Evolution of the Helvetic Continental margin

Transcription

1 Evolution of the Helvetic Continental margin Christoph Klein , M.Sc. Georesources Management Supervisors: Prof. Janos Urai Prof. Ralf Littke RWTH Aachen University

2 Table of Contents 1. Introduction Evolution of Pangaea Evolution of the Helvetic Continental margin... 6 Triassic/Jurassic... 6 Cretaceous Conclusion List of references Introduction In 1911, Nowak proposed the term of the Helveticum (from lat. Helvetia). Firstly described in Switzerland it is the most external and lowermost superunit and equivalent to upper Bavarian layers and the Dauphinois in France. The Helveticum is a geologic subdivision of the Alps and described as marine sediments from upper Cretaceous and Tertiary origin. Nowadays, formations of the Helveticum can be found in the northern Alpine mountain ranges where outcrops are present at the surface. The Helvetic nappes were detached from their former crystalline basement of Penninic origin and massively thrusted over the Infrahelvetic complex and the external massif of the Alps. In Switzerland, Germany and Austria the Helvetic nappes are also partly thrusted over the Molasse Basin. Over the top of the Helvetic nappes the Penninic nappes were overthrusted. The region is tectonically defined to the South by Ultrahelvetic respectively Flysch formations and to the North by sediments of the folded Molasse (see Figure 1). For the reason that the Mesozoic orogeny of the Alps was controlled by subduction, collision and closure of two oceanic domains, results of bathymetric research showed that the Tertiary Helveticum was originally deposited at the southern margin of the European plate. The sedimentation was subdivided in the northern Helvetic facies and the southern Helvetic facies. Those facies are divided by the Intrahelvetic barrier, which is regionally different in its thickness as well as in its composition.

3 Figure 1: Palaeogeographic map of the Alps, present-day situation of the superunits of the Alps. Note: not a precise map in tectonical matter (After Froitzheim et al., 1996). 2. Evolution of Pangaea The evolution of the Alps is a typical example of a collisional belt and was strongly influenced by the processes that were present during the breaking apart of the ancient Pangaean continent. By splitting apart a lot of different zones of sedimentation were created which were for example deep ocean basins, shallow marginal seas and barrier zones. In the late Triassic 230 M years ago, the Pangaean continent was forced to split along a rifting system. This rift was induced from the Tethys Ocean between Arabia and Greece-Italy and led to a breaking apart between Gondwana and Laurasia (Wegener, 1920). In the following the Palaeotethys was closed by subduction, so that the Turkish island was connected to the Laurasian Continent (see Figure 2). Presumably 170 M years ago, in the eastern parts the rifting system was relocated to the North and cut off the islands of Greece-Italy from the Laurasian Continent, which was the birth of the Piemont Ocean. It consists mainly of oceanic lithosphere and is defined mainly by a classic alpine ophiolitic basement. In westerly direction the rift evolved further in the direction to Mexico and began to separate North America from Africa-South America. The created graben was the ancestor of the modern Atlantic Ocean and made a connection to the Neotethys Ocean (see Figure 3). The central Atlantic Ocean was born approximately 120 M years ago, when North America and Africa drifted more and more apart. Iberia began to separate from North America, which was the consequence of the mid-ocean ridge of the Atlantic Ocean as well as of two transform faults, which formed northerly and southerly of Iberia. Additional transform faults propagated on both sides of Greenland. The situation of the southern European Continent is shown in Figure 5. Iberia shows an appendix, the Briançonnais-Microcontinent, which is framed to the North by the Valais Ocean and on the southern side by the Piemont-Ocean. (Pfiffner, 2009)

4 Figure 2: Tectonic situation of the Pangaean continent in the Early Triassic approximately 237 M years ago. Birth of the growth of a rift system between Gondwana and Laurasia coming from the Tethys Ocean (Scotese, 1997). At that time the Dauphinois-Helveticum continental margin began to form on the southeast border of the European Continent. In between the Helvetic Continental margin and the South-, and Eastalpine formations laid the Penninic Ocean. Those three areas are the most important for the looks of our Alps nowadays (Pfiffner, 2009). The Piemont Ocean was then influenced by multiple transform faults, which are a hint for an opening in an asynchronous matter as a consequence of the counter clockwise movement of Africa. The Briançonnais-Microcontinent, which largely consist of platform sediments with stratigraphic gaps, separated in the following from Iberia. Figure 3: Tectonic situation around 200 M years ago. Birthplace of the Central Atlantic Ocean and the Gulf of Mexico as a consequence of the breaking apart of North America from Africa. Palaeotethys has already been closed (Scotese, 1997).

