Seismic faulting and fluid interactions: a structural study from carbonated fault damage zones within ultramafic rocks

Faults and shear zones within the metamorphic ultramafic rocks of the Voltri Massif (Ligurian Alps, NW Italy) often exhibit significant or complete carbonation of the host rocks and are locally associated with gold mineralization hosted in quartz veins (e.g., in the Lavagnina Lakes area). Here, a specific fault zone part of a larger regional system (i.e., the Bisciarelle Fault Zone) displays distinct structural characteristics linked to a fluid-assisted multistage brittle deformation in serpentinized peridotites, possibly indicating paleoseismic activity. Within the fault rocks, cataclasite and breccias are present along with saponite-bearing gouge, featuring layers of coseismic spherulitic grains interspersed in silica/chalcedony veins and cement. Spherulites consistently crystallize as concentric bands of fibrous Fe-dolomite and display multiple layers of radial crystal growth regularly alternating with darker, oxides-rich concentric bands. The concentric growth of spherulites is evident from the microtextural relations between successive bands, which depart radially from the spherulite cores, made of a submillimetric nucleus of carbonates or single grain of the host rock (e.g., relicts of fragments of fault core). In this study, we present a multiscale analysis of this fault zone, integrating field observations, microstructural examination, SEM-EDS investigation, and electron backscattered diffraction (EBSD). The primary focus is on the microstructures within the fault core and the significance of distinctive carbonate spherulite layers in conjunction with silica/chalcedony cement and veins. Our findings reveal that these structures are indicative of the interaction between CO2-rich fluids released during both coseismic and interseismic phases of faulting. This interaction occurs during cycles involving fluid pressure build-ups, faulting events, fluid flushing, and the subsequent precipitation and sealing of minerals during seismic failure of the fault.