Swiss J Geosci (2015) 108:3–34 DOI 10.1007/s00015-015-0188-x

Tectonics of the Monte Rosa and surrounding nappes (Switzerland and Italy): Tertiary phases of subduction, thrusting and folding in the Pennine Alps

1 1 2 Albrecht Steck • Henri Masson • Martin Robyr

Received: 6 May 2014 / Accepted: 11 February 2015 Ó Swiss Geological Society 2015

Abstract The Monte Rosa basement fold nappe, sur- metamorphism. Early extrusional structures have been rounded by other continental units of the Brianc¸onnais s.l. transposed by the younger thrust structures. The NW-di- domain and ophiolites of the Piemont Ocean, represents a rected thrust of the Alps was accompanied since about major structure of the Pennine Alps situated at the border 35 Ma by ductile dextral shear and backfolding in the of the Canton Valais (Switzerland) and Italy. The Central zone of dextral transpression between the converging Alps were formed during the collision and SE-directed European and Adriatic plates. underthrusting of the European below the Adriatic plate by successive underthrusting, detachment and accretion of the Keywords Switzerland Á Italy Á Central Alps Á Tertiary Austroalpine Sesia continental crust, the Piemont oceanic fold nappes Á Alpine orogeny crust and the continental Brianc¸onnais–Europe plate bor- der. The 90–60 Ma Sesia high-pressure metamorphism, Abbreviations followed by the 50–38 Ma Zermatt-Saas Fee and Monte Ant Antrona ophiolitic unit Rosa high-pressure metamorphism, and since 40 Ma by Be–Ru Berisal–Ruginenta the Barrovian regional metamorphism, reveal a long-last- CB Cimes Blanches ing Alpine evolution during convergence of both plates. DB Dent Blanche The superposition of the ultra-high pressure Zermatt-Saas 2 DK Seconda zona diorito-kinzigitica Fee ophiolites by the continental Cimes Blanche unit of Fu Furgg the Brianc¸onnais domain and the medium pressure ophi- Go Gornergrat olitic Tsate´ nappe is explained by delamination and tec- MR Monte Rosa tonic flake detachment of the Cimes Blanches from the Por Portjengrat Brianc¸onnais crust and its south directed thrust over the Se Sesia Zermatt-Saas Fee and Tsate´ ophiolites. The main ductile SM Siviez-Mischabel deformational structures, related to the NW-directed nappe Ts Tsate´ ophiolitic unit emplacement, were generated after 40 Ma under green- Tu Tuftgrat schist to amphibolite facies Barrovian orogenic Va Valpelline ZH Zone Houille`re ZS Zermatt-Saas Fee ophiolitic unit Editorial handling: A. G. Milnes.

& Albrecht Steck [email protected] 1 Introduction 1 Institut des Sciences de la Terre, Ge´opolis, Universite´ de Lausanne, 1015 Lausanne-Dorigny, Switzerland The Monte Rosa unit is a recumbent gneiss fold nappe of 2 Institut fu¨r Geologie, Baltzerstrasse 1 ? 3, 3012 Bern, the Penninic zone of the Central Alps exposed in the over Switzerland 4000 m high mountain range at the border of Switzerland 4 A. Steck et al. and Italy (Figs. 1, 2, 3, 4). The geological investigation of Fig. 1 Structures of the Central Alps, modified after Steck (2008),c the Monte Rosa nappe started in the 20th century by the Steck et al. (2013), Tektonische Karte der Schweiz (2005) and Cavargna-Sani et al. (2014). The Monte Rosa nappe is situated at the famous works on the Pennine Alps by Argand (1911, 1916) western border of the Lepontine tectonic half-window. CL Centovalli and Hermann (1937). Argand defined the gneiss fold line, MFL Malenco-Forno-Lizun nappe, RKG Rote Kuh-Gampel nappes of the Pennine Alps after the discovery of the fault, 2DK Seconda zona diorite-kinzigitica, ZH Zone Houille`re; (1) nappes of the Central Alps by Schardt (1903) during the Monte Leone basement, (2) Mesozoic and Paleogene sedimentary cover of the Monte Leone, Campo Tencia, Adula and other lower perforation of the Simplon railway tunnel. Geological Penninic nappes, (3) Pizzo del Vallone nappe, (9) Brusson window, mapping of the six map sheets at a scale of 1:25,000: St. (10) Canavese Permian-Liassic sediments, (11) Scaredi unit (amphi- Niklaus, Simplon, Randa, Saas, Zermatt and Monte Moro bolite facies grade Ivrea basement), (12) Chiavenna ophiolites, (13) by Bearth (1953, 1954a, b, 1964, 1972, 1978) and Mat- Salarioli Carboniferous-Triassic sediments, (14) Tuftgrat Mesozoic cover of Stockhorn basement, (15) Teggiolo Mesozoic-Paleogene terhorn by Bucher et al. (2004) laid a solid framework for cover of Antigorio nappe, (16) Mesozoic cover of Portjengrat unit, future geological studies. Bearth (1939, 1952, 1956, 1957a, (17) Cristallina Mesozoic cover of Sambuco basement, (18) Bosco b, 1958, 1962, 1967, 1973) discovered and described, in his unit, (19) Mesozoic-Paleogene Brianc¸onnais cover, (20) Stalden- study of the Monte Rosa region the high-pressure regional Berisal-Ruginenta basement, (29) Geisspfad peridotite, (30) Cima d’Agaro peridotite, (31) Moncucco peridotite, (32) Albogno peri- metamorphism followed by the Barrovian metamorphism dotite, (33) Finero peridotite. Note on the Simano nappe: Recent work of the Central Alps. Dal Piaz et al. (1972) discussed the by the authors has demonstrated West of the Ticino river the tectonic evolution and Eoalpine high-pressure metamor- subdivision of the classical Simano nappe into two tectonic units of phism of the Monte Rosa—Piemont Ocean—Sesia tran- different origin, called Verzasca and Campo Tencia. The prolongation of this separation East of the Ticino is still uncertain and for this sect. Gosso et al. (1979) proposed a structural model of the reason we provisionally maintain the name Simano Monte Rosa nappe. Milnes et al. (1981) analysed the in- terference pattern of post-nappe folds and concluded by a et al. (1999, 2001), Sartori et al. (2006) and Steck (2008). model of the nappe stack. Lacassin (1984), (Lacassin and The Mont Fort, Ruginenta, Portjengrat, Camughera, Siviez- Mattauer 1985) discovered a kilometre-size NW-directed Mischabel units and the Zone Houille`re are subdivisions of sheath fold in the Gornergrat zone at the roof of the Monte the Grand Saint-Bernard nappe of Argand (1911). These Rosa nappe and estimated a displacement of 20 km for a continental units are, together with the Monte Rosa nappe 1 km wide shear zone. Sartori (1987, 1990) presented a attributed to the Brianc¸onnais (sensu lato) paleogeographic detailed study of the structure and stratigraphy of the domain limited to the north by the Valais basin and to the Combin zone, followed by a synthesis of the structure and south by the Piemont Ocean (Tru¨mpy 1980; Escher 1988; stratigraphy of the Grand Saint-Bernhard nappe by Escher Sartori et al. 2006). The Swiss National Foundation Project (1988). Dal Piaz (2001) wrote a historical review of the 20, reflexion seismic survey of the Zweisimmen-Zermatt geology of the Monte Rosa massif completed with personal and -Mattmark transect performed new data on the deep comments. Pleuger et al. (2005, 2008) propose a model of geological structures of the Central Pennine Alps (Fig. 4; the complex Alpine structural evolution of the Monte Rosa Marchant et al. 1993; Steck et al. 1997). Recent personal nappe. The metamorphic evolution of the Zermatt-Saas Fee field survey during the revision of the 1:25,000 map sheet ophiolites and Monte Rosa basement nappe have been in- Saas of the Swiss geological Survey, originally mapped by vestigated by Bearth (1967), Dal Piaz (1992, 2001), Meyer Bearth (1954a, b), led the authors to reconsider the Alpine (1983), Barnicoat and Fry (1986), Ganguin (1988), Pfeifer structural framework and to propose an updated description et al. (1991), Colombi (1989), Cartwright and Barnicoat of the Monte Rosa and surrounding units and to conclude (2002), Pawlig and Baumgartner (2001), Engi et al. (2001, on a model of their structural evolution based on litho- 2004) and Bucher et al. (2005). The radiometric dating of logical, stratigraphic, structural, metamorphic and radio- the 50–38 Ma high- to ultra-high-pressure metamorphism metric age data (Figs. 2, 3, 4; Table 1). The crucial point of of the Zermatt-Saas Fee unit (Gebauer 1999; Amato et al. our fieldwork was the distinction and description of the 1999; Engi et al. 2001, 2004; Lapen et al. 2003; De Meyer different tectonic units and their stratigraphy exposed in the et al. 2014) followed after 40 Ma by the medium-pressure Monte Rosa transect of the Penninic Alps. The following Barrovian metamorphism of the Central Alps (Frank 1983; main questions are discussed in this study: Hunziker et al. 1992; Markley et al. 1998; Engi et al. 2001; Janots et al. 2009) reveal a long lasting and complex 1. Which stratigraphic observations permit the recon- structural evolution by deep underthrusting, extrusion and struction of the paleogeography preceding the Alpine younger ductile folding and thrusting. Numerous master- continental collision of Europe and Adria? studies by students of the University of Lausanne investi- 2. Which metamorphic and radiometric data constrain the gated the geology of the Monte Rosa and its surrounding Alpine tectonic history? units. The Middle Penninic units of the Valais Alps have 3. Which deformational structures and geometric features been described in Escher (1988), Escher et al. (1997), Steck explain the Alpine tectonic evolution? etnc fteMneRs n urudn nappes surrounding and Rosa Monte the of Tectonics

A STRUCTURES OF THE CENTRAL ALPS OS AR VERRUCANO RN O nz H rie R B E of HELVETIC MESOZOIC L D F a e 2 VALAIS CALCSCHISTS k IE ak F2 BÜNDNERSCHIEFER e N L o A f a AUSTROALPINE Th r un N F

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e H AAR D 2 d L n I a 20 K W 19 NPF Scopi Carassino is F2 D Le F R 6 TA E L T A ucomagno IN H A A L N T LUCOMAGNO F n ADULA R e T 6 1 E O F nd Chièra 7 PL N H 2 le in O F6 -H I N b e G o D n d 2 rn 19 E F6 GOMSor en b A D H bl le G L F - n nd TAMBO N O 2 te or 17 e E bi H F SIMANO in 05 AVERS D - 2 U Al 4 - 7 3 Z RN r S

1 s … F SAMBUCO O TENC a n MP IA p 2 A p U STE A S F r F T 6 20 C F L p u 19 R 2 S 2 E o n GA HI Ti C V c r g E S 10 E in t F LC N F4 N o 04 TT 2 rn A U F V T F A C 2 F ER IN 2 o IS D 6 F h A 15 M ZA A 6 is L N rn a 07 l A 1 luhho g S E V E df V G n g C T T a e I B F ia A ic F W M 1 F 4a r in R F E E 4b ANTIGORIO z o N N a O F 2 L 2 2 R R O A 20 I O U E s 19 E L c D H 29 G RKG Y E a V CI F F 12 LD A T 20 MA 5 FORC 2 O 20 I M N G e LU PL R O N 12 N A IS W r MM N U M G A RARO O 18 I 10 15 z A OL JÄGER Z Rône F 20 R CHRÜ C a T - 6 O A i A L. 19 N s c G

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19 I BOS g e R 20 C F u E i 3 a Moesa 5 b B IO 10 re r P s TE ü 10 si A c M Ve m V k A r NA A 20 N gel O RG Z R etto N MA E A H 3 A F 24 V R 4 A S NT C IVIEZ ZH R-S L F2 15 30 I 10 F4 BERNIN -M N IGORI D Salmone IS 20 E 15 TE C D O A BELLINZONA LI L N A H I 10 MERGOSCIA A D N A R F O NALE T O O 6 T TO B F A I F 2 19 S B 4 E S P er O F L 6 Zicch Ticino F era o 2 E 1 as 10 V ROSSO M 19 ORSELINA 31 Com TONALE LINE 38 F C L 01 6 07 32 3 1 3 ro 10-1 E l o ne TÉ e F6 gn Fi A H b a a A R C 40 h AT eja R TS c 16 NGR Pr E N s JE A H 1 A i RT G M 20 O N - F6 L P TA ALPINE STRUCTURES B O EN T TR 31 IN E N U G N A U 23 IN E A M 13 R L D C tectonic boundary 15 22 F6 20 11 O gho r n 10 L itta FURGG L fault FRILIHORN M 30 A F G 30 3 Toce I O R 25 STOCK syncline TSATÉ HORN M P E o 14 n N d 46 11 23 GORNERGRAT e E e antiformal syncline lli C r 14 io - MONTE ROSA ne g T zo F g anticline T n 5 a CIMES BLANCHES Va A 10 10 E M synformal anticline R o AS FEE V g ZERMA I a N SA F2-F3 fold axial trace (NW-verging) 2DK E L N - I A F4 fold axial trace (SW- to S-verging) 12 L N 10 O o t E R F5 fold axial trace (S- to SE-verging) t 23 e T F l S o S 23 5 i c E F6 fold axial trace or crest line c o

B V TSATÉ A 22 fold axis and plunge 23 2DK F5 N 50 km A

9 C A LINE AOST SESIA modified after Steck (2008), Steck et al. (2013), Cavargna-Sani et al. (2014) 5 6 A. Steck et al.

2 Lithological description of the tectonic units Fig. 3 Tectonic map of the Monte Rosa region, after Wetzel (1972),c Laduron (1976), Steck (1990), Steck et al. (1999, 2001) and Keller et al. (2005), completed with unpublished maps of Master degree 2.1 The Monte Leone nappe geologists from the University of Lausanne and personal observa- tions. Black lines correspond to lithological boundaries that may be Steck (2008) divides the Moncucco unit of Bearth (1957a) stratigraphic, intrusive or tectonic. Complements to the legend: 20 into a lower element to be correlated with the Monte Antrona metabasites, 20a calcschist and marble, 20b serpentinite and peridotite, 20c gabbros, 21 Zermatt-Saas Fee metabasites, 21a Leone nappe of the Lepontine Alps and an upper, new sediments, 21b serpentinites. Granite gneiss of the Siviez-Mischabel, Ruginenta nappe (Fig. 3). Bearth (1957a), Laduron and Camughera and Ruginenta units are distinguished by contrasted Merlin (1974), Laduron (1976) and Milnes et al. (1981) darker colours investigated the former Camughera and Moncucco units. The Monte Leone nappe is characterised by garnet-two- mica-plagioclase gneiss intruded by the porphyritic, two nappe (Milnes in Steck et al. 1979). A 50 m large fold micas Moncucco granite dated by the Rb–Sr whole rock hinge, cored by siliceous marbles, separates at Alpe method at 271 ± 4.8 Ma (Bigioggero et al. 1981). This Pradurino the paragneiss of the Monte Leone nappe from unit, exposed in the core of the Vanzone fold, is con- the paragneiss, the Moncucco peridotite and the upper sidered as the southern continuation of the Monte Leone garnet-two micas-plagioclase schist of the Ruginenta

