EVOLUTION OF THE PENNINIC DISTAL DOMAIN. By Sebastian Thronberens (283135)

Transcription

1 EVOLUTION OF THE PENNINIC DISTAL DOMAIN By Sebastian Thronberens (283135)

2 INHALTSVERZEICHNIS 1. Introduction Penninic Distal Domain Depositinal Evolution Tectonic Evolution Stratigraphy Valais-Trough Brianҫonnais terrane Piemont-Ocean Conclusion References... 12

3 1. INTRODUCTION The Penninics constitute the only alpine tectonic unit extending along the whole alpine mountain belt. In the eastern Alps the penninic Rhenodanubian Flysh strikes from E to W. Further to the south the Tauern Window and the further west located Engadiner Window expose penninic units through Austroalpine nappes. In the central Alps the Penninics generally strike from NE to SW and build up the Prealps, separated geographically from the central penninic units and are located between the Molasse Basin and the Helvetics. In the western Alps the Penninics extend with N to S striking. FIG 1 TECTONIC MAP OF THE ALPS (SCHMID, FÜGENSCHUH ET AL. 2004). The Penninic domains derive from their paleogeographic depositional environments (Fig 1) and are separated into the lower Penninic nappes of the Valais-Trough, the middle Penninic Nappes of the Brianҫonnais domain and the upper Penninic nappes comprising depositions of the former Piemont- Ocean (Pfiffner et al. 2010).The stratigraphic units of the distal domains of the Piemont-Ocean and the Valais-Trough imply a relatively similar lithological sequence, which will be clarified in the ongoing chapters PENNINIC DISTAL DOMAIN The whole orogeny of the Alps and therefore the evolution of the penninic distal domain are a product of a classical Wilson cycle that started with the opening of the alpine Tethys between Eurasia in the north of Africa and Adria in Mesozoic times (Kissling 2008). It began with the breakup of the Pangaea Supercontinent which last from Triassic to early Cretaceous times. At the border of Jurassic to Triassic time. The alpine Tethys is subdivided in three large ocean basins along a large spreading zone, which also contains oblique striking transform faulting.

4 FIG 2 PALGEOGRAPHICAL RECONSTRUCTION FOR THE LATE CRETACEOUS (SCHMID, FÜGENSCHUH ET AL. 2004). One of those transform faults strikes NW to SE along the western border of the former European continent and separates the Brianҫonnais and Corsica-Sardinia domain from the Iberian microplate, situated in the west (after a model from Frisch 1993). The Ligurian-Piemont- Ocean opens in the western part of the ridge, while the eastern part consists of the Penninic-Ocean. But depending on the literature the Penninic and Piemont-Ocean are expressions for the same domain and will be called the Piemont-Ocean in this paper. The alpine Tethys Ocean separates the European and the Adriatic continents. The Corsica-Sardinia and the Brianҫonnais microcontinents are located at the southern end of the European continent. In the south-east the Austroalpine is separated from the Brianҫonnais terrane by the Penninic Ocean (Fig 2). In the north of the Brianҫonnais domain, at the distal area of the European margin, the Valais-Trough is located, which rifting phase started during the late Jurassic, followed by spreading which is suggested to have lasted from Barremo-Aptian to Albo-Cenomanian with an aperture of 200 km for each phase (Stampfli and Mosar 1998). The rotation of Iberia from Turono to Senonian time is thought to have leaded to the closure of the Valais Ocean DEPOSITIONAL EVOLUTION In the Turonian time the distal domains of the Penninicum were located in deep oceanic areas, such as the Valais-Trough and the Piemont-Ocean. Even the Brainconnais continent seemed to be flooded in a shallow marine (pelagic) environment. Subduction under the Adriatic plate was certainly taking place. Induced by the subduction process, first alpine nappes were uplifted. Even the southern parts of the Piemont-Ocean have been concerned by the subduction. The erosion of the uplifted alpine nappes caused embankments into the Piemont-Ocean from south eastern direction, leaded to the deposition of the Gets-, Simme- and Arblatsch-Flysch. Those deposition set up the southern Penninic Flysch basin.

