Seismic faulting and CO2-rich fluid interactions: Evidence from carbonate spherulitic grains in ultramafic fault damage zones

Faults and shear zones in the metamorphic ultramafic rocks of the Voltri Massif (Ligurian Alps, NW Italy) are often accompanied by intense or complete carbonation of the host rocks. One of these fault zones, hosted in peridotite and serpentinite, is characterized by distinctive structural features that can be related to a fluidassisted multistage brittle deformation, indicative of paleoseismic activity. The fault rocks contain cataclasite and breccias with layers of coseismic spherulitic grains of dolomitic composition, silica/calchedony veins and cement, and saponite bearing gouge. The fault is part of a system of regional structures associated with gold mineralization in quartz veins and various stages of hydrothermal alterations. Here we present a multiscale study of this fault zone combining field observations, microstructural analysis, SEM-EDS, and electron backscattered diffraction (EBSD). Focus is on the microstructures of the fault core and the significance of the characteristic carbonate spherulite layers in association with silica/chalcedony cement and veins. Our results show that these structures are indicative of the interaction between CO2-rich fluids released during coseismic and interseismic phases of faulting, i.e., during cycles of fluid pressure build-ups, faulting, fluid flushing, and mineral precipitation and sealing during seismic failure of the fault.