Early deformation of ultrahelvetic mélanges in the helvetic nappes (Western Swiss Alps)

The Ultrahelvetic mélanges represent an important clue for understanding the Tertiary Alpine orogeny, particularly in the external Alps of western Switzerland. The remarkable architecture of the Upper and Lower Ultrahelvetic units is classically explained by an important phase of gravity sliding and orogenic sliding accompanied by the formation of chaotic complexes and olistostromes (wildflysch), a phenomenon called 'diverticulation'. A structural analysis reveals evidence for early phases of deformation, despite strong overprinting by late Alpine deformation. These early fabrics are observed in foliated pebbly-mudstone (a tectonic breccia, part of the mélange matrix), in disrupted flysch sequences and in dark shales. They attest to brecciation related to a complex pattern of veining, with several generations of calcite cement and several stages of failure during lithification, and they show the important role of fluids during the deformation. They present numerous analogies with the fabrics described in association with thrusts in accretionary prisms. Considering the tectonic fabrics and the relations among the infra-Prealpine mélanges, a tectonic model to explain the emplacement of the Ultrahelvetic units is proposed in which the décollements developed in three weak levels. The early fabrics evolved within a basal thrust formed in a final stage of the Valaisan subduction.