5 The Brianconnais domain had pelagic conditions at that time. This was proven by the deposition of pelagic marly limestones at that time (source). The Valais-Trough was already filled with the Jurassic and Cretaceous Bündnerschiefer. During the Turonian the Prättigau Flysch started to deposit. From the northeast the Rhenodanubian Flysch got embanked into the basin. FIG 3 CROSS-SECTION THROUGH THE PREALPINE KLIPPEN NAPPES (PFIFFNER ET AL. 2010). Until the Eocene the subduction under the Adriatic continent propagated northward. The first distal european margin was subducted in such depth that it metamorphism processes took place. The Penninic Valais-Trough was part of one large basin complex located between the northern alpine foreland and the central Alps at that time. The basin comprised also the Helvetics and the Dauphinois and was a typical foreland basin created by the crustal bent caused by the overthrustig Adriatic continent, which also uplifted the alpine foreland and induced its erosion. Basin infill was delivered from south-western Alps creating the Niesen Flysch, but also from the central part of the Alps, depositing the Prättigau Flysch. The foreland basin is defined as the North Penninic Flysch basin.

6 FIG 4 CROSS-SECTION OF WESTERN ALPS AND A SIMPLIFIED PALINSPASTIC MODEL (STAMPFLI & MOSAR 1998). In the Oligocene the Penninic domains were subducted and developed as nappe stack when first back thrusting induced by the continent-continent collision of the European and the African continent caused uplift of the nappe stacks. One part of the Penninic domain, comprising the Gets-, Simme- and Arblatsch Flysch got detached on an Mesozoic detachment horizon and was shifted northwards generating the Klippen nappe (Fig 3), while the other Penninic units got deeply subducted and were highly metamorphic overprinted. 2. TECTONIC EVOLUTION The Penninic domains are results of rift tectonics in the Alpine Tethys which seem to be related to the Pangaea break-up and the opening of the central Atlantic and might have started in late Triassic age. (Schmid et al. 1987; Hunziker et al. 1992). During the Jurassic the Alpine Tethys developed into subsiding rim basins with thinned cold lithosphere. Rifting, like the one causing the Valais- and Piemont-Ocean, separated the rim basins. The absolute beginning of this rifting phase is suggested to have occurred during the Sinemurian, followed by a thermal uplift phase in the Toarcian (Favre and Stampfli, 1992). Then a phase of seafloor spreading succeeded in the early-middle Jurassic (Bill et al. 1997). Except from the mentioned rim basins, thermal subsidence during the Bajocian led to progradation of carbonate platforms (Stampfli, 1998). So the former rift shoulders, including even the Brianҫonnais domain got sinked submarine as a result. In the late Jurassic the Iberian plate got separated from Newfoundland due to Atlantic rifting, what caused a rifting between France and the Brianҫonnais domain, the Valais rift, which is thought to have opened km from late Jurassic to the Aptian (Stampfli, 1998). Thermal subsidence is suggested to have started in the Valanginian by Gradstein et al (1995). Further spreading is thought

7 to have occurred from Barremo-Aptian to the Albo-Cenomanian and a closure of the Valais-Ocean during the Albian. Stampfli (1998) suggests an opening of the Valais-Ocean of 200 km during rifting and the same amount during the spreading phase. The closure was induced by the rotation of Iberia from Turono-Senonian time might have last till Lutetian time, proofed by age dating of sediment influx (Homewood, 1983). The following subduction caused the Valais domain to become the lower Penninic suture zone which got underplated below the northern part of the Brianҫonnais domain probably during the middle Eocene. The Brianҫonnais domain contains a crystalline basin which is set up of the Suretta and the Tambo nappes (Fig 4). Whereas the Suretta nappe is of pre-alpine age and derived from an early Permian subvolcanic intrusion called the Roffna granite and constitutes the frontal part of Suretta, the Tambo nappe comprises a porphyritic granite complex, which intruded during early Perm (Stampfli, 1998). A part of the Mesozoic cover unit was detached from its basement on a sequence of initially interlayered anhydrite-dolomite deposits and incorporated into the accretionary prism and generated the Préalps Médiannes, which comprise an external Penninic unit today also known as the Klippen nappe (Wissing, 2003). The subduction of the Penninic domain was completed during Oligocene and the Penninic nappe stack got back thrusted by the continent-continent collision which was followed by the slab-break off induced uplift and deformation of the stack.

