Giessen nappe
Encyclopedia
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
nappe
Nappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...
in the southeastern part of the Rhenish Massif in western Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. The nappe is an "alien" (allochthon
Allochthon
thumb|right|250px|Schematic overview of a thrust system. The hanging wall block is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a [[window |window]]...
ous) unit in the Rhenohercynian zone
Rhenohercynian Zone
The Rhenohercynian Zone is in structural geology a fold belt of west and central Europe, formed during the Hercynian orogeny . The zone consists of folded and thrusted Devonian and early Carboniferous sedimentary rocks that were deposited in a back-arc basin along the southern margin of the then...
of the Hercynian orogeny, it was thrusted
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
over the usual slightly metamorphosed
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
and Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s in this zone.
The Giessen nappe is often correlated with the Harz nappe in the Harz
Harz
The Harz is the highest mountain range in northern Germany and its rugged terrain extends across parts of Lower Saxony, Saxony-Anhalt and Thuringia. The name Harz derives from the Middle High German word Hardt or Hart , latinized as Hercynia. The legendary Brocken is the highest summit in the Harz...
and similar units in the Bohemian Massif
Bohemian Massif
The Bohemian Massif; or Český masiv; is in the geology of Central Europe a large massif stretching over central Czech republic, eastern Germany, southern Poland and northern Austria...
in Czechia. These units form the rare erosional leftovers of small microcontinents that were during the Hercynian orogeny squeezed between the paleocontinents Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
to the south and Laurussia to the north.
Outcrop area
Giessen nappe outcrops cover circa 300 km2 from the western edge of the VogelsbergVogelsberg
Vogelsberg is a municipality in the Sömmerda district of Thuringia, Germany....
over Braunfels
Braunfels
Braunfels is a town in the Lahn-Dill-Kreis in Hesse, Germany. It is located on the German Framework Road.- Location :The climatic spa of Braunfels lies at a height of some 100 m above the Lahn valley...
, Wetzlar
Wetzlar
Wetzlar is a city in the state of Hesse, Germany. Located at 8° 30′ E, 50° 34′ N, Wetzlar straddles the river Lahn and is on the German Timber-Framework Road which passes mile upon mile of half-timbered houses. Historically, the city has acted as the hub of the Lahn-Dill-Kreis on the north edge of...
and Gießen
Gießen
Gießen, also spelt Giessen is a town in the German federal state of Hesse, capital of both the district of Gießen and the administrative region of Gießen...
, almost till Marburg
Marburg
Marburg is a city in the state of Hesse, Germany, on the River Lahn. It is the main town of the Marburg-Biedenkopf district and its population, as of March 2010, was 79,911.- Founding and early history :...
. The river Lahn
Lahn
The Lahn River is a -long, right tributary of the Rhine River in Germany. Its course passes through the federal states of North Rhine-Westphalia , Hesse , and Rhineland-Palatinate ....
flows along the northern part of the nappe until she crosses the nappe between Gießen and Wetzlar to continue flowing westwards to the Rhine.
Lithology and stratigraphy
The Giessen nappe consists of a thin sequence of slightly metamorphosed Lower and Middle Devonian slateSlate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
s and radiolarite
Radiolarite
Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians. This term is also used for indurated radiolarian oozes and sometimes as a synonym of radiolarian earth...
s covered with Upper Devonian to Lower Carboniferous greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
s. At some spots along the base of the Giessen nappe alien tectonic slices of metabasalt occur. Chemical analysis of these rocks shows they were originally mid ocean ridge basalts. Especially the Upper Devonian to Lower Carboniferous greywackes are clearly different from the rocks of this age of the Rhenohercynian basin, that now form the footwall of the Giessen nappe. This difference in lithology led German geologist Franz Kossmat
Franz Kossmat
Franz Kossmat was an Austrian-German geologist, for twenty years the director of the Geological Survey of Saxony under both the kingdom and the subsequent German Republic....
in 1927 to the conclusion that the Giessen nappe is an allochthonous unit.
The base thrust of the Giessen nappe consists of cataclasite
Cataclasite
Cataclasite is a type of cataclastic rock that is formed by fracturing and comminution during faulting. It is normally cohesive and non-foliated, consisting of angular clasts in a finer-grained matrix.- Types of cataclasite :...
s, breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s and mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
s and forms a semi-horizontal thrust plane. Locally the thrust plane can be deformed by late stage folding and thrusting. Erosion has separated some small pieces of the nappe from the central mass, forming small klippe
Klippe
thumb|right|350px|Schematic overview of a thrust system. The shaded material is called a [[nappe]]. The erosional hole is called a [[window |window or fenster]]. The klippe is the isolated block of the nappe overlying autochthonous material....
s in the vicinity of Braunfels and Wetzlar. Internally, the rocks of the nappe are isoclinally folded
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
and cut by shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
s.