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 -1- (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. -2- 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 -3- 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 -6- 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