8 FIG 5 CROSS-SECTION ALONG THE VALAIS-TROUGH FROM NW TO SE IN EARLY CRETACEOUS (PFIFFNER ET AL. 2010). FIG 6 STRATIGRAPHIC TABLE OF THE PENNINIC DOMAIN (PFIFFNER ET AL 2010).

9 3. STRATIGRAPHY The stratigraphic evolution of the three distal penninic units, the Valais-Trough, the Brianҫonnais continent and the Piemont-Ocean is strongly connected to the different facies environments due to their different water depth. The deep oceanic domains of the Valais-Trough and the Piemont-Ocean a build up similar containing a volcanic basement, covered by Mesozoic Bündnerschiefer with a Flysch sequence on top which have been deposited from middle Mesozoic age to Paleogene (Fig 5) VALAIS-TROUGH The following information presented here concerning the Valais domain is derived from the research of Steinmann 1994 and Steinmann & Stille 1999 given in Pfiffner et al (2010). The Valais domain can be stratigraphically subdivided in a basal unit comprising the Paleozoic crystalline such early Mesozoic units, the Bündnerschiefer (or Schistes Lustrés) representing Mesozoic deposits and the Cenozoic Flysch sediments on top. The basal unit consists of a mélange of gneisses, Permian clastics and Triassic dolomites and evaporates. Furthermore it comprises Liassic limestones and pillow basalts from the oceanic crust (Steinmann & Stille 1999). The base deposited on thinned continental crust which used to be the former continental margin but also oceanic crust in the center of the trough (Fig 5). The generation of the oceanic crust was induced by the extensional blocktectonic during spreading which was followed by overthrusting due to the compressional regime of the alpine orogeny. The overlaying sediments of the Mesozoic Bündnerschiefer measure thicknesses of 1000 m. Three units of Bündnerschiefer nappes belong to the Valais domain: the Tomül-, the Grave- and Prättigauand the Vals and Piz Terri-Bündnerschiefer (Schmid, 1996). The following description refers to the Tomül-Bündnerschiefer from the Mittelbünden area (Fig 6), which was investigated by Steinmann It starts with the Bärenhorn-Formation which deposited from Malm to early Cretaceous and consists of an alternating sequence of cm thick sandy-limy layers and 5-10 cm thick schists. Locally turbiditic sandstone layers occur. The Bärenhorn-Formation is overlain by the middle Cretaceous Nollaton-Formation. This formation has a significant higher clay content compared to the Bärenhorn-Formation. The lower part comprises thick argillites. The upper part also comprises argillites but also turbiditic intercalations. Those units are overlain by the Nollakalk-Formation. This unit was deposited during the Cenomanian and consists of sandy to shaly limestones. The top unit of the Valais-Trough was deposited mainly during Cenozoic time, but started in late Cretaceous times with thick limestones layers, marly and sandy limestones and marls. Those units are overlain by the Prättigau Flysch sediments from the Paleocene to early Eocene (Trümpy et al 1970). It consists of turbiditic limestones at the bottom, overlain turbiditic limestone, sandstone and argillites. The flysch sediments of the Eocene derive from prograding embankments from a southern source area BRIANҪONNAIS TERRANE The upper penninic Brianҫonnais domain did not develop in a deep oceanic basin like in the north and south penninic domain. It is buildup of the crystalline Suretta and Tambo nappes to the east with the Mesozoic cover of the Schams and Starlera nappes in the north. The following description is based on the research of Sartori et al (2006) presented in Pfiffner et al (2010). Similar to the Valais- Trough, the crystalline basement comprises Paleozoic gneisses, covered by the sedimentation of Permian quartzite and middle Triassic dolomites and upper Triassic evaporites which consist of a

