A. G. MILNES Geological Institute, Swiss Federal Institute of Technology, Zurich,

Structural Reinterpretation of the Classic Simplon Section of the Central

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 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 (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 , 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 (in part after Bearth, 1972). east of the Simplon Pass, , 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

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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 (Fig. 2) and cover rocks remains speculative. I think the and is conspicuously absent from the traditional geologic column on Table 1 can be used as a profiles. The block to the north is downthrown working hypothesis. relative to that to the south, but some lateral

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movement component must probably be as- cross section (Fig;. 2, ni3). The moderate to sumed. It is strange that this structure was not steep northward dip of the axial planes, in- recognized by the early workers since it pcsed dicated also by the minor fold measurements one of the main obstacles to the completion of (see Milnes, 1968, Fig. 4a), fits well with a zone the tunnel excavation. It completely halted of intense folding in the tunnel 8 to 9 km from penetration from the north side, due to the the Brig entrance as noted by Schardt, the extremely fractured state of the rocks and to official tunnel geologist (Schardt, 1898-1906). the numerous hot springs (temperatures 30° to 50°C, initial average total emission 300 Monte Leone and Wanclfluhhorn 1/sec), and retarded the completion of the Fold Systems project by a whole year (Schmidt, 1908, p. 94- One of the most interesting results of recent 103). Recent borings in Alpe Veglia, related to work has been the recognition that a phase of a proposed and now abandoned hydroelectric major isoclinal folding affected the basement scheme, revealed chaotic relations inside the nappe pile after it had been formed and that zone unrelated to the geology on either side. this is largely responsible for the regional struc- One core over 150 m long consisted entirely of ture as now exposed. The spectacular fold on fault breccia (U. Ragni, 1971, personal the 1,000-m-high east face of Monte Leone commun.). (Fig. 3) lias long been admired, but never con- sidered as more than an isolated problematic Rebbio Fold Zone feature. It is now seen to be a minor fold Another major structure which is absent (axial trace mj, Fig. 2) on the limbs of a huge from or misinterpreted in the traditional pro- isoclinal structure (axial trace M) which has files is the antiform-synform pair known as the been traced for many kilometers northeast- Rebbio fold zone (Milnes, 1968). Minor folds ward from the Simplon area (Milnes, 1973). of this zone deform the main foliation in the The same structure can also be identified in the basement gneiss, but no new axial planar southeast (axial trace W, Fig. 2) and has been structure is developed. In places, the folds also followed again northeastward to the Wandfluh- deform an earlier aggregate elongation linea- horn fold hinge, where the Antigorio and tion, although generally they are coaxial with Monte Leone nappes are folded together (Hun- it. The deformation of the axial plane of one of ziker, 1966, p. 497-501; Hall, 1972). Some the major Monte Leone folds by the Rebbio other folds of the same generation have also folds is well defined on the Simplon tunnel been identified (mi and. mj in the northwest,

MONTE LEONE

Figure 3. View of the east face of Monte Leone, (in the core of the fold) are shaded black; in nature showing the post-nappe isoclinal fold mj (looking west, they form a prominent brown horizon on the pre- down the plunge of the fold axis). The cover rocks dominantly gray-weathering cliff face. Heights in separating the Monte Leone basement from the Bsrisal meters.

