Structural Reinterpretation of the Classic Simplon Tunnel Section of the Central Alps
A. G. MILNES Geological Institute, Swiss Federal Institute of Technology, Zurich, Switzerland Structural Reinterpretation of the Classic Simplon Tunnel Section of the Central Alps ABSTRACT tained, and to some extent also the traditional names. However, it will be shown that the The basement nappe concept arose during nappes were not emplaced simultaneously and the excavation of the Simplon tunnel in the that all were subjected to at least two sub- Central Alps. Recent detailed structural and sequent phases of deformation resulting in petrologic investigations in this classic area major folding. One of these was a phase of have confirmed the concept in general outline. isoclinal folding of regional importance which A complicated structural history has been de- caused widespread inversion of the original duced, however, with two phases of major structural succession. On the basis of these new folding and a period of large-scale faulting after surface findings, an attempt is made to cor- the phase of nappe emplacement. The earlier relate surface with tunnel geology, and a new of the subsequent fold phases was of regional cross section along the tunnel trace is presented. importance and caused widespread inversion of the original structural succession. During this PETROGRAPHIC AND phase, the basement rocks were in their most STRATIGRAPHIC CONSIDERATIONS ductile state and suffered the main part of their The petrography of the area to the southeast internal deformation. of the Simplon Pass (Fig. 1) has recently been described in great detail (Ragni, 1960; Milnes, INTRODUCTION 1964; Wieland, 1966). Two general petro- At 7:20 a.m. on Friday, February 24, 1905, graphic complexes are recognized: "crystalline the last rock barrier in the Simplon tunnel was basement"—consisting of gray-weathering broken through, and a new concept in Alpine quartzo-felspathic gneiss, schistose gneiss, and geology was born, the basement nappe.
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