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TRANSCRIPT
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Student paper
Field trip - Alps 2013
Evolution of the Penninic nappes - geometry & P-T-t history
Kevin Urhahn
Abstract
Continental collision during alpine orogeny entailed a thrust and fold belt system. The
Penninic nappes are one of the major thrust sheet systems in the internal Alps. Extensive
seismic researches (NFP20,...) and geological windows (Tauern-window, Engadin-window,
Rechnitz-window), as well as a range of outcrops lead to an improved understanding about
the nappe architecture of the Penninic system. This paper deals with the shape, structure
and composition of the Penninic nappes. Furthermore, the P-T-t history1 of the Penninic
nappes during the alpine orogeny, from the Cretaceous until the Oligocene, will be
discussed.
1 The P-T-t history of the Penninic nappes is not completely covered in this paper. The second part, of
the last evolution of the Alpine orogeny, from Oligocene until today is covered by Daniel Finken.
1. Introduction
The Penninic can be subdivided into three partitions which are distinguishable by their
depositional environment (PFIFFNER 2010). The depositional environments are situated
between the continental margin of Europe and the Adriatic continent (MAXELON et al.
2005). The Sediments of the Valais-trough (mostly Bündnerschists) where deposited onto a
thin continental crust and are summarized to the Lower Penninic nappes (PFIFFNER 2010).
The Middle Penninic nappes are comprised of sediments of the Briançon-micro-continent.
The rock compositions of the Lower- (Simano-, Adula- and Antigori-nappe) and Middle-
Penninic nappes (Klippen-nappe) encompass Mesozoic to Cenozoic sediments, which are
sheared off from their crystalline basement. Additionally crystalline basement form separate
nappe stacks (PFIFFNER 2010). Rocks of the Upper Penninic nappes encompass
predominantly ophiolitic rocks (Platta-nappe) and oceanic sediments of the Piemont Ocean
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(PFIFFNER 2010). A more detailed discussion about the evolution of the depositional
environment is covered by the paper "The evolution of the Penninic distal domain" by
Sebastian Thronberens.
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Table of Contents
Abstract ...................................................................................................................................... 1
1. Introduction ............................................................................................................................ 1
2. Geometry of the Penninic units ............................................................................................. 4
2.1. Eastern Alps ..................................................................................................................... 4
2.1.1. Internal structures and shape of the Penninic nappes ............................................. 5
2.2. Western and Central Alps ................................................................................................ 6
2.2.1. Internal structures and shape of the Penninic nappes ............................................. 6
3. P-T-t history .......................................................................................................................... 12
4. Conclusion ............................................................................................................................ 13
References ................................................................................................................................ 14
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2. Geometry of the Penninic units
The Penninic nappes forms an irregular narrow band which containing more changes in
architecture along strike direction, as compared to the Helvetic system. Additionally there
are three geological windows (Rechnitz-window, Tauern-window, Engadin-window) within
the Eastern Alps and Cliffs of Penninic units at the northern front of the Central Alps
(Fig.1). The basal overthrust fault separates the Penninic nappes from the Helvetic system at
the western Alps. In the following the main focus will be on the western and central part of
the Alps, due to the more interesting internal structures of the Penninic units in these
regions.
Figure 1: Tectonic map of the Alps including cross section lines. (Penninic units colored purple) (Pfiffner 2010)
2.1. Eastern Alps
The narrow band of exposed Penninic rocks at the northern front of the Eastern Alps
consists of cretaceous rhenodanubic flysch (PIFFNER 2010). At the Tauern-window are
bünderschists and ophiolites exposed (Fig.2). These sediments, as well as the rhenodanubic
flysch, originate from the Penninic ocean (PIFFNER 2010). The last witnesses of the Briançon-
micro-continent are observable in the Engadin-window (PIFFNER 2010).