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wi in the southeast, Fig. 2). Of these, the Berisal, Monte Leone, and Antigorio Nappes Crevola fold (wi) is particularly interesting The phase of regional isoclinal folding is not since it is well exposed and easily accessible from yet well enough understood to enable an ac- the main Simplon road, the cover rocks in its curate reconstruction of the basement nappes core providing the decorative marble from the as they were after their initial emplacement. Crevola quarries (compare Cinque, 1939; This is particularly true of the Berisal and Trommsdorff and Wenk, 1965). Monte Leone units (the core and upper limb of The regional structure is that of a recumbent the regional isocline), although in the regions of synform facing southwest (facing here refers constrictional deformation earlier structures to the top and bottom of a structural succession, are still preserved. Also in the less strongly a previously existing pile of basement sheets), deformed Antigorio nappe, it is difficult to the axial plane represented now by traces M separate the detailed effects of the two phases— and W. The major fold axes vary southeast- the nappe-forming and the subsequent iso- trending to east-trending and are generally clinal folding. Analysis of minor structures curved (Fig. 1; Milnes, 1968, Fig. 1—"edge of within the nappe (Milnes, 1964) and petro- Monte Leone granite gneiss"). The units now fabric analysis of quartz (Milnes, 1970) both lying above the axial plane of this synform are indicated the development of an early foliation thus structurally inverted, implying that: (1) (possibly during the nappe-forming move- The upper surface of the traditional Monte ments) followed by a phase of deformation with Leone nappe is really part of the base of the the compressive axis normal to it (possibly nappe, now upside-down; (2) the Berisal base- during deformation on the limb of the isoclinal ment nappe was structurally lower than the fold). However, these seem to be related to Monte Leone in the original nappe pile; and (3) the structurally highest levels in the changes in conditions in the rock (from Simplon structure are to be sought in the little- penetrative to nonpenetrative deformation in know areas to the south and southwest, where the case of the minor structures, from cata- the axial traces M and W impinge against the clastic flow to syntectonic recrystallization in Simplon-Centovalli fault. the case of the quartz), rather than to two quite separate deformative phases. This region In general, the deformation during this phase is probably not favorable for studying the of isoclinal folding seems to have been of mechanism of basement nappe formation, as is flattening type, with the development of a being attempted elsewhere (see Elliott, 1970). penetrative foliation parallel to the axial plane of the folds, particularly in the basement Lebendun Nappe rocks. This means that good fold hinges are few The earliest event in the Simplon region and far between, and that the axial planes have seems to have been the overthrusting of the to be traced often for long distances parallel to Lebendun nappe. Its basal plane is everywhere the foliation in rather homogeneous gneisses. an extremely sharp contact and is markedly dis- Where good major fold hinges are visible, the cordant to lithologic units within the nappe rocks are generally B-tectonites with a strong (Milnes, 1964, Fig. 5; Joos, 1969, Fig. 6), and it elongation of mineral aggregates parallel to probably represents a thrust plane or slide. The major and minor fold axes. Such fold hinges tunnel profile supports what the surface find- seem to have been preserved partly because the ings suggested, that this thrust plane, and the deformation was of constrictional type and thus Lebendun nappe as a whole, became wrapped a new foliation failed to develop. All gradations round the frontal part of the Antigorio nappe between pure constriction and pure flattening during the latter's emplacement (Fig. 2). The exist (Milnes, 1968), and in intermediate states recent controversy concerning the "origin" of the new foliation may have been only partially the Lebendun or the direction of movement on or imperfectly developed. Such a situation is this early thrust plane (compare Joos, 1967), quite apparent in the low glaciated hills east of however, will remain unresolved until the Crevola. The rocks were obviously extremely succeeding structural events are understood far ductile during this phase of deformation and better than they are at present. much of the strain now visible in the rocks must have occurred at this time, and not, as ACKNOWLEDGMENTS previously assumed (Milnes, 1968), during the My special thanks are due to P. Bearth for nappe-forrjiing movements. his constant interest in my work over many

