Geology of the Alps Part 2: The Penninic Nappes
Lecture notes by Nikolaus Froitzheim
The Penninic superunit is subdivided into the Upper, Middle, and Lower Penninic Nappes and the Subpenninic Nappes. The Upper Penninic Nappes originate from the area of the Piemont-Ligurian Ocean, the Middle Penninic Nappes from the Briançonnais spur of the Iberian continent, and the Lower Penninic Nappes from the Valais Ocean. The Subpenninic Nappes come from the distal part of the European continental margin, close to the Valais Ocean. Therefore the Penninic Nappes are partly of oceanic, partly of continental origin. Metamorphosed Penninic units occur in the internal parts of the Alps and unmetamorphic ones in the external part, close to the front of the Alps. The former were buried in the course of southward subduction and accreted at the base of the Alpine orogenic wedge (underplating) whereas the latter were already detached close to the surface and accreted to the tip of the orogenic wedge (frontal accretion). In general, the more northerly derived nappes were thrust under the more southerly derived ones, so that the rule is “the higher, the more from the south”. This arrangement, however, was disturbed and partly inverted by nappe refolding (particularly in the internal part) and by out-of-sequence thrusting” (particularly in the external part). Therefore, the paleogeographic restoration is in some cases still unclear or controversial.
Chapter 6: The Upper Penninic nappes in the Pennine (Valais) Alps
The Upper Penninic Nappes comprise oceanic crust and its sediment cover from the Piemont-Ligurian Ocean, also known as Alpine Tethys. In addition, they comprise some nappes built of continental crust: the Sesia Nappe (=Sesia-Lanzo Zone) and the Dent Blanche Nappe in the western Central Alps and Western Alps, and the Margna Nappe in the Eastern Alps. These three nappes originate from a continental fragment or microcontinent in the Piemont-Ligurin Ocean, termed “Cervinia” or “Margna-Sesia fragment”. E. Argand, the pioneer of geologic research in the Pennine Nappes, originally called the Sesia and the Dent Blanche nappes Penninic, and R. Staub parallelized them with the Upper Penninic Margna Nappe in Grisons. Later, the same R. Staub changed his mind, erroneously correlated Sesia and Dent Blanche with the Lower Austroalpine Err and Bernina Nappes, and hence called them Austroalpine. This affiliation has persisted in large parts of the literature up to the present day, and has even become popular science knowledge (“Le Cervin - est-il africain?”). The Margna Nappe in Grisons is, however, overlain by ophiolites of the Platta Nappe, and ophiolite slivers also occur structurally above the Sesia Nappe, along the Periadriatic Line. As the boundary between Penninic and Austroalpine is per definition drawn above the uppermost and most southerly-derived Piemont-Ligurian ophiolites, we treat Sesia, Dent Blanche and Margna as Penninic Nappes. Hence, we distinguish continental and oceanic Upper Penninic Nappes.
In the classical Pennine Alps (Valais Alps), the type area of the Penninic superunit, the Upper Penninic Nappes are represented by the oceanic units Zermatt-Saas Nappe and Tsaté Nappe, by the continental units of the Dent Blanche and Sesia nappes, as well as some minor continental slivers (e.g. Etirol-Levaz sliver) and metasediment sheets (Cimes Blanche Nappe) (Fig. 6-1, 6-2).
The Zermatt-Saas Nappe comprises oceanic crust (serpentinite, gabbro, pillow basalt) and its former cover of radiolarite and calcareous to pelitic sediments, with Alpine eclogite-facies metamorphism (Fig. 6-3). The metamorphism reached ultrahigh-pressure conditions in the area of Lago di Cignana in Valtournenche; coesite and microdiamond were found there. The age of eclogite-facies metamorphism is between 50 and 40 Ma, that is, Eocene. The intrusion of gabbros and thus the formation of oceanic crust has been dated at ca. 160 Ma, Middle Jurassic. Slivers of continental crust which are found along the top of the Zermatt-Saas Zone and are lithologically similar to rocks of the Sesia Nappe (Etirol-Levaz sliver and others) were subducted and metamorphosed together with the ophiolites in the Eocene; they may be interpreted as extensional allochthons, that is, they were already detached from the continental margin during the continental break-up in the Jurassic by low-angle normal faults. If so, the Zermatt-Saas ophiolites, at least partly, represent a former ocean-continent transition zone.
The Tsaté Nappe, the second and structurally higher oceanic unit in the Upper Penninic Nappes of the type area, comprises similar rocks as the Zermatt-Saas Nappe: Serpentinite, gabbro, basalt, and a sedimentary cover with radiolarite at the base, followed by thick calcschists. Just like the Zermatt-Saas Nappe, it has been strongly disrupted and sheared, so that the original geometric relations have hardly been preserved. In contrast to the eclogite-facies, partly ultrahigh-pressure metamorphic Zermatt-Saas Nappe (25-30 kbar), however, the Tsaté Napope only records pressures of 13 to 18 kbar at maximum. Therefore, about 12 kbar are “missing” along the Zermatt-Saas/Tsaté boundary.