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2.1.1. Internal structures and shape of the Penninic nappes
Figure 2 shows the nappe structures in the western part of the Tauern-window. The
Penninic basal overthrust contact was passively ductile deformed. The basal overthrust, as
well as the internal overthrusts, are running parallel to the Helvetic sediments below
(PIFFNER 2010). The Glockner-nappe-complex consists of metamorphic sandstones,
claystones and limestones in the footwall (PIFFNER 2010). In the hanging wall prasinites are
also common (PIFFNER 2010). The lower parts of the Glockner-nappe complex are similar to
the Lower Penninic bündnerschists in the Central Alps (PIFFNER 2010). The upper parts are
similar to sequences of the Piemont ocean in the Central Alps (PIFFNER 2010). The Penninic
nappes show a remarkable "Schuppenbau", meaning these nappes are intensively
interleaved with each other. The Matrei-zone emphasize the "Schuppenbau", because this
zone comprises a mélange of ophiolites, bündnerschists and fragments of Austroalpine units
(PIFFNER 2010). This mélange has been formed due to intensive shear stress within the
subduction zone (PIFFNER 2010). The Matrei-zone represents the former plate boundary
Figure 2: Cross section of Tauern-window (Pfiffner 2010)
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between the Adriatic continental margin and the Penninic ocean and is similar to the Arosa-
zone in the Central Alps (PIFFNER 2010).
2.2. Western and Central Alps
In the Western Alps the Lower Penninic units pinch out southward and finally got
replaced by the Vocontian Basin. The Mid-Penninic nappes form a broad band (Briançon).
The Upper Penninic nappes are mainly exposed in the east of the Western Alps. At the
eastern part of the Central Alps the Penninic units border to the Sesia-zone (Austroalpine
nappes) to the east as well as to the Ivrea-zone (South Alpine System) to the south. In the
following the Geometry of the Penninic units, from east to west, will be discussed.
2.2.1. Internal structures and shape of the Penninic nappes
The deep tectonic structures in the eastern part of the Central Alps are shown in figure 3.
In the north of the cross section crystalline basement of Helvetic nappes and also the Aar-
and Gotthard-massif, as well as the Lucomagno-Leventina dip deeply southward.
Figure 3: Cross section along east traverse of the NFP20 seismic line (Pfiffner 2010)
These Helvetic units are overlayed by Lower Penninic nappes (Simano- and Adula-nappe-
complex). The rock composition of these crystalline nappes point to the former thin
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European continental margin. These crystalline basement represents the former substratum
of the Valais trough. The Adula-nappe complex contains internal overthrusts, which are
indicated by thin marble lineaments (PFIFFNER 2010). These lineaments are point to a
development of a complex interleaved structure during formation in great depth within the
subduction channel (Pfiffner 2010). The former sedimentary charge of the Valais trough was
sheared off northward from their crystalline basement. These Mesozoic sediments, mostly
consists of Bündnerschists, form the Grava- and Tomül-nappe, likewise the Vals-, Aul- and
Chiavenna-nappe. Several overthrusts are isoclinal folded, which indicated a polyphase
tectonic (Pfiffner 2010). Ophiolites at the base of the Tomül-, Aul- and Chiavenna-nappe
represent small pull-apart-basins of the Valais trough (Pfiffner 2010). In the south the Lower
Penninic units were large-scale refolded. The overturned limb form a steep shear zone
(Insubric line), which belongs to the Periadriatic Lineament and separates the Austroalpine
System from the South Alpine units. The intensive sheared rocks within the shear zone show
back thrusts, as well as a dextral shear component (Pfiffner 2010). The Bergell-Intrusion
ascend along the shear zone. This intrusion consists of Tonalites and Granodiorites.
The Lower Penninic nappes are overlayed by Mid-Penninic units. In figure 3 the Schamser-
nappe represents the Mid-Penninic Mesozoic sediments, which are also sheared off from
their crystalline basement. The Schamser-nappe is bounded on all sides by folded tectonic
contacts. The internal structure of the Schamser-nappe contains two fold limbs of different
compositions. One limb consists of Jurassic breccia sequences, which were deposited at the
Briançon continental margin close to syn-sedimentary normal faults (PFIFFNER 2010). The
laterally rapid changes had been carried through the alpine deformation and led to an
overturned limb consisting of breccias (PFIFFNER 2010). The Sulzfluh- and Falknis-nappe are
similar to the Schamser-nappe (PFIFFNER 2010). The Tambo- and Suretta-nappe are mostly
composed of pre-Triassic crystalline basement and contain remnants of autochthonous
cover including quartzites, evaporites, dolomites and marble (PFIFFNER 2010). The contact
between the crystalline Suretta-nappe and the Mesozoic sediments on top, likewise the
overthrust contact to overlaying Avers-Bündnerschists, are overprinted by tight folds
(PFIFFNER 2010). The fold-axis-planes are changed their dipping direction from northward
into southward at deeper levels. The refolding gives a indication of another deformation
phase.
