Tectono-metamorphic evolution of an evaporitic décollement as recorded by mineral and fluid geochemistry: The “Nappe des Gypses” (Western Alps) case study

Abstract

Evaporites play a major role on the structuration of collisional orogens especially when they act as décollement units. However, their exact pressure-temperature-deformation (P-T-d) paths are poorly documented. In this study, the first direct P-T-d constraints of the "Nappe des Gypses" formation (western French Alps) have been established. An innovative association of structural geology, petrography, crystallochemistry, and detailed study of both fluid inclusions and stable isotopes (C, O) analysis has been applied to this evaporitic facies. Geochemical analysis shows that the "Nappe des Gypses" formation has recorded the three typical metamorphic and deformational events of the Alps (namely D1, D2 and D3). These different constraints allow the determination of the first determination of the P-T path for this unit. Metamorphic peak conditions of the "Nappe des Gypses" are at 16.6 ± 2.3 kbars and 431°C ± 28°C. This formation was buried at similar conditions than the oceanic units. During the exhumation path, the D1-D2 transition is reached at 350°C ± 20°C and 6.5 ± 1.8 kbars and the D2-D3 transition is assumed to be at 259°C ± 24°C and 2.0 ± 1.0 kbars (Strzerzynski et al., 2012). Peak P-T conditions overlap those of the median Liguro-Piemontese units but are different from those of the Briançonnais units. It implies 1) an active and crucial role of the "Nappe des Gypses" during the exhumation of the Alpine oceanic complex. And 2) confirms the allochthonous and more distal origin of the European Thetysian passive margin of the "Nappe des Gypses" formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 kbars and 200 and 300°C, allows to discuss pore pressure excess and its mechanical consequences on the exhumation process. This process is very likely to amplify the "décollement" effect of the evaporites and allow the nappe stack formation. This illustrates the role of this formation as a décollement surface. This difference of evolution highlights the major role of the evaporitic formations on the exhumation and structuration of a collisional chain. Such methodology could contribute to decipher the role of evaporites in the structural context of other collisional chains such as Himalaya, Pyrenees or Zagros.

The North Pyrenean Frontal Thrust: structure, timing and late fluid circulation inferred from seismic and thermal-geochemical analyses of well data

“…The different T h values observed in a same fluid inclusion plane as well as the absence of post-entrapment modification suggest the cooling of a relatively continuous fluid flow from one source which is recorded in a single generation of fluid inclusions. Such an effect has already been observed in the décollement layer within Upper Triassic evaporites of the Alps (Barré et al, 2020), as reflecting the cooling of a single fluid circulation during the retrograde metamorphism of the Alpine collision. The occurrence of Triassic evaporites within the NPFT fault zone is inferred from borehole data and our structural interpretation (Figs.…”

Multiple sulfur isotopes signature of Thermochemical Sulfate Reduction (TSR): Insights from Alpine Triassic evaporites

“…Sulfides (pyrite and minor chalcopyrite) and elemental sulfur (Fig. 2; Barré et al, 2017) are observed in direct association with anhydrite and recrystallized carbonates (Barré et al, 2020), corresponding to a typical TSR paragenesis (e.g., Machel et al, 1995). In addition, the characteristic intermediate-valence sulfur species required for TSR to occur, S3 − and Sn 2− (polysulfides), have been identified by in-situ Raman analyses of heated fluid inclusions (Barré et al, 2017).…”

“…Here, we used 34 S/ 32 S fractionation factors at relevant temperatures between aqueous sulfates and aqueous H2S from Eldridge et al (2016) and between aqueous sulfates and aqueous elemental sulfur (S8) from Eldridge et al (2021) to determine the equilibrium temperatures of the different reduced sulfur compounds (Table 2). Using this method, the apparent precipitation temperatures of some sulfides are higher (up to 980 °C; Table 2) than the metamorphic peak temperature of the Nappe des Gypses formation (431 ± 28 °C; Barré et al, 2020) despite petrographic criteria indicating the phases to be syngenetic (Fig. 2).…”

“…4), slightly lower than those of sulfates. Moreover, the metamorphic path of the Nappe des Gypses formation was characterized by high temperatures of 137-431 °C throughout its history (Barré et al, 2020), and the occurrence of elemental sulfur in fractures and deformed anhydrite bedding (Fig. 2B, C, E) implies that it formed during the tectonic evolution of the Nappe des Gypses.…”

“…During the subduction-collision path of the Alps, the Nappe des Gypses formation acted as a major decollement that was crucial to the structuring of the Alps. The formation underwent three metamorphic and deformational events typical of the Alps (namely D1, D2, and D3), which favored fluid-rock interactions (see Barré et al, 2020, for a complete description of the tectonic evolution of the Nappe des Gypses). Sulfides (pyrite and minor chalcopyrite) and elemental sulfur (Fig.…”