Tectonic evolution of the Engi Slates, Glarus Alps, Switzerland

Abstract

We present a geological map, profiles and the results of a detailed structural analysis of the Early Oligocene Engi Slates southwest of the village of Engi in the Sernft Valley in canton Glarus (Switzerland). In this area, the Engi Slates are folded on a deca- to hectometer scale into tight NW-vergent folds with sharp hinges. This took place during the Plattenberg F₁ folding phase. No axial plane foliation was formed. The F₁ folds are unconformably cut-off by a 16-25° NE dipping thrust, along which Eocene marls were emplaced onto the folded Engi Slates. We refer to this thrust as Riedboden Thrust and correlate it with the base of the upper, chaotic part of the Wildflysch Nappe of Oberholzer (1942). This is a (probably tectonic) mélange of rocks from the underlying Sardona, Blattengrat and North Helvetic Flysch Units. It lies as a 5 to 100 m thick, more or less continuous nappe below the Glarus Thrust. A younger, SE-dipping tectonic foliation (Plattenberg F₂ foliation) cuts both through the folded Engi Slates, the Riedboden Thrust and the Eocene marls. This foliation is the axial plane foliation to meter-scale open F₂ folds. It developed parallel to the overturned limbs of the F₁-folds.