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Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province
Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slipârelated deformation on upperâcrustal faults in basaltic rocks is closely associated with fluidârock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basaltâderived fault rocks from three passively exhumed fault zones in the Faroe Islands. Faultârock petrology is determined by optical petrography and automated phase identification based on micrometerâscale chemical maps from scanning electron microscope Xâray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by Xâray fluorescence. The fault rocks show intense fluidâmediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing alongâfault percolation of carbonated meteoric water, increasing fluidârock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressureâsolution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluidâmediated alteration by CO2 and water, while other species are redistributed within the fault zones.</p