Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example from the Faroe Islands, North Atlantic Igneous Province

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Abstract
Fault rock petrology exerts an important control on the permeability structure and mechanical properties of fault zones. Slip-related deformation on upper-crustal faults in basaltic rocks is closely associated with fluid-rock interaction, altering the distribution of physical properties within the fault. Here we present quantitative descriptions of the geochemical and petrological evolution of basalt-derived fault rocks, from three passively exhumed fault zones in the Faroe Islands. Fault-rock petrology is determined by optical petrography and automated phase identification based on micrometer-scale chemical maps from scanning electron microscope X-ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X-ray fluorescence. The fault rocks show intense fluid-mediated 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 along-fault percolation of carbonated meteoric water, increasing fluid-rock ratios. Our results suggest the only mobile species within the fault zones are Ca, Si, Al, which are leached during 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 cements in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure-solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid-mediated alteration by CO2 and water, while other species are redistributed within the fault zones.
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