10 thick layer of anhydrite. The blonde dolomites of the Keuper overlay those evaporites. The sedimentation of those sediments is thought to a phase of subsidence. The opening of the Piemont- Ocean in the early Jurassic induced an uplift of the Brianҫonnais continent, which led to a lack of sedimentation and erosional environment. Following thermic subsidence led to 1500 m of limestone oolithes in north western Sub-Brianҫonnais, while sedimentation of conglomerates and coal seams in the south eastern part are an evidence for continental depositional environment. During Dogger and Malm a further stop of subsidence, connected to the opening of the Valais-Trough in the NW induced a stop in sedimentation and caused erosion. Again the following thermic subsidence led to the deposition of 30 to 300 m thick limestone, partly oolithic. The early Cretaceous facies is mainly pelagic when the thin bedded limestones of the Calcaire plaqueté were deposited. Those were covered by the pelagic limestones and marls of the Couches rouges which locally overlay Malm limestones transgressively. The Couches rouges were deposited during late Cretaceous to Paleogene but stopped in the Paleocene. In Eocene the starting subduction of the Piemont-Ocean in the accretionary prism led to down flexing which induced further subsidence of the Brianҫonnais domain. This process led to the deposition of the Médianes flysch sediments (Stampfli, 1998) PIEMONT-OCEAN The sediments of the former Piemont-Ocean constitute the suture zone between the Penninic domains and the in the eastern Alps extending Austroalpine nappes. The stratigraphic sequence presented here is derived from the work of Dietrich (1970) and Marthaler (1984) and mostly referred to the Combin-Zone. Sedimentation in this domain started with opening of the Piemont-Ocean in a domain of massifs of the old continental margin with Triassic quarzites from a reducing environment with local dolomites like in the Evoléne- and Frilihorn-Formation (Marthaler 1984) and a pre-triassic crystalline at its base during early Jurassic. The oldest rocks were generated by the rifting and comprise serpentinites derived from mantle peridotites, gabbros and basalts (Pfiffner et al 2010). The basalts and pillow basalts developed to today s ophiolites of the early to middle Jurassic, due to the obduction processes induced by the back thrusting processes due to continent-continent collision. They are covered by sequences of thin layers of radiolarites, phyllites, calcareous schists and layers of breccia. The radiolarite layer becomes more abundant during the late Jurassic when the sequences stop. The sequence starts with the pillow basalts and is consistently followed by the deposition of radiolarites, which were deposited below CCD (Pfiffner et al 2010). Metamorphism changed the radiolarites to a sericite-chlorite-quartzite. In early Cretaceous thin bedded phyllitic limestone and phyllites had been deposited, which lithological compositions are very similar to the one of the Bündnerschiefer. The sediments of the Jurassic and Cretaceous are thought to have derived into marble and phyllitic marbles of the Frilihorn-Formation (Marthaler, 1984). Interbedded mantle vulcanites are an evidence of ongoing spreading processes at this time, 133 Ma after Dietrich (1970). On top the nappe the Gurnigel-Flysch has been deposited from the late Cretassic Maastrichtian until the late Eocene and measures 800 m in thickness. It consists of conglomeratic layers of dolomite, marble and quartzite pebbles, but also alternating sequences of sandy layers and turbiditic shale layers. Sandstone and shale were deposited during the Paleogene on a deep sea alluvial cone.the source area of those flysch sediments is thought to have been situated on the Brianҫonnais continent (Pfiffner et al 2010).

11 4. CONCLUSION The Evolution of the Penninic distal domain as it exists today can be more or less chronological isolated in-between the time from the late Triassic age to the Oligocene from the start of its tectonic evolution until the subduction of the nappe stack was completed. Following metamorphism and back thrusting formed the Flysch and Bündnerschiefer sediments which are recently exposed and derived from the Mesozoic to Cenozoic distal depositions of the former Alpine Tethys. Large scale tectonic rifting events like the break off Pangaea and the Atlantic rifting influenced the tectonic processes which led to the generation of the marine basins of the Valais- and the Piemont- Ocean, both separated by the former Iberian Brianҫonnais continent. All of those Penninic domains contain a pre-triassic crystalline basement, but due to the rifting and thinning of the continental crust the oceanic north and south Penninic domains also contain oceanic crust. The following marine facies from the Mesozoic time comprises the Bündnerschiefer. They can be subdivided into Tomül, Grava and Prättigau and the Vals and Piz Terri Bündnerschiefer nappes associated with the Valais- Ocean domain and the Avers Bündnerschiefer nappe which deposited in the Piemont-Ocean domain. The late Cretaceous to Paleogene flysch nappe comprises embankments due to turbiditic processes during subsidence induced by the subduction. One of the most common flysch sediments of the Valais domain is the Prättigau flysch, but also the Rhenodanubian Flysch in the north east and the Gurnigel Flysch of the Piemont-ocean constitute to the important Flysch sediments of the Penninics. A special external unit is the displaced Klippen (Fig 3) nappe which has been detached from its basement during the subduction progress during the Oligocene and contains the Mesozoic units of the Brianҫonnais domain.

12 REFERENCES Bill, M.; Bussy, F.; Cosca, M; Masson, H.; Hunziker, J. C. (1997): High-precision U-Pb and 40Ar/39Ar dating of an Alpine ophiolite (Gets nappe, French Alps). In Eclogae Geologicae Helvetiae (90), pp Bistacci, A.; Dal Piaz, G.; Massironi, M.; Zattin, M.; Balestrieri, M. (2001): The Aosta-Ranzola extensional fault system and Oligocene-Present evolution of the Austroalpine-Penninic wedge in the northwestern Alps. In International Journal of Earth Sciences (Geol Rundsch) 90 (3), pp Bucher, S.; Schmid, S. M.; Bousquet, R.; Fügenschuh, B. (2003): Late-stage deformation in a collisional orogen (Western Alps): nappe refolding, back-thrusting or normal faulting? In Terra Nova 15 (2), pp Caron, C.; Homewood, P.; Wildi, W. (1989): The original Swiss flysch: a reappraisal of the type deposits in the Swiss prealps. In Earth-Science Reviews 26 (1-3), pp Ceriani, S.; Fügenschuh, B.; Schmid, S. M. (2001): Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs. In International Journal of Earth Sciences (Geol Rundsch) 90 (3), pp Decarlis, A.; Dallagiovanna, G.; Lualdi, A.; Maino, M.; Seno, S. (2013): Stratigraphic evolution in the Ligurian Alps between Variscan heritages and the Alpine Tethys opening: A review. In Earth-Science Reviews 125, pp Dietrich, V. (1970): Die Stratigraphie der Platta-Decke: fazielle Zusammenhänge zwischen Oberpenninikum und Unterostalpin: Geologisches Institut der Eidg. Technischen Hochschule und der Universität Zürich Egger, H.: Zur paläogeographischen Stellung des Rhenodanubischen Flysches (Neokom-Eozän) der Ostalpen. In Jb. Geol. B.-A 1990 (133.2), pp Favre, P.; Stampfli, G. (1992): From rifting to passive margin: the examples of the Red Sea, Central Atlantic and Alpine Tethys. In Tectonophysics 215 (1-2), pp Frisch, W. (1979): Tectonic progradation and plate tectonic evolution of the Alps. In Tectonophysics 60 (3-4), pp Frisch, W.; Vavra, G.; Winkler, M. (1993): Evolution of the Penninic Basement of the Eastern Alps. In J.F Raumer, Franz Neubauer (Eds.): Pre-Mesozoic Geology in the Alps: Springer Berlin Heidelberg, pp Fügenschuh, B.; Loprieno, A.; Ceriani, S.; Schmid, S. M. (1999): Structural analysis of the Subbriançonnais and Valais units in the area of Moûtiers (Savoy, Western Alps): paleogeographic and tectonic consequences. In International Journal of Earth Sciences 88 (2), pp Galster, F.; Cavargna-Sani, M.; Epard, J.-L.; Masson, H. (2012): New stratigraphic data from the Lower Penninic between the Adula nappe and the Gotthard massif and consequences for the tectonics and the paleogeography of the Central Alps. In Tectonophysics 579, pp Gradstein, F. M.; Agterberg, F. P.; Ogg, J. G.; Hardenbol, J. (1995): A Triassic, Jurassic and Cretaceous Time Scale. In Society for Sedimentary Geology (SEPM), p. 32. Homewood, P. (1983): Palaeogeography of alpine flysch. In Palaeogeography, Palaeoclimatology, Palaeoecology 44 (3-4), pp Hunziker, J.; Desmons, J.; Hurford, A. (1992): Thirty-two years of geochronological work in the Central and Western Alps: a review on seven maps: Université de Lausanne (Mémoires de géologie, Lausanne). Kissling, E. (2008): Deep structure and tectonics of the Valais and the rest of the Alps. In Bulletin Angewandte Geologie (13(2)), pp Kissling, E. (1993): Deep structure of the Alps what do we really know? In Physics of the Earth and Planetary Interiors 79 (1-2), pp Marthaler, M. (1984): Géologie des unités penniques entre le val d'anniviers et le val de Tourtemagne (Valais, Suisse). Birkhäuser, Bâle. Maxelon, M.; Mancktelow, N. S. (2005): Three-dimensional geometry and tectonostratigraphy of the Pennine zone, Central Alps, Switzerland and Northern Italy. In Earth-Science Reviews 71 (3-4), pp

13 Pfiffner, O. A. (2010): Geologie der Alpen. 2 nd ed. Bern, Stuttgart, Wien: Haupt (UTB, 8416). Sartori, M.; Gouffon, Y.; Marthaler, M. (2006): Harmonisation et définition des unités lithostratigraphiques briançonnaises dans les nappes penniques du Valais. In Eclogae geol. Helv. 99 (3), pp Schmid, S. M.; Pfiffner, O. A.; Froitzheim, N.; Schönborn, G.; Kissling, E. (1996): Geophysical-geological transect and tectonic evolution of the Swiss-Italian Alps. In Tectonics 15 (5), pp Schmid, S.; Zingg, A.; Handy, M. (1987): The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone. In Tectonophysics 135 (1-3), pp Schmid, S. M.; Fügenschuh, B.; Kissling, E.; Schuster, R. (2004): Tectonic map and overall architecture of the Alpine orogen. In Eclogae geol. Helv. 97 (1), pp Stampfli, G.; Borel, G. (2002): A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrons. In Earth and Planetary Science Letters 196 (1-2), pp Stampfli, G.; Mosar, J.; Marquer, D.; Marchant, R.; Baudin, T.; Borel, G. (1998): Subduction and obduction processes in the Swiss Alps. In Tectonophysics 296 (1-2), pp Steinmann, M. (1999): Geochemical evidence for the nature of the crust beneath the eastern North Penninic basin of the Mesozoic Tethys ocean. In Geologische Rundschau 87 (4), pp Steinmann, M. Christoph M., C. (1994): Die nordpenninischen Bündnerschiefer der Zentralalpen Graubündens. Trümpy, R. (1970): Aperçu général sur la géologie des Grisons: Impr. Louis-Jean. Wissing, S.; Pfiffner, O. (2003): Numerical models for the control of inherited basin geometries on structures and emplacement of the Klippen nappe (Swiss Prealps). In Journal of Structural Geology 25 (8), pp