Student paper Field trip - Alps 2013
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
(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.
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
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). -4-
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
Figure 2: Cross section of Tauern-window (Pfiffner 2010) 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
-5- 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
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.
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 -8- 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
-9- 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
-10- 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) -11-
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 -12- 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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