5 During the Cretaceous period a western oriented convergent plate movement of Europe and Africa- Arabia led to the closure of the Piemont-Ocean by subduction and was followed by the closure of the Valais-Ocean in the Cenozoic by collision of the Briançonnais-Microcontinent and the Adriatic continental margin (see Figure 5), later European continental margin (Dietrich, 1989). Complex tectonic movement led to the evolution of the Alps, which induced an even more complex geometry of the resulting rock formations. Figure 4: Late Cretaceous outlook of the earth s tectonic situation. Growth of the Atlantic Ocean, counter clockwise movement of the African continent and beginning closure of the Piemont-Ocean and collision of the Briançonnais- Microcontinent (Scotese, 1997). Figure 5: Detailed outlook on the situation of the European Adriatic continental margin in the Early Cretaceous with the Briançonnais-Microcontinent and the Piemont Ocean (Penninic), (Pfiffner, 2009).

6 3. Evolution of the Helvetic Continental margin a. Triassic/Jurassic Triassic sediments are prevalent sandstones and conglomerates. Being in the Triassic a continental stage, the Helvetic margin became a shelf in the Early Jurassic due to the opening of the Piemont Ocean, accompanied by the lateral spreading of both continental margins. The sequence of the sedimentary deposition was influenced by tectonic events and led in the Lower Jurassic (Lias) to a deposition of breccia, lime stones and sandstones. Additionally, dolomite can be found throughout this formation. The accumulation environment was shallow marine, which is proofed by ripple marks as well as cross lamination (Trümpy, 1975). On top of the dolomite layers a series of clay was formed due to lacustrine sedimentary conditions. In the middle of the Jurassic (Dogger) distractive geotectonics continued and induced huge fluctuation in the thickness of the different iron rich sandstone and breccia layers. In the following Late Jurassic (Malm) limestone was deposited from a deep sea environment and built formations that reach up to 500 m in thickness. Above that, limestone with alternating stratification of clay was deposited in a neritic depositional environment, which is explained and illustrated by Oolithes (Schmid, 2004). b. Cretaceous Figure 6: Strongly shortened geographic profile of the Alps at the end of the Early Cretaceous. Crosses: continental crust, white: sediment cover, black: oceanic crust, vertical ruling: lithospheric mantle (Froitzheim, 2012) Figure 6 shows the situation of the Alps at the end of the Early Cretaceous. The evolution of the Helvetic shelf in the Cretaceous can be separated into two sequences. The first sequence was a shallowing-upwards-sequence as a result of the uplift of the European continental shelf beginning in the Late Cretaceous. As a result of the uplift the Helvetic Ocean experienced a regression towards the South, which led to a fall dry of the northern Helvetic sedimentation area. Clay Formations ( Marnes Bleues ) were deposited, which can be dated to a worldwide anoxic event and have their maximum of thickness in the centre of the Vocontian Basin (Steinmann, 1994). Due to large scale tectonic changes it came in the Lutetian to an inversion of the continental shelf movement, which led

