tectonics of the simplon massif and Lepontine gneiss dome …

 

 

1661-8726/08/020515-32
DOI 10.1007/s00015-008-1283-z
birkhäuser Verlag, basel, 2008

swiss J. Geosci. 101 (2008) 515–546

tectonics of the simplon massif and Lepontine gneiss dome:
deformation structures due to collision between the underthrusting
European plate and the Adriatic indenter

Albrecht Steck

Key words: Alpine tectonics, simplon massif, nappe tectonics, transpression, continental collision

AbstrAct

rEsuME

this study presents a review of published geological data, combined with orig-
inal observations on the tectonics of the simplon massif and the Lepontine
gneiss dome in the Western Alps. New observations concern the geometry
of the Oligocene Vanzone back fold, formed under amphibolite facies condi-
tions, and of its root between Domodossola and Locarno, which is cut at an
acute angle by the Miocene, epi- to anchizonal, dextral centovalli strike-slip
fault. the structures of the simplon massif result from collision over 50 Ma
between two plate boundaries with a different geometry: the underthrusted
European plate and the Adriatic indenter. Detailed mapping and analysis of
a complex structural interference pattern, combined with observations on the
metamorphic grade of the superimposed structures and radiometric data, al-
low a kinematic model to be developed for this zone of oblique continental
collision. the following main Alpine tectonic phases and structures may be
distinguished:

1. NW-directed nappe emplacement, starting in the Early Eocene (~50 Ma);
2. W, sW and s-verging transverse folds;
3. transpressional movements on the dextral simplon ductile shear zone since

~32 Ma;

4. formation of the bergell – Vanzone backfolds and of the southern steep
belt during the Oligocene, emplacement of the mantle derived 31–29 Ma
bergell and biella granodiorites and porphyritic andesites as well as intru-
sions of 29–25 Ma crustal aplites and pegmatites;

5. formation of the dextral discrete rhone-simplon line and the centovalli
line during the Miocene, accompanied by the pull-apart development of the
Lepontine gneiss dome – Dent blanche (Valpelline) depression.

It is suggested that movements of shortening in fan shaped NW, W and sW di-
rections accompanied the more regular NW- to WNW-directed displacement
of the Adriatic indenter during continental collision.

cette étude représente une revue de données géologiques publiées, complé-
tées par des observations originales sur la tectonique du massif du simplon
et du dôme de gneiss du Lépontin. De nouvelles observations sont rappor-
tées sur la géométrie du rétro-pli de Vanzone, formé à l’Oligocène sous des
conditions du faciès amphibolite et sur sa racine située entre Domodossola et
Locarno, où elle est coupée avec un angle aigu par le décrochement dextre,
epi- et anchi-zonale de la faille du centovalli, datée du Miocène. Les struc-
tures du massif du simplon résultent de la collision de la plaque européenne
sous-charriée et de l’éperon de la plaque adriatique, depuis 50 Ma. ce sont
deux bordures de plaques avec une géométrie très différente. Des levés géo-
logiques détaillés et l’analyse des structures superposées, combinés avec des
observations sur le degré du métamorphisme et des datations radiométriques,
permettent de conclure sur un model cinématique de cette zone de collision
continentale, de direction oblique et en transpression. Les phases et structures
tectoniques alpines suivantes sont distinguées:

1. Dès l’Eocène inférieur (~50 Ma), mise en place des nappes, dans une di-

rection NW.

tre du simplon.

2. Formation de plis transverses de vergence W–sW.
3. Mouvements transpressifs dans la large zone de cisaillement ductile et dex-

4. A l’Oligocène, formation des plis de rétrocharriage bergell – Vanzone, ver-
ticalisation des racines, mise en place des intrusions du bergell, de biella
et d’andésites porphyritiques d’origine mantélique datées de 31–29 Ma et
intrusion d’aplites et de pegmatites, d’origine crustale datées de 29–25 Ma,
5. Au Miocene, formation des lignes tectoniques dextres rhone-simplon et
des centovalli, accompagnée dans une structure d’extension et de trans-
pression, du soulèvement du dôme de gneiss du Lépontin et de l’abaisse-
ment de la dépression de la Dent blanche (Valpelline).

Nous concluons que le poinçon adriatique s’est déplacé selon un chemin ré-
gulier de direction NW à WNW, accompagné des déformations de raccourcis-
sement NW–sE, W–E et sW–NE.