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The sediments of the Piemont Ocean in figure 3 comprise the Avers-Bündnerschist, the
Arblatsch-Flysch and the Platta-nappe. The Platta-nappe mostly consists of ophiolites. Their
overthrust contacts are also folded. The fact that the Mid- and Upper-Penninic units raise up
southward, as well as outcrops in the east reveal that also the Mid- and Upper-Penninic units
were hit by a large back fold (PFIFFNER 2010). In general, the Penninic units show a syncline
structure, due to the dipping massifs in the north and the back folding in the south
(PFIFFNER 2010).
The Penninic units also had been faulted by large faults and strongly folded and back
folded which illustrates figure 4. In the north of figure 4 the Helvetic units are dipping
southward, as well as in figure 3. The Rhone-Simplon-Fault dissected the base of the Mid-
and Lower-Penninic units and get them in contact with the Bündnerschists of the Sion-
Courmayeur-Zone. The Rhone-Simplon-Fault is a normal fault at the Simplon-pass, a strike-
slip fault at the Rhonetal and a reverse fault at Savoyen (PFIFFNER 2010). The changing
nature of the Rhone-Simplon-Fault results from the orientation of the fracture plane.
Figure 4: Cross section along west traverse of the NFP20 seismic line
The Bernard-nappe-complex is located above the Rhone-Simplon-Fault and comprises the
Pontis-, Siviez-Mischabel- and Mont Fort-nappe. These nappes consists of pre-Triassic
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crystalline basement and were overlayed by Permo-Triassic quartzites (PFIFFNER 2010). The
internal structure of the Bernard-nappe-complex is characterized by large isoclinal folds and
large-scale south-tilting back folds. The Zone Houillère consists of crystalline basement,
which is overlayed by thick sequences of Carboniferous sediments and represents the lowest
Mid-Penninic unit (PFIFFNER 2010). The name results from encountered coal seams. The
northern part of the Zone Houillère and the overlaying Pontis-nappe are intersected by
normal faults, which are related to the Rhone-Simplon-Fault. The southern part of the
Monte Rosa-nappe as well as the underlaying units forms a large south-tilting back fold. The
Aosta-Ranzola-Fault intersect the back-fold-structure. The Monte Rosa-nappe mostly
consists of the variscan Monte Rosa Granite. The southern steep belt is composed of a
coarse-grained biotite-K-feldspar-oligoclase augengneiss, which possesses an age of
302±6Ma (STECK et al. 2013).
The ophiolite-bearing Upper Penninic Zermatt-Saas-Fee- and Antrona-nappe get wedged
in the Mont Fort-, Siviez-Mischabel- and Monte Rosa-nappe. The Zermatt-Saas-Fee-nappe is
overlayed by the Tsaté-nappe, which comprises Mesozoic sediments of the Piemont Ocean.
The Tsaté-nappe possess Jurassic basaltic pillow lavas (PFIFFNER 2010), which had been
complicated folded. The Dent Blanche-nappe on top formed cliffs of Austroalpine rocks.
Cliffs in the west of Switzerland and in Chablais of Savoyen were built of several Penninic
nappes including the Klippen-, Gurnigel-, Simmen- and Niesen-nappe. The main part took the
Klippen-nappe. The Klippen-nappe was detached from their crystalline basement and
obducted northward onto the Subalpine molasse. The internal structure differs between the
northern- and the southern-part. The northern-part of the Klippen-nappe is characterized by
large-scale folds and show a more plastic deformation behavior. The southern-part describes
a "Schuppenbau" (PFIFFNER 2010). These variations of internal structures were emphasize
by the designations "Médianes plastiques" and "Médianes rigides" for the northern-
respectively the southern-part of the Klippen-nappe (PFIFFNER 2010). The disparity is based
on differences in stratigraphic sequences. Huge sequences of evaporites filled the cores of
large anticlines, additionally Jurassic marly limestones favored the formation of folds in the
northern part. The stratigraphic sequences had also triggered the formation of faults.
