Finite strain patterns in some Alpine nappes

Abstract

The patterns of finite strain in some Alpine nappes and thrust sheets are reviewed, together with their tectonic evolution. Glarus Nappe. The finite strain pattern, approximately that of heterogeneous simple shear, was acquired during Upper Oligocene times when the Helvetic thrust block was deformed together with its infrastructure and basement. The Glarus Thrust was formed during Miocene times and modified by Pliocene to Recent differential uplift. Morcles Nappe. This large recumbent fold nappe overrode the Aiguilles Rouges Massif and its cover towards the northwest. The finite-strain pattern resulted from successive diagenetic compaction, multilayer buckling and heterogeneous simple shear. High strains occur along the inverted limb of the nappe and fold hinges are oriented subperpendicular to the NW stretching lineation. Final emplacement of the nappe, during the Miocene, was perhaps a later event. Chartreuse Massif. 33% E-W shortening was developed across this fold and thrust belt during post-Helvetian times by décollement along the Oxfordian shales, in response to thrusting and uplift in the Belledonne basement. The Oxfordian shales had earlier accommodated a pre-Helvetian (probably Oligocene) deformation in the basement and its Trias-Bajocian cover, when N030° trending, subhorizontal folds, cleavage and a stretching lineation (18% N030° stretching) were developed. Digne Nappe. Detached along the Keuper evaporites, the cover was displaced westwards in two stages: (1) late Oligocene, with folding, cleavage development, formation of the frontal slices and an overall E-W shortening of 20% and (2) late Miocene-Pliocene, after emplacement of the Embrunais Nappes in the east, producing the Digne Thrust. It is concluded that for these Alpine nappes geological criteria must be used to establish their evolution. If the internal deformation can then be assigned to part of this evolution, the pattern of finite strain can be used to quantify and elucidate that part. A maximum of information is gained if the incremental deformation history can also be obtained from study of syntectonic fibres.