Introduction

the simplon massif and the Lepontine gneiss dome in the cen-
tral Alps represent one of the best studied zones of continental
collision, which has been investigated for over a 150 years (e.g.

studer 1851; Gerlach 1869; schardt 1903; schmidt & Preiswerk
1905; Argand 1911, 1916; Preiswerk et al. 1934; bearth 1956a & b;
Wieland 1966; Hunziker 1969; Hunziker & bearth 1969; Milnes
1973, 1974; Milnes et al. 1981; Frank 1983; steck 1984, 1990;
Merle et al. 1989; Lacassin 1989; Mancktelow 1985, 1990, 1992;

Institute of mineralogy and geochemistry, Anthropole, university of Lausanne, cH-1015 Lausanne, switzerland. E-mail: Albrecht.steck@unil.ch

tectonics of the simplon massif and Lepontine gneiss dome 515

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steck & Hunziker 1994; steck et al. 1999, 2001; Engi et al. 2001;
Keller et al., 2005; Handy & Oberhänsli, 2004; schmid et al.
2004; Maxelon & Mancktelow 2005; Handy et al. 2005; Kon-
rad-schmolke et al. 2006; babist et al. 2007). the simplon mas-
sif and the Lepontin gneiss dome represent a unique region in
the western Alpine arc where deformational structures of its
deepest level of the nappe stack are exposed (Plate 1, Fig. 1).
the continuous outcrop conditions of the central Alps allow
a study of the rock-rheology, dependent change in structural
style between a superficial, more rigid, orogenic lid and a deep-
seated, very ductile, tectonic stage of the Alps. the complexity
of the fold and fault interference pattern in this zone of conti-
nental collision exposed in the central Alps (Plate 1) results
mainly from the following:

1. the different and complex geometry of the European plate

boundary and the Adriatic indenter (Fig. 2); and

2. the change from greenschist facies to higher amphibolite
facies grade of the synorogenic barrovian metamorphism,
implying a strong change with depth in the ductile proper-
ties of the deformed rocks.

Different models have recently been published to explain the
complex structures and the tectono-metamorphic history of the
Lepontine Alps, and these need to be discussed (Milnes 1974;
Milnes et al. 1981; steck, 1984, 1990; steck & Hunziker 1994;
Grujic & Mancktelow 1996; steck 1998; Keller et al. 2005, 2006;
Maxelon & Mancktelow 2005). New observations are presented
in the present work concerning the geometry, kinematics and
metamorphism of the Vanzone back fold and the centovalli
line. the aim of this study is to review already published data,
completed by new observations, and to suggest an updated
model for the formation of this complex zone of continental
collision exposed in the simplon area.

Geological setting

the simplon area is the region of Eurasia, where schardt
(1903), schmidt & Preiswerk (1905) and Argand (1911) discov-
ered and proposed the model of the nappe structures of the
Alps. this generalisation of the nappe structure model to an
entire mountain range happened 36 years after the description
of the Antigorio recumbent fold by Gerlach (1869), and over
20 years after the discovery of the Glarus thrust by bertrand
(1884). the geology of the central Alps is illustrated in recent
geological and tectonic maps (1: 500’000) of switzerland, pub-
lished by the Federal Office for Water and Geology (2005) and
in an overview map of the Alps by schmid et al. (2004). More
detailed tectonic maps in a scale of 1 : 100’000 have been com-
piled by steck et al. (1999, 2001) and berger & Mercolli (2006).
An updated structural map of the central Alps is represented
on Plate 1 and the main tectonic units of the simplon region are
listed on table 1 (modified after steck et al. 2001 and berger &
Mercolli 2006). the central Alps are composed of pre-triassic,
poly- and mono-cyclic crystalline basement units and their Al-
pine metamorphosed Mesozoic – cenozoic sedimentary cover

Fig. 2. Late Jurassic and Late cretaceous plate geometry preceding the ter-
tiary collision of Eurasia, Iberia and Adria (modified after stampfli et al. 2001;
bernoulli et al. 2003; Masson 2002; schmid et al. 2004 and Masson et al. sub-
mitted; AA = Austroalpine, se = sesia zone, tz = tizia). Note, that the sesia
zone (se) has been overprinted by its high pressure metamorphism during a
late cretaceous phase of subduction (babist et al. 2007; Hunziker et al. 1992;
ruffet et al. 1997; rubatto et al. 1999).

series, Jurassic ophiolites of the Piemont basin (Alpine tethys
ocean) and Oligocene intrusives and volcanites, situated along
the Periadriatic line. New units have been defined in the core
of the Vanzone fold. the following units have been identified
below the Antrona ophiolites (Plate 1, Fig. 3, 4):