Overthrusts preferred formed along discontinuities within the stratigraphic sequences due
to syn-sedimentary faults (PFIFFNER 2010). The rigid feature in the southern part results
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from the lack of plastic Early- and Middle-Jurassic sediments as well as from the mechanical
properties of the Triassic carbonates (PFIFFNER 2010).
Figure 5: Cross section along ECORS-CROP seismic line in the western part of the Central Alps (PFIFFNER 2010)
The deep tectonic structures in the western part of the central Alps are shown in figure 5.
The Mont Blanc Massif is overlayed by sediments of the Dauphinois and thick sequences of
Lower Penninic units. The Versoyen-nappe consists of argillaceous shales and prasinites,
whereas the Moûtier- and Petit-St.-Bernard-nappe were composed of Mesozoic and
Cenozoic clastic sediments (Bündnerschists) (PFIFFNER 2010). The Middle Penninic units
above are comprising the Zone Houillère, Ruitor- and Gran Paradiso-massif and are mostly
consist of crystalline basement. These nappes represent the former upper crust of the
Briançon-micro-continent. Thin sequences of Permo-Triassic quartzites overlay the
crystalline basement of the Ruitor-nappe. The highest Middle Penninic units is the Gran
Paradiso-massif, which shows an interesting dome-shaped appearance (Fig.5). This
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phenomenon is interpreted as a "collapse-structure" (PFIFFNER 2010). Due to the plate
collision, the nappe stack was compressed and formed a orogeny. In fact of their own
weight, the nappe stack started to "deliquesce". This induced horizontal stretching in the
upper parts of the orogeny (PFIFFNER 2010). Thus nowadays we can observe the dome-
shape-appearance. The Upper Penninic units cover the Middle Penninic nappes and dip
vertically in the east. They are composed of calcareous shales and contain metabasites.
Furthermore, they were overprinted by a high-pressure-metamorphism. Nowadays they
were common as a mixture of blueschist- to eclogite-facies metabasites, likewise high-
pressure metasediments (PFIFFNER 2010). In the northwest of Gran Paradiso-massif, the
basal overthrust of Upper Penninic nappes was complicated deformed due to overthrusts
within the Middle Penninic units compared with young folding events (PFIFFNER 2010). The
contact between the Adriatic continental margin and the Lower Penninic units (Lanzo-
nappe) was intensively interleaved (Fig.5) due to several kinematics within the subduction
channel (PFIFFNER 2010). Rocks of the Geneiss Minuti-, the Sesia-, Lanzo-, and Canavese-
zone belong to the former subduction channel. The steep position of its results from late
folding of the Penninic nappes. There are also two important shear zones within the
subduction channel. Both led to an relative uplift of the Penninic units. The Gressonney-
shear zone is folded around the Gran Paradiso-massif. Furthermore at the Insubric line the
Canavese-zone got overthrusted onto the Adriatic mantle and lower crust.
Figure 6: Cross section along the southern part of the Western Alps (PFIFFNER 2010)
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The deep tectonic structures in the southern part of the Western Alps are shown
in figure 6. Remarkable in figure 6 is the Dora-Maira-massif in the east-south-eastern part.
Parts of the Adriatic mantle were pushed into the crust of the Briançon-micro-continent. The
uncommon core of Adriatic mantle-rocks and associated unusually high position of those
induced a positive gravity abnormality and correspond to the Ivrea-body (PFIFFNER 2010).
On the Adriatic part the Adriatic mantle was shortened due to ESE-tilting reverse faults. The
crystalline basement of the Middle Penninic nappes formed complex structures and had
been interleaved with the Upper Penninic nappes (Queyras-nappe and Viso-Ophiolite). The
Queyras-nappe consists of high-pressure metamorphic Mesozoic Bündnerschists and
Metabasalts and contain a substratum of continental crust. The tectonic contact between
the Bündnerschists and the continental crust got intensively folded. The continental crust of
the Dora-Maira-massif is encased by ultra-high-pressure rocks (Eclogites) (PFIFFNER 2010).
The most western parts of the Middle Penninic units comprises Mesozoic sediments above
sediments of Permo-Carboniferous age and continental crust in the footwall. This
sedimentary sequence equals the already mentioned Zone Houillère (PFIFFNER 2010). By
contrast with the situation in the Central Alps, the Middle Penninic units got in contact with
the Dauphinois due to the lack of Lower Penninic units. The Lower Penninic units pinch out
southward and got replaced by the Vocontian Basin. The Dauphinois comprises the Pelvoux-
and Belledonne-massif. Both contain a thin sequence of Jurassic sediments in the hanging
wall (PFIFFNER 2010). In the case of the Pelvoux-massif the Jurassic sediments got overlayed
by Cenozoic Flysch-sediments.
