Seismic cycle recorded in cockade-bearing faults (Col de Teghime, Alpine Corsica)

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.