Rhenohercynian Zone
Encyclopedia
Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
a fold belt of west and central Europe, formed during the Hercynian orogeny (about ). The zone consists of 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 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...
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 early 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 that were deposited in a back-arc basin
Back-arc basin
Back-arc basins are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the...
along the southern margin of the then existing paleocontinent Laurussia.
The Rhenohercynian Zone presently forms a narrow zone through western and central Europe, from Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
in the west to the Harz mountains of central 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...
in the east, including the Rhenish Massif (Ardennes
Ardennes
The Ardennes is a region of extensive forests, rolling hills and ridges formed within the Givetian Ardennes mountain range, primarily in Belgium and Luxembourg, but stretching into France , and geologically into the Eifel...
, Taunus
Taunus
The Taunus is a low mountain range in Hesse, Germany that composes part of the Rhenish Slate Mountains. It is bounded by the river valleys of Rhine, Main and Lahn. On the opposite side of the Rhine, the mountains are continued by the Hunsrück...
, Eifel
Eifel
The Eifel is a low mountain range in western Germany and eastern Belgium. It occupies parts of southwestern North Rhine-Westphalia, northwestern Rhineland-Palatinate and the south of the German-speaking Community of Belgium....
and Hunsrück
Hunsrück
The Hunsrück is a low mountain range in Rhineland-Palatinate, Germany. It is bounded by the river valleys of the Moselle , the Nahe , and the Rhine . The Hunsrück is continued by the Taunus mountains on the eastern side of the Rhine. In the north behind the Moselle it is continued by the Eifel...
). The total length of this basin (the Rhenohercynian Basin) could have been more than 2500 km. In the east the basin merges with the East-Silesian basin of southern Poland
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...
. The sedimentary rocks are often weakly metamorphic
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...
(greenschist facies). Most geologists consider the South-Portuguese zone to be a continuation of the zone to the west.
If the Rhenohercynian Basin was a continuous feature or rather a string of temporaneously interconnected smaller basins is not well-understood, because in many places the Devonian and Carboniferous rock strata are covered with younger deposits. Parts of the basin have their own names, like the Cornwall basin in Cornwall, the Munster basin in Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
or the Rhenisch basin in Belgium and Germany.
Tectonic structure and metamorphism
Foreland
Foreland is the easternmost point of the Isle of Wight. It is located three miles east of the town of Brading, and due south of the city of Portsmouth on the British mainland. It is characterised by a pub called the Crab and Lobster and various beach huts plus a beach cafe and a coast guard...
of the Hercynian orogen. It has a lower grade of metamorphism than the Saxothuringian Zone
Saxothuringian Zone
The Saxothuringian Zone or Saxothuringicum is in geology a structural or tectonic zone in the Hercynian or Variscan orogen of central and western Europe. Because rocks of Hercynian age are in most places covered by younger strata, the zone is not everywhere visible at the surface...
to the south, meaning its rocks have generally been at smaller depths and under lower temperatures. The Subvariscan Zone north of the Rhenohercynian Zone was untouched by Hercynian metamorphism. During the Hercynian orogeny, the Rhenohercynian zone was folded and thrusted internally. It was thrusted over the foreland to the north (the London-Brabant Massif and other Avalonia
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...
n terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s). From the south it was overthrusted by the Mid-German Crystalline High
Mid-German Crystalline High
The Mid-German Crystalline High is a structural high in the Paleozoic geology of Germany. The high forms a northeast-southwest oriented zone through Germany, but actual rock outcrops are sparse since Paleozoic basement rocks are in most of central Germany overlain by younger sedimentary rocks...
, part of the Saxothuringian Zone.
The metamorphic grade increases towards the south or southeast. The southern edge of the Rhenish Massif lies in the Northern Phyllite Zone, which has a higher grade than other parts of the zone.
The Devonian basin
The Rhenohercynian basin was situated north of the Rheic OceanRheic Ocean
The Rheic Ocean was a Paleozoic ocean between the large continent Gondwana to the south and the microcontinents Avalonia and others to the north...
, the ocean that spread between the continents of Laurussia (north) and 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,...
(south) from the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
onwards. The southern margin of Laurussia was formed during the Caledonian orogeny
Caledonian orogeny
The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...
of the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
period, about 420 million years ago. In the Gedinnian/Lochkovian (Early Devonian) the southern part of the Caledonian mountain belt became a region of north-south extension
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
. An elongated basin was formed parallel to the continental margin. It separated the London-Brabant Massif to the north from the Normannian and Mid-German Highs to the south.
In the Middle Devonian (from 390 million years ago) a subduction zone existed south of Laurussia, where oceanic lithosphere
Oceanic lithosphere
Oceanic lithosphereOceanic lithosphere is typically about 50-100 km thick , while the continental lithosphere has a range in thickness from about 40 km to perhaps 200 km; the upper ~30 to ~50 km of the typical continental lithosphere is crust...
of the Rheic Ocean subducted beneath the Mid-German/Normannian highs. Volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
above the subduction zone created a cordillera
Cordillera
A cordillera is an extensive chain of mountains or mountain ranges, that runs along a coastline . It comes from the Spanish word cordilla, which is a diminutive of cuerda, or "cord"...
-type mountain chain, the Ligerian cordillera. In the Siegenian/Pragian and Emsian the Rhenohercynic basin was a back-arc basin behind this cordillera. Tectonic subsidence
Tectonic subsidence
Tectonic subsidence is the sinking of the Earth's crust relative to neighboring crust or other point of reference. Components of plate movement create several environments in which subsidence occurs including, passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins,...
in a system of horsts and graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
s together with basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic volcanism resulted in the creation of new oceanic lithosphere. In the Middle Devonian a second basin, the Saxothuringian/Armorican basin, developed south of the Rhenohercynian basin. To the west some crustal convergence took place, and the Normannian High was partly thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
ed over the sedimentary basin fill of the Rhenohercynian basin.