3. P-T-t history
The pressure-temperature-time pathway of the Penninic nappes passed through different
metamorphic facies conditions. It is difficult to generalize the pressure-temperature-time
pathway in all for the Penninic units, although the orogenetic events in the Western-,
Central- and Eastern Alps were similar, however, they occurred in different time stages. The
variance in chronology of events results from the anti-clockwise rotation of the Adriatic plate
during the continent-continent-collision. Nevertheless the major events for the Western-,
Central- and Eastern-Alps were conform and can be summarized. In the following the
Suretta-nappe act as example case to explain the main pressure-temperature-time history of
the Penninic nappes from the Cretaceous until the Oligocene. In the Cretaceous the
convergence of Europe and the Adriatic plate was East-West oriented. Thus in the east only
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the oceanic crust of the Piemont ocean was partly subducted under the Adriatic plate. The
subduction led to a Cretaceous blueschist-metamorphism of Upper Penninic units. In the
other parts of the Penninic only sedimentation took place. In the Early Eocene (Lutetian 49-
42Ma) the Middle Penninic units got also subducted. The Suretta-nappe has been rapidly
subducted and a pressure-dominated blueschist-metamorphism has been overprinted. The
age of pressure-dominant metamorphism within internal domains vary between 50-35Ma in
the Western Alps and 49-42Ma in the Central Alps (PFIFFNER 2010). The pressure-dominated
metamorphism changed over time into a temperature-dominated metamorphism. These
metamorphic front shift over time to more external domains (PFIFFNER 2010). In the Late
Eocene the Penninic units experienced compressional conditions due to the continent-
continent-collision of Europe and the Adriatic plate. Furthermore, they got overthrusted by
Austroalpine units (PFIFFNER 2010). Thus the rocks experienced a pressure-dominated
eclogitic- to blueschist-metamorphism in the Central Alps. In the Oligocene the Bergell-
Intrusion ascend along steep reverse faults and induced regionally a amphibolite-
metamorphism. A more detailed discussion about the evolution of intrusive bodies within
the Alps is covered in the paper "Alpine granites" by Jacqueline Engmann. The second part,
of the last evolution of the Alpine orogeny, from the Oligocene until today is covered by
Daniel Finken.
4. Conclusion
The Penninic nappes have a complex internal structure and were mostly compost of
bündnerschists, ophiolites and crystalline basement. The Mesozoic sediments are mostly
sheared off from their crystalline basement. The crystalline basement also formed separate
nappes. The Penninic units experienced three major stages of metamorphic impacts over
time due to several orogenetic events. First subduction and exhumation of the nappes,
followed by overthrusting by Austroalpine units and at least the impact due to the ascent of
intrusion bodies along fault systems. The intensive folding and characteristic large scale back
folding reflects an intensive polyphase tectonic. In compare to other alpine sheet systems,
only the Penninic units possess ultra-high-pressure metamorphic rocks. The rather rapid
exhumation rates without accompanying heating were the most important factors for the
preservation of high-pressure rocks (RING 1992). The Penninic units of the Central Alps are
the structurally deepest rocks exposed in the Alps today (MAXELON et al. 2005).
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References
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zone, Central Alps, Switzerland and Northern Italy. Elsevier.
Pfiffner, O. A. (2010). Geologie der Alpen. Haupt UTB.
Ring, U. (1992). The Alpine geodynamic evolution of Penninic nappes in the eastern Central Alps:
geothermobarometric and kinematic data. J. metamorphic Geol., pp. 33-53.
Schmid, S., Pfiffner, O., Froitzheim, N., Schönborn, G., & Kissling, E. (1996). Geophysical-geological
transect and tectonic evolution of the Swiss-Italien Alps. In Tectonics, Vol. 15 (pp. 1036-
1064).
Steck, A., Torre, F. D., Keller, F., Pfeifer, H. R., Hunziker, J., & Masson, H. (2013). Tectonic of the
Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central
Alps. Swiss J Geosci.