– the camughera unit composed of porphyritic granite
gneisses, that are intrusive into metapelites and amphibo-
lites,

– the ruginenta unit, named after a village in the Antrona
valley, comprising a basement of similar composition of the
camughera basement, which is stratigraphically overlain
by the sediments of the salarioli unit (Fig. 4; the “salari-
oli mulde” of bearth 1956b, consisting of Permo-Mesozoic
sediments), composed of 10–180 m thick black graphite-
rich slates, sandstones and microconglomerates of probable
carboniferous age, followed by 1–10 m of Permo-triassic

tectonics of the simplon massif and Lepontine gneiss dome 517

.

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Fig. 4. stratigraphy of the carboniferous and Permo-triassic sedimentary cover of the ruginenta unit (Antrona valley). the name “salarioli unit” is proposed
for the whole carboniferous – triassic sedimentary sequence that lies in a normal stratigraphic position on the ruginenta basement and forms a tectonic contact
with the higher camughera basement. Note that bearth (1956b) and (Keller et al. 2005) used the term “salarioli Mulde” for the Permo-triassic sediments, that
form, after their interpretation, a syncline (Mulde) in the camughera unit.

phengitic quartzites and above this, boudins of 5–100 m
thick triassic dolomites with less than 10% of calcite and
quartz. the contact to the higher camughera gneisses is tec-
tonic; on the Pso d’Ogaggia it is marked by several meters
of cornieules.

– the Moncucco unit, within which we include the up to
400 m thick Moncucco peridotites, a sequence of meta-
pelites, paragneisses, and amphibolites, an approximately
50 m wide fold hinge containing white siliceous marbles,
outcropping at the Alpe Pradurino (Paleozoic or Meso-
zoic?; bearth 1956b), and, in the core of the isoclinal Van-
zone fold, the Moncucco porphyritic bi-mu-granite-gneiss,
which is up to 700 m wide and has an age of 271 ± 4,8 Ma,
(rb-sr, biggogero et al. 1981).

the distinction and correlation of the units in the Vanzone fold
below the Antrona ophiolites is similar to the proposition of
Milnes in steck et al. (1979; Milnes & Müller in trümpy 1980):
the Moncucco unit of this study is attributed by Milnes to the

Monte Leone nappe. the rugginenta unit is named Moncucco
unit and the camughera unit of this study camughera unit
following the propositions of bearth (1957). the two latter
units correspond to the southern root of the bernhard nappe
of Argand (1911, 1916; Milnes in steck et al. 1979). Keller et
al. (2005) correlate their camughera-Moncucco unit with the
Monte Leone gneiss to the north of the Valle bognanco. bearth
(1957) and Keller et al. (2005) propose that the bernhard or
siviez-Mischabel nappe to the north of the Antrona ophio-
lites is a root less unit with no southern continuation in the
camughera-Moncucco units. the proposed separation of the
ruginenta and Moncucco units of this study is arbitrary and
it cannot be ruled out that the two units actually belong to the
same tectonic unit, as proposed by Argand (1911). It is also pos-
sible that the siliceous marbles of Alpe Pradurino represent a
relict of Mesozoic sediments marking the boundary of these
two tectonic units, as proposed on the geological and tectonic
maps of switzerland, 1: 500’000 of the Federal Office for Wa-
ter and Geology (2005). In this case, the Moncucco and Mon-

tectonics of the simplon massif and Lepontine gneiss dome 519

dei peridotites belong to the upper unit, i.e. the newly defined
ruginenta unit. On Plate 1 and Fig. 1, the camughera unit is
correlated with the siviez-Mischabel nappe, whereas the rugi-
nenta unit is correlated with the Pontis-berisal nappe and the
Moncucco unit with the Monte Leone nappe. this correlation
is based largely on the tectonic position within the nappe pile
(after unfolding of the last phases) and lithological affinity. It
is also possible that a part of the Moncucco unit represents a
lateral equivalent of the bosco – Isorno – Orselina zone, which
is characterised by numerous ultramafic bodies (cima d’Agaro,
Albogno etc., 5 ant 6 on Plate 1).