7 to a deepening-upwards-sequence (Stampfli, 1998). The continental basement subsided and forced a transgression of the Helvetic Ocean towards North. Evidence is given by pelagic carbonate deposits during the Cenomanian. Clay stone again, as well as siliceous limestone (e.g. Schrattenkalk) experienced a huge increase in thickness and were replaced by calcareous slate in southern direction. In the following the Helvetic Ocean was able to regain the northern Helvetic sedimentation area. In the Late Cretaceous mostly limestone was deposited, which shows a decreasing thickness to the distal end (Blaha, 2012). In the Palaeocene and Early Eocene the Helvetic shelf experienced a phase of uplifting again, which can be shown by shallow-water sediments such as nummulite and corallinacea (Faupl, 2000). Those sediments are then overlain by pelagic deposits of terrigenous flysch sediments and pass finally into the Molasse sediments of the Alpine foreland basin. With the beginning of the Late Oligocene in the post-collisional stage the Helvetic-Dauphinois domain was strongly deformed, when the orogeny propagated onto the proximal European margin (i.e. Glarus thrust, see Error! Reference source not found.). Former inactive rift systems were reactivated and inverted. Surprisingly, the orogeny then became bivergent with a southern and northern foreland and yet higher rates of erosion, due to the uplift of the Central Alps (Burkhard, 1998). The Helvetic and Ultrahelvetic nappes are cover sheets and minor folds, mainly of Mesozoic carbonate and Palaeogene flysch composition. Those rocks were detached along Triassic evaporites, Middle Jurassic and Lower Cretaceous shales (Piaz, 2012). Figure 7: Picture of the Glarus thrust, which can be seen as a horizontal line in the rocks. Older Permo-Triassic rock on top of younger Cretaceous lime stones as well as Palaeogene flysch and molasses. Taken in August Conclusion After the separation of Gondwana from Laurasia the orogeny of the Eastern Alps and the evolution of the Helvetic Continental margin were mainly induced by subduction, collision and closure of a convergent plate boundary of the Late Jurassic to Early Cretaceous closure of a Tethys Ocean and the closure of the Penninic Ocean (Sissingh, 2006). The Helvetic represents the southern shelf and partly the European continental margin. The analysis of the different layer types of the actual Helvetic nappes lead to different sedimentary settings. On top of the Variscan basement follows an autochthonous Permian-Mesozoic cover, marine sandstones and dolomite (evaporites). Followed by

8 the northward-overthrusted Helvetic nappes consisting of alternating sequences of limestone and clay. Those are a consequence of a series of the ocean s transgression and regression, due to subsidence and uplift of oceanic and continental crust. Those nappes were overlain by discontinuous Ultrahelvetic nappes (Froitzheim, 2012). Active plate tectonics and folds have then resulted in deformation and compression of the created stratigraphy. 5. List of references Blaha, U., (2012): Reconstruction of the Tectonic Evolution of the Helvetic Zone between Lake Chiemsee and Saalach River, East Upper Bavaria, Germany. Geophysical Research, Vol. 14, EGU Burkhard, M. and Sommaruga, A., (1998): Evolution of the western Swiss Molasse basin: structural relations with the Alps and the Jura belt. In: Mascle, A. et al. (eds), Cenozoic Foreland Basins of Western Europe. Geol. Soc. Special Publ., 134, Dietrich, D., Casey, M. (1989): A new tectonic model for the Helvetic nappes; Geological Society London, Special Publications 45, Faupl, Peter, Wagreich, Michael (2000): Late Jurassic to Eocene Palaeogeography and Geodynamic Evolution of the Eastern Alps. Froitzheim, Nikolaus (2012): Geology of the Alps Part1: General remarks; Austroalpine nappes. Steinmann Institut, Universität Bonn. Nowak, J., (1911): Über den Bau der Kalkalpen in Salzburg und im Salzkammergut. Bull. Acad. Sei. Cracovie, ser. A, Cracovie Pfiffner, A., (2009): Geologie der Alpen, Haupt Verlag Bern. Piaz, Giorgio V. Dal, (2012). Geological outline of the Alps, Episodes, Vol. 26, no. 3, Dipartimento di Geologia, Paleontologia e Geofisica, Università di Padova. Schmid, S.M., Fügenschuh, B., Kissling, E. & Schuster, R. (2004): Tectonic map and overall architecture of the Alpine orogen; Eclogae geol. Helv. 97, Scotese, C.R. (1997): Continental Drift Flip Book, 7th edition, Arlington, Texas, 80 pp. Sissingh, W. (2006): Syn-kinematic palaeogeographic evolution of the West European Platform: correlation with Alpine plate collision and foreland deformation, Netherlands Journal of Geosciences, Vol 85 2, pp Stampfli, G. M. et al., (1998): Subduction and obduction processes in the Swiss Alps, Tectonophysics 296 (1998) Trümpy, Rudolf, (1975): Penninic-austroalpine boundary in the Swiss Alps: A presumed former continental margin and its problems, American Journal of science, Vol. 275-a, Pp Wegener, Alfred (1920): Die Entstehung der Kontinente und Ozeane. 2. Aufl., 135 S., Friedrich Vieweg & Sohn, Braunschweig