Swiss J Geosci (2013) 106:427–450 DOI 10.1007/s00015-013-0135-7 Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps Albrecht Steck • Franco Della Torre • Franz Keller • Hans-Rudolf Pfeifer • Johannes Hunziker • Henri Masson Received: 9 October 2012 / Accepted: 29 May 2013 / Published online: 17 July 2013 Ó Swiss Geological Society 2013 Abstract The Lepontine dome represents a unique region Adriatic indenter. The transverse folding F4 was followed in the arc of the Central and Western Alps, where complex since 30 Ma by the pull-apart exhumation and erosion of the fold structures of upper amphibolite facies grade of the Lepontine dome. This occurred coevally with the formation deepest stage of the orogenic belt are exposed in a tectonic of the dextral ductile Simplon shear zone, the S-verging half-window. The NW-verging Mont Blanc, Aar und Gott- backfolding F5 and the formation of the southern steep belt. hard basement folds and the Lower Penninic gneiss nappes of Exhumation continued after 18 Ma with movement on the the Central Alps were formed by ductile detachment of the brittle Rhone-Simplon detachment, accompanied by the N-, upper European crust during its Late Eocene–Early Oligo- NW- and W-directed Helvetic and Dauphine´ thrusts. The cene SE-directed underthrust below the upper Penninic and dextral shear is dated by the 29–25 Ma crustal-derived aplite Austroalpine thrusts and the Adriatic plate. Four underthrust and pegmatite intrusions in the southern steep belt. The zones are distinguished in the NW-verging stack of Alpine cooling by uplift and erosion of the Tertiary migmatites of the fold nappes and thrusts: the Canavese, Piemont, Valais and Bellinzona region occurred between 22 and 18 Ma followed Adula zones. Up to three schistosities S1–S3, folds F1–F3 and by the exhumation of the Toce dome on the brittle Rhone– a stretching lineation XI with top-to-NW shear indicators Simplon fault since 18 Ma. were developed in the F1–F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Keywords Switzerland Á Italy Á Alpine orogeny Á Alpe Bosa and Wandfluhhorn anticlines and synclines over- Tertiary fold structures Á Gneiss dome Á Geochronology print the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the 1 Introduction The Lepontine gneiss dome of the Central Alps represents an Editorial handling: A. G. Milnes. exceptional outcrop of a collisional mountain belt. The Le- pontine dome is defined by the area of Tertiary amphibolite Electronic supplementary material The online version of this facies metamorphism and consists of basement nappes sepa- article (doi:10.1007/s00015-013-0135-7) contains supplementary material, which is available to authorized users. rated by their Mesozoic cover series of the deepest tectonic level of the Central Alpine structural culmination (Fig. 1). This A. Steck (&) Á H.-R. Pfeifer Á J. Hunziker Á H. Masson dome structure is unique in the Western and Central Alpine arc. Institut des Sciences de la Terre, Universite´ de Lausanne, Another dome structure with an amphibolite facies gneiss core baˆtiment Geopolis, Dorigny, 1015 Lausanne, Switzerland e-mail: [email protected] is exposed in the Tauern window of the Eastern Alps (e.g. Schmid et al. 2004). In the Lepontine dome the transition from F. Della Torre the more simple thrust structures of the high tectonic lid to the Dr. P. Ammann SA, 6616 Losone, Switzerland deepest tectonic amphibolite facies units with their ductile fold F. Keller structures can be studied in continuous profiles. Complex fold Lu¨ssirain 59, 6300 Zug, Switzerland interference structures are exposed in a tectonic half-window 428 A. Steck et al. BRENNER FAULT 1 TAUERN 2 EUROPE 1 E IC IN T LP LT E ES A U LV CI A A E FA T O F H ST IC L R IA R HI U C N A’ 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 H AC ME G IS F V DO L E E S T M G LI P A R O C L E 2 R B N G Z N A I TI A E H N S SS E P O G CA IM Be 3 IMPLON M P I R - S FA E A P E U A L Ad ÔN LT H TO ALE FAULT R Fig 2 S E G U E T O H L R C U N S E S N E A L C E A N F IL N I L O A E U U B A Z IG L C N A B Q A T A E Ê N V T V S É F S E N E E T I D O C V SA M IF A