Tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between low-grade metamorphism and tectonic phases

The frontal Penninic units (FPU) of the Western Alps underwent a complex tectono-metamorphic evolution during Alpine orogeny that can be divided into three major stages: a first one pre- to syn-collisional (D1 and D2), a second one (D3) and third one (D4) post collisional. This study aims to integrate detailed structural mapping with metamorphism (Kübler index and K-white mica b-cell dimension analysis) and thermochronology (fission track analysis). Metamorphic conditions in the FPU range from high anchizonal to epizonal conditions. The highest metamorphic temperatures (∼300^°C) were deduced for the Houiller unit and the Cheval Noir unit. Structural analysis and fission track ages indicate that the temperature peak in the Houiller unit was reached during D1, i.e.in the Eocene. The metamorphic peak in the Cheval Noir unit was reached during D2 due to overthrusting of the more internal Subbriançonnais and Houiller units.

Kübler indices and fission track data show a vertical gradient within the Cheval Noir unit with the higher temperatures recorded at the base of the stratigraphic series. This metamorphic gradient was successively folded during D3 in the Oligocene.

The K-white mica b-cell dimension values do not show major differences within the FPU units. They present a clear bimodal distribution interpreted to be due to the superposition of the effects of the two stages of deformation. The Kwhite mica b-cell dimension values from the FPU are higher than those recorded in the Dauphinois domain indicating that the FPU were subjected to higher pressures compared to the Dauphinois domain. Metamorphism in the Dauphinois domain, which is affected by one major phase of deformation (i.e.D3) only, is related to the overthrusting of the FPU along the syn-D3 Roselend thrust.

According to our tectonic reconstruction the first two phases of deformation (D1 and D2) developed during subduction of the Valaisan units, within the accretionary wedge. Our data indicate that metamorphism affecting the FPU south of Moûtiers is mainly related to the first stage (D1 and D2) when the FPU formed the frontal and upper part of the Alpine accretionary wedge.

The second stage (D3), characterized by thrusting and folding, developed during the Oligocene in a post-collisional scenario and led first to low T–low P metamorphic overprint and finally to a partial exhumation of the FPU already in the Late Oligocene.