc basins are poorly understood; nevertheless these type of sediments play a key role as they can prevent or facilitate the propagation of seismic ruptures to the surface. Field observations and petrophysical analyses carried out on extensional fault systems exposed in poorly lithified sediments of the Crotone forearc basin in south Italy have documented (1) the widespread occurrence of foliated cataclastic sands in the fault cores, and (2) the local occurrence of ~0.5-1cm thick, discontinuous black gouge layers with up to 1 m displacement along the main slip zones. Mercury-injection porosity analysis indicates that mean pore size in undeformed protolith is ~100-300 micron, in the foliated material is ~20-80 micron, and in the black gouge is ~2 micron. XRD analyses performed on both the protolith sands and the black gouges show the presence of pyrophyllite in the gouge, a phyllosilicate which generally forms in deep diagenetic/anchizone conditions. No pyrophillite was detected in the undeformed host sediments. The integration of field observations, microstructural, petrophysical and mineralogical analyses suggest that such gouges could have formed during coseismic rupture propagation at shallow crustal depths (<1 km). To test this hypothesis, we have performed two sets of sliding experiments on both fine and coarse undeformed sands using a high-velocity rotary shear apparatus at room humidity and temperature. The undeformed materials are mostly composed of quartz, and subordinately by feldspar, plagioclase and phyllosilicate minerals. Each sand has been sheared at constant sub–seismic and seismic slip rates (0.1-1-10 mm/s and 0.1-0.5-1.3 m/s, respectively) with three different values of normal loads (Sigman=1, 7 and 14 MPa). All the experiments were arrested at 1.2 m of total slip, i.e. a value comparable with that accommodated by the studied natural gouge layers within the fault zones. The experimental results show that at Sigman=1 MPa, for both fine- and coarse-grained sands, the friction coefficient is independent from the sliding velocity . AtSigman≥7 MPa, the friction coefficient is independent from the applied normal stress. In particular, at sub-seismic slip rates (100 micron/s to 10 cm/s) both the fine- and coarse-grained sands show strain hardening behavior and a frictional coefficient at steady state of 0.6-0.7 and 0.5-0.6, respectively. At seismic slip rates (>0.5 m/s), the onset of dynamic weakening is observed only in the fine sands, as the friction coefficient drops from a peak value of 0.7-0.8 to a value of 0.3- 0.4 recorded at 0.8-1.2 m of sliding displacement. These results suggest that the initial grain size and the evolution of the grain/pore size distribution during shearing, can also play a role in the dynamic weakening mechanisms in such materials. Finally, the grain size distributions, mineralogical compositions (XRD data, bulk and clay-size fractions), porosity data, and microstructural features of natural and experimental fault gouges will be compared to identify diagnostic markers of coseismic rupture propagation in poorly lithified sediments deformed in tectonically active area at very shallow crustal levels.