BRENNER FAULT 1 TAUERN 2

EUROPE 1 E IC IN T P LT E S L U V CIE A A A L FA O F E T C LT ÖTZTAL A R H IS I U R I CH N A T R 1 S I F A U N S E N IN U C D E N I J R HAR R D A A TT G E A G A N O P G U R G N I O IES 1 E TERN H AC ME G IS F V DO 6 GAS L E E S G IT M L P A R 2 C O E L R B N G Z A I TI A E 3 H N S SS E P O G CA IM Be 3 IMPLON M P I R - S FA E40-22 Ma A P E U A L Ad ÔN LT H TONALE FAULT S R E 40-35 Ma G E U H MR T O C L N R E U S S N 50-40 Ma A E C A N F L E O L N I L Z I A E B 5 U L U C N IG Q T A B A V A Ê N T V F É S E N E T I D E O C SA S M I F E TH SESIA S A FAULT V IC AOST Fig 3 H Bi A C S 3 N S 4 E N A N N E 90 - 60 Ma C Fig.4 M 1 E ILANO BELT N E E O R R Tr D G C E I L G L E N B I N É S N E IN I E 5 C H A

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T Simplon detachment zone: H A R R G thrust fault U EN ductile Simplon shear zone S TE T RA strike-slip fault S 34 - 18 Ma normal fault 100 km brittle Rhone-Simplon fault (ductile foot wall) 18 - 3 Ma back fold AS 2014

Fig. 2 Late Cretaceous and Tertiary structures and metamorphism of red): Mantle derived 32–29 Ma (Adamello 42–29 Ma) dioritic the Central and Western Alps, modified after Frey et al. (1999), Steck magmatism, 4,(violet): Late Cretaceous-Early Paleocene 90–60 Ma et al. (2001, 2013) and Oberha¨nsli (2004) with locations of the eclogite facies metamorphism of the Sesia zone, 5,(blue–violet): structural map of the Monte Rosa region (Fig. 3) and geological Eocene 50–38 Ma eclogite facies metamorphism of the Middle and Zweisimmen-Monte Rosa section (Fig. 4), 1, (green): Late Eocene- Upper Penninic nappes, 6,(blue): Late Eocene eclogite facies Miocene greenschist facies metamorphism, 2,(yellow): Late Eocene– metamorphism of the Adula nappe Oligocene 40–28 Ma amphibolite facies metamorphism, 3,(magenta

Tectonics of the Monte Rosa and surrounding nappes… 7

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I R E D Z 8 p 30° F

D 22 6 3 90 100 22 X 21 23 4 34° 16° 8 A. Steck et al.

R Ô 2 Monte Rosa H NW N Weisshorn Dufourspitze SE E - 4506 m 4634 m Zweisimmen Rawil S Sierre F I F3 HES 5 M E LANC CH ES B P LAN CIM 5km DENT B 4 L 22 F RAT F 5km 19 RN G 1 F 5 O 8 THO N ER 4 7 PLAT OR RN N 23 ILIH GO FR F L F4 11-12 5 L E T T x . B A RN S II DK WILD 6 A R O LI A HO H FURGG L T RN H G K DE T C MON É K x S C F O LEN IVIEZ-MISCHABEL IS 21 U T T T S 4 0m 0m M E F BHORN T F ZON 3 LAU AR RAHELVETIC DOLDEN 4 A 3 HE A NIESEN ULT HOR N S OS 20 E N P F OR R FO E M IH E SESIA L R 4 9 LL T N D MO R E N I PR N 7 NI -MES T O EA A OU OZOIC E S M 6 F 23 O L LPES OCHTHONOUS P C N Z 4 T RAUT P - ST 8 R F T E PA C E ALDE O 6 E ARB N H L S 7 ONIF - RUGINENTA G E EROU N TA F IO S EUBRÜC MIT -16 6 I C V IOL A K 14 NA AR C . 13 O AL O N S ZONE HOUI H R S PLAMMIS I A T B A O A N N M A -C L J C ADRIATIC PLATE O MONTE LEONE A GASTERN J U L B E 6 Ä. R LĖRE CH. MA R YE 4 P 6 21 UR ? LANZO LEGENDE O EUROPEAN CONTINENT IOL O AAR GG RI E H TE IGO N HELVETIC NAPPES Jurassic - F T O ELV 6 RN 1 AN Z ISHO N AUTOCHTHONOUSOligocene GL ZOIC A 3 2 MESO ERAMPIO V Triassic N V ETIC AL ENDU IS LEB 7 Carboniferous BER «X»-NAPPE 2 1 UPPER CRUST: Paleozoic ULTRAHELVETIC F5 F5 and older basement s.l F GOTTHARD 6 ULTRAHELVETIC: Mesozoic-Eocene . VALAIS BASIN s. str. Sion-Courmayeur: Upper Cretaceous-Eocene EU Niesen: Mesozoic-Eocene ROP BRIANÇONNAIS s.l. PLATFORM: EAN Prealps: Triassic-Eocene UP PE 8 Permo-Mesozoic-Eocene R CR UST 7 Carboniferous Zone Houillère 4 Niouc - Stalden - Ruginenta 6 Siviez-Mischabel Paleozoic and older basement 13 Furgg Permian - Upper Jurassic 11-12 Monte Rosa Paleozoic and older basement EU FAISCEAU VERMICULAIRE ROP 19 Frilihorn Permo-Mésozoic EA 18 Cimes Blanches: Permo-Mesozoic N LO WE 17 Gornergrat: Permo-Mesozoic R CR UST 9,14-16 Stockhorn basement and Portjengrat nappe MOHO PIEMONT OCEAN BASIN 20 MAN Antrona ophiolites TLE 21 Zermatt-Saas Fee ophiolites 22 Tsaté ophiolites 23 AUSTROALPINE: Sesia Upper Crust 28 ADRIATIC PLATE: II Zona Diorito-Kinzigitica 10 km AS 2014

Fig. 4 Schematic geological profile of the Zweisimmen-Rawil-Sier- completion of the F3 phase of folding. As curvature is a geometrical re-Weisshorn-Monte Rosa transect of the Swiss-Italian Alps. The property that is not conserved by strain (even homogeneous), this Zweisimmen-Zermatt transect is based on reflexion seismic data of trace has been transformed by younger deformation in such a way that the National Foundation 20 deep seismic project (Escher et al. 1997; today it is no more a locus of maximum curvature. As for the Berisal Steck et al. 1997). In the frontal part of the Monte Rosa nappe, the fold, the Monte Rosa border was already curved before the F5 phase, axial trace of the Mondelli fold is in fact a « paleo-axial » trace that so that the point of its intersection with the Berisal axial trace is also does not correspond any more today to the definition of an axial trace. not a point of maximum curvature of this borderline. (3) Pizzo del This line marks the emplacement where the axial trace was after the Vallone nappe

nappe (Fig. 3). The siliceous marbles may represent a of garnet-two micas-plagioclase gneiss or a staurolite remnant of a Mesozoic sequence separating two Alpine and kyanite bearing schist intruded by granite, whereas basement nappes. Carrupt (2003) described the less de- its lower part consists of the Moncucco peridotite with a formed stratigraphic column of the Monte Leone nappe thin decametric horizon of garnet-two mica schist at its front, the Mesozoic (-Tertiary?) Holzerspitz series in the base. It is not excluded that the Moncucco peridotite Binn valley. The Monte Leone unit is paleogeographically represents a distinct nappe, a possible equivalent to the situated to the north of the Valais basin and classically Geisspfad peridotite in the Binn valley (29 on Fig. 1). attributed to the European continent (e.g., Steck et al. The Salarioli sedimentary sequence starts with 40 m of 1999, 2001). graphite and anthracite bearing, black staurolite-kyanite micaschist, sandstone and conglomerate attributed to the 2.2 The Ruginenta nappe Upper Carboniferous in the Vallone di Trivera near Prabernardo (Antrona valley), reaching 180 m in the The newly defined Ruginenta nappe (Steck 2008)con- Vanzone fold hinge on the Salarioli pass. The Car- sists of pre-Upper-Carboniferous basement strati- boniferous is in the Vallone di Trivera overlaid by about graphically overlaid by the Upper Carboniferous and 10 m of greenish-white, tabular phengite quartzite of Permo-Triassic sediments of the Salarioli series (Fig. 5). Triassic or Permo-Triassic age and 5 m of Triassic This sedimentary sequence originally described as a dolomite-limestone marble (Steck 2008). The Ruginenta synclinal structure, the so-called ‘‘Salarioli-Mulde’’ or nappe with its normal sedimentary cover, the Salarioli Salarioli syncline of Bearth (1957a) represents actually series, is considered as the southern root of the Berisal the normal autochthonous stratigraphic cover of the zone, the Upper and Lower Stalden zones and the Ruginenta basement. The basement is mainly composed Carboniferous Zone Houille`re. Tectonics of the Monte Rosa and surrounding nappes… 9

Table 1 Nomenclature and tentative correlation of Alpine structures observed by various authors in the Monte Rosa region and in the Lepontine gneiss dome

Correlation of Alpine Structures in the Central Alps Monte Rosa region Lepontine dome

Pleuger, Nagel, Jens, Huber, Ramsay & Simpson This paper Steck 1984, 2008, 1985, Jansen & Froitzheim 2008 Age Age Steck et al. 2013 Maxelon & Mancktelow 2005 Metamorphism Metamorphism Folding phase F6: Folding phase F6: Folding phase D4, FA4: 10 Ma Glishorn anticline Glishorn anticline Glishorn anticline 10 Ma Masera syncline Masera syncline Masera syncline 18 Ma

Simplon line 18 Ma greenschist f. greenschist f. Vanzone phase Folding phase F5: Folding phase F5: Southern steep belt, FA4 30 Ma Vanzone anticline Vanzone anticline

34 Simplon dextral shear DII,XII - DIV,XIV Simplon dextral shear DII,XII - DIV,XIV Folding phase F4: Mischabel phase no equivalent no equivalent Mischabel fold

Deformation phase DI Folding phase F4, NW-SE Folding phase FA3 Malfatta phase with top-to-NW shear XI SW-verging folds: Maggia, with N-S fold axis: and SW-verging folds. It Alpe Bosa, Wandfluhhorn Maggia fold comprises three schistosities and folding phases Deformation phase DI with top-to-NW shear XI. It Barrovian metamorphism Tertiary Structures Tertiary Barrovian metamorphism comprises three schistosities amphibolite facies and folding phases Deformation phase D2 Mattmark phase schistsity S3, F3 schistosity S3, F3 schistosity S2, FA2

higher greenschist facies schistosity S2, F2 schistosity S2, F2 Deformation phase D1 schistosity S1, FA1 schistosity S1, F1, schistosity S1, F1 not described 38 Ma 38 Ma greenschist f.

eclogite eclogite lineation E-W eclogite eclogite Mergoscia and Cimalunga Mergoscia and Cimalunga >50 - 38 Ma HP metamorphism HP Cretaceous - Middle Eocene

2.3 The Camughera—Verosso—Siviez-Mischabel Triassic dolomitic and calcitic marble, 25 m of black nappe dolomite and dolomite breccia, 70 m of Upper Jurassic calcite marble, 25 m of Upper Cretaceous to lower Eocene The basement gneisses of the Camughera and Verosso phyllitic marble and ends with 15 m of a black Middle- units are similar and interpreted as the root of the Siviez- Eocene flysch. This series has a typical Brianc¸onnais sensu Mischabel nappe (Argand 1911; Escher et al. 1988, 1993). stricto affinity (Ellenberger 1953). The autochthonous These three gneissic basements are linked together through Carboniferous to Mesozoic stratigraphic sequence of the the Preja F5 synform and Balmahorn F5 antiform (Fig. 3). lower limb of the Siviez-Mischabel nappe outcrops in the Their Variscan and older basement mainly consists of Mattertal, south of Visp in an inverse position testifying to garnet-two micas-plagioclase gneiss and schist containing the fold nappe geometry of this nappe (Fig. 4; Escher 1988; pre-Alpine migmatite structures, for instance in the Tor- Genier et al. 2008). A thin band of metasediments of rente Ovesca bed below Antronapiana. The old basement is probable Triassic to Cretaceous age is pinched into the intruded by porphyritic, two micas granite gneiss. U–Pb gneiss along the southern border of the Verosso element dating of zircon from the Randa granite yields an age of (Cima d’Azoglio; Carrupt and Schlup 1998). Its stratigra- 269 ± 2 Ma (Bussy et al. 1996). Sartori (1987, 1990), phy has an Ultrabrianc¸onnais affinity. It is an open question Escher (1988) and Genier et al. (2008) investigated the if this Azoglio series and the adjacent gneiss still belong to Carboniferous and Mesozoic cover of the Siviez-Mischabel the Siviez-Mischabel nappe. nappe in the Visp-Zermatt valley (‘‘Matter-Tal’’, Fig. 3). Sartori (1990) described in the Barrhorn area (Mattertal) 2.4 The Monte Rosa nappe the stratigraphic sequence of the sedimentary cover of the Siviez-Mischabel nappe (Figs. 3, 5). It consists of up to The Monte Rosa nappe consists of an old basement com- 150 m of Carboniferous greyish quartzite, arkose and posed of garnet-two micas-plagioclase gneiss and schist graywacke, about 200 m of Early Permian quartzitic to and some amphibolite layers, intruded by at least two conglomeratic green or white micaschist, 100 m of Middle generations of granites (Bearth 1952, 1953, 1957b). The 10 A. Steck et al.

black flysch MIDDLE EOCENE Plattjen NW orange Gornergrat SE colloured UPPER phyllitic CRETACEOUS calcareous sandstone JURASSIC? marble marble with dolomite lenses MIDDLE TRIASSIC

white or ...... grey UPPER ...... marble JURASSIC

...... black marble, breccia, MIDDLE graphitic schist, LOWER ferriferous JURASSIC dolomite TRIASSIC ...... paleokarst, metabauxite . dolomite ~300 m breccia UPPER TRIASSIC mica quartzite black dolomite tabular quartzite

...... dolomite and limestone . . . . wildflysch ...... MIDDLE ...... TRIASSIC . . .. . conglomeratic ...... quartzite ...... PERMO- ...... pelite . . . calcschist ...... TRIASSIC CRETACEOUS ...... «série grise» ...... calcschist UPPER ...... white ~200 m ...... MIDDLE JURASSIC V V CRETACEOUS ...... quartzite tabular UPPER calcschist ...... marble ...... CRETACEOUS dolomitic ...... breccia LOWER-MIDDLE ...... quartzitic marble JURASSIC white microbreccia mica schist albitic UPPER yellow siliceous UPPER calcite