The pre-Alpine Cretaceous paleogeography

In this study we refer to investigations and models by trümpy
(1980), stampfli et al. (2001); bernoulli et al. (2003) and schmid
et al. (2004) for the Late cretaceous paleogeography of the
west-Mediterranean area, modified after Masson (2002; Mas-
son et al. 2008; Fig. 2) prior to closure of the Piemont ocean
(Alpine tethys) and continental collision of Eurasia with the
Adriatic indenter. the opening of the Piemont ocean is con-
strained by radiometric dating: the gabbroic blocks of the
Gets nappe flysch have an age of 166 ± 1 Ma (u-Pb zircon age,
bill et al. 1997), the gabbros of the Zermatt-saas Fee ophiol-
ites ages of 164 ± 2.7 Ma (Allalin gabbro) and 163.5 ± 1.8 Ma
(Mellichen gabbro, rubatto et al. 1996), the Antrona ophiolites
an age of 163.1 ± 2.4 Ma, 158 ± 17 Ma and 155.6 ± 2.1 Ma and
the Misox zone that of 161.0 ± 3.9 Ma (Liati et al. 2005). these
ages suggest progressive opening of the Piemont ocean, from
the Middle to the Late Jurassic and from s to N. A cretaceous
u-Pb age of 93.4 ± 1.7 Ma of magmatic zircons has been found
by Liati & Froitzheim (2006) in an eclogitic metabasite of their
newly defined balma unit in the roof of the Monte rosa nappe
(2 km to the E of Point 2 on Figure 7). the authors suggest that
the balma unit belongs to the Valais basin. In this study it is sug-
gested that either the dated metabasites belong to the Zermatt
– saas Fee ophiolites or to the Furggzone. the age testify in the
first case of a cretaceous continuation of basaltic magmatism
in the Piemont ocean or represents in the second case basal-
tic dikes intruding the Paleozoic to upper Jurassic Furgg sedi-
ments as observed at the passo della Preja (steck et al. 2001).
It is also important to note that the gabbro intrusions, within
the ophiolites of the Versoyen zone are in tectonic contact with
the cretaceous sion-courmayeur-tarantaise flysch unit and
have been dated at 334 ± 4 Ma (sHrIMP u-Pb age on zircon,
bussy et al. 2005; Masson et al. 2008). the Versoyen ophiol-
ites represent the Paleozoic basement that is stratigraphically
overlain by the triassic dolomites and Liassic calcschists of the
Petit st. bernhard tectonic unit. the carboniferous Versoyen
ophiolites and its Mesozoic sedimentary cover are together
overprinted by an Alpine eclogite facies metamorphism (bous-
quet et al. 2002) and form the newly defined Versoyen-Petit
st. bernhard unit (Masson et al. 2008). A wildflysch (Méchan-
deur Formation) with elements of the latter unit is exposed in
the Versoyen valley on the tectonic contact between the Pa-

leozoic ophiolites of the Versoyen-Petit st. bernhard nappe
on top and the sion-courmayeur-tarantaise calcschist unit of
the Valais zone below. this wildflysch is interpreted (Masson
et al. 2008) as the youngest sediment of the sion-courmayeur-
tarantaise unit. this wildflysch may be a southern equivalent
of the mélange of Visp (Visp wildflysch on Fig. 5; Masson 2002,
2006; Masson et al. 2008). the ophiolites from Visp (Valais,
switzerland) represent blocks and olistoliths in the polygenic
Visp olistostrome (mélange, wildflysch, Masson 2002; ensemble
du Furgguwald, steck 1987; Visp wildflysch on Fig. 5). It fol-
lows that the Late cretaceous (?) to Eocene Valais basin was
an important marine wrench basin with a thick succession of
Late cretaceous (?) to Priabonian flyschoid sediments (calc-
schists or “bündnerschiefer”), with Middle Jurassic ophiolites
only in Eastern switzerland, in the Misox zone (161.0 ± 3.9 Ma;
Liati et al. 2005), and in the ramosch zone of the Engadine
window (Florineth & Froitzheim 1994), where the Valais trench
basin reaches the Piemont ocean with its oceanic crust (trümpy
1980). In conclusion, the Late cretaceous (?) to Priabonian
Valais basin, of western switzerland, was not the cretaceous
ocean with an oceanic crust, as proposed in classical and recent
paleogeographic models of the Alps (trümpy 1980; Froitzheim
et al. 1996; stampfli et al. 2001; schmid et al. 2004). the root of
the sion-courmayeur-tarantaise flysch is situated in the Val-
ais suture on the limit of the European (Helvetic) and brian-
çonnais domains (Plate 1, Fig. 1 & table 1). An other ophiolite
sequence, composed of meta-peridotites, -gabbros and -basalts
is associated with the continental rocks of the Moncucco and
bosco-Isorno-Orselina units, in the latter with carbonate sedi-
ments. the age of this ophiolites is unknown, probably Paleozic
and/or Mesozoic. these units are rooting north of the Monte
Leone gneiss and sion-courmayeur-tarantaise flysch in the
Valais suture (table 1, Fig. 1 and Plate 1). the subduction, ex-
humation and accretion of the sesia zone as a part of the Adri-
atic margin occurred in the Late cretaceous before the closure
of the Piemont ocean (Konrad-schmolke et al. 2006; Handy et
al. 2005; babist et al. 2007). the sesia high-pressure metamor-
phism was dated of 65–68 Ma (Hunziker et al. 1993; Duchène et
al. 1997; rubatto et al. 1998, 1999; babist et al. 2007).

