Giessen ceiling

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The Giessen Nappe in the geological map of the southeastern Rhenish Slate Mountains

The Giessen Nappe is a geological unit of the southeastern Rhenish Slate Mountains . Due to its sequence of rocks, it represents a foreign body among the surrounding geological units and reached its current position as a tectonic blanket during the Variscan mountain formation through extensive thrust . Similar rock units are exposed in the southern Kellerwald and in the Harz as well as in the Czech Republic . These geological units are the remains of the upper micro- or continental plate involved in the Variscan continental collision in the Carboniferous , otherwise rarely preserved in Central Europe, and thus represent an important testimony to the mountain-forming processes.

location

The closed distribution area of ​​the Gießener Nappe is about 300 km 2 and extends at the west foot of the Vogelsberg for about 40 km from Braunfels via Wetzlar and Gießen almost to Marburg . Its northern part is accompanied in the east by the Lahn before it crosses the Gießener blanket between Gießen and Wetzlar and flows to the west towards the Rhine .

Rocks and sequence of layers

The rock content of the Gießen ceiling is composed of slightly thick clay , pebble and alum slates from the Lower and Middle Devonian and Grauwacken from the Upper Devonian and Lower Carbons . Above all, the Grauwacken of the Oberdevon stand in clear contrast to the clay and limestone that were deposited at the same time in the geological units immediately below the Giessen ceiling. This finding gave the first reason to classify the Gießener Grauwacken as a non-local rock body. In some places at the base of the Giessen Nappe there are tectonic scales made of basalts , which, due to their chemical composition, are considered MOR basalts .

structure

The base thrust of the ceiling structure is characterized by the occurrence of heavily stressed cataclasites and mylonites formed at relatively low temperatures . Today it lies almost horizontally, but is locally deformed by later folding and thrusting processes. Rock bodies at the edge were separated from the bulk of the ceiling by erosion and now lie in front of it as cliffs , for example at Braunfels and Steindorf, a district of Wetzlar. The rocks within the ceiling are tightly folded in places and criss-crossed by shear zones and faults.

History of exploration

Because of the Gießener Grauwacken, which deviates from the surrounding area, these have been interpreted by some geologists as a non-local geological unit since the beginning of systematic geological mapping, for example in 1927 by the geology professor Franz Kossmat who worked in Leipzig . Its special character has also been described by other geologists who carried out their research in this area, above all Johannes Ahlburg and Wilhelm Kegel in the first half of the 20th century. These interpreted the deviations as the product of narrow, differently designed deposition spaces that were directly next to each other. Since then, numerous studies have dealt with various individual questions. The interpretation of the unit as a tectonic blanket took a back seat to the notion that the Gießener Grauwacken was a special deposit area that remained essentially in connection with the surrounding units.

Discussions about the nature of the Gießener Grauwacken became more lively again after 1980 through various summarizing scientific essays, after more recent geological investigations supported the theory of a non-local origin with new results. Although numerous details have not yet been clarified due to the poor outcrops and the complicated geological conditions, the concept of a tectonic blanket for this geological unit has meanwhile become established. This is also followed by the representation in official maps: "Since the Giessener Grauwacke is stored in the hanging wall of pelagic rocks of the same age but different facially, it is interpreted as the erosion residue of a ceiling pushed over from the south."

With reference to the Giessen nappes, other parts of the south-eastern Rhenish Slate Mountains and the units of the southern Kellerwald and Harz Mountains associated with them are assumed to have a non-local origin. Separated from the Gießen ceiling are the allochthonous units of lesser extent located northwest in the Gladenbacher Bergland - the Steinhorn or Lohra ceiling as part of the Frankenbach scaling zone - as well as the elongated Hörre ceiling, which is characterized by the absence of volcanic rocks. Due to the distribution pattern of the dated zirconia ages in their sediments, these units are also assigned an Armorica Terran according to the preferred plate tectonic model , or a hinterland belonging to Gondwana before the collision with Laurussia .

Individual evidence

  1. ^ Roland Walter et al.: Geology of Central Europe. 5th edition. Schweizerbarth'sche Verlagsbuchhandlung, Stuttgart 1992, ISBN 3-510-65149-9 , p. 179.
  2. ^ A b H. Huckriede, K. Wemmer, H. Ahrendt: Palaeogeography and tectonic structure of allochthonous units in the German part of the Rheno-Hercynian Belt (Central European Variscides) . In: International Journal of Earth Sciences . tape 93 , no. 3 , June 2004, p. 414-431 , doi : 10.1007 / s00531-004-0397-4 .
  3. ↑ Jiří Kalvoda et al: Tectonostratigraphic development of the Devonian and Carboniferous in the Brunovistulian terrane, Czech Republic. In: M. Aretz, H.-G. Herbig (Ed.): Carboniferous Conference Cologne. From platform to basin. (= Cologne Forum for Geology and Paleontology. Volume 15). Cologne 2006, ISBN 3-934027-18-0 , pp. 53–54 ( muni.cz ).
  4. ↑ Wolfgang Dörr: Stratigraphy, material inventory and facies of the Giessener Grauwacke (eastern Rhenish slate mountains). (= Geological treatises Hessen. Volume 91). Wiesbaden 1990, DNB 910500126 .
  5. ↑ a b F. Kossmat: Structure of the Varist mountain building. In: Treatises of the Saxon Geological State Institute. Issue 1, Leipzig 1927, 39 pp.
  6. ^ J. Grösser, W. Dörr: MOR basalts in the eastern Rhenish slate mountains. In: New Yearbook for Geology and Paleontology. Monthly Hefts, Volume 12, 1986, pp. 705-722.
  7. ↑ a b M. Birkelbach among others: The geological development of the eastern Lahnmulde (excursion April 1988). In: Annual reports and communications from the Upper Rhine Geological Association. Volume 70, Stuttgart 1988, pp. 43-74.
  8. ↑ J. Ahlburg: About the spread of the Silurian, Hercyn and Rhenish Devons and their relationship to the geological structure in the eastern Rhenish Mountains. In: Yearbook of the Prussian Geological State Institute . Volume 40, Issue 1, Berlin 1921, pp. 1-82.
  9. ^ W. Kegel: Outline of the geology of the Lahnmulde. Explanations of an overview map and profile of the Lahnmulde left by Johannes AHLBURG. (= Treatises of the Prussian Geological State Institute. New series. Issue 86). Berlin 1922, DNB 580347923 .
  10. ^ W. Kegel: Geology of the Dillmulde. In: Treatises of the Prussian Geological State Institute, New Series, Issue 160, Berlin 1934, 48 pp.
  11. ^ W. Engel et al: Nappe Tectonics in the Southeastern Part of the Rheinisches Schiefergebirge. In: H. Martin, FW Eder (Ed.): Intracontinental Fold Belts. Heidelberg 1983, pp. 267-287.
  12. ^ Geological overview map 1: 200,000, sheet CC 5510 Siegen. Federal Institute for Geosciences and Natural Resources, Hanover, description , accessed on January 11, 2016.
  13. ↑ Katja Eckelmann, Heinz-Dieter Nesbor, Peter Königshof, Ulf Linnemann, Mandy Hofmann, Jan-Michael Lange, Anja Sagawe: Plate interactions of Laurussia and Gondwana during the formation of Pangea - Constraints from U – Pb LA – SF – ICP – MS detrital zircon ages of Devonian and Early Carboniferous siliciclastics of the Rhenohercynian zone, Central European Variscides . In: Gondwana Research . tape 25 , no. 4 , May 2014, p. 1484–1500 , doi : 10.1016 / j.gr.2013.05.018 . , also as (PDF senckenberg.de ).

Coordinates: 50 ° 34 ′ 50.2 "  N , 8 ° 35 ′ 13.9"  E