~ 100 m dolomite JURASSIC TRIASSIC gneiss marble dolomite breccia breccia LOWER sandy black dolomite (Carnian) sandy MIDDLE sandy marbleJURASSIC ? calcite V V prasinite .. . . yellow dolomite calcite V ...... MIDDLE yellow dolomite marble JURASSIC dolomite ...... calcite marble marble LOWER ...... TRIASSIC graphitic LOWER ...... calcschist CRETAC.? and TRIASSIC ...... TRIASSIC marble JURASSIC calcite marble ...... white LOWER U.JURAS. «schistes lustrés» limestone ...... tabular quartzite cornieule yellow ...... nodular calcschist L.JU. V V ...... quartzite ...... TRIASSIC . . . . yellow dolomite ...... mica quartzite . . . phyllitic dolomite TRIASSIC white marble UPPER ...... PERMO- . . . + calcite marble metachert ...... gneiss albitiques . . . cornieule TRIASSIC ...... phengite quartzite ...... TRIASSIC . . quartzite . . . metabasite ...... arkose Mn,Fe JURASSIC black schist (metachert) ...... graywacke . . .V....V. ..V...... PERMO- ...... + + granite V V prasinite, V V ...... metabasite CARBONIF. prasinite, ...... + pillow lava ...... V V pillow lava ...... 310±20 Ma + VARISCAN MIDDLE ...... CARBONIFERO...... + + + + + + ...... CARBONIFEROUS VARISCAN granite and + gabbro JURASSIC + MIDDLE ...... + + + gabbro ...... graphitic schist ...... and 270±4 Ma + OLDER + OCEANIC JURASSIC ...... micaschist and + + + ...... and ...... OLDER + BASEMENT CRUST + OCEANIC ...... conglomerate + + . . . . . conglomerate ...... BASEMENT VARISCAN 164±2.7 Ma CRUST ...... basic volcanites + + paragneiss ...... and 163-5±1.8 Ma ...... + serpentinite ...... OLDER 163.1±2.4 Ma ...... + BASEMENT 155,156±2.1 Ma ...... paragneiss 93.45±1.7 Ma 87±20 Ma + + granite paragneiss serpentinite + + + + + + paragneiss VARISCAN + + Randa and VARISCAN granite + + + OLDER + and 269±2 Ma BASEMENT OLDER BASEMENT

Salarioli Barrhorn Cimes Blanches Gornergrat Tuftgrat Furgg Grundberg Zermatt-Saas Tsaté COVER Ruginenta Siviez-Mischabel Stockhorn Monte Rosa Portjengrat Antrona Tracuit BASE

BRIANÇONNAIS (sensu lato) PLATFORM PIEMONT OCEAN

Fig. 5 Stratigraphic columns of the main tectonic units in the Monte Steck et al. 1999, 2001; Sartori et al. 2006; Steck 2008) and master Rosa region (modified after Marthaler 1984; Sartori 1987, 1990; students works of the University of Lausanne Escher 1988; Vannay and Allemann 1990; Carrupt and Schlup 1998;

Rb–Sr whole rock age from a sample of the Monte Rosa convinced by this proposition. The gneisses with some granite indicate a first phase of granitic intrusion at amphibolite boudins are attributed to the old Monte Rosa 310 ± 20 Ma (Frey et al. 1976). U–Pb zircon geochro- basement. Wetzel and Bearth suggest that the Stelli my- nology from the porphyritic Mezzalama granite yields lonite zone separates a higher Monte Rosa from a lower younger ages of 270 ± 4 Ma (Pawlig and Baumgartner Macugnaga sub-unit of the Monte Rosa nappe. High- 2001). Engi et al. (2001) suggest an U–Pb monazite age at pressure mineral assemblages of phengite, talc, chloritoid 330 Ma for the main Monte Rosa granodiorite mass. and kyanite in undeformed rocks and in white schist have Bearth (1952, 1957b) distinguished the about 25 km long been interpreted by Pawlig and Baumgartner (2001)as and up to 2 km vide Alpine shear zone, the so-called products of the Eocene high-pressure metamorphism ‘‘Stelli zone’’, named after the Stellihorn peak in the overprinting former aluminium rich pre-Alpine metaso- northern part of the Monte Rosa gneisses. It represents an matically transformed granite. The interpretation by Bearth Alpine mylonitised zone of ortho- and paragneisses with (1952) of this aluminium-rich white schist as the product of amphibolite boudins and rare kyanite and chloritoid bear- intense Alpine deformation has therefore to be revised. ing white schist. It is named according to Pleuger et al. (2008) the Stellihorn shear zone on the geological and 2.5 The Furgg series, composed of the Permo- structural maps (Fig. 3). It occupies the northern border of Carboniferous Cavalli and Mesozoic Preja the Monte Rosa nappe between the Loranco valley, the formations Stellihorn and Mattmark, where it continues in a southern direction to the west of the Monte Moro pass and Argand (1911) defined the Furgg syncline or Furgg zone as Macugnaga. Wetzel (1972) proposed a continuation of the the whole sequence of ophiolites and sedimentary rocks Furgg series into Bearth’s Stelli zone. We are not that separates the Monte Rosa basement nappe from the Tectonics of the Monte Rosa and surrounding nappes… 11

Portjengrat basement nappe in the Furgg and Loranco lake dam (Fig. 3). Wetzel (1972) postulates that the eastern valleys. This Furgg zone continues to the west through the branch of the Furgg zone separates the Monte Rosa nappe Mattmark area and separates at the Stockknubel, near the at the Seewjinerberg in a higher Monte Rosa and a lower Gornergrat, the basement gneisses of the Stockhorn from Macugnaga complex. This zone corresponds on the the Monte Rosa unit (Fig. 10; Bearth 1953). The exact 1:25,000 map ‘‘Monte Moro’’ of Bearth (1954b) to a zone definition and significance of the Furgg zone are still a of albite-muscovite schist, strongly affected by the Stelli source of debate (Bearth 1952; Wetzel 1972; Jaboyedoff mylonite zone, which is herein attributed to the Monte et al. 1996; Dal Piaz 2001; Keller and Schmid 2001). The Rosa basement. Wetzel’s (1972) attribution of this zone of whole Furgg zone is strongly affected by Alpine shearing paragneiss to the Furgg series is questionable (Steck et al. and folding. The Furgg zone does not represent a ‘‘tectonic 1999, 2001). me´lange’’ as previously discussed by Froitzheim (2001)or A complete section through the sedimentary sequence of Pleuger et al. (2005). New observations in the Loranco the Furgg series is exposed at the Passo della Preja (Fig. 6; valley and at the Passo Della Preja allow the distinction Steck et al. 2001). The layers of meta-graywacke, meta- between isoclinal folds and slices of Antrona amphibolite, arkose, mica quartzite and micaschist with numerous am- serpentinite and calcschist from the Furgg series, which is a phibolite boudins, of the Cavalli Formation show a tectonic sequence of Upper Paleozoic to Upper Jurassic sediments, contact with the Antrona ophiolite. These meta-graywackes crosscut by basic dikes. We call them the Upper Paleozoic are overlain by 20 cm of Triassic quartzite and about 3 m Cavalli and Mesozoic Preja Formations of the Furgg of dolomite and cornieule, followed by 4 m of a graphitic series (Figs. 6, 7, 8, 9). sandy calcite marble, 6 m of a brown weathering siliceous The Zermatt-Saas Fee ophiolites form at the Stock- marble and 15 m of a white calcite marble attributed re- knubel to the east of the Gornergrat an isoclinal slice or spectively to the Lower, Middle and Upper Jurassic of the fold in the Upper Paleozoic graywackes of the Furgg series Preja sedimentary Formation. The whole sequence is cross (Fig. 10). Centimetre to decametre bands of marbles of the cut by up to 40 cm thick amphibolite dikes that are strongly Preja Formation follow the strongly sheared contact be- affected by boudinage (Jaboyedoff et al. 1996; Figs. 6, 7, 8, tween the Upper Paleozoic Cavalli Formation and the 9). The attribution of the different members of the Monte Rosa basement in the Furgg and Mattmark valley sedimentary sequence is based on lithostratigraphic sections. Two branches of the Furgg zone separated by a analogies and a precise dating is difficult due to the lack of Monte Rosa paragneiss zone are exposed at the Mattmark fossils. The interpretation of the meta-graywacke with its

F5 NW P F4 X SE B R A E C J amph K A F P A ibolite O E N R T L T M R D T D R IA O R N s N O I e A

I A

A L rp S e S O S nt

S M S gra i F4 I ni

I I C ywac te C T E m ke et L ab OW as JU ER ite MI RA Passo della Preja JU DD SS RA LE IC SS 2322 m IC U M J P J ID U P U D R ER R LE AS A S J LO S IC U W S calcite marble X R E IC T AS R R SI IA C sandy S graphsan m P c ar ERM o dstone bl qu rn itic mar e a ieu N graywackertz le TIO ite ble MA OR se F rp JA A en RE am N tin P S p ite IE hi T I N R bo R LL IO E lit O A T S e N AV A G A C RM G O FO UR PH F IO 100 m LIT E A. STECK 2013

Fig. 6 Geological cross-sections of the Furgg Series composed of the Cavalli and Preja Formations at the Passo della Preja, forming a F4-F5 fold interference structure (Swiss map coordinates: 652.15 km/106.45 km/altitude 2322 m) 12 A. Steck et al.

grayw acke

e cl gr og ay iti w c ac g ba ke ra s y it w e a c k e 20 cm

Fig. 7 Branching eclogitic basic dikes in Permian graywacke of the the stratigraphic layering when jumping to a higher stratigraphic Cavalli Formation, west of lake Cavalli (Swiss map coordinates: level. Boudinage of the crosscutting basic dike reveals of weak Alpine 650.65 km/104.15 km/altitude 1860 m). The branching sill crosscuts deformation

2 m

Fig. 8 Outcrop of the Furgg Series at the northern end of lake Cavalli (Swiss map coordinates: 651.15 km/104.85 km/altitude 1490 m). Typical boudinage and fold structures of eclogitic basic dikes in Permian meta-arkose and -graywacke of the Cavalli Formation mafic boudins as sedimentary me´lange of Middle Jurassic types of inclusions exist, which would indicate the erosion age discussed by Jaboyedoff et al. (1996) is improbable. of an ophiolitic oceanic or continental crust. The Upper All boudins are uniformly amphibolite (retroeclogite) Paleozoic age, probably Permian, proposed by Bearth imbedded in a siliciclastic greywacke matrix. No other (1952) and Wetzel (1972) for the herein defined Cavalli Tectonics of the Monte Rosa and surrounding nappes… 13

basement relationship. The Alpine deformation is in this NW SE region very intense and no sedimentary structures, char- acteristic of an autochthonous sedimentary contact are observable. The significant difference in lithology between the Furgg series in the Loranco valley and Passo della Preja and the autochthonous Mesozoic series on the Portjengrat basement at the Grundberg in the Saas valley (Fig. 5) gives evidence that both series have a distinct paleogeographic origin. Lenses of the Cavalli Formation are exposed all along the outer contact and also inside the Monte Rosa nappe (Wetzel 1972; Dal Piaz 2001). A thin band of mi- caschist with amphibolite boudins, attributed to the Cavalli Formation is intercalated between the Monte Rosa gneiss and the Antrona ophiolites in the Ovesca riverbed at Vil- ladossola. It is suggested that the Furgg sedimentary series has been overthrust to the north from an original pa- leogeographic position situated on the southern part of the Monte Rosa basement.

2.6 The Portjengrat nappe

The basement gneisses of the Portjengrat nappe are similar to those of the Monte Rosa, Siviez-Mischabel, Camughera and Ruginenta nappes. Bearth (1939) discussed the com- plex and still controversial relationship of the Portjengrat unit with the Monte Rosa and Siviez-Mischabel nappes. 1 m Masson (2002) proposed that it belongs to neither of their units but it forms an independent small continental block, originally stuck in paleogeography between the Antrona Fig. 9 Triassic dolomite of the Preja Formation cross-cut by about 30 cm thick basic dikes in the ridge 150 m NW of the Passo della and Zermatt-Saas Fee branches of the Piemont Ocean. The Preja (Swiss map coordinates: 652.06 km/106.52 km/altitude Monte Rosa and Portjengrat units differ through their 2370 m; Fig. 5 in Jaboyedoff et al. 1996) Alpine high-pressure mineral assemblages (3T phengite polytype) from the greenschist to amphibolite facies Formation is more plausible (Steck et al. 2001). This age is Siviez-Mischabel basement gneiss with its 2 M-phengite also supported by an Rb–Sr whole rock isochron of polytype (Frey et al. 1983; Vincent Baudraz, personal 275 ± 20 Ma (Frey et al. 1976). The graywackes of the communication). The boundary between the two units is Cavalli Formation overlie the old Monte Rosa basement uncertain. It may be situated in a zone of mylonite that and Permo-Carboniferous granites on Alpine deformed crosses the western ridge of the Weissmies (Huang 1936). contacts. Wetzel (1972) suggests an autochthonous base- The basement of the Portjengrat nappe is overlain at the ment-cover relation. The amphibolitic dykes are late—or Grundberg above Saas-Grund by its autochthonous Meso- post—Jurassic. Figures 7 and 8 illustrate how the Upper zoic sedimentary sequence studied by Vincent Baudraz Paleozoic Cavalli Formation typically looks like near the (personal communication). The Grundberg series starts Cavalli Lake. Figure 7 shows only little deformed with a Triassic sequence composed by up to 2 m of basal branching basic dikes cross cutting the stratigraphic lay- arkose, 2 m of dolomitic marble and cornieule (rauh- ering of the Paleozoic greywacke. The outcrop on the wacke). 50 cm of a nodular calcschist and 1 m of a pure northern end of the Cavalli Lake (Fig. 8) illustrates the calcite marble, attributed to the Jurassic, separate the Tri- widespread strong Alpine deformation by folding and assic sequence from the probable Late Cretaceous se- boudinage. The Upper Paleozoic sediments of the Furgg quence composed by 80 m of calcschist and 9 m of a series with their amphibolite boudins show near the An- sedimentary me´lange (wildflysch; Fig. 5). Basaltic dikes dolla refuge in the Loranco valley a sharp contact with the occur in both the gneissic basement and the sedimentary Paragneiss of the Portjengrat unit, which was interpreted by cover (marbles) of the Portjengrat unit in the Val Bognanco Keller and Schmid (2001) as an autochthonous cover- (Carrupt and Schlup 1998). 14 A. Steck et al.

