Abstract
Polygonal Faults are ubiquitous features, characterized by covering large parts of basin with a typical polygonal pattern. In the last decade, different mechanical models for the generation of polygonal faults have been proposed; however, as they are commonly not directly observable, their formation remains a matter of debate. We found polygonal fault structures being exposed close to the surface in marine soft sediments at 5 m depth at the western coast of Guadeloupe. The structures are associated with fault-bound thermal springs and clearly visible at the sea bottom due to preferential precipitation of sulfur minerals and growth of cyano-bacteria. In a multidisciplinary study involving a team of hydrogeologists, marine micro-biologists, and structural geologists, we study the genesis of polygonal faults in this setting. We suggest that SiO2 precipitation due to fluid flux from thermal springs increases the cohesion of the most permeable soft sediments. Dewatering of the underlying layers causes the formation of polygonal faults at a depth of <1 m. These polygonal faults then act as channels for hot fluids, resulting in accumulation of sulfur and the development of cyano-bacteria mat at the surface. This site is a unique worldwide exemple of actively degazing polygonal faults and offers the unique opportunity to study the formation of polygonal faults in situ. Furthermore, it provides views on how polygonal faulting may interact with the biosphere, shaping the environment for marine biota. These results have been obtained in the frame of GEOTREF program (See Lebrun et al., Poster), which is a multidisciplinary innovative platform for exploration and development of high-enthalpy GEOThermal energy in Fractured REservoirs. In that frame, this study contributes to a better understanding of fluid migration and drainage evolution.