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 P-T path determination for this unit. Metamorphic peak conditions of the “Nappe des Gypses” are at 16.6 ± 2.3 kbar 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 kbar and the D2-D3 transition is assumed to be at 259 °C ± 24 °C and 2.0 ± 1.0 kbar (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 Tethyan passive margin of the “Nappe des Gypses” formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 kbar 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.