WSW Hohtälli Stockhorn ENE 3286 m 3532m 3500 m Gornergrat 3500 m X S2 3089 m S2 X S2 2 T NIT I SS N U 3000 m E N EI OR S IT U GN H - Z E S3 CK X T T IT O Z R A B T A L S U R A G Q R C 7 I TUFTGRAT C T CS I E I Stockknubel S N Z FURGG UNIT S T A R R I A S3 1a 1b S3 U R O T 6 G Q 3 8 5 4 S3 X TL S3 X 7 2000 m N S2 2000 m X

Schistosity (S2 and S3 in Gornergrat unit, S1, S2 and S3 in Stockhorn unit) Stretching lineation XI Foldaxes F3 Equal-area net, lower hemisphere modified after ROBYR 1998

Fig. 10 Geological section through the Gornergrat and Stockhorn Triassic dolomite, 4 Tuftgrat Jurassic marble, 5 Gornergrat Permo- (after personal communication by Martin Robyr 1988), other Triassic quartzitic albite gneiss, 6 Gornergrat Triassic quartzite and structural data are represented on stereogram 3 of Fig. 16. 1a Furgg limestone (TL), 7 Zermatt-Saas Fee ophiolite, 8 Zermatt-Saas Fee Permo-Carboniferous graywacke with basic boudins (Cavalli F.), 1b calcschist (CS) Furgg Mesozoic marble (Preja F.), 2 Stockhorn paragneiss, 3 Tuftgrat

2.7 The Stockhorn nappe intercalated in the calcschist at the base of the Zermatt- Saas Fee ophiolites is tentatively attributed to the The Stockhorn nappe consists of Paleozoic basement schist Gornergrat nappe. Two generations of garnet support the or gneiss composed of meta-pelite, meta-arkose and meta- poly-metamorphic history of this garnet-muscovite schist graywacke overlain by the Mesozoic Tuftgrat sedimentary (Caroline de Meyer personal communication). Caroline de series (Figs. 5, 10). The basement schist contains a Meyer interprets the garnet-muscovite schist layer as an greenschist facies assemblage of white mica, garnet, isoclinal fold or tectonic slice of the Gornergrat nappe. quartz, chlorite, and ±chloritoid, ±albite. The Tuftgrat The Gornergrat nappe is characterized by a normal series starts with cornieule (rauhwacke). These Triassic stratigraphic sequence. A 330 m thick quartzitic and sediments are overlain by *10 m of sandy marble with conglomeratic albite-white-mica schist and tabular white- phyllitic intercalations, a few meters of calcibreccia of mica quartzite sequence is attributed to the Permo-Triassic probable Jurassic age and over 20 m of dark brown calc- that has a definite Brianc¸onnais affinity. This sequence is schist of unknown age. Bearth (1953), Escher (1988) and overlain by 10 m of limestone and thinner boudinaged Escher et al. (1988) describe the Stockhorn unit and the layers of dolomite, that also show a strong lithological higher Gornergrat nappe envelope as a SW-verging fold of affinity with the Middle Triassic carbonates of the Bri- the Monte Rosa nappe. But the study of the Stockknubel anc¸onnais domain. They are followed by a few tens of region suggests that the Stockhorn nappe is separated from metres of calcareous sandstones. All these rocks are the Monte Rosa basement by the Furgg zone (Figs. 3, 10). crosscut at Swissmap coordinates 639,192/103 821 by mafic dikes (Fig. 5, south of Plattjen), where Caroline de 2.8 The Gornergrat nappe Meyer (personal communication) observed albite-horn- blende symplectites which probably testify of retrograde The Gornergrat nappe is exposed at the type locality and greenschist facies recrystallization of former omphacite. below the Mittaghorn in the Saas valley, as a 300 m thick Also the high Si content of the phengite corroborates an band bounded along its top by the Zermatt-Saas Fee early high-pressure metamorphism of the Gornergrat ophiolites and along its base by the Portjengrat gneiss nappe. The crystallisation of tremolite in the siliceous (Figs. 3, 5). A 10 m thick layer of garnet-muscovite schist dolomite marbles indicates conditions at the limit of Tectonics of the Monte Rosa and surrounding nappes… 15 greenschist and amphibolite facies of the younger Barro- *2 m of mica quartzite and *3 m of tabular quartzite vian metamorphic overprint. attributed to the Lower Triassic, over 10 m of an alterna- tion of yellow dolomite and grey calcite marble of the 2.9 The Cimes Blanches nappe Middle Triassic, over 10 m of Upper Triassic black dolo- mite, dolomite breccia ending with yellow siliceous dolo- Argand (1916) distinguished the Cimes Blanches and Fri- mite, over 6 m of an alternation of albite micaschist, lihorn nappes as the ‘‘Faisceau vermiculaire’’. The Cimes nodular and siliceous marble and breccia attributed to the Blanches and Frilihorn nappes comprise the same strati- Lower Jurassic and over 4 m of Upper Cretaceous (?) graphic sequence as the Gornergrat nappe but occur in a yellow siliceous calcite marble. different tectonic position (Figs. 3, 4, 5, 11). Vannay and Allemann (1990) studied the stratigraphic series of the 2.10 The Frilihorn nappe Cimes Blanches nappe at the Cimes Blanches type locality (Figs. 3, 5). They propose a lithostratigraphic correlation The Frilihorn nappe (Marthaler 1984) presents a strati- with the well-known and paleontologically dated members graphic sequence similar to the Cimes Blanches nappe. It of the Barrhorn series of the Siviez-Mischabel nappe is separated from the Cimes Blanches nappe and inter- (Sartori 1987, 1990; Fig. 5). The series is composed from calated in the calcschist of the ophiolitic Tsate´ nappe base to top of over 5 m of a Permo-Triassic albite gneiss, (Figs. 3, 4, 11).

SW NE Ober Gabelhorn Weisshorn Zinalrothorn Schalihorn Unter Gabelhorn Mettelhorn Platthorn

F 10 3 DENT BLANCHE NAPPE thrusts 3 10 thrust S - S 7 1 3 te era 3 om 8 gl n co s 1 7 F3 F u ro 3 e 1 TSATÉ if prasinit n e 8 o rb F a 4 5 C TSA 7 4 TÉ calc F2 2 SIVIEZ-MISCHABEL NAPPE sch F ist 4 basement gneiss 1 4 MIS thrusts + S CHA 6 CB 5 1 BE L BA thrusts FRILIHORN CK ZERMA ca F TT-SA lcs thrusts + S1 OL 6 e AS ch D clogite FE ist 2 F4 E 9 Zermatt

Fig. 11 Structures of the Mischabel backfold, view from Sunnegga 9 eclogitic Zermatt-Saas Fee ophiolitic nappe, 10 Dent Blanche above Zermatt. Geological structures modified after Sartori (1987, basement nappe. Red lines are multiple folded thrust planes, 1990), Sartori et al. 2006, Steck (1989) and Steck et al. (1999, 2001). F2 = axial surface of second-phase folds, F3 = axial surface of 1 Basement gneiss of the Siviez-Mischabel nappe, 2, 3 its sedimen- third-phase folds, F2 and F3 fold axes are oriented parallel to a NW- tary cover, 2 Carboniferous conglomerates, 3 Barrhorn Mesozoic- directed stretching lineation with top-to-NW shear indicators, Eocene series, 4, 5 Frilihorn Mesozoic series, 4 Frilihorn quartzite, 5 F4 = axial surfaces of Mischabel back fold with its 22°W-plunging Frilihorn dolomite, : = younging upwards stratigraphic polarity, 6 fold axis (stereogram (2) on Fig. 16), (1) stereogramm Alterhaupt and CB Cimes Blanches nappe (Permo-Mesozoic series), 7 calcschist of (6) Zermatt-Saas Fee on Fig. 17 the ophiolitic Tsate´ nappe, 8 prasinite, meta-basite of the Tsate´ nappe, 16 A. Steck et al.

2.11 The Antrona and Zermatt-Saas Fee ophiolite ophiolites that separate the Gd. Saint Bernard basement nappes (here Siviez-Mischabel basement) from the Dent Blanche basement, well aware that it comprised several distinct The map reveals the superposition of isoclinal folds and tectonic units. The Tsate´ nappe is only one of these units. large tectonic slices in the internal structure of the Alpine Although it is the thicker one, it is recommended to deformed Antrona and Zermatt-Saas Fee unites. These maintain Argand’s original definition of the Combin zone ophiolitic units are composed of serpentinite, eclogite, (Tru¨mpy 1980) and to apply the name Tsate´ nappe, intro- amphibolite, prasinite, meta-gabbro, marble, chert (meta- duced by Sartori (1987) and Escher et al. (1988), to the part radiolarite), garnet-micaschist and calcschist. The serpen- of it made of ophiolite-bearing calcschist (‘‘schistes tinites preserve in the Antronapiana region, at Alpi di lustre´s’’). Cama, Pt. 1827 m, a large relictic body of peridotite. The The composition of the Tsate´ nappe is similar to the serpentinites are mainly composed of antigorite, with a rim Zermatt-Saas Fee and Antrona nappes, however with a of talc, chrysotile, actinolite, chlorite and magnetite larger proportion of metasediments. Its internal structure is (‘‘pierre ollaire’’) at the contact with the amphibolite or complex. It is dismembered into numerous thrust-sheets prasinite. Intrusive mafic dikes in the serpentinites are and tectonic slivers that are interpreted as a former accre- transformed into rodingite, composed of epidote, vesu- tionary prism formed during subduction along an active vianite, actinolite, diopside and grossular. Pillow lava margin (Marthaler and Stampfli 1989; Steck 1989). The structures are observed in the amphibolite and eclogite theoretical original stratigraphic column would consist of (Bearth 1967). Calcite marbles of probable Upper Jurassic an oceanic floor (ophiolite) overlain by oceanic sediments age that contain detrital graded bedding structures overlie (Fig. 5; Marthaler et al. 2008). The ophiolites are repre- serpentinite or amphibolite with primary sedimentary sented by serpentinites, metagabbros and metabasalts with contacts. This feature testifies to the complexity of the pillow lava structures. They have never been dated and are Middle and Late Jurassic extensional structures of the ascribed to Middle to Late Jurassic by analogy with the Piemont oceanic floor. Piemontite quartzite schist and gabbros of the Zermatt-Saas Fee nappe (see above) and of black chert are also observed (Fig. 5). The calcschist with the Gets nappe in the Prealps (Bill et al. 1997). The detrital ophiolite elements is likely Cretaceous. U–Pb metasediments consist of marble and chert ascribed to the SHRIMP dating on zircon yields in the Zermatt-Saas Fee Late Jurassic and above all of thick masses of Cretaceous nappe an age at 164 ± 2.7 Ma for the Allalin gabbro and calcschist partly dated by Cenomanian-Turonian for- an age at 163–5 ± 1.8 Ma for the Mellichen gabbro (Ru- aminifera (Marthaler 1984). Intercalations of metabasites batto et al. 1998). The same method gives for the Antrona in the calcschist are abundant: it is difficult to discriminate ophiolites in the Loranca valley ages ranging from between magmatic recurrences, tectonic slices, blocks 163.1 ± 2.4 Ma to 155 and 156 ± 2.1 Ma (Liati et al. (olistolites) or volcanoclastic resediments. The Tsate´ nappe 2005) testifying to the Middle Jurassic age of the Piemont differs from the eclogitic Zermatt-Saas Fee and Antrona oceanic floor. An U–Pb SHRIMP age on zircon of nappes by its lower pressure and temperature metamorphic 93.4 ± 1.7 Ma (Cenomanian; Liati and Froitzheim 2006) assemblages indicating high-pressure greenschist facies was obtained in an eclogite of the Balma unit exposed in conditions (Balle`vre and Merle 1993; Reddy et al. 1999; the upstream part of the Valsesia, north of Alagna (Pleuger Negro et al. 2013). et al. 2005). This age is compatible with an U–Pb SHRIMP age on zircon of 87 ± 20 Ma (Late Cretaceous) in an 2.13 The Dent Blanche-Sesia nappe amphibolitic eclogite from Andolla in the Loranco valley (Liati et al. 2001). The post-Late Jurassic basaltic intru- The Austroalpine Dent Blanche nappe and its southern sions in the Furgg and Portjengrat series provide an addi- root, the Sesia zone (Figs. 1, 3 and 4), consist in the study tional argument supporting magmatism during Cretaceous. area of a polycyclic basement gneiss intruded by Palaeo- It is premature to say if these Cretaceous ages represent a zoic gabbros (Mont Collon gabbro: U/Pb zircon age of separate magmatic pulse or the end of an on-going mag- 284.2 ± 0.6 Ma, Monjoie et al. 2007; Anzasca gabbro: 288 matic activity from Middle Jurassic to Late Cretaceous. ?2/-4 Ma, Bussy et al. 1998) and younger crosscutting granites (Mont Collon pegmatitic granite: U/Pb zircon age 2.12 The Tsate´ ophiolite nappe of 282.9 ± 0.7, Monjoie et al. 2007). The gabbros at the base of the Dent Blanche thrust are strongly deformed and The term ‘‘Combin zone’’ has been often used since Bearth metamorphosed under greenschist facies conditions. Dis- (1953) in the Alpine literature for this unit. However Ar- criminating these gabbros from the underlying meta-gab- gand (1909, 1911) had defined the ‘‘Combin zone’’ as the bros of the Tracuit zone (Tsate´ nappe) can be challenging entire composite set of Mesozoic metasediments and (Bearth 1953; Sartori 1987; Figs. 3, 11). The metamorphic Tectonics of the Monte Rosa and surrounding nappes… 17

LA mineral assemblage of quartz, albite, epidote, actinolite, ADU ~84 Ma Santonian PE UST R URO HR A ESABBI RT T N A RD VOLCAN ≥ 40 MaDE US chlorite, titanite, ±biotite, with some Fe-riebeckite (not HA UN THR O TA TT PIO LA SIN ER SCHAMSMB ET O M ONE DU BA UND TA R G RA IO re A IS IS SU N E R O utu LA ALA Mg-glaucophane) indicates greenschist facies conditions. V GO f VA e V A TI ☼ tur S E a AN UN E fu I M ND ON A C8 E Some bands of dolomite and calcitic marble in the Sesia BE ALL N -3 G HELVETIC R LE L V NE EU N O50 ID DE EO AY O ≥ R O E L RM Ç T T ZZ T U N S IC gneiss are interpreted as remnants of its Mesozoic PI ON CO RE TA N U N M - LÈ EN IA R A ION IL IN R OTH S OU G L R E E H RU E B E C sedimentary cover (Venturini 1995). The rocks of the Sesia N N - AB MD O SUB- ZO E CH N LD IS A IEU TA M R S BRI. S Z E E E IE H AT H P E zone preserve an Alpine metamorphic gradient evolving IV UG R C F N S M G N S IO A R LA A T C NE B A UC R S N E -S D from greenschist facies conditions in the study area to O E RN T R E T B G IM O R O A A F T U a C IH O H S A S M IL F K O N S M T 0 R T C R G O A R N -6 amphibolite facies of the Barrovian regional metamor- F N O E G R A E O 0 O T T R T -S Z M 9 M S N U N T T e IE T O F A A T r P S PREPIEMONM R A tu e U N S phism of the Central Alps toward the NE (Fig. 3). The G fu r IA R SI P N M tu E TH A L E R É fu H S R B J E T C E A T N E E Sesia zone underwent farther south, (Fig. 2), out of the Z A D S N R S LA S N E O T U R P B V E T IA A N S N H IA limits of the geological map (Fig. 3), eclogitic high-pres- T GETS E E A T D S C U R