TH N S A FAULT A IC AOST H Bi C C S E S N N N E MI N 1 E LANO BELT O E E Tr D R R E G C L G I L E N B I É N N IN E H P P EM ILI U A T A MONFERRATO HR D U 1 THRUSTS S TS PELVOUX A ADRIA 1 PE NN IN D E I G S N stretching lineations of E 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 18 - 3 Ma back fold Monte Rosa e - de pl Biasca Splügenpass lbit nblen agiocla SW a hor se A albi NE 7- her N17- te – n1 Zicc anticlin hor A e MAGGIA nble A nde A’ 0 m Ve LEVENTIN 0 m S RAMPIO rza SURETT SCH PLAT VE sc TAMBO AMS TA ? a a ADULA -AVER OPHIOLITE nt A S MONTE ROSA A icli A SIMANO ? ne ANTRON TOCE CULMINATION WandfluhhornMaggia synclin syncline MONTEANTIGORIO LEONE TICINO e 50 km CULMINATION Rhône-Simplon fault AS 2013 Fig. 1 Oligocene and Neogene structures and metamorphism of the 0–3 ? epidote ? hornblende = oligoclas An [17 ? hornblende Central and Western Alps, modified after Frey et al. (1999), Steck et al. (Wenk and Keller 1969; Steck 2008). The Adamello gabbros, diorites (2001) and Oberha¨nsli (2004) showing the location of the structural map and granites (Ad), 32–29 Ma Bergell tonalites and granites (Be), of the central Lepontine Alps (Fig. 2) and the cross-section A–A’ (see 31–30 Ma Biella pluton (Bi) and Traversella diorite (Tr) are Oligocene also Fig. 8). The zones of the Late Cretaceous and Eocene high-pressure mantle-derived intrusions located along the Insubric line, the limit metamorphism are not represented. It is suggested that the anchizone- between the European and Adriatic plates (Beccaluva et al. 1983; greenschist facies limit, corresponding to a temperature of about 300 °C Reusser 1987; Romer et al. 1996; Berger et al. 2012; Kapferer et al. represents an important rheological boundary in the quartz-rich granitic 2012). The dextral Rhoˆne-Simplon fault continues to the west in the European crust that controls the position of the frontal NW verging and Chamonix zone and after new observations by Daniel Egli (personnel en echelon Belledonne, Mont Blanc and Aar F2 basement folds in the communication) also along the Penninic front to the east of the Mont zone of NW–SE directed Alpine compression between the European and Blanc massif. (1) Oligocene–Miocene greenschist facies metamor- Adriatic plates. The greenschist-amphibolite facies boundary which phism, (2) Oligocene–Miocene amphibolite facies metamorphism, (3) delimits the Lepontine dome is defined by the reaction: albite An Mantle derived dioritic magmatism of the deeply eroded gneiss dome located between the Toce (2) to discuss a revised tectonic and kinematic model in the and Ticino rivers, limited to the south by the Canavese and light of recent field investigations, together with a compi- Tonale faults of the Insubric line. lation of structural data collected by the authors. The The aims of this paper are (1) to describe and decipher classic geological documents of the early twenty century the geometry of the structures of the Lepontine Alps and (Preiswerk et al. 1934; Niggli et al. 1936; Wenk 1955) have Tectonics of the Lepontine Alps 429 been completed with detailed geological maps at a scale of to south these are: (1) the Helvetic Aar and Gotthard folds, 1:25,000, i.e. Campo Tencia (Keller et al. 1980), Bellin- (2) the Lower Penninic units detached from the upper zona (Baechlin et al. 1974) and Valli Vigezzo, Fenecchio e European crust, (3) the Valais basin calc-schists, (4) the Basso Isorno (Bigioggero et al. 1981a, b), and numerous Middle Penninic units attributed to the Brianc¸onnais domain, Ph.D. theses (Burckhardt 1942; Hasler 1949; Kobe 1956; (5) the Upper Penninic Antrona and Zermatt–Saas oceanic Knup 1958; Hunziker 1966; Reinhardt 1966; Wieland crust elements of the Piemont ocean, (6) the Sesia zone of the 1966; Keller 1968; Bianconi 1971; Colombi 1989; Brou- Austroalpine domain and (7) the Southern Alps forming the wer et al. 2005; Burri 2005). In addition, the tectonic northern margin of the Adriatic plate. As indicated in models by Merle et al. (1989), Grujic and Mancktelow Table 1, four deep underthrust zones are distinguished in the (1996), Maxelon and Mancktelow (2005), Berger et al.