Intercalated between the Zermatt-Saas and Tsaté nappes isthe Cimes Blanches Nappe, a thin sheet mainly comprising quartzite (Lower Triassic) and marble (Triassic, Jurassic) (Fig. 6-4, 6-5). The metamorphic grade of this nappe is similar to the one of the Tsaté Nappe, in any case considerably lower than for the Zermatt-Saas Nappe. The Tsaté Nappe, the Cimes Blanches Nappe, and the Frilihorn Nappe (another thin sheet of quartzite and marble within the Tsaté Nappe) together form the Combin Zone. Age and facies of the sediments in the Cimes Blanches Nappe show that these rocks were deposited on continental, not oceanic crust. North of the Dent Blanche Nappe (Fig. 6-1, 6-2), the Cimes Blanches Nappe rests directly on the Middle-Penninic St.Bernard Nappe, since the Zermatt-Saas Nappe does not occur in this area. The position of the Cimes-Blanche Nappe, sandwiched between the oceanic series of the Zermatt-Saas and Tsaté nappes, may be explained in two ways: either (1) these sediments belonged to the cover of the St.Bernard Nappe (Briançonnais) and were emplaced in their present position by a southeast-directed backthrust or low-angle normal fault, or (2) they were originally part of the cover of the Sesia and Dent Blanche nappes (Cervinia), were detached from these in the course of northwest-directed thrusting of Tsaté over Sesia/Dent Blanche, were thereby emplaced on the Zermatt Saas Nappe and, further northwest, on the St. Bernard Nappe, and were in a second step buried under the Dent Blanche and Sesia nappes when these were thrust “out-of-sequence” towards northwest. At the moment, this second possibility appears more likely as it is better supported by structural observations, in particular, the transport sense of shear zones. It implies that the origin of the Tsaté Nappe was southeast of the Sesia and Dent Blanche Nappes.
The Sesia and Dent Blanche nappes are two parts of a once continuous thrust sheet which has been disrupted by erosion (and possibly normal faulting) in the hinge area of a large antiform, the Vanzone antiform. Both parts comprise three types of pre-Mesozoic basement: (1) gneiss, metabasite and marble with late Variscan granulite-facies metamorphism, that is, former Variscan lower crust (Valpelline series in the Dent Blanche Nappe, Seconda Zona Dioritico-Kinzigitica, or 2DK, in the Sesia Nappe); (2) Variscan upper crustal basement with abundant orthogneisses (e.g., Arolla series in the Dent Blanche Nappe); (3) Permian gabbros, for example at the Matterhorn (Fig. 6-7). In addition, there are thin layers of rocks from the former Mesozoic cover, intensely folded together with the basement. The Alpine metamorphism reached eclogite facies in the Sesia Nappe (at ca. 70 to 65 Ma) and blueschist facies in the Dent Blanche Nappe, followed by widespread greenschist-facies overprint.
Chapter 7: The Middle, Lower and Sub-Penninic nappes in the Pennine Alps
In the Pennine Alps, the Middle Penninic (Briançonnais) is represented by the St.Bernard Nappe and the Lower Penninic (Valais) by the Sion-Courmayeur Zone and the Antrona Ophiolites. The Sion-Courmayeur Zone is characterized by easily erodible calcschists, mainly of Cretaceous age; therefore, the deep Valais Rhone Valley follows this zone. The mountains to the south, incised by the tributary valleys of the Rhone – Saastal, Mattertal, Turtmanntal, Val d’Anniviers, Val d’Hérens and Val de Bagnes – are formed by the less erodible gneisses of the St.Bernard Nappe. The Dom, the highest mountain entirely located in Switzerland (4545 m), is also part of the Bernhard Nappe. The Monte Rosa Nappe, forming the mountains further southeast (Dufourspitze, 4634 m), belongsin my opinion to a deeper, Subpenninic tectonic level (Fig. 7-1).
The St.Bernard Nappe (German: Bernharddecke) is formed by three subordinate sheets (Fig. 7-2). These are, from top to base - in map view from south to north – the Mont Fort Nappe, the Siviez-Mischabel Nappe, and the Zone Houillière Nappe. In addition, the Pontis Nappe, formed by Triassic quartzite and marble, has been identified between the Siviez-Mischabel Nappe and the Zone Houillière Nappe; new investigations have shown, however, that these Triassic rocks belong to the cover of the Zone Houillière Nappe, whereby the Pontis Nappe has become obsolete. Only at the western end of the Pennine Alps, another nappe is sandwiched between Siviez-Mischabel Nappe and Zone Houillière Nappe: the Ruitor Zone, formed by Variscan basement rocks. In an early stage, the partial nappes of the St.Bernard Nappe were recumbent folds with a basement core and normal as well as inverted limbs of Mesozoic cover rocks, that is, fold nappes. This original geometry was later dissected by important shear zones. The contact between the Mont Fort Nappe and the structurally higher, Upper Penninic Tsaté Nappe is isoclinally folded; rocks of the Tsaté Nappe are found in isoclinals synforms inside the Mont Fort Nappe (Fig. 7-2), suggesting that the Tsaté Nappe was thrust over the Briançonnais before the fold nappes formed. This is typical for the structural style of the Penninic Nappes in the Pennine Alps: The original, subduction-related thrusts were folded aound the large fold nappes like the Monte Rosa and St.Bernard nappes when these formed. In a further stage after the formation of the fold nappes, large backfolds formed, most notably the Mischabel Backfold which folded the Siviez-Mischabel Nappe back towards south over the front of the Zermatt-Saas Nappe (Fig. 7-2).