O D sure metamorphism during Late Cretaceous (90–60 Ma; S A AS 2013 Dal Piaz et al. 1972; Hunziker et al. 1992; Duche`ne et al. 1997; Rubatto et al. 1998, 1999; Babist et al. 2007). The Fig. 12 Tentative paleogeographic interpretation of the Alpine ‘‘Seconda Zona Diorito-Kinzigitca’’ klippe and the South- Tethys in the Central Alps before the Tertiary collision of the converging Europe and Adria plates (modified after Steck et al. 2013); ern Alps are not described herein. the Sabbione volcano of middle Jurassic age (Carrupt 2003) stands at the western continuity of a northern branch (Orselina-Bombogno) of the Valais basin (denuded continental mantle window) 3 Structural and metamorphic evolution

3.1 Introduction Froitzheim (2001), Pleuger et al. (2005), and Herwartz et al. (2008) proposed a different paleogeographic model. The Middle Penninic nappes consist of Variscan and older They suggest that the Monte Rosa unit formed the southern polymetamorphic basement gneiss intruded by Late Pa- margin of the European (Helvetic) crust separated to the leozoic granites and covered by their Mesozoic to Tertiary south by the Valais basin from the Brianc¸onnais terrain, the sedimentary series. The composition of the Grundberg, latter being separated by the Ligurian basin (Piemont Tuftgrat, Gornergrat, Cimes Blanches, Barrhorn and Sal- ocean) from the Adria margin. The Antrona ophiolites are arioli series and their Paleozoic basements suggests a attributed to the Valais basin. We agree that the highly common Brianc¸onnais (s.l.) continental origin (Escher deformed and metamorphosed sedimentary cover of the 1988; Steck et al. 1999, 2001; Sartori et al. 2006). The Monte Rosa nappe (Furgg series) is not specific and could Upper Paleozoic to Upper Jurassic Furgg sediment cover of be attributed to various paleogeographic positions. But the the Monte Rosa nappe is to strongly deformed and meta- correlation of the Mesozoic sediments of the Antrona morphosed that it doesn’t allow its attribution to a specific ophiolites with the flyschoid sediments of the Valais basin paleographic domain. The Upper Penninic Tsate´, Zermatt- is very questionable. No equivalents of the Sion-Cour- Saas Fee and Antrona units are composed of ophiolites of mayeur flysch (or contourite) series exist in the Antrona the Middle Jurassic-Cretaceous Piemont oceanic crust and nappe. On the other hand, the Mesozoic series of the An- serpentinites of the denuded mantle with their (Middle? -) trona and Zermatt-Saas Fee nappes are quite similar. Late Jurassic and Cretaceous-Paleocene oceanic floor Froitzheim and collaborators also contest the southern sediments (partly expelled to the Prealps). Paleogeographic rooting of the Brianc¸onnais Siviez-Mischabel nappe reconstructions by Tru¨mpy (1980), Escher et al. (1997), through the Verosso and Camughera gneiss situated below Stampfli et al. (2001) and Bernoulli et al. (2003) suppose the Antrona nappe as proposed by Argand (1911) and by that the Middle Jurassic-Cretaceous Piemont oceanic basin Milnes in Steck et al. (1979), (Milnes et al. 1981) and in the separates the Brianc¸onnais s.l. continental crust to the north geological sections of Escher et al. (1988, 1993), (Steck (including the Subbrianc¸onnais Stalden-Berisal-Ruginenta, et al. 1999, 2001; Steck 2008; Fig. 4). the Brianc¸onnais s. str. Camughera-Siviez-Mischabel and The Tertiary Alpine structures result from the collision, Cimes Blanches, and the Prepiemont Mont Fort, Stock- the SE-directed subduction, the extrusion and accretion of horn, Monte Rosa and Portjengrat) from the Austroalpine upper crustal slices detached from the European and Bri- Sesia and Southalpine Adriatic continental blocks to the anc¸onnais plate below the Adriatic plate (e.g., Tru¨mpy south (Fig. 12). The complexity of the geometry of the 1980; Laubscher and Bernoulli 1982). Figure 12 suggests Piemont oceanic basin or basins may account for the pre- the paleogeographic situation at the Santonian, some sent-day cartographic discontinuity between the Zermatt- 84 Ma ago, before the Alpine collision of the two con- Saas Fee and Antrona ophiolitic units. verging plates. The proposed regional distribution of the 18 A. Steck et al. different units is tentative. Structural, metamorphic and extrusion of the Piemont oceanic crust and mantle and the geochronological constraints enable the temporal distinc- southern Brianc¸onnais (Prepiemont) continental Mont Fort, tion of successive phases of underthrusting, extrusion and Portjengrat, Monte Rosa units between 50 and 38 Ma (Lapen collision in the Alpine thrust belt during the Adria and et al. 2003; Bucher et al. 2005). The occurrence of coesite in Europa plate convergence. Three regional metamorphic the Zermatt-Saas Fee ophiolites in the Lago Cignana region events (Figs. 2, 3) are followed by two episodes of mag- testifies that these rocks were subducted down to at least matic activity. A high-pressure metamorphic event oc- 90 km depth (Reinecke 1998). Angiboust et al. (2009) de- curred during the Late Cretaceous-Paleocene (90–60 Ma) termined homogeneous peak burial conditions with tem- in the southern Sesia zone (Fig. 2; Hunziker et al. 1992; peratures around 540 ± 20 °C and pressures of 23 ± 1 kbar Duche`ne et al. 1997; Ruffet et al. 1997; Rubatto et al. 1999; throughout the 60 km wide Zermatt-Saas ophiolite unit. Konrad-Schmolke et al. 2006; Babist et al. 2007). Jan Bucher et al. (2005) calculate the P–T conditions at Pleuger (personal communication on the Swiss Tectonic *2.5–3.0 GPa and *550–600 °C. The latter suggest that the Group excursion March 2014) dates synkinematic crys- antigorite breakdown in the serpentinites that released large tallised Alpine white mica in Sesia mylonites of the ex- amounts of dehydration water in the subducted serpentinites truding Dent Blanche-Sesia nappe with the 40Ar/39Ar slab (Ulmer and Trommsdorff 1995) facilitated exhumation method at 63.09 ± 0.77 Ma. This age corroborates K–Ar of the Zermatt-Saas eclogites and blueschists. Lapen et al. ages obtained by Zingg and Hunziker (1990), (Zingg et al. (2003) describe a prograde garnet growth between 50 and 1990) from the Montalto region. Then the rocks of the 38 Ma associated with P–T conditions ranging from 1.0 to Zermatt-Saas Fee, Tsate´, Portjengrat and Monte Rosa units 2.8 Gpa and 410 to 620 °C. These data involve average burial were metamorphosed at high-pressure to ultra-high-pres- rates ranging from 0.47 ± 0.03 cm/a at 1.0 Gpa to sure conditions during the Eocene (50–38 Ma; Gebauer 0.23 ± 0.02 cm/a at 2.8 Gpa followed by an extremely rapid 1999; Amato et al. 1999; Engi et al. 2001; Lapen et al. exhumation rate of 10–26 km/Ma. Buoyancy forces acting on 2003; De Meyer et al. 2014). Finally a Late Eocene-Early the low-density serpentinites (D = 2.5–2.6) of the ophiolites Miocene (40–18 Ma) Barrovian regional metamorphism and on the Monte Rosa and Portjengrat granite (D = 2.7) occurred in the Central Alps (Hurford 1986; Hunziker et al. probably played a major role in the rapid extrusion of these 1992; Steck and Hunziker 1994; Markley et al. 1998; Engi units. Several models suggest buoyancy-driven transport of et al. 2001, 2004; Rubatto et al. 2009; Janots et al. 2009). UHP rocks from the subduction conduit (e.g., Chemenda Following these three metamorphic episodes, magmatic et al. 1995; Butler et al. 2013). The garnet and glaucophane activity is responsible for the intrusion of Oligocene mineral assemblage indicates in the Tsate´ nappe high-pres- (32–29 Ma) mantle derived andesitic dikes (including the sure greenschist facies conditions that did not exceed Traversella, Biella and Bergell intrusions, Figs. 1, 2) along 0.8–1.0 GPa of pressure for a maximum temperature of the Insubric line and for the 29–25 Ma crustal aplite and 400 ± 50 °C (e.g., Balle`vre and Merle 1993; Reddy et al. pegmatite dikes in the southern steep belt between Do- 1999; Negro et al. 2013). These P–T conditions clearly con- modossola and the Bergell (Beccaluva et al. 1983; Romer trast with the ultra high-pressure eclogitic conditions ob- et al. 1996; Scha¨rer et al. 1996; Rubatto et al. 2009). served in the Zermatt-Saas Fee nappe (26–28 kbar and The Alpine accretionary wedge is characterised by distinct 600 °C, Reinecke 1998) testifying that these two parts of the domains of similar pressure and temperature conditions but Piemont oceanic crust subducted to different depths before different ages (Fig. 2). The proposed depth of metamorphism their extrusion and accretion to the Sesia and Adria inden- is deduced from p-t estimates. Our observations do not allow ter. Engi et al. (2001) propose pressures of 10–15 kbars and a distinction between the lithostatic pressure and the tectonic temperatures of 600–750 °C for the high pressure metamor- overpressure suggested in the models of Petrini & Podlad- phism of the Monte Rosa gneisses and pressures of chikow (2000), Mancktelow (2008) and Schmalholz & 10–12 kbars and temperatures of 600–730 °C for the Bar- Podladchikov (2013). Between 90 and 30 Ma, the Sesia zone, rovian metamorphism of the Monte Rosa gneiss situated to the Piemont oceanic crust, the Brianc¸onnais continental crust, the East of the Ossola valley. the Valais basin and the southern European crust were suc- Relicts of an eclogitic schistosity and mineral lineation are cessively subducted, extruded and accreted in the huge zone locally preserved at Mellichen and the Spitze Flue in the of Alpine subduction and collision as the result of the upper Ta¨sch valley (Fig. 3; Steck 1989; Ring and Merle southeast-directed European plate underthrusting below the 1992). Steck (1989) suggested, that the eclogitic E–W Adriatic plate (Fig. 12). The Late Cretaceous to Early Pale- trending mineral lineation could belong to the same phase as ocene Sesia underthrusting, extrusion and accretion at the the W-directed Cretaceous shear structures of the Austrian front of the Adriatic plate occurred to the south of the Piemont Alps (Ratschbacher 1987). The mineral lineation is oriented ocean basin, before its closure during the Paleocene-Early parallel to the younger stretching lineation XIV. An E–W Eocene. This was followed by the underthrusting and rapid mineral lineation was also described in eclogite boudins of Tectonics of the Monte Rosa and surrounding nappes… 19 the Adula nappe situated to the east of the Lepontine dome by related to the extrusion of the Late Cretaceous and Eocene Meyre and Puschnig (1993), Partzsch (1998), Kurz et al. high-pressure rocks are observed. Intense Oligocene and (2004) and Cavargna-Sani et al. (2014). The similar orien- Neogene deformation and crystallisation obliterated gener- tation of the eclogite lineation in the Western, Central and ally early schistosity. The deformational structures reported Eastern Alps raises the question of a common Cretaceous– on the tectonic and structural maps and cross-sections of the Eocene deformation phase. However, no shear indicators Monte Rosa region (Figs. 3, 4, 13, 14, 15, 16, 17) result

SW 3 4 5 6 7 NE 5km AT 5km NERGR GOR Hohtälli STOCKHORN ZERMATT-SAAS FEE

E A AAS FE F T-S U N RMAT GORNERGRAT R ZE G T G PORTJENGRAT R VEROSSO MONTE ROSA O F3 N ONE 2 0m A 0m MONTE LE

5km NA 5km LEG I RHÔNE-SIMPLON L. IMA F3 LL F3 C DE N O NE ATÉ O TS EE M Z S F A N AA N A VA T-S O ER M AT R H ON 0m M T TE ER MONTE ROSA G RUGINENTA L 0m 1 Z N U EO A M N A E C F5 3 4 5 6 7

Fig. 13 SW–NE oriented structural cross-sections through the Monte Rosa nappe (for locations, see Fig. 3)

1 2 SE 5km 5km NW S CHE LAN S B Z ME ERMA CI TT - SAA S FEE MONTE ROSA É C T IMES F3 A BLA TS Dufourspitze NCH 5km 0m ES 0m 5km 3

GO F D RN 4 EN F E FR PLA T BL 4 RG IL TTHO ANCHE F RA IHO RN 5 T RN E MONT ROSA SIVIEZ-MISCHABEL FU R S G MI II DK É G T O ZERMATT SC T G O R HA C N T BE A E U L K R 5km S H G F F T F O R T SESIA A 4 5 R T G 4 0m N R 0m MONDEL A LI F3 Trifthorn T F F5 U F3 F5 R F G P 4 G O 20 R T G SESIA MONTE ROSA J R II DK A E UN NT N D 5 0m R G B 0m V 6 O E A N R R MR CA A N A T G Z MUGHERA O 5km ZS N 5km F3 E F F5 5 F F5 F5 5

T F 20 A U R A R B G VEROSSO 25 6 G A 4 N N

28 23 T P G E L 6 R J SESIA R M 5km MR O T 6 0m RUGINENTA R 6 N E A V O SOUTH A J H A P CANAVESE LI A N O ALPINE ZS A Z F5 O F R N 5 N F3 E A CAMUGHERA N

N 6 T VEROSSO

E R 4 4 3 20 6 MR O 0m M N 0m 7 NE O SESIA A CANAVESE LINE EO N L V ZS T E A T A E N N 6 L O

Z F E M 10 km 3 O O

N N

E 1 E RHÔNE-SIMPLON LINE 2

Fig. 14 SE–NW oriented vertical cross-sections through the Monte Rosa nappe, seen in the 46°SW-plunging Vanzone fold axis direction (for locations, see Fig. 3). (II DK) Seconda Zona Diorito-Kinzigitica, (MR) Monte Rosa nappe 20

