Secco, Michele

Moderate to large earthquakes often nucleate within and propagate through carbonates in the shallow crust. The occurrence of thick belts of low-strain fault-related breccias is relatively common within carbonate damage zones and was generally interpreted in relation to the quasi-static growth of faults. Here we report the occurrence of hundreds of meters thick belts of intensely fragmented dolostones along a major transpressive fault zone in the Italian Southern Alps. These fault rocks have been shattered in-situ with negligible shear strain accumulation. The conditions of in-situ shattering were investigated by deforming the host dolostones in uniaxial compression both under quasi-static (strain rate ∼10−5 s−1) and dynamic (strain rate >50 s−1) loading. Dolostones deformed up to failure under low-strain rate were affected by single to multiple discrete extensional fractures sub-parallel to the loading direction. Dolostones deformed under high-strain rate were shattered above a strain rate threshold of ∼120 s−1 and peak stresses on average larger than the uniaxial compressive strength of the rock, whereas they were split in few fragments or remained macroscopically intact at lower strain rates. Fracture networks were investigated in three dimensions showing that low- and high-strain rate damage patterns (fracture intensity, aperture, orientation) were significantly different, with the latter being similar to that of natural in-situ shattered dolostones (i.e., comparable fragment size distributions). In-situ shattered dolostones were thus interpreted as the result of high energy dynamic fragmentation (dissipated strain energies >1.8 MJ/m3) similarly to pulverized rocks in crystalline lithologies. Given their seismic origin, the presence of in-situ shattered dolostones can be used in earthquake hazard studies as evidence of the propagation of seismic ruptures at shallow depths.

Few fault rocks are known to be associated undoubtedly with seismic faulting. Here, we investigated the formation mechanism of cockade breccias found in transtensional faults cutting marbles and quartzites from the Col de Teghime area (Alpine Corsica, France). Field surveys coupled with detailed microanalytical investigations indicated that: (i) the core clasts of the cockades are composed of host rock fragments >310 μm in size that are suspended in the slipping zones and arranged in inverse grading; (ii) the concentric rims of the cockades show a cyclic zoning made of saddle dolomite + Mg-calcite + goethite + anatase; (iii) the cockade-bearing veins are associated with minor fault veins filled with fine fragments (<300 μm in size) cemented by the same minerals of the cockade rims. We propose that the cockade-bearing faults formed at shallow crustal depths (<2 km) and recorded the main phases of the seismic cycle: (1) co-seismic fragmentation of the wall rocks in presence of fluids; (2) co-seismic fluidization of the rock fragments resulting in elutriation of the finer particles and formation of residual porous and well-sorted slipping zones, where cockades will nucleate. Inverse grading resulted from co-seismic shaking and shearing; (3) post-seismic to inter-seismic cementation by deposition of carbonate-rich rims due to slow mineral pressure growth, resulting in the suspension of the clasts within the slipping zones. The formation mechanism of cockade breccias proposed here provides an alternative view of earthquake-related processes in fluid-rich environments at shallow crustal depths.