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years, and for numerous discussions and critical 1969, Zur Geologie und Pétrographie der comments. The present synthesis of work in this Monte Giove-Gebirgsgruppe im östlichen area was made possible by a research grant from Simplon-Gebiet (, Italia): Schweizer. the Schweiz. Nationalfonds. Mir.eralog. u. Petrog. Mitt., v. 49, p. 277-322. Milnes, A. G., 1964, Structure and history of the REFERENCES CITED Antigorio nappe (Simplon group, north Italy) [Ph.D. thesis]: Basel, Universität Basel, 90 p. Badgley, P. C., 1965, Structural and tectonic 1968, Strain analysis of the basement nappes principles: New York, Harperand Row, 521 p. in the Simolon region, northern Italy: Bearth, P., 1956, Geologische Beobachtungen im Internat. Geôl. Cong., 23d, Prague 1968, Grenzgebiet der lepontinischen und pen- Rept., v. 3, p. 61-76. ninischen Alpen: Ecologae Geol. Helvetiae, 1970, Petrofhbric analysis of quartz in the v. 49, p. 279-290. Central Alps—A progress report [Habil. • 1972, Geologischer Atlas der Schweiz: thesis]: Zürich, Eidg. Technische Hochschule, 1:25,000. Simplon sheet: Basel, Schweizer. 41 p. Geol. Komm, (in press). 1973, Post-nappe folding in the western Bearth, P., Nabholz, W. K., Streckeisen, A., and Lepontine Alps, Italy: Ecologae Geol. Hel- Wenk, E., 1967, Exkursion Nr. 20. Simeon- vetiae (in press). pass: Brig-: Geol. Führer der Ragni, U., 1960, Studio geologico-petrografico dei Schweiz, v. 5, p. 336-350. complessi metamoriici della zona di Alpe Cadisch, J., 1953, Geologie der Schweizer Alpen: Veglia (Massiccio de:l Sempione, Alpi Lepon- Basel, Wepf, 480 p. tine) [Ph.D. thesis]: Milano, Università Cinque, F., 1939, Il "Marmo di Crevola" ed i suoi Degli Studi, 720 p. minerali: Soc. Italiana Sci. Nat. Atti., v. 78, Schardt, H., 1904, Die wissenschaftlichen Ergeb- p. 204-223. nisse des Simplondurschstichs: Wintherthur, Elliott, D., 1970, The flow of basement into nappes Schweizer. Naturi. Gesell. Verh., 39 p. [abs.]: Am. Geophys. Union Trans., v. 51, p. 1898-1906, .Rapports trimestriels au Conseil 431. Fédéral Suisse sur l'état des travaux du Friz, C., 1963, La serie stratigrafica del ricoprimento percement du Simplon. Renseignements penninico inferiore del Lebendun in alta Val géologiques: , Rept. 17-19. Formazza (Alpi Lepontine): Mem. Acc. Schmidt, C., 1908, Die Geologie des Simplonge- Patavina, Sc. Lett, ed Arti, v. 75, no. 2, p. birges und des Simplontunnels: Basel, Rein- 319-333. hardt, 108 p. Hall, W.D.M., 1972, The structural geology and Schmidt, C., and Preisw:rk, H., 1908, Erläuterun metamorphic history of the Lower Pennine gen zur geologischen Karte der Simplongruppe nappes, Valle di Bosco, Ticino, Switzerland 1:50,000: Bern, Schweizer. Geol. Komm., 72 [Ph.D. thesis]: London, Imperial College, 220 P- P- Stella, A., 1906, Cenni geologici sulle nuove linee di Heim, A., 1921, Geologie der Schweiz. Band 2. Die accesso al Sempione: Roma, Soc. Italiana per le Schweizer Alpen: Leipzig, Tauchnitz, 1C18 p. Strade Ferrate del Mediterraneo, Serv. delle Higgins, A. K., 1964, The structural and meta- Costruzioni, 42 p. morphic geology of the area between Nufenen- Trommsdorff, V., and Wenk, H.-R., 1965, Die pass and Basodino, Tessin, S. Switzerland Regelung des Dolomites von Crevola (Sim- [Ph.D. thesis]: London, Imperial College, plon-Gruppe). Ergebnisse und Probleme: 260 p. Schweizer. Minerolog. u. Petrog. Mitt., v. 45, Hunziker, J. C., 1966, Zur Geologie und Geo- p. 551-569. chemie des Gebietes zwischen Valle Antigorio Wieland, H., 1966, Zur Geologie und Petrographie (Provincia di Novara) und Valle di Campo der Valle Isorno (Novara, Italia): Schweizer. (Kt. Tessin) : Schweizer. Mineralog. u. Petrog. Mineralog. u. Petrog. Mitt., v. 46, p. 189-303. Min., v. 46, p. 473-552. 1970, Polymetamorphism in the Monte Rosa, western Alps: Ecologae. Geol. Helvetiae, v. 63, p. 151-161. Joos, M. G., 1967, Ueber die Herkunft der Leben- MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY dunserie: Ecologae Geol. Helvetiae, v. 60, p. 7, 1972 151-155. REVISED MANUSCRIPT RECEIVED JUNE 6, 1972

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