Re-examining Argandʹs view of the Siviez–Mischabel nappe

We present the results of field work in the frontal part of Siviez–Mischabel nappe near Chandolin (Valais, Switzerland). Mapping reveals packages of interleaved cover and crystalline basement strata. We focus on monocyclic units (cover units and the Randa orthogneiss) in order to understand the structural geometry of the Siviez–Mischabel nappe. Sedimentary `way-upʹ criteria consistently indicate that cover units are upright, even for strata previously mapped as overturned. In general, primary foliation (S1) parallels or dips more steeply to the south than bedding throughout the study area. An L1 lineation lies in the plane of foliation and trends north–south. Kinematic indicators in the Randa orthogneiss consistently yield a top-to-the-north sense of shear, consistent with the orientation of L1. These relations indicate that the frontal part of the Siviez–Mischabel is a series of thrust imbricates, rather than a large-scale recumbent fold as originally suggested by Argand in 1916.