Masoch, Simone (2018) Formation of cockade breccias in extensional brittle faults (Col de Teghime, Alpine Corsica). [Magistrali biennali]
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It is well-known that fluid migration in the Earth’s upper crust is strongly controlled by the structure of fault zones. Importantly, fluid migration and pore fluid pressure variations in fault networks control the nucleation and evolution of earthquake sequences. Vein filling in fault zones is one of the most impressive geological signatures of the interaction between fluids and fault zone rocks. A relatively common fault vein filling fabric is the spectacular cockade breccia, consisting of fragments of wall- and fault rocks rimmed and sealed by concentric layers of fluid-precipitated minerals. Consequently, the formation of cockade breccia requires rock fragmentation and cementation in the presence of fluids under particular physical and chemical conditions that may occur during different phases of the seismic cycle. This thesis discusses the structure and the mechanism of formation of cockade breccia hosted in the slipping zones of the Miocene in age extensional brittle faults that cut quartzites and impure dolomitic marbles of the Schistes Lustrés Complex from Alpine Corsica (France). Original structural geology field surveys and detailed microstructural (optical cathodoluminescence and scanning electron microscopy; micro-tomography; image analysis) and mineralogical/geochemical (micro-Raman spectroscopy, X-ray powder diffraction, Energy-dispersive X-ray spectroscopy) investigations of the fault rocks indicated that: (a) core clasts of the cockades derive from the wall rocks, have rounded shape and are well-sorted with the fraction finer < 310 µm in diameter almost completely absent; (b) the core clasts of the cockades are suspended (i.e., do not touch each other) in the slipping zones; (c) in some slipping zones, the core clasts are arranged in inverse grading; (d) the concentric layers (systematically four in total) rimming the clasts of the cockades have strong mineralogical zoning consisting of alternate/rhythmic precipitation of saddle dolomite, Mg-calcite and Fe- and Ti-oxides/hydroxides; (e) cockade-breccia are cut but also kinematically associated with veins made of ultrafine (size < 200 µm) wall rock clasts cemented by the same mineral assemblage of the rims of the cockades; (f) the cockade breccia are cut by dolomite-bearing veins and partly sealed by late precipitation of calcite. The above findings allowed me to propose the following model for the formation of the cockade-bearing faults. The model links the formation of the cockade microstructures numbered (a) to (f) to different phases of the seismic cycle: (1) co-seismic fragmentation of the wall rocks (a) in presence of CO2- and Fe-rich fluids which promoted also the rounding of the clasts (abrasion and chemical wear); (2) co-seismic fluidization of the rock fragments associated to fluid pulses migrating in the fault zone. Fluidization resulted in elutriation of the fine particles, which were deposited in distal veins (e), and formation of a residual very porous and well-sorted clast assemblage (a) which will make the core of the cockades. Inverse grading (c) and rounded shape (a) of the cores resulted by shaking (Brazil-Nut Effect) and co-seismic shearing of the clasts; (3) post-seismic to interseismic cementation by deposition of concentric carbonate-rich rims (d) around the core clasts of the cockades. Rim deposition was probably due to slow (years to centuries?) mineral pressure growth processes associated to the ingression of fluids with variable composition in the porous clast assemblage. Pressure growth resulted in the progressive lift of the clasts and in their "suspension" in the cockade assemblage (b). The precipitation of saddle dolomite and the late and partial sealing of the cockade breccia by calcite cement (f) suggest that the cockade breccia formed at shallow depths in the crust (< 2 km). Based on this conceptual model, cockade breccias are particular fault rock assemblages which record the passage of seismic ruptures in the presence of pressurized migrating fluids. Given the scarcity in the current literature of fault rock assemblages possibly associated to seismic faulting, the results of this study may allow us a better comprehension of earthquake-related processes at shallow crustal depths and find application in seismic hazard studies
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