Mont Fort Nappe and Siviez-Mischabel Nappe are formed by Variscan metamorphic basement and post-Variscan cover. The basement includes various gneisses, schists, and amphibolites, partly also Variscan eclogites. In the Siviez-Mischabel Nappe, the basement was intruded by a Permian granite which has been metamorphosed and deformed in the Alpine orogeny (Randa Orthogneiss). The cover comprises Permian schistose clastic metasediments (conglomerate, sandstone, pelite) intercalated with metavolcanites of the same age (rhyolites and other), followed by Lower Triassic quartzite. The most complete Mesozoic succession is found in the normal limb of the Siviez-Mischabel Nappe: The Barrhorn Series (Fig. 7-3, 7-4). Here the quartzites are followed by calcareous schist (“Röt”), calcite marble (Middle Triassic) and dolomite (Upper Triassic). The Liassic is missing, instead Middle Jurassic breccias rest unconformably on Upper Triassic. This hiatus is typical for the Briançonnais nappes of the Western Alps and for the nappes of the Préalpes Médianes Rigides (see chapter 8); the succession in the latter area is almost identical to the Barrhorn Series. The Upper Jurassic is represented by calcite marble, again unconformably overlain by Couches Rouges (pelagic marly limestone) of Late Cretaceous to Early Tertiary age; this is also typical for the Briançonnais. The succession is completed by Eocene flysch. The Barrhorn Series has partly been detached from the basement of the Siviez-Mischabel Nappe and sheared back towards Southeast, so that its southern end forms an isoclinal, southward-closing fold core inside the Tsaté nappe (Fig. 7-3, 7-4).
The Zone Houillière (coal-bearing zone) is the lowermost, most northerly located sub-nappe of the St.Bernard Nappe, copping out on the lower southern slopes of the Rhone Valley. It comprises metamorphosed, coal-bearing, clastic sediments of Late Carboniferous and Permian age, overlain by Triassic quartzite and calcite marbles. East of Val d’Anniviers, the Zone Houillière wedges out; further east, however, similar succession reappear (Lower Stalden Zone). The Alpine metamorphism of the St.Bernard Nappe is of greenschist facies; relics of glaucophane in the Siviez-Mischabel and Mont-Fort nappes indicate an earlier, high-pressure stage (blueschist facies). The Alpine metamorphism cannot be older than Eocene because it affected Eocene sediments of the Barrhorn Series.
At its southeastern end, the St.Bernard Nappe is intensely folded together with the Antrona Ophiolites. The latter comprise serpentinite, metabasalt, and metagabbro (with eclogite relics) as well as their former sedimentary cover of calcschists. U-Pb dating of zircon from metagabbros yielded a Jurassic protolith age, similar to the ophiolites of the Zermatt-Saas Nappe. Originating in the main outcrop area of the Antrona Ophiolites, a thin layer of these rocks, sandwiched between gneisses, extends first towards South and then, bent around the hinge of the southwest-plunging Vanzone Antiform, towards Northeast. This ophiolite layer is not the “root” of the Antrona Ophiolites, but a narrow synformal fold which has been termed “Antrona-Mulde” long ago (“Antrona synf.” In Fig. 7-2). Another layer of ophiolites can be followed towards southwest; steeply northwest-dipping, it separates the Monte Rosa Nappe to the Southeast from the Portjengrat and Stockhorn units to the Northwest (Fig. 7-5, 7-6). These two units consist of gneiss with a Mesozoic metasediment cover and are treated as parts of the St.Bernard Nappe. The boundary zone between the Monte Rosa Nappe and the Portjengrat and Stockhorn units, known as the Furgg Zone, is characterized by a strong shearing deformation; in addition to the above-mentioned ophiuolites, it comprises meta-arkoses (Permian), quartzite and marble (Mesozoic), as well as a typical association of amphibolite boudins (with eclogite relics) and leucocratic gneisses, probably representing strongly deformed Variscan basement; for one of the amphibolite/eclogite boudins, an Early Paleozoic protolith age was determined. The thin layer of ophiolites extends, locally interrupted, around the southwestern end of the Monte Rosa Nappe to the southern flank of the Monte Rosa massif. The Stockhorn Unit wedges out towards southwest so that there, the ophiolite layer is directly overlain by the ophiolites of the Zermatt-Saas Nappe and the two can hardly be distinguished. On the southern side of the massif, a thin layer of gneiss (Stolemberg Unit) occurs again between the ophiolite layer (termed Balma Unit) and the Zermatt-Saas Ophiolites. The Stolemberg Unit is interpreted as representing the St.Bernard Nappe (Fig. 7-7). Eclogites of the Balma Unit yielded an age of 93 Ma for the gabbroic protolith (U-Pb zircon) and an age of ca. 45 to 42 Ma (Lu-Hf garnet) for the eclogite metamorphism. The protolith age is the same as in the Chiavenna Ophiolite at the western end of the Eastern Alps; both together represent the youngest remnants of oceanic spreading in the Alps. Antrona, Balma, and Chiavenna ophiolites are interpreted to represent oceanic crust of the Valais Ocean; the suture of this ocean therefore roots south of the Monte Rosa Nappe. If this is true, the Monte Rosa Nappe, being structurally deeper than the suture, originates from the northern margin of the Valais Ocean, that is, from the European continental margin (Subpenninic).
To the Northeast, the Antrona Ophiolites abut against the Simplon Fault, an important, southwest-dipping, Miocene-age normal fault comprising thick mylonites in the footwall (northeast), topped by a layer of cataclasite (Fig. 7-1, 7-2). Further north, units from the Valais Oceanbasin reappear on both sides of the Simplon Fault (Sion-Courmayeur Zone). The Antrona Ophiolites and the Sion-Courmayeur Zone were probably connected originally, representing the Valais Ocean suture, and were disrupted by the displacement along the Simplon Fault.
The Sion-Courmayeur Zone consists of several individual thrust sheets formed by flysch-type schists and mélanges of Cretaceous and Early Tertiary age. Foraminifera of late Middle Eocene age were found in schist of the Pierre-Avoi Unit. The mélanges, which probably represent sedimentary breccias and mega-breccias (debris flows), contain clasts of gneiss, Mesozoic sediments, basalt, gabbro, and serpentinite.
The Monte Rosa Nappe (Subpenninic) comprises Variscan (Carboniferous-age) and post-Variscan (Permian-age) granitoids intruded into an older, amphibolites-facies metamorphic to migmatitic basement. This basement only locally carries relics of a Permian to Lower Triassic sediment cover of meta-arkoses and –sandstones. The Monte Rosa NAppe was affected by Alpine (Eocene) eclogite-facies metamorphism; eclogites occur mainly as boudins in the structurally highest parts of the nappe. During exhumation, the rocks were overprinted first by amphibolite-, then greenschist-facies metamorphism. In spite of its somewhat bizarre “bird’s head” shape in cross-section, the Monte Rosa Nappe as a whole represents a large antiformal fold (Ragno Antiform), around which the higher tectonic units have been folded. The Ragno fold was deformed by younger folds, of which the Vanzone Antiform is most obvious (Fig. 7-2). A narrow “neck” of the Monte Rosa Nappe can be followed towards Northeast to Ascona (Fig. 9-1); there, the nappe disappears in the subsurface, due to the axial plunge of the Ragno Antiform.
Chapter 8: The Préalpes and the klippen of Central Switzerland
The Preálpes represent a large outlier (klippe) of Penninic nappes near the northern front of the Alps, resting in an allochthonous position on the external zone (Fig. 1-3). The Préalpes are laterally subdivided into the Préalpes du Chablais, located west of the Rhone Valley mainly in France, and the Préalpes Romandes, east of the Rhone Valley in western Switzerland. East of the eastern end of the Préalpes, some smaller, isolated klippen occur in Central Switzerland, of which the Mythen klippe is most famous. Also towards the Southwest, the chain of klippen is continued by some smaller klippen in the French Western Alps (Savoye). The most complete succession of the Penninic nappes is exposed in the Préalpes Romandes (Fig. 8-1).
Some remarks about flysch
The term “flysch” will play an important role in this chapter. Flysch means clastic successions with a high proportion of turbidites, deposited at great water depth along convergent plate boundaries. In the Alps, flysch deposits are generally Cretaceous or Tertiary in age (except for the Upper Carboniferous Hochwipfel Formation in the Carnic Alps which is related to the Variscan Orogeny). The flysch successions were partly deposited on oceanic crust during the subduction of an oceanic plate, partly on thinned continental crust during ongoing subduction of a continental margin after the beginning of continent collision. Depending on composition of the source area, flysches in the Alps are predominantly carbonatic or quartz-dominated (siliciclastic). Flysch with a high proportion of chromium spinel shows that ophiolites cropped out in the source area because chromium spinel is a frequent component of ultrabasic rocks like serpentinite. When flysch is deposited along a convergent plate boundary, the turbidites are normally shed from the slope of the upper plate and accumulate on the lower plate (Fig. 8-2, profile A). The youngest parts of a flysch succession frequently contain “exotic” blocks and slivers of older rocks; such a deposit is termed “wildflysch”. The reason is that the deposition area of the flysch has moved, piggy-back on the downgoing plate, so close to the source area that slides and debris flows from the continental slope can reach it (Fig. 8-2, profile B). After this stage, the sedimentation soon ends because the flysch is thrust under the front of the accretionary wedge (profile C). During this underthrusting, at least the upper parts of the flysch unit are strongly sheared, so that it often becomes difficult to distinguish in the “wildflysch” between sedimented slide blocks and tectonic mixture (tectonic mélanges). When the flysch is detached from its underlying crust, it forms a flysch nappe in the accretionary wedge. The accretionary wedge continues to grow by accretion of more units, whereby the flysch nappe under consideration moves up in the wedge (profile D). Parts of it may slide back into the trench (profile E). In this way, older flysch may be “recycled” in younger flysch. Affiliation of flysch units to their original deposition area is often difficult and questionable, also in the case of some flysch units in the Préalpes.
Sequence of nappes in the Préalpes Romandes
The base of the Préalpes Romandes is formed by the Ultrahelvetic Nappes which in turn rest on the Molasse to the Northwest and on the Helvetic Nappes to the Southeast. The Ultrahelvetic units will be described later; for the moment we only mention that large parts of the Ultrahelvetic are claystones of Jurassic and Cretaceous age; these incompetent rocks form a décollement horizon on which the Penninic Nappes were thrust towards north. To the Southeast, the lowermost Penninic nappe of the Préalpes Romandes is the Niesen Nappe (Fig. 8-1, 8-3). It comprises small slivers of gneiss at the base, overlain by only locally preserved Triassic, Jurassic and Cretaceous strata, and an up to 1.5 km thick flysch succession, the Niesen Flysch. It rests unconformably on the older formations and is Maastrichtian to Eocene (Lutetian) in age (Fig. 8-4). The Niesen Flysch was shed from the South; the unconformity at its base testifies of a pre-Maastrichtian tectonic phase. The flysch was probably deposited at the northern margin of the Valais Ocean; the Monte Rosa Nappe may represent the former basement under the Niesen Flysch.
At the NW border of the Préalpes, the Gurnigel Nappe rests on the Ultrahelvetic and Molasse units, in a similar structural position as the Niesen Nappe to the SE (Fig. 8-3). It consists of Maastrichtian to Middle Eocene flysch. Further east in central Switzerland, the Schlieren Flysch represents the prolongation of the Gurnigel Nappe. Both nappes, together with the Wägital Flysch still farther east, are also termed “Subalpine flysches”. Their paleogeographic origin is dubious. Since flysch units similar to the Gurnigel Flysch occur also in a higher tectonic position above the Brianconnais-derived Préalpes Médianes Nappe (Saane Nappe; see below), Gurnigel and Schlieren flysch are often interpreted to be derived from the Piemont-Ligurian Ocean. If this is correct, their present-day position below the Préalpes Médianes Nappes would have to be explained by an out-of-sequence thrust at the base of that nappe (cf. Fig. 1-8). On the other hand, an origin of Gurnigel and Schlieren Flysch from the Valais Ocean basin is also possible; this would not require out-of-sequence thrusting.
In the internal, southeastern part of the Préalpes, the Zone Submédiane rests on the Niesen Nappe or, where the latter is lacking, directly on the Ultrahelvetic. This is not a coherent succession of rocks but rather a tectonic mixture (mélange) of different rocks, often evaporites (gypsum, anhydrite, cargneule) of Middle and Late Triassic age, which formed the décollement horizon of the overlying Préalpes Medianes Nappes and were mixed with material of the underlying nappes during northward thrusting of the Préalpes Médianes Nappe (shown in orange in Fig. 8-3). The Zone Submédiane also comprises slate, radiolarian-rich limestones, and basalt, i.e., indicators of oceanic crust. It is therefore interpreted as the suture of the Valais Ocean. Above follows the Préalpes Médianes Nappe (=Klippen Nappe). This unit is derived from the continental crust of the Brianconnais spur. The sediment succession reaches from Lower Triassic to Eocene. The Préalpes Médianes Nappe is subdivided into Médianes Plastiques to the NW and Médianes Rigides to the SE. The Médianes Plastiques are folded whereas the Rigides are not, but rather represent “rigid” sheets bounded by thrust faults. This results from the stratigraphy of both parts: The deposition area of the Médianes Rigides was a high during the Jurassic and the Cretaceous whereas the northwestern part of the deposition area of the Plastiques was a basin, the “Cancellophycus Basin”, named after the Cancellophycus Beds, basinal marls of Middle Jurassic age. On the Rigides high, carbonates were deposited during phases of subsidence but non-deposition or erosion prevailed in the absence of subsidence. Therefore, the sediment succession of the Rigides is almost exclusively formed by carbonates and lacks the alternation of competent and incompetent layers which is necessary for the formation of folds. The largest hiatus in the Rigides spans Late Triassic to Middle Jurassic. In the Late Liassic the area was uplifted above sea level and deeply eroded, so that terrestrial to shallow marine strata of Late Middle Jurassic age (Mytilus Beds) lie unconformably on Middle Triassic Limestone and Dolomite. In contrast, sedimentation in the Plastiques basin was almost continuous and resulted in a succession of carbonatic and marly sediments which favoured folded. To the Northwest, another high bordered the “Cancellophycus Basin”. This high is only relictic in the Préalpes but is better preserved in the Central Swiss klippen (Mythen Klippe).
In the French Western Alps, the Préalpes Médianes Rigides find their prolongation in the Briançonnais nappes sensu stricto, the Préalpes Médianes Plastiques in the Subbriançonnais which is structurally deeper and located more to the West. The Gastlosen chain (Fig. 8-1, 8-5) forms the northwesternmost part of the Rigides, located between two deep post-nappe synforms filled by higher tectonic units. The Préalpes Médianes Rigides display the same facies as the Barrhorn Series of the Siviez-Mischabel Nappe in the Pennine Alps (see chapter 7); therefore they are interpreted as detached parts of the sedimentary cover of this nappe. In a similar way, the Préalpes Médianes Plastiques are correlated with the Zone Houillière in the Pennine Alps.
In the southeastern part of the Préalpes, the Breccia Nappe rests on the Préalpes Médianes (Rigides) Nappe. It comprises Upper Triassic (dolomite, slate, coquinitic limestone), Liassic limestone and slate, a lower succession of sedimentary breccias with components of Triassic dolomite and Liassic limestone (Upper Liassic and Dogger?), slate with radiolarite layers (Lower Malm), an upper brecia succession (Upper Malm?), cherty limestones (Lower Cretaceous), slate with quartzite layers (Aptian-Albian), Couches Rouges, i.e. pelagic marls (Late Cretaceous to Eocene), and “wildflysch”. The Jurassic breccias were shed from fault scarps located to the Northwest. These may have been related to normal faults at the continental margin between the Briançonnais (NW) and the Piemont-Liguria Ocean (SE).
The Nappe Supérieure, or Simme Nappe sensu lato, overlies the Breccia Nappe or, where the latter is absent, directly the Préalpes Médianes Nappe (Fig. 8-3, 8-5). It is composed of four minor nappes, from base to top: the Saane Nappe, the Dranses Nappe, the Simme Nappe sensu stricto, and the Gets Nappe. These are partly made up of sub-nappes themselves. The Saane Nappe (Nappe de la Sarine) comprises slivers of Maastrichtian and Paleocene flysch similar to the Gurnigel Nappe; this is the reason why the Gurnigel Nappe is assumed to originate from the South Penninic (Piemont-Ligurian) domain. The Dranses Nappe consists of red shale, followed by calcareous flysch of Late Cretaceous age (Campanian and Maastrichtian), similar to the Helminthoides Flysch of the Western Alps (see chapter 13) and containing the same characteristic trace fossils (Helminthoides). The Simme Nappe (sensu stricto) consists of two parts, separated by a thrust. The lower part comprises shaly-sandy successions of Albian age, sandstone of Turonian age, and a pelitic complex containing olistoliths of cherty limestone (Dogger), radiolarite (Malm), and pelagic calpionella limestone (Upper Malm to Lower Cretaceous). The upper part is Late Cretaceous flysch (Cenomanian and younger) with characteristic, polymictic conglomerate layers (Mocausa Conglomerate). The Gets Nappe comprises slate and micritic limestone at the base (Upper Malm and Lower Cretaceous), followed by sandy flysch (Late Cretaceous), an olistostrome with components of basalt, gabbro, and granite, and finally the Late Cretaceous Hunsrück Flysch (Coniacian to Campanian). The gabbro was dated at ca. 166 Ma (Dogger). Saane, Dranses, Simme, and Gets Nappe are assumed to originate in the Piemont-Ligurian Ocean. In the case of the Simme Nappe, conglomerate pebbles and olistoliths show strong similarities with successions of the Southern Alps.
The klippen of Central Switzerland
Several small outliers of Penninic nappes in Central Switzerland represent the eastern prolongation of the Préalpes Romandes: the klippen of the Giswiler Stöcke, the Stanserhorn, the Buochserhorn, the Mythen, and, at the eastern end, the Iberg Klippen. The Klippen Nappe is exposed in synclines of the underlying Helvetic Nappes and exhibits partly the facies of the Préalpes Plastiques Rigides, partly the one of the Plastiques. The most eye-catching and popular klippe, due to its remarkable morphology, is the Mythen massif (Fig. 8-6). It probably originates from a high delimiting the Préalpes Médianes Basin to the Northwest. The Klippen Nappe at the Mythen is made up of thin Triassic and Dogger but mainly of massive Malm limestone, unconformably overlain by Upper Cretaceous pelagic marls of the Couches Rouges. In the nineteenth century, the fact that a mountain consisting of Jurassic limestone is rising above Cretaceous sediments led to the interpretation that the mountain must have stood out as a cliff from the Cretaceous sea where the marls were deposited. Later, the Jurassic limestones turned out to be relics of an eroded nappe, ‘’swimming’’ on the Cretaceous sediments. Nevertheless, the term klippe is still used since then, now meaning an isolated nappe remnant in a tectonic sense.
The most complete succession of nappes can be found at the Iberg Klippen where not only the Middle Penninic Klippen Nappe is exposed but also the ophiolite-bearing, Upper Penninic Arosa Zone and, as the uppermost unit, a remnant of the Austroalpine Northern Calcareous Alps (Fig. 8-7). The base of the section in the area is formed by Cretaceous to Tertiary sediments of the Helvetic Drusberg Nappe. Above follows a flysch complex (“S” in Fig. 8-7). The lower part of this flysch complex is the Upper Cretaceous to Eocene Iberg Wildflysch which contains components of basement and sedimentary rocks. The latter can be traced back to a Briançonnais-type sediment succession. The Wildflysch is overlain by Eocene-age Schlieren Flysch, followed at the top by Late Cretaceous Schlieren Flysch. As mentioned above, a South Penninic origin is often assumed for both the Schlieren Flysch, whose main outcrop area is further west, and for the Gurnigel Nappe. This assumption, however, is challenged by the fact that the Iberg Wildflysch contains Briançonnais components and is situated beneath the Schlieren Flysch. Therefore, Rudolf Trümpy suggested an origin from the North Penninic Valais Basin for the Schlieren Flysch and Gurnigel Nappe (Eclogae geologicae Helvetiae, 2006). The Klippen Nappe, overlying the flysch complex, mainly comprises Malm limestone and Couches Rouges. Above follows the Arosa Zone, consisting mainly of pillow basalts with radiolarite cover, typical for the Piemont-Ligurian Ocean, tectonically mixed with Austroalpine Triassic rocks. The uppermost nappe is made up of Austroalpine Mesozoic units, mainly Raibl Group (Carnian) and Hauptdolomit (Norian). This nappe is presumably the western prolongation of the Lechtal Nappe and hence of the Northern Calcareous Alps.
Chapter 9: The Lepontine Nappes
The Lepontine Nappes belong to the Subpenninic units and are exposed in a dome-like structure, the Lepontine Dome in the Central Alps. The outcrop area of the Lepontine Nappes is separated to the North from the Gotthard Massif and its sedimentary cover by the Penninic Basal Thrust (Fig. 9-1: PBÜ). The steep orientation of the thrust front is caused by post-thrusting deformation of the nappe pile. This area is called Northern Steep Belt. In the South, the Lepontine Nappes are bounded by the Insubric Line, a section of the Periadriatic Line (Fig. 9-2). In this area, the Insubric Line has a steep northward dip. Most activity of the fault took place in Oligocene times, when the movement was first a relative uplift of the northern block, i.e. the Lepontine nappe pile, followed by dextral strike-slip (Fig. 9-3). The uplift of the northern block was very significant; rocks of the Lepontine Nappes which experienced upper amphibolite facies conditions and partly migmatization (Fig. 9-4), were juxtaposed to unmetamorphic units of the Southern Alps. Depending on which geothermal gradient is assumed, this implies 20 km or more of relative uplift. The uplift of the northern side is either interpreted as indicating a back-thrust or, alternatively, an originally south-dipping normal fault rotated into its present, north-dipping orientation later on. Immediately north of the Insubric Line, the nappe units are in a steep position; this zone is described as the Southern Steep Belt or as the root zone (Wurzelzone) of the Lepontine Nappes (Fig. 9-5). Approaching from the North, one can nicely observe how the nappes and their internal foliation progressively steepen towards the South and finally disappear in the ground immediately north of the Insubric Line. To the West, the Lepontine Nappes are bounded by the Simplon Fault, a southwestward dipping normal fault comprising a wide mylonite belt in the lower plate, overlain by cataclasites and the actual fault surface. The Simplon Fault was active in the Miocene, with a maximum movement rate between 18 and 15 Ma and a reduced rate probably until 3 Ma. Instead of joining the Insubric Line to the Southeast, as one would expect from the large-scale geometry (and as has been erroneously assumed by some), the Insubric Line turns to the Northeast towards the center of the Lepontine Dome where it becomes less distinct and finally disappears. As a consequence, the outcrop area of the Lepontine Nappes is open to the Southwest; the Moncucco and Camughera nappes belong to the Lepontine Nappes and, in a sense, also the Monte Rosa Nappe (see chapter 7). The eastern boundary of the Lepontine Nappes is also formed by a Miocene normal fault, which displays a northeastward-dipping mirror-image orientation with respect to the Simplon Fault: the Forcola Fault. Compared with the Simplon Fault, its displacement and the thickness of its mylonite belt are minor. The Forcola Fault disappears to the South under the Quaternary fill of the Mera Valley and, just like the Simplon Fault, does not join the Insubric Fault. To the North, the Forcola Line becomes indistinct within the Misox Zone, the zone of Mesozoic sediments and ophiolites between the Adula Nappe, the easternmost Lepontine Nappe, and the overlying Tambo Nappe.
Each of the Lepontine Nappes comprises a core of Variscan basement rocks, mainly orthogneisses, paragneisses, and amphibolites, and a metamorphic sedimentary cover of conglomerates and quartzites (Permian and Lower Triassic), dolomites (Middle and Upper Triassic), and metamorphic limestones, marls, and breccias which are probably Jurassic in age (Fig. 9-6). In some cases, most notably the Adula Nappe, basement and sedimentary cover together have been intensely and isoclinally folded or imbricated (Fig. 9-7). The basement and its cover originate from Europe’s southern continental margin, north of the Valais Ocean. In some cases (Maggia Nappe, Soja Nappe), an origin from the Briançonnais has been proposed, either based on comparison of sediment facies (in the case of the Soja Nappe), or on structural arguments, as for the Maggia Nappe. Only for the Berisal Nappe, which is the extension of the Bernhard Nappe in the footwall of the Simplon Fault, a Briançonnais origin is almost certain (“Be” in Fig. 9-1).
The Mesozoic cover of the crystalline nappes is overlain or enveloped by Bündnerschiefer, i.e. calcareous mica schists, originating from marly, shaly, and sandy series of probably Cretaceous age. The Bündnerschiefer locally contains ophiolithe complexes including serpentinite, gabbro, and basalt. The ophiolithes are derived from the oceanic crust of the Valais Ocean. The Bündnerschiefer partly represents the former sedimentary cover of the ocean crust (Lower Penninic), partly the cover of the distal European continental margin (Subpenninic). Because of the difficult distinction between the various types of sediment cover, both oceanic and continental Bündnerschiefer as well as the Triassic cover of the crystalline nappes are depicted in grey in Fig. 9-1.
The Lepontine Dome is structurally divided by the Maggia Transverse Zone (Maggia-Querzone), a north-south zone of steep foliation, into two sub-domes, the Simplon Sub-dome to the West and the Ticino Sub-dome to the East. The Maggia Transverse Zone more or less coincides with the Maggia Nappe in Fig. 9-1.The structurally deepest units are exposed in the Verampio Window (“V” in Fig. 9-1) in the core of the Simplon Sub-dome; the deepest gneiss exposed here may be the southern prolongation of the Gotthard Massif. Like onion skins, the higher Lepontine Nappes surround it. The culmination of the Ticino Sub-dome coincides with the Ticino valley (Valle Leventina); the gneiss of the Leventina Nappe is exposed on the lower valley slopes, overlain by gneiss of the next higher unit, the Simano Nappe, on both sides of the valley. The Adula and Monte Rosa Nappes represent the highest units, symmetrically arranged at the Eastern and Western end of the Lepontine Dome, respectively. Together with the Bellinzona-Dascio Zone, both nappes comprise rocks with an Alpine (Eocene) high-pressure metamorphism, in contrast to the other tectonically deeper nappes situated in between. This can be explained by the Monte Rosa and Adula nappes representing the southernmost, most distal part of the European continental margin, the part which descended most deeply into the subduction zone. A “tail” of the Monte Rosa Nappe can be followed towards east into the Southern Steep Belt, getting thinner and finally disappearing near Ascona. It represents a tight, upright, post-nappe antiform . This “tail” is interveined with Alpine pegmatites (Fig. 9-8).
The Adula Nappe (Figs. 9-9, 9-10) consists of extremely flattened layers of ortho- and paragneiss, intercalated which thin seams of Mesozoic sediments. Boudins and lenses of eclogites can be found especially in the paragneiss layers (Fig. 9-11, 9-12), recording increasing metamorphic pressure from north to south. In the southern part of the nappe, especially in the part known as the “Cima Lunga Lappen” (“CL” in Fig. 9-1), also garnet peridotites occur, such as those of Alpe Arami, Cima di Gagnone, and Monte Duria (Fig. 9-13). They display Eocene pressures of more than 3 GPa (30 kbar). The eclogites do not represent Mesozoic ocean floor rocks mixed into the older gneisses by a mélange mechanism but mafic constituents of the Variscan basement which has been subducted en bloc in the Alpine orogeny. Eclogites at Trescolmen bear Alpine garnets (ca. 37 Ma) with Variscan cores (ca. 336 Ma), clarifying the poly-metamorphic character of these rocks.
After elogite-facies metamorphism during subduction, the high-pressure nappes were rapidly exhumed and the entire Lepontine nappe stack then overprinted by Barrow-type, amphibolite facies metamorphism, referred to as the Lepontine Metamorphism. The corresponding metamorphic isograds extend across nappe boundaries, for example across the boundary between Adula Nappe (with eclogite relicts) and Simano Nappe (without ecligite relicts). The isograds more or less image the shape of the Lepontine Dome, however very asymmetrically: The maximum conditions (sillimanite zone with with partial melting, Fig. 9-4) can be found immediately north of the Insubric Line. From here, the temperatures decrease rapidly towards the South but much more slowly towards north.
The history of deformation of the Lepontine Nappes is complex. Overprinting relations reveal that some areas experienced up to 7 deformation phases. The paleogeographic origin of some nappes is still ambiguous because of the multiple folding. The first regionally developed deformation phase led to north-vergent shearing, isoclinal folding and nappe stacking (F1). In the Adula Nappe, this phase is called Zapport Phase (Fig. 9-7). It took place after the peak of eclogite-facies metamorphism. The second phase caused refolding of the nappes into large scale isoclinal recumbent folds (F2; in the Adula Nappe: Leis Phase). A pronounced stretching lineation formed parallel to the fold axes, reflecting orogen-parallel (approximately SW-NE oriented) stretching (Fig. 9-14). The third phase (F3) led to the formation of the North-South trending Maggia Transverse Zone (Fig. 9-15) and the fourth (F4) formed the Northern and the Southern Steep Belt, by rotating the foliation and nappe boundaries. As an example, Fig. 9-5 shows an F2-fold, probably steepened by F4.
In the present-day cross section, the Adula Nappe with its high-pressure relicts is sandwiched between two nappes which underwent much lower pressure conditions during the Alpine Orogeny: The Simano Nappe below and the Briançonnais-derived Tambo Nappe above (Fig. 9-16). Fig. 9-17 shows a reconstruction accounting for this situation. It assumes that the lithosphere of the Piemont-Ligurian Ocean and that of the Valais Ocean were subducted in two distinct subduction zones, and that the exhumation of the Adula Nappe out of the northern subduction zone resulted from the extraction of the middle plate – that is, its sinking into the deeper mantle, between ca. 37 and 35 Ma (“slab extraction”). Later on, the ongoing subduction of the European Plate caused the underplating of the deeper Lepontine Nappes below the Adula Nappe, but at much shallower depths.
Kathrin Wellnitz.
Geology of the Alps Part 1: Gemeral remarks; Austroalpine nappes
Figure sources:
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