Syntectonic fluid flow and deformation mechanisms within the frontal thrust of a foreland fold-and-thrust belt. Example from the Internal Jura, Eastern France

Structural mapping and microstructural analyses are combined with stable, clumped, and radiogenic isotopes to unravel deformation mechanisms and syntectonic fluid flow during upward propagation along the frontal thrust (Fuans thrust) of the internal part of the Jura fold-and-thrust belt. The Fuans thrust cuts through Upper Jurassic-Lower Cretaceous carbonate deposits, branches into two splays, and generates a system of fault-propagation folds. Folding was accomodated by layer-parallel shortening and layer-parallel shear, which generated flexural slip along bedding surfaces, stylolites, and foliated fabric in marly limestone interbeds. Thrusting was characterized by cycles of mutually overprinting pressure-solution and brittle processes, such as frictional sliding and fracturing/veining. This generated foliated tectonites in marls and limestones without significant cataclasis of the host rock. Pressure-solution and micrometer slip along slickenfibers are evidence of aseismic slip, in agreement with the ongoing aseismic growth of anticlines in the Jura fold-and-thrust, while crackle-like vein texture may account for episodic seismic fracturing. Geochemical data show that Fuans thrust acted as a conduit for the downward infiltration of meteoric-derived fluids, which channeled along foliated tectonites during syntectonic fluid flow. Calcite mineralizations precipitated at temperatures between 54 °C and 10 °C, showing an increased contribution of organic carbon from soils in colder paleofluids. This indicates a progressive opening of the system, characterized by a more efficient and extensive meteoric fluids circulation. Considering the highest recorded temperature of 54 °C and assuming the modern geothermal gradient of 30 °C/km, thrusting in the study outcrop initiated at a burial depth of at least ∼1.4 km.