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. Al- schist (with or without biotite, garnet, stauro- though the correlation between tunnel and lite), with amphibolite and ultramafic rocks surface geology was open to various interpreta- locally important; and "sedimentary cover" — tions (compare Schardt, 1904; Schmidt, 1908), mainly brown-weathering calcareous schist and the fact of large sheets of pre-Triassic basement impure marble (called variously "Biindner- lying on top of younger, presumably Mesozoic, schiefer" or "calcescisti"), with thin but locally cover rocks had been conclusively demon- important associations of dolomitic, gypsum- strated. The Simplon structure provided a key bearing, and quartzitic rocks (referred to as to the understanding of the Alpine internal "Trias" in the older literature, for instance, zone from the Mediterranean to the Hungarian Schmidt and Preiswerk, 1908). The cover is plain (Heim, 1921, p. 507), and the Simplon considered to represent Mesozoic sediments, profile has been reproduced in textbooks and metamorphosed during the Alpine orogeny, in field guides up to the present day (for instance, the present area under amphibolite facies con- Cadisch, 1953, Fig. 46; Badgley, 1965, Fig. ditions. The basement represents the eroded 3-31; Bearth and others, 1967, Fig. 1). De- stumps of earlier metamorphic and granitic tailed remapping has now been completed complexes on which the cover sediments were over a wide area to both sides of the tunnel deposited. The relative age of basement and trace (Milnes, 1968, 1973; Bearth, 1972) and cover, on which the whole basement nappe the time is ripe for a reconsideration of this concept depends, has been definitely proved at classic cross section. In the following, the basic only one locality (loc. X, north of Varzo, Fig. concept of a series of basement nappes is re- 1), where boulders of basement granite are Geological Society of America Bulletin, v. 84, p. 269-274, 3 figs., January 1973 269 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/1/269/3443615/i0016-7606-84-1-269.pdf by guest on 02 October 2021 Figure 1. Geologic-tectonic map of the area south- and Italy (in part after Bearth, 1972). east of the Simplon Pass, Lepontine Alps, Switzerland Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/1/269/3443615/i0016-7606-84-1-269.pdf by guest on 02 October 2021 Figure 2. Structural profile along part of the lection (specimens every 10 m or less, 500 thin sections, (Schardt, 1898-1906) and its southern approaches recumbent synform; mi, m2, im, and wi = axial traces Simplon tunnel and its southeastward extension (for and unpub. notes, deposited in the Geological Museum, (Stella, 1906). The lines in the ornamentation indicate of major folds related to the regional synform; line of section, see Fig. 1), based on detailed surface Swiss Federal Institute of Technology, Ziirich, Switzer- the trace of the main foliation. M-W = postulated axial base of Monte Leone nappe, — base of mapping, a review of the Schardt Simplon tunnel col- land), and the official reports on the tunnel itself trace of the regional Monte Leone/Wandfluhhorn Lebendun nappe. MILNES, FIGURE 2 Geological Society of America Bulletin, v. 84, no. 1 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/1/269/3443615/i0016-7606-84-1-269.pdf by guest on 02 October 2021 STRUCTURAL REINTERPRETATION OF SIMPLON TUNNEL, CENTRAL ALPS 271 enclosed in marble of the cover (Milnes, 1964, TABLE 1. GEOLOGIC COLUMN p. 18-19). calcescisti {?Lower Jurassic) The cover/basement boundary is generally ------------- unconformity well defined and easily mapped, and con- stitutes the main petrographic feature on which Cover Trias an unravelling of the structure depends. ------------- unconformity Within the two main complexes mappable scisti bruni (Permian and lithologic boundaries are only locally present, Carboniferous) gradational and intercalated transitions are the general rule, and lithologies are not necessarily Transitional psephite, psarrniite constant parallel to the main penetrative folia- ----------or - intrusivunconformite contacy t tion or to the cover/basement contact. This crystalline rocks should be emphasized because too often in these Basement (high-grade metamorphic rocks deeper zones of the Alps the structure has been and granite) deduced on the basis of petrographic criteria Note: Since this column is seen through the mists of two or three phases of intense deformation and a long period with little regard for the actual structural rela- of Alpine metamorphism, there is small wonder that its out- tions. In Figure 1, the structure has been line remains unclear. brought out by coloring the cover black and distinguishing various basement units. No MAJOR STRUCTURAL EVENTS attempt has been made to decide which cover Structural analysis has now been carried out "belongs" to which basement, since criteria for over a large part of the western Lepontine determining whether a given contact is a de- Alps, of which the Simplon is the western flank, formed stratigraphic boundary or an early and a complicated deformational history is slide are generally absent (with the exception emerging (Higgins, 1964; Hall, 1972; Milnes, of the contact at loc. X mentioned above). The 1973). In the region o£ the Simplon tunnel, uniform signatures, however, should not belie the structural history can be summarized as the petrographic complexity of the units. follows, starting with the youngest event: One important unit, the Lebendun nappe, 5. Faulting (Veglia and Simplon-Centovalli falls outside the above generalizations, or rather fault zones) shows cover in some places, and in other places, 4. Local folding (Rebbio fold zone) basement characteristics. For this reason, the 3. Regional isoclinal folding (Monte Leone Lebendum is referred to here as "transitional." and Wandfluhhorn fold systems) It is made up of two lithologic members which 2. Basement nappe emplacement (Berisal, are not found in the other two complexes: Monte Leone, and Antigorio nappes) gray-weathering psephitic and psammitic 1. Early thrusting (Lebendun nappe) gneiss, and brown-weathering, heterogeneous, This sequence of events has been built up noncalcareous to calcite-bearing schist (known mainly on the basis of detailed surface mapping as "scisti bruni"). The brown schist presents a and on qualitative observation of the type, areal real problem where it occurs in direct contact distribution, and relative ages of minor struc- or intercalated with the calcareous schist of the tures and strain markers, and to a much lesser cover. Features used to distinguish it are the extent on the geometric-statistical treatment of occurrence of thin conglomeratic horizons, the orientation data. development of disorientated biotite por- phyroblasts, and the low content or absence of Veglia and Simplon-Centovalli Fault Zones carbonate (according to unpublished data from The Simplon-Centovalli fault zone, separat- Schardt, the mean CaCC>3 content of the "scisti ing the Simplon nappe complex (lower Pen- bruni" in the Simplon tunnel was 7 percent nine) from the upper Pennine St. Bernard [204 analyses] whereas that of the adjacent nappe (Fig. 1), does not intersect the tunnel cover calcareous schists was 52 percent [97 trace and has been discussed elsewhere (Bearth, analyses]). In spite of some recent advances in 1956, p. 280; Hunziker, 1970, p. 158), so it will this direction (for instance, Friz, 1963; Joos, not be treated further here. The Veglia fault 1969, p. 286), stratigraphy in the Lebendun zone, however, intersects the tunnel

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