Alpine continental collision and orogenic metamorphism

rock deformation and metamorphism are interdependent pro-
cesses that characterise a zone of continental collision. Apart
from composition the mechanisms of deformation depend on
the pressure – temperature – time conditions, and the age of
the deformational events may be constrained by radiometric
dating of metamorphic minerals and synorogenic magmatic
rocks. N-NW-directed fold nappes and thrusts are early Alpine
structures related to continental collision (Fig. 5). these nappes
have been formed by the detachment of the upper part of oce-
anic and continental crust of the Austro-alpine continental do-
main (sesia zone), the Piemont oceanic crust, and the brian-
çonnais and European continental crust, during underthrusting
to the sE below the Adriatic plate. these units were subducted

520 A. steck

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tectonics of the simplon massif and Lepontine gneiss dome 521

table 1. List of the Alpine tectonic units of the simplon area represented on Plate 1, modified after steck et al. (1999, 2001), Geologische Karte der schweiz,
1: 500’000 (2005); berger & Mercolli (2006).













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to different depths before their detachment, extrusion and ac-
cretion in the Alpine orogenic belt. Eclogite facies mineral as-
semblages, with coesite in the Zermatt – saas-Fee nappe, testify
that in an early stage of subduction, some units reached a depth
of over 90 km before their very rapid extrusion (trommsdorff
1991; reinecke 1998; Engi et al. 2001; 2004; bucher et al. 2003)
in a so-called tectonic accretionary channel (tAc), a term ap-
plied by Engi et al. (2001). both the resultant structures and
the mechanism of extrusion in this Early-Alpine tectonic ac-
cretionary channel are complex and still not well understood.
the great volume of low-density serpentinites in the Zermatt
– saas-Fee zone may be responsible for an important support
by buoyancy driven extrusion of this high-pressure unit. two
periods of high-pressure metamorphism have been dated in
the transect of the Western Alps considered here: the sesia
zone metamorphism of Late cretaceous age (Hunziker et al.
1992; Duchène et al. 1997; ruffet et al. 1997; rubatto et al. 1999;
babist et al. 2007) and Alpine middle Eocene metamorphism
of the ophiolitic Zermatt-saas Fee and Antrona, the Monte

rosa, Portjengrat and cimalunga-Adula units dated 50–40 Ma
(becker 1993; Gebauer 1999; Amato et al. 1999; Engi et al. 2001;
Lapen et al. 2003; Mahlen et al. 2005). the whole Alpine nappe
stack was then overprinted by a younger late Eocene barro-
vian regional metamorphism. this Alpine nappe stack consists
of (from top to bottom) the Austroalpine Dent blanche nappe,
the upper Penninic ophiolitic tsaté, Zermatt-saas Fee and
Antrona nappes of Piemont ocean origin, the Middle Penninic
continental Monte rosa, Portjengrat-Furgg, cimes blanches-
Frilihorn, siviez-Mischabel, Pontis-berisal and Zone Houillère
nappes of the briançonnais domain, and the Lower Penninic
cretaceous – Priabonian Valais calcschists, the continental
Monte Leone, bosco-Isorno-Orselina (with ophiolites), Leb-
endun-bombogno, Maggia, someo-cimalunga-Adula, Antigo-
rio-Pioda di crana-simano, Verampio, Leventina-Lucomagno
nappes and the Helvetic Gotthard, tavetsch, Aar and Gastern
gneiss folds and their sedimentary cover nappes (Plate 1, Fig. 1
and table 1). the greenschist-facies metasediments of the Va-
lais basin contain relicts of a blueschist-facies event related

522 A. steck