. l . b l M n b 2 630 X 640 r n 650 O F II 70° o r 660 Alpine structures 4 h o 30° D 65° 35° h II X + N I + 18 - 3 Ma D 34 -18 Ma e 38° IIII t e T 80° o s 30° R 30° d 22° i ? la X N H E II p c HOR 110 A Ô E D e o LM LEON I g + i A l 45° B D N NW-directed thrusting o IV e 84° D 50° t E i O NIE IV = 30° -S IMPL N LI with stretching lineation lb 60° a 60°

F 40° 5

35° 90° 64° 16° A F6 Domo 30° X Saas Fee 25° A HER I E 90° J G SIVIEZ-MISCHABEL RE U 65° Fold axis H 35° PORTJENGRAT 70° P F M NE F and plunge C D D granite gneiss 5 A LEO 5 F I 40° L 40° 25° NTE 2,3 N FO F paragneiss ASCO F Cavalli A C MO anticline 4 70° CAM 2 lake N

A 28° EL 22° O L FURGG B N F 76°

A 40° 5 R 22°

B 60° MISCHABEL

H R T A

C F T ves 75° T 20° S 40° 5 N N O ca F BACK FOLD I F 34° 50° 4

M O 2 N 54° A E 70° Villadossola

40-35 Ma 11 26° E N H I 0°

90° 77° D 50° 37° 66° D G 23° D 20° 13 G D I X IV Antronapiana 73° II I A U F3 F 50° 44° MONCUCCO 100 4 T 30° R 70°X 100 SW-directed extension 10° 22° T II 16° I 40° Mattmark lake SALARIOLI and dextral shear with M 24° 80° 18 5 62° 40° 37° F2 stretching lineation 15° 34° O N E . Z Stellihorn 22° D IIII 22° X

II 45° 20° F

20° 3 70°

36° de Vogogna

Zermatt 40°R a len e

20° A z nb nd BACK FOLDS E n r e

50° D A ho bl H n

35° IV X . + or

X S T 7 h 55° . Fold axis

30° I 1

IV > +

F n e

5 46° t Loro F 40° A o and plunge

X 3 TUFTGRAT e id I 80° as ep cl + A 34° STOCKHORN MONDELLI o 3 I 46° F BALMAHORN 30° N lig 0- 5 40° OR o n S VANZONE IH ANTICLINE A E L e E I L D bit E I 45° al anticline 30° 30° T 46° S N S 26° I T 42° syncline A L R G Vanzone 30° R 46° NE Macugnaga E F R S

3 O MONTE ROSA

90 G A 48° 90 24° E . CIMES BLANCHES granite gneiss Mondelli 80° C I V DIIIIII 32° T A amphibolite facies I N E X G A SE-directed back trusting E E 32° I I C F Z S 4634 m IN with stretching lineation A Monte Rosa L I N 48° . ? IC 81° -K . . A T O XIII S 37° greenschist facies N MONTE ROSA 70° IT - A R T E IO N T MONTE ROSA N granite gneiss D fault thrust fault 24° A A A 7° O COL ET T N paragneiss Z 80° M O . S M 5° N Z F A A P R 3 V 84° L

D ND X E IIII III. A 20° CO DIV Z E 44° Schistosity dip 26° S D N 44° W-directed extension C IIII IM R with streching and dextral underthrusting ALE F 24° 24° Cimalegna GN 34° 4 10° E lineation turned A 58° with stretching lineation H pitch angle 20° to horizontal T. O T Se T XIV 25° F5 r menza57° T U 34°F E 3 2° L O dextral shear F IO 6 TSATÉ Alagna C S 10 Km al. et Steck A. 37° C MASERA 50° 650 O extension F 630 8° B A. STECK, H. MASSON, M. ROBYR 2014 5 Tectonics of the Monte Rosa and surrounding nappes… 21 b Fig. 15 Structural map of the Monte Rosa and surrounding nappes, post-date the S-directed Cimes Blanches flake detachment Figs. 10 (Gornergrat section) and 11 (Mischabel backfold) illustrate and overthrust and the younger retrograde deformation complex interference structures only observable at a local scale phases DIII and DIV (Figs. 15, 16, 17, 18, 19, 20, 21).

3.2.1 The NW-directed thrusting and deformation period mainly from the Late Eocene to Neogene Alpine continental DI collision. The superposition from base to top of the high- pressure oceanic Zermatt-Saas Fee, continental Cimes Three schistosities S1–S3 developed during the NW-di- Blanches, oceanic Tsate´ and continental Dent Blanche rected thrusting period DI. The oldest schistosity S1 is sub- nappes is an enigmatic Alpine structure; it will be discussed in parallel to the stratification and thrust planes. It is difficult the kinematic model of chapter 4. to recognise and is often obliterated by the younger de- formations. The view on the Mischabel backfold above 3.2 Middle Eocene to Neogene underthrusting Zermatt illustrates the three phases of deformation related of the European Plate below the Adriatic to the NW-directed thrusting DI (Fig. 11). Only the thrust indenter, nappe formation, post-nappe folding of the Cimes Blanches on the Zermatt-Saas Fee nappe is and dextral transpressional deformations older and related to the S-directed flake detachment (see chapter 4). The thrust surfaces S1 are folded by second F2 In this chapter we discuss the main NW-directed trusting and third F3 folds prior to the F4 Mischabel backfold. The phase DI, the Mischabel and Neubru¨ck F4 backfolds, the distinction of the three schistosities S1–S3 is very difficult younger SW-oriented high-temperature dextral shear phase in the gneisses and in the basal mylonitic thrust zone of the DII structures and related Vanzone backfolding F5, that Dent Blanche nappe (Fig. 11) and only the main

1 MONDELLI FOLD F3 2 MISCHABEL FOLD F 4 3 GORNERGRAT FOLDF3 4 MEZZALAMA DOME 5 RIO DI MENTA FOLD F3

Alpine structures 6 VANZONE FOLD F5 . l . b l n b 2 M r n 630 F X 70° 640 o r 650 660 II o h 30° O D 4 D 65° h 38° I 35° + II X + I N e NW-directed thrusting t e o s 80° d 30° R T 30° 22° i la TICLINE with stretching lineation X ? AN H II p c RN 110 D e O o H E IIII 45° A Ô g M E + i L N l A D N LEO o B e IV 50° it 84° E IE D = 30° - LON IN IV lb 60° S IMP L a 60°

30° X

I F 40° 5 Fold axis 64° 16° 35° 90° A Domo and plunge 25° A HER F6 E 90° J G F SIVIEZ-MISCHABEL 35° RE U 65° 2,3 H 70° P anticline E PORTJENGRAT F M NE F

C 5 O 5 D IN A LE I 40° L 40° 25° NTE e N 7 IC F F A C MO T 4 70° 2 N c

A 28° N 22° O MISCHABEL L A FURGG 76° o N X F

F L IV 40° 5 R T

4 B E 60° BACK FOLD A R 75°

B D F T T T 20° A 40° IV 5 N N Villadossola H 34° 50°

O F E N C A 70°

S 54° 2 26° E I N 0° H I 5

D M 90° II D 7 37° 66° 50° G 20° G SW-directed extension X 23° 2 I A ONCUCCO U73° F3 50° 44° M 7 MELLICHEN BASITE and dextral shear with 100 T 30° R 70°X 100 10° F T II stretching lineation 16° 22° 4 I 40° M 24° 80° 17 18 7 ONE 62° 40° 37° 15° Z . 34° Stellihorn 22° X

II 22° 40°

20°

VANZONE 20° F 70° e

R 3 36° nd

Zermatt A ble e 45° E n d 20° or n Vogogna

BACK FOLDS 50° H 6 h ble

D S + n

IV X r

35° 7 o

Fold axis I 1 h 55° . Loro

X 1 > +

IV n F A te

and plunge F 5 F 46° e o

46° 40° s d 5 X 3 TUFTGRAT 30° la pi A I 80° oc e I ig + 34° STOCKHORN MONDELLI ol -3 S anticline 30° 17 N n0 40° OR A E 3 H ANTICLINE e E 30° I it I syncline L lb L D 45° 46° Vanzone a S 30° E I N 26° I T 42° T A L R S RG 30° NE 46° Macugnaga E F OR SE-directed back trusting 3 G

D E S

90 A 48° 90 IIIIII . with stretching lineation N E 24° I Mondelli 80° C CIMES BLANCHES L I V 32° C T . I I A E ? T AN E X D G . X F N 32° I IIII I C III S 4634 m A Z A Monte Rosa E I N 48° fault thrust fault N 81° -K . A O O Equal-area net, lower hemisphere S Z 70° IT - 37° MONTE ROSA N R T A IO N T V D W-directed dextral 24° A A Schistosity A 7° CO TT N S D underthrusting with 80° LME O . IV M 4 5° Z P stretching lineation MEZZALAMA GRANITE F A L X

R 3 84° D Stretching lineation

D N X A I E IIII III. 20° CO X Z 26° SE 44° N Stretching lineation X IV R Schistosity dip II C 44° IM ALE F E with streching dextral shear 24° 24° G 34° 4 10° NA H lineation turned X 58° pitch angle Stretching lineation IV T to horizontal 20° O extension T U 25° F5 57° T Fold axis MASERA E O 34°F3 2° L SYNCLINE Alagna IO S TSATÉ C 10 Km Axial surface AS 2013 F 37° C 6 50° 650 O F 630 8° B 5

Fig. 16 Structural map 1 of the Monte Rosa nappe with stereograms of typical Alpine structures 22 A. Steck et al.

1 ALTERHAUPT, TSATÉ 2 ALPHUBEL FOLD F3 3 DENT BLANCHE THRUST 4 CIMES BANCHES 5 VALTOURNANCHE, Z-S ++ TSATÉ + + +++ ++ ++ + +++ + +++ + Alpine structures 6 ZERMATT, ZERMATT-S. . l . b l n b 2 X r n 620 630 F II 70° 640 o r 650 660 4 h o 30° D 65° D 35° h II X + 38° I I + e NW-directed thrusting t e o s 30° 80° 30° d a X 22° i ? l ANTICLINE + with stretching lineation II p c N 110 + D R e O IIII o 45° AH g M + i L l BA D e o IV

50° it 84° = lb 60° a 60° + + 30° X F + 40° + + I 5 16° 35° Fold axis 90° ++ 25° F6 A ++++ and plunge SIVIEZ-MISCHABEL 90° EJ + ++++ F2,3 35° PORTJENGRAT 70° PR F anticline TSATÉ NE 5 ++ 40° LI 40° 25° 7 IC F G F A + 28° T 4 22° 70° 2 N

AN FURG O 22° N X F +

MISCHABEL EL IV 60° 40° 5 R R T F BACK FOLD B D F 4 20° A 40° IV 34° 5 N H F 50° DI C 54° O 2 A 26° IS 2 CCO H E M 50° 7 37° 90° D H X G 23° 20° II C X I A ONCU I 50° 44° M SW-directed extension 100 N 10° T 30° 100 7 RIFFELALP, ZERMATT-S. A 16° 22° F IT and dextral shear with L 4 24° 40° 80° B 18 M X stretching lineation 17 40° 37° ONE II 15° 34° Z . T Stellihorn 22°

N Alterhaupt

1 40° E X 20°

II D 22° 45° 20° F 70°

30° R 3 36° VANZONE Zermatt A

20° 50° E

D H +

S

BACK FOLDS 6 35° IV X

10° I

F + Fold axis 5 46°

F 40° e X Riffelalp 3 TUFTGRAT s + 30° a and plunge F T I 80° cl 46° SATÉ 34° go 5 17 STOCKHORN MONDELLI li + o 30° N ite 7 40° OR ANTICLINE lb anticline Matterhorn 30° IH a L D 46° Vanzone 30° L I 45° syncline 4478 m E 26° +

X AT 42° T IV GR S o M. Cervino R A X 30° NE Macugnaga 46° I I F OR 20° 3 G S SE-directed back trusting 90 E 90 D 24° IN Mondelli 80° E + IIIIII with stretching lineation 3 CIMES BLANCHES L S 32° IC . FEE ? T X AS N 32° I DIIII N A 4634 m A . . X Breuil -S I III 20° TT Monte Rosa E Z A N 81° N M O I fault thrust fault ER Z 70° -K Z 37° MONTE ROSA N O Equal-area net, lower hemisphere IT VA R 3166 m 24° IO XII 7° CO TTA D W-directed dextral 80° LME A Schistosity underthrusting with Cimes Blanches MEZZALAMA GRANITE 5° N . D F3 84° O

IV stretching lineation DIIII Z X X 15° 4 20° DA III. Stretching lineation I ON 44° 26° C S X CIMAL E X IV Valtournanche D S P Stretching lineation II F L I 24° 24° EGNA 34° 4 10° dextral shear X 58° A 5 I 20° N Stretching lineation X O IV F T R extension X 25° 5 57° T E II 34°F E MASERA 10° TSATÉ 3 2° L H Fold axis F3 Alagna IO T SYNCLINE C 37° C U 10 Km O Fold axis F4-5 620 50° O F DB F 630 8° 650 B + 5 S 6

Fig. 17 Structural map 2 of the Monte Rosa nappe with stereograms of typical Alpine structures schistosity (S2 and/or S3) was represented on Figs. 3, 15, 2008; Steck and Hunziker 1994; Lacassin 1984, 1989; 16 and 17. Note that in the Alpine literature the main Lacassin and Mattauer 1985; Steck et al. 2013). Also the schistosity and the main phase of NW-verging folding and main deformational structures of the Stellihorn shear zone thrusting are often noted FA1, S1 and FA2 and S2 in the northern Monte Rosa nappe belong to this phase of (Table 1; Huber et al. 1980; Ramsay in Steck et al. 1979; NW-directed thrusting DI (Lacassin 1984, 1989; Steck Klein 1978; Grujic and Mancktelow 1996; Maxelon and 1984, 1990; ‘‘Mattmark phase’’ of Pleuger et al. 2008; Mancktelow 2005). We don’t follow this simplification Table 1). It is suggested that these DI phase deformational (e.g., Steck 1984, 1990). The distinction of the parallel structures have been formed by ductile detachment of the SE–NW stretching lineations as L1, L2 or L3 and their upper Brianc¸onnais and European crust during their Ter- attribution to the schistosities S1, S2 and S3 is only rarely tiary SE-directed underthrusting below the Adriatic plate possible. For this reason the SW–NE oriented stretching (Steck 1990; Epard and Escher 1996; Escher and Beau- lineations, related to the three schistosities with top-to-NW mont 1997). Fold axes were reoriented parallel to the main shear indicators, are designated together as XI (XI corre- SE-NW direction of extensional shearing by rock internal sponding to the great axis of the deformation ellipsoid). rotation. Such F2 and F3 folds, with their NW-oriented They are related to the first period of NW-directed axes like the Platthorn F3 fold to the north of Zermatt thrusting DI that includes several phases of NW-directed (Fig. 11, Alterhaupt fold, Stereogram 1 on Fig. 17; Sartori thrusting and folding (Table 1). The second S2 and third 1987) and the Stockhorn-Gornergrat F3 fold south of S3 schistosities with their NW–SE-oriented stretching Zermatt (Figs. 3, 10, 13, 14 and stereogram 3 on Fig. 16) lineation called XI and associated top-to-NW shear indi- have a same SW-vergence. This reveals a component of cators are the main deformational structures of the Pen- dextral transpression during the NW-directed overthrusting ninic nappes (Figs. 3, 15, 16, 17; Steck 1980, 1984, 1990, (Steck 1989). The Cimalegna F3 fold to the north of Tectonics of the Monte Rosa and surrounding nappes… 23

SW NE

XII

4 cm Monte Rosa gneiss

Fig. 18 Monte Rosa granite gneiss from Villadossola with stretching lineation XII, view on the schistosity plane (Ovesca river-bed, Swiss map coordinates: 663.3 km/102.0 km/250 m)

SW NE

S, XII

2 cm Monte Rosa granitegneiss

Fig. 19 Monte Rosa granite gneiss from Villadossola in Ovesca type deformed K-feldspar porphyroclasts with K-feldspar growth in riverbed with stretching lineation XII, surface perpendicular to the pressure shadows indicate dextral shear deformation under amphibo- schistosity plane and parallel to the stretching lineation XII. Sigma- lite facies conditions

Alagna-Valsesia characterised by low angle NW- or SE- Only the Mondelli F3 recumbent anticline, in the centre of plunging fold axis and stretching lineation has a similar the Monte Rosa nappe is NE-facing (Figs. 3, 15 and SW-vergence as the Platthorn and Gornergrat folds and stereogram 1 on Fig. 16). Mondelli is the name of a hamlet belong likely to the same deformation phase (Figs. 3, 11, and chapel in the Anzasca valley situated on the axial trace 13, 14, 15; Gosso et al. 1979; Pleuger et al. 2005, 2008). and with a good view on the huge fold hinge. The axial 24 A. Steck et al.