Carboniferous compression
The Rhenohercynian basin disappeared when the continent Gondwana collidedContinental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
with Laurussia in the course of the Carboniferous period (the Hercynian orogeny). The sedimentary rocks in the basin were thrusted in a series of piggyback basins over the northern foreland (the London-Brabant Massif). These rocks now form the 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...
sequences of Cornwall, the Ardennes
Ardennes
The Ardennes is a region of extensive forests, rolling hills and ridges formed within the Givetian Ardennes mountain range, primarily in Belgium and Luxembourg, but stretching into France , and geologically into the Eifel...
, the Eifel
Eifel
The Eifel is a low mountain range in western Germany and eastern Belgium. It occupies parts of southwestern North Rhine-Westphalia, northwestern Rhineland-Palatinate and the south of the German-speaking Community of Belgium....
and the Harz.
From the Frasnian
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian epoch. It lasted from 385.3 ± 2.6 million years ago to 374.5 ± 2.6 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage...
age (380 million years ago) the mafic volcanism ended and the basin came locally under compressional
Compression (geology)
In geology the term compression refers to a set of stresses directed toward the center of a rock mass. Compressive strength refers to the maximum compressive stress that can be applied to a material before failure occurs. When the maximum compressive stress is in a horizontal orientation, thrust...
stress, which led to folding and thrusting in the sedimentary rocks. Somewhere near the end of the Devonian, a subduction zone developed under the Mid-German/Normannian highs and Rhenohercynian crust began to subduct. This was the short Bretonnic phase of the Hercynian orogeny. It was followed, from the Tournaisian
Tournaisian
The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from 359.2 ± 2.5 Ma to 345.3 ± 2.1 Ma...
(early Carboniferous, 355 million years ago) till the end of the Visean
Viséan
The Visean, Viséan or Visian is an age in the ICS geologic timescale or a stage in the stratigraphic column. It is the second stage of the Mississippian, the lower subsystem of the Carboniferous. The Visean lasted from 345.3 ± 2.1 to 328.3 ± 1.6 Ma...
by a new period of extension.
During the Sudetic (main) phase of the Hercynian orogeny (330-320 million years ago, Late-Visean and Namurian
Namurian
The Namurian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 326 and 313 Ma . It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Namurian is named for the Belgian city and province of Namur where strata of this age...
/Serpukhovian
Serpukhovian
The Serpukhovian is in the ICS geologic timescale the uppermost stage or youngest age of the Mississippian, the lower subsystem of the Carboniferous. The Serpukhovian age lasted from 328.3 Ma tot 318.1 Ma...
) compressional tectonics had the upper hand again. In the Namurian age full scale continental collision between Laurussia and Gondwana resulted in the destruction of the last oceanic crust of the basin. Its sedimentary fill was, however, not (totally) subducted but instead thrusted northward. During the later part of the Carboniferous period (Westphalian
Westphalian (stage)
The Westphalian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 313 and 304 Ma . It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Westphalian is named for the region of Westphalia in western Germany where strata...
and Stephanian
Stephanian (stage)
The Stephanian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 304 and 299 Ma . It is a subdivision of the Carboniferous system or period and the regional Silesian series...
) the Rhenohercynian zone formed the foreland of a relatively fast developing Hercynian mountainbelt to the south. Isostatic
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
subsidence of the foreland resulted in the development of a deep foreland basin
Foreland basin
A foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
. This filled with the products of erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
in the Hercynian mountains and the contemporaneously uplifted London-Brabant Massif to the north. During the Westphalian, the basin was completely filled and rose above sea level.
Stratigraphy
The Rhenohercynian basin was filled with Devonian and Carboniferous sediments. Sedimentation was often disrupted by tectonic phaseTectonic phase
A tectonic phase or deformation phase is in structural geology and petrology a phase in which tectonic movement or metamorphism took place. Tectonic phases can be extensional or compressional in nature....
s, but nevertheless the total thickness of the sediments can in some places be more than several kilometers.
When a foreland basin was formed in the Rhenohercynian zone, this was filled with upper Carboniferous (Pennsylvanian
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
) flysch
Flysch
Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...
and molasse
Molasse
The term "molasse" refers to the sandstones, shales and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse is deposited in a foreland basin, especially on top of flysch, for example that left from the rising Alps, or erosion in the Himalaya...
sediments. The Namurian is characterized by flysch, in the Westphalian this gradually grades into molasse and other continental deposits, among which the thick coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
layers of the Belgian coal measures.
Literature
; 1989: Tectonostratigraphic units in the Variscan belt of Central Europe, in: (eds.): Terranes in the Circum-Atlantic Paleozoic orogens, Geological Society of AmericaGeological Society of America
The Geological Society of America is a nonprofit organization dedicated to the advancement of the geosciences. The society was founded in New York in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitchcock, John R. Proctor and Edward Orton and has been headquartered at 3300 Penrose...
Special Paper 230, pp. 67–90.; 1992: Phanerozoic structures and events in central Europe, in: (eds.): A Continent Revealed - The European Geotraverse, 297 pp., Cambridge University Press, ISBN 052142948X, pp. 164–179.; 2000: The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution, in: (eds.); Orogenic Processes, Quantification and Modelling in the Variscan Belt, Geological Society of London, Special Publications 179, pp. 35–61.; 2001: The Variscan collage and orogeny (480±290 Ma) and the tectonic definition of the Armorica microplate: a review, Terra Nova 13, 122-128.; 2003: Gondwana-derived microcontinents – the constituents of the Variscan and Alpine collