Evolution of fault permeability during episodic fluid circulation : evidence for the effects of fluid–rock interactions from travertine studies (Utah–USA).

International audienceFaults are known to be important pathways for fluid circulation within the crust. The transfer properties along faults can evolve over time and space. The Little Grand Wash and Salt Wash normal faults, located on the Colorado Plateau, are well known examples of natural CO2 leakage systems from depth to surface. Previous studies dated and established a chronology of CO2-enriched fluid source migration along the fault traces and linked the aragonite veins observed close to Crystal Geyser to CO2-pulses. However, multiple circulation events recorded along a given fault segment deserve to be studied in minute detail in order to unravel the chronology of these events, precipitation processes and associated mechanisms. A combination of structural geology, petrography, U/Th dating, oxygen and carbon isotope analysis were used to study the fault related CO2-enriched paleo-circulations in order to build a conceptual model of CO2-circulation along the faults. This study resulted in the precise descriptions of the features attesting CO2-enriched fluid circulation by a characterization of their relationship and architecture at the outcrop scale. These features are witnesses of a large range of circulation/sealing mechanisms, as well as changes in fluid chemistry and thermodynamic state of the system, providing evidence for (i) the evolution of the fluid through a pathway from depth to the surface and (ii) different cycles of fault opening and sealing. Large circulation events linked with fault opening/sealing are observed and calibrated in nature with millennial circulation and sealing time-lapses. Numerical modelling indicates that such sealing timescale can be explained by the introduction of a fault sealing factor that allows modifying permeability with time and that is calibrated by the natural observations