W E

F2

F1

Vanzone F5

F3 10 cm

Fig. 20 ‘‘Mushroom-shaped’’, multiple Type 2 fold interference F5 overprinting older F1-2-3 folds (Ovesca river-bed outcrop near pattern (after Ramsay 1967) in hornblende-biotite-plagioclase-gneiss Ruginenta, Swiss map coordinates: 657.0 km/99.6 km/575 m) of the Ruginenta unit, from the hinge zone of the Vanzone anticline

SW NE

10 cm

Fig. 21 Same locality as Figs. 18 and 19. Amphibolite of the chlorite growth in a clock-wise rotated extensional joint and drag Antrona zone on the northern border of the Monte Rosa gneiss. folds demonstrate the continuation of dextral shear under retrograde Boudinage of a competent plagioclase rich layer with quartz and greenschist facies conditions during DIV deformation surface schistosity is a third schistosity S3 refolding two The NW-directed thrusting of the Dent Blanche nappe is older schistosities S1 and S2 in the Rio di Menta fold younger than the youngest sediments of its footwall, i.e., (stereogram 5 on Fig. 16; Reinhardt 1966; Steck 1984). the Middle Eocene flysch of the Siviez-Mischabel nappe. Tectonics of the Monte Rosa and surrounding nappes… 25

The NW-directed thrusting of the Siviez-Mischabel nappe Camughera, Zermatt-Saas Fee and Monte Rosa units in the was dated by Markley et al. (1998) with the 40Ar-39Ar hinge of the Vanzone anticline, between Antronapiana and dating on synkinematic white micas from the Triassic of the village of Vanzone. The stretching lineation XII is ori- the Siviez-Mischabel nappe at 40–35 Ma. ented parallel to the 46°SW-plunging Vanzone fold axis F5 (Steck 1984, 1990; Steck and Hunziker 1994; Figs. 3, 15, 3.2.2 The Mischabel backfold, F4 16, 18, 19, stereograms 4, 5 and 6 on Fig. 16 and stere- ograms 5 and 7 on Fig. 17). The axial trace of the Vanzone The Mischabel backfold F4 (Figs. 11, 14 and stereogram 2 anticline continues from the evident fold hinge to the north on Fig. 16) deforms the pre-existing thrust planes, S1–S3 of the village of Vanzone in a NE-direction, crosses the schistosity and XI stretching lineation. Milnes et al. (1981) Ovesca river at the bridge branching to Ruginenta (Fig. 20), and Mu¨ller (1983) studied the geometry of the Mischabel continues through the marble fold hinge of Alpe Pradurino, anticline and Mittaghorn syncline. These S-verging folds then passes south of the Moncucco summit, in the centre of are coeval with the NW-directed frontal thrusts of the the Moncucco granite gneiss to the ‘‘Calvario’’ church south Middle and Lower Penninic nappes (Escher et al. 1988, of Domodossola (Fig. 3; Milnes et al. 1981; Steck et al. 1993; Escher and Beaumont 1997). Neither dextral shear 1999; Pleuger et al. 2005; Steck 2008). These field obser- structures, nor a stretching lineation parallel to the F4 fold vations are different from the structural interpretation of the axis are observed in the Mischabel backfold. Pleuger et al. Vanzone fold published by Keller et al. (2005), that has to (2008) attributed NW- or SE-plunging stretching lineations be revised. A similar dextral shear zone (also in blue on with top-to SE shear indicators of backthrusting in the Figs. 15, 16, 17) is exposed parallel to the southern contact Vanzone fold hinge to his ‘‘Mischabel phase’’. Another of the Monte Rosa nappe and forms top-to-SW detachment backfold that deforms the limit of the Sion-Courmayeur structures in the porphyritic Mezzalama granite gneiss and unit and the Zone Houille`re situated at Neubru¨ck between the overlying Zermatt-Saas Fee ophiolites (stereogram 4 on Stalden and Visp is attributed to the same phase of back- Fig. 16; Le Goff (1986) in Balle`vre and Merle 1993; Steck folding F4 (Fig. 4). The north-closing syncline exposed at 1990). These dextral ductile shear zones DII developed the Passo della Preja, belongs probably to the same back- between ca. 34 and 18 Ma (Figs. 3, 15; Steck and Hunziker folding phase F4 as the Mischabel backfold (Fig. 6). 1994; Romer et al. 1996; Scha¨rer et al. 1996; Markley et al. 1998; Steck 2008; Steck et al. 2013). The dextral shearing 3.2.3 SW-striking dextral-shearing DII, backfolding F5 DII, the Vanzone backfolding and the steepening of the and formation of the southern steep belt southern root of the Penninic nappes were formed under of the Central Alps high temperature amphibolite facies conditions (Steck and Hunziker 1994; Engi et al. 2001; Keller et al. 2005; Steck The NW-directed thrusting parallel to the stretching lin- 2008; Steck et al. 2001, 2013). The synmetamorphic Van- eation XI was overprinted by SW-striking dextral ductile zone anticline (Figs. 2, 3, 4, 15, 16, 17; Steck et al. 2001) shear zones during a younger phase of dextral transpression folded the greenschist facies–amphibolite facies isograd, characterized by a SW-plunging stretching lineation XII defined by the reaction: albite An0-3 ? epidote ? horn- (Figs. 18, 19). The main dextral shear zone during this DII blende = oligoclase An17 ? hornblende (Wenk and Keller deformation period is the Simplon ductile shear zone (rep- 1969; Steck et al. 2001). A marked change in the style of the resented in blue on Figs. 2, 15, 16, 17). It crosses the whole F5 Vanzone anticline is observed from the SW to the NE. Penninic and Austroalpine nappe stack from the Canavese Whereas the Vanzone anticline forms an open concentric line near Locarno and the Centovalli and Valle Vigezzo to F5-fold north of Alagna-Valsesia, the fold geometry is the east, through the Simplon pass and reaches the Rhone transposed into a squeezed fold with parallel fold limbs to valley west of Visp in the front of the Siviez-Mischabel the east in the Toce valley (Figs. 1, 3, 15, 16, 17; Steck nappe (Steck 1980, 1984, 1990; Steck and Hunziker 1994; 2008). This modification in the geometry of the fold results Steck 2008; Steck et al. 2013). During on-going dextral from squeezing related to an increase of rock ductility from shearing, the whole Penninic nappe stack was deformed by the low temperature, greenschist facies conditions on the the younger SE-facing Vanzone F5 anticline, termed Ar- roof of the Monte Rosa nappe, to the high temperature, gand’s ‘‘Insubric phase’’ of backfolding in the Alpine lit- amphibolite facies grade rocks in the Toce valley. The ‘‘golf erature (Figs. 3, 4, stereogram 6 on Figs. 16, 20). The club’’ shaped geometry of the Monte Rosa nappe results Vanzone fold deforms the axial trace of the Mischabel from this increase of deformation intensity from the large backfold (Figs. 3, 20 and geological Sect. 4 on Fig. 14). frontal fold to the root zone of the nappe (Figs. 1, 2, 3, 14, The Vanzone F5 anticline developed during a late phase of 15, 16, 17). The cooling from west to east of the Monte Rosa the still on-going dextral ductile shearing DII. This dextral, nappe by uplift and erosion to temperatures lower than top-to-SW shear zone collectively overprints the Ruginenta, 650–600 °C was dated with LA PMMS U–Pb monazite 26 A. Steck et al. ages by Engi et al. (2001) at 45–40 ± 3 Ma at Macugnaga, 1994; Campani 2011; Steck et al. 2013). A steep NW- 42–40 ± 1 Ma at Antronapiana and 33–31 ± 1 Ma in the plunging stretching lineation XIII associated with top-to-S Toce Valley. Nearly isothermal decompression induced thrust structures developed during backthrusting DIII on sillimanite crystallization in the high-grade staurolite and the Canavese inverse fault (Figs. 3, 15, 16, 17). The am- kyanite bearing rocks west of Domodossola (Keller et al. phibolite facies dextral shear DII is extended toward the 2005). 40Ar-39Ar-ages on hydrothermal muscovite of gold- south in the southern steep belt of the Central Alps. Here quartz veins indicate farther cooling of the Vanzone anti- the dextral shearing (DIV) develops under retrograde cline below 300 °C at 31.6 ± 0.3 Ma in the Brusson win- greenschist facies to non-metamorphic conditions of de- dow (9 on Fig. 1) and 32.4 Ma near Quarazza, 29.0 Ma at formation (represented in red on Figs. 15, 16, 17). Another Pestarena (these three localities are situated to the SW of the greenschist facies to anchizonal dextral shear and un- map Fig. 3). The same method of dating yields farther to the derthrust zone DIV, ending with brittle shear structures and NE consistent younger ages of 24.5 Ma at Alpe Trivera fault gauges (kakirites, Svenonius 1892 in Higgins 1971), south of Antronapiana and 11.6 Ma on the Rhone-Simplon crosses the Alps from the Centovalli and Valle Vigezzo to line in the Zwischbergen Valley above Gondo (Fig. 3; the east of Domodossola (Domo on Fig. 3), through the Pettke et al. 1999). These 40Ar-39Ar muscovite ages cor- Bognanco and Loranco valleys to the region of Zermatt to roborate the K/Ar and Rb/Sr white mica and biotite cooling the west (Fig. 3, DIV on Figs. 3, 15 and stereogram 7 on ages already obtained by Hunziker (1969), Hunziker and Fig. 16). Illites, from nine fault gauges of the Centovalli Bearth (1969), Hunziker et al. (1992), Steck and Hunziker Line were dated with the K–Ar method of 14–4 Ma by (1994) and Steck et al. (2013) on the same profile. The rapid Surace et al. (2011), completing the 9.1–8.3 Ma ages of cooling of the amphibolite facies Vanzone anticline in the Zwingmann and Mancktelow (2004). The over two kilo- Toce transect to 650 and 300 °C at 32–30 and 10 Ma re- metre wide dextral zone of shear and underthrusting DIV in spectively, was accompanied by uplift, tectonic denudation the southern Sesia zone and the Ivrea zone south of the and erosion of the Lepontine dome structure. This uplift Canavese line, designated as ‘‘schisti di Fobello e Rimella’’ phase occurred contemporaneously with dextral shearing to the south–west of the Toce valley, was mainly formed along the ductile Simplon shear zone DII (34 to \25 Ma). under retrograde greenschist facies to non metamorphic The timing of this dextral shearing is inferred from the ages conditions (Fig. 15). The mylonites are younger than the of the mantle derived andesite and tonalite (32–29 Ma) and Oligocene intrusives (Schmid et al. 1989; Steck 1990). It is synkinematic crustal aplite and pegmatite intrusions in the probable that this zone of mylonites overprints also older southern steep belt between Domodossola and Locarno. deformational structures of the Canavese line that are not Dextral shear occurred before the andesite intrusions and distinguishable from the younger mylonites. The Canavese continued before, during and after the synkinematic aplite line that separates Cretaceous eclogites of the southern and pegmatite intrusions dated at 29–25 Ma (Steck and Sesia zone from Alpine greenschist facies structures in the Hunziker 1994; Romer et al. 1996; Scha¨rer et al. 1996; Southern Alps border was certainly active since Cretaceous Steck et al. 2013). time. These older movements are attesded both by geo- chronology (see above Sect. 3.1) and by the sedimentation 3.2.4 Brittle normal detachment on the Rhone-Simplon of the Late Cretacous flysch of the Simme nappe in the line and continuation of dextral shear DIII and DIV Prealps. The Dent Blanche depression to the west, the under retrograde metamorphic conditions Lepontine dome to the east, separated by the Simplon ductile shear zone DII and the younger Rhone-Simplon low On-going cooling by uplift and erosion of the Lepontine angle detachment DIV form together a pull-apart structure dome at temperatures below *300 °C marked the onset of in the zone of dextral transpression between the European brittle normal faulting on the Rhone-Simplon line that is plate to the north and the Adriatic indenter to the south superimposed to the earlier Simplon ductile shear zone DII (Fig. 1; Steck 2008). The inclination, increasing from north (blue on Figs. 2, 15, 16, 17; Simplon dextral shear DII and to south of the 20°SW-plunging Hu¨bschhorn fold at the DIV on Table 1). In contrast, a ductile shear DIV still Simplon pass, the 30°SW-plunging Balmahorn and occurs in the hot footwall of the Simplon shear zone 46°SW-plunging Vanzone F5 folds result from the Central (represented in red on Figs. 2, 15, 16, 17). This brittle low- Lepontine dome uplift since 35–25 Ma dated by radio- angle normal fault detachment on the Rhone-Simplon line metric cooling ages (Hurford 1986; Hunziker et al. 1992; was dated by the Rb–Sr and K–Ar methods on muscovite Steck and Hunziker 1994; Pettke et al. 1999: Engi et al. and biotite and the zircon and apatite fission track method 2001; Rubatto et al. 2009; Steck et al. 2013). Figure 22 at 18–3 Ma (Hurford 1986; Hunziker and Bearth 1969; represents the view from Croppo on the amphibolite facies Soom 1990; Steck 1984, 1990, 2008; Mancktelow 1985, Vanzone F5 anticline in the Moncucco summit, the Preja 1990, 1992; Hunziker et al. 1992; Steck and Hunziker F5 syncline in the Bognanco valley and the large Toce Tectonics of the Monte Rosa and surrounding nappes… 27

SW NE

V an zo n e a n t LINE ic LON l P IMP in re E-S e ÔN ja H F Moncucco s R 5 y n Bognanco c l in ULMIN e TOCE C ATION + F 3 + 4 5 + + NAPPE NDUN 2 + An BE trona 6 3 LE 3 6 5 + 7 MO F un N 1 e T 1 ANTI in ML 5 6 it E GORI cl L O N E n + Camughera E AP PP y Ruginenta unit O PE 1 NA s NE NE a unit F6 N LEO r + 3 AP NTE se PE MO Ma Domodossola Valle Vigezzo Toce Masera CENTOVALLI LINE

+

Croppo

Fig. 22 View from Croppo on the Toce alluvial plain near the greenschist facies Masera F6 syncline and the large Toce Domodossola, the amphibolite facies Vanzone F5 anticline in the culmination of the Lepontine gneiss dome situated in the foot wall of Moncucco summit and the Preja F5 syncline in the Bognanco valley, the 30–35°SW-dipping Rhoˆne-Simplon normal fault culmination of the Lepontine gneiss dome situated in the subduction, an extensional event occurred between 80 and footwall of the 30–36° SW-dipping Rhone-Simplon normal 60 Ma leading to the reactivation of former crustal-scale fault. Figure 22 illustrates also the position of the Masera thrusts as extensional detachments (e.g., Combin fault). F6 syncline that was formed under retrograde greenschist This extensional event would be followed by a Mesoalpine facies conditions after cooling by uplift and erosion of the phase of convergence between 45 and 40 Mas. New ra- amphibolite facies Vanzone F5 anticline (Steck 2008; diometric dating (Hunziker et al. 1992; Duche`ne et al. Steck et al. 2013). 1997; Ruffet et al. 1997; Rubatto et al. 1999; Steck and Hunziker 1994; Markley et al. 1998; Engi et al. 2001, 2004; Konrad-Schmolke et al. 2006; Babist et al. 2007; 4 Model of the Tertiary kinematics of the Central Rubatto et al. 2009; Janots et al. 2009) suggests a con- Alps tinuous succession of phases of subduction, extrusion and accretion in the Alpine accretionary wedge during the Already Argand (1916) (Figs. 1, 3, 4, 11, 14) observed the Adria-Europa convergence since about 90 Ma with a mi- superposition, from base to top, of the oceanic Zermatt- gration of the zones of subduction, extrusion and accretion Saas Fee unit, the continental Cimes Blanches and Frili- from south to north (Fig. 12). However, the normal fault horn, the oceanic Tsate´ and continental Dent Blanche models of Platt (1986) and Balle`vre and Merle (1993)do nappes. Numerous authors discussed this remarkable not account for the presence of the continental Cimes Alpine tectonic structure (Argand 1916;Gu¨ller 1947; Blanches unit at the limit of the ultra-high pressure Zer- Bearth 1953, 1954a, b, 1964, 1978; Sartori 1987; Platt matt-Saas Fee with the greenschist facies Tsate´ nappes. An 1986; Escher 1988; Marthaler and Stampfli 1989; Vannay alternative tectonic model that takes into account the and Allemann 1990; Ring and Merle 1992; Balle`vre and presence of the Cimes Blanches nappe is developed here- Merle 1993). Platt (1986) and Balle`vre and Merle (1993) after. It is based on the Brianc¸onnais affinity of the Triassic suggest that the exhumation of the high-pressure Zermatt- of the Gornergrat and Cimes Blanches units (see above Saas Fee unit in the footwall of the Combin normal fault Sect. 2.8). Note that the supposed SE-directed thrust of the permitted the juxtaposition of these high-pressure meta- Cimes Blanches nappe is not confirmed by deformational morphic rocks with rocks from shallower levels (herein the structures. For this reason it cannot be excluded that the Tsate´ ophiolite nappe, Escher et al. 1988). Platt (1986) Brianc¸onnais type Cimes Blanches and Gornergrat units suggests that the formation of detachment faults occurred represent continental allochthons inside of the Piemont during on-going convergence in an orogenic accretionary Ocean. The detailed mapping of the Cimes Blanches region wedge. In contrast, Balle`vre and Merle (1993) argue that, by Vannay and Allemann (1990) and of the Mischabel following the 100–90 Ma Eoalpine phase of deep backfold by Sartori (1987, 1990) (Fig. 11) and personal 28

NW MOLASSE Gur Pr Ni Wi Do Be SM DB Ts ZS 2DK SOUTHERN ALPS SE MOLASSE R-S SM SOUTHERN R-S ZH LI LI ALPS ~100 km MR Se Gas MR Aar Se MOHO ~30 km Gas Aar 30 Ma to present:MOHO Dent Blanche depression-Lepontine dome pull-apart, Rhône-Simplon line, Ts 2DK LI Pegmatites DB ZS MOHO Jura thrust and Aar back fold 29-25 Ma ~30 km MR Se Bi BIELLA Gr. MOHO Aar ZH ~30-25 Ma Be SM MOHO 31 ± 0.2 Ma Dextral transpression, MOHO TAVEYANNE And. Argands Insubric phase of backfolding, 32.5 ± 0.3 Ma Helvetic nappes and 32-25 Ma ~30 km DB CB Ts Ca SOUTHERN Insubric line magmatism Va LI ALPS NW-directed thrust of Valpelline, Dent Blanche and Aar CB Ts Tsaté on Cimes Blanches and Siviez-Mischabel, ~34 Ma SM Se followed by south directed Mischabel back fold, ZH Be-Ru Fu Por followed by dextral transpression and MR Ant ZS accompained by 40-22 Ma Barrovian metamorphism 1) Delamination and SE-directed 1) CB 2) thrust of Cimes Blanches flake Ts VaDB on Zermatt Saas Fee and Tsaté ~40 Ma 2) NW-directed thrust of Tsaté and Fu SM Por Dent Blanche on Cimes Blanches Ant Se MR ZS Gets CB SOUTHERN CB ALPS SE-directed subduction of Portjengrat, Antrona, Va and Monte Rosa - Furgg, their Por Ts 50-38 Ma high pressure metamorphism, DB SM Va Fu followed by their buoyancy-driven extrusion SM Ant Se DB MR during folding and accretion ZS Ant Ts Fu Se MR Por

ZH EUROPE Go Tu CB MR Fu Ant Gets ADRIA After accretion of Dent Blanche-Sesia below ZS the Southern Alps (Adriatic plate): ~50 Ma Va SE-directed subduction, medium MOHO Por DB pressure metamorphism, extrusion Ts SM ZS and accretion of Tsaté, 50-38 Ma subduction and ultra-high pressure EUROPA - ADRIA convergence Se al. et Steck A. metamorphism of Zermatt-Saas Fee Tectonics of the Monte Rosa and surrounding nappes… 29 b Fig. 23 Model of the Tertiary kinematics of the Central Alps. The E. The spectacular S-verging Mischabel F4 fold (Figs. 3, figure is not to scale. The relative paleogeographic position of the 11, 14, 15 and stereogram 2 on Fig. 16) exposed north Cimes Blanches, Portjengrat and Monte Rosa continental units and the geometry of the Piemont oceanic basin are tentative of Zermatt reflects a younger phase of S-directed backfolding conjugate to the frontal NW-directed observations (Steck 1989) are used to propose a new Penninic and Helvetic thrusts (Escher et al. 1993; kinematic model. Escher and Beaumont 1997). The Mischabel back fold The proposed model (Fig. 23) is based on the assump- deforms the pre-existing thrusts and schistosities S1-S3 tion that in the Late Cretaceous, the Brianc¸onnais (s.l.) with their associated NW–SE-trending stretching lin- continental was separated by the Piemont ocean from the eation XI, at the southern contact of the Siviez- Austroalpine Dent Blanche-Sesia continent to the south Mischabel nappe. (Fig. 12). A. The Adria–European plate convergence starts with the Late Cretaceous to Early Paleocene subduction, high- 5 Conclusion pressure metamorphism, buoyancy driven extrusion and accretion of the Sesia zone to the Adriatic indenter Table 1 proposes a tentative correlation of the Alpine (Hunziker et al. 1992; Duche`ne et al. 1997; Ruffet Structures observed in the Monte Rosa region and the et al. 1997; Rubatto et al. 1998; Dal Piaz et al. 2001; Lepontine gneiss dome. The Early Alpine (Late Cretaceous Konrad-Schmolke et al. 2006; Babist et al. 2007; and Eocene, so-called ‘‘Eoalpine’’) phases of SE-directed Handy et al. 2010). subduction, extrusion and accretion of the Sesia, Tsate´, B. It is followed by the Eocene subduction to various Zermatt-Saas Fee, Monte Rosa and Portjengrat units are depths of the Piemont oceanic floor (Gebauer 1999; inferred from P–T estimation, radiometric dating of high- Amato et al. 1999; Lapen et al. 2003). The southern pressure metamorphic minerals and the tectonic position of Tsate´ nappe was overprinted by a high-pressure the different units (e.g., Balle`vre and Merle 1993; Reinecke greenschist facies metamorphism at a pressure of 1998; Gebauer 1999; Amato et al. 1999; Lapen et al. 2003; 9 kbar and temperature of 400 °C (Balle`vre and Merle Bucher et al. 2005). The E–W directed stretching lineation 1993), whereas the northern Zermatt-Saas Fee ophio- in eclogites of the upper Ta¨sch valley testifies of a phase of lites were metamorphosed under ultra-high pressure high-pressure deformation (Steck 1989). The lineation has conditions of over 26–28 kbar and 600 °C (Reinecke the same orientation as an eclogite mineral lineation ob- 1998; Bucher et al. 2005), then extruded by buoyancy served in the Adula nappe (Meyre and Puschnig 1993; forces and accreted to the Sesia-Adria plate margin. Pleuger et al. 2003, 2008; Kurz et al. 2004; Cavargna-Sani C. The less-dense Cimes Blanches (and Frilihorn) conti- et al. 2014) and a Cretaceous E–W extension observed in nental flake was, during on-going Adria-Brianc¸onnais- the Eastern Alps (Ratschbacher 1987). Other deformational Europe convergence, detached by crustal delamination structures allowing a better understanding the mechanism from its Brianc¸onnais crust and overthrust toward the of deep underthrusting and extrusion were unfortunately south on the extruded eclogite facies Zermatt-Saas Fee not observed. Normal fault shear structures that testifie of ophiolites and southern Tsate´ ophiolites. The Etirol- the extrusion oft the Monte Rosa nappe, as documented in Levaz unit with its high-pressure metamorphism other high-pressure metamorphic units such as the Adula exposed farther south has a different, probably Aus- nappe (Cavargna-Sani et al. 2014) or in the Tso Morari troalpine, origin (Dal Piaz 1992; Steck et al. 2001). nappe of the NW Indian Himalaya (Epard and Steck 2008) D. The southern part of the Brianc¸onnais flake was during are missing and probably obliterated by younger ductile NW-directed thrusting imbricated as Frilihorn nappe deformation. The main ductile deformational structures of into the Tsate´ ophiolites, followed by the NW-directed the Monte Rosa nappe were created under greenschist to Tsate´ and Dent Blanche thrust over the Cimes amphibolite facies metamorphism of the Lepontine gneiss Blanches and Zermatt-Saas Fee nappes (Figs. 3, 11). dome. The Late Eocene and Oligocene Penninic nappe The continuing NW-directed Dent Blanche and Tsate´ emplacement is documented by deformational structures, thrusts accompanied the NW-directed emplacement of such as schistosities, stretching lineations, shear indicators the Siviez-Mischabel fold nappe dated at 40–35 Ma by and folds in ductile deformed rocks at greenschist to am- Markley et al. (1998) with Ar40/Ar39 ages on synkine- phibolite facies conditions. Radiometric ages indicate that matic crystallized white mica. The Siviez-Mischabel the Barrovian regional metamorphism of the Central nappe was thrust during this phase over the Zone Alpine nappe stack occurred during the ‘‘Meso-Neo- Houille`re, the Valais calcschists (Sion-Courmayeur Alpine’’ orogenic phase between 40 and 18 Ma (Hunziker nappe) and the Lower Penninic Monte Leone nappe. et al. 1992; Markley et al. 1998; Janots et al. 2009). The 30 A. Steck et al.

NW-directed nappe thrusting was followed by high tem- Bearth, P. (1954b). Map sheet 1349: Monte Moro, Geologischer Atlas perature dextral shear and SE-directed Vanzone backfold- der Schweiz 1329, Bundesamt fu¨r Landestopographie, Landes- geologie, Wabern, Schweiz. ing and high-temperature and retrograde deformation Bearth, P. (1956). Geologische Beobachtungen im Grenzgebiet der associated to cooling by uplift and erosion of the rising Lepontinischen und Penninischen Alpen. Eclogae Geologicae Lepontine gneiss dome. Dextral transpression in the Cen- Helvetiae, 49(2), 279–290. tral Pennine and Lepontine Alps, the zone of collision of Bearth, P. (1957a). Die Umbiegung von Vanzone. Eclogae Geolo- gicae Helvetiae, 50(1), 161–170. Europe and Adria, was since 33 Ma accompanied by the Bearth, P. (1957b). Erla¨uterungen zu Blatt Saas und Monte Moro. Insubric Line mantle and crustal derived magmatism and Geologischer Atlas der Schweiz 1329 und 1349, Bundesamt fu¨r by the frontal NW-directed Helvetic thrusts (Kirschner Landestopographie, Landesgeologie, Wabern, Schweiz. ¨ et al. 1996; Steck 2008; Steck et al. 2013). Bearth, P. (1958). Uber einen Wechsel der Mineralfazies in der Wurzelzone des Penninikums. Schweizerische Mineralogische und Petrographische Mitteilungen, 38(2), 363–373. Acknowledgments Michel Balle`vre, Robin Lacassin and Yves Bearth, P. (1962). Versuch einer Gliederung der alpinmetamorphen Gouffon are thanked for their critical reading of the manuscript. The Serien der Westalpen. Schweizerische Mineralogische und unpublished master thesis works of Anne Crespo-Blanc (1984), Anne- Petrographische Mitteilungen, 42(1), 127–137. Marie Bruttin (1985), Janine Wahli (1985), Pascal Vinard (1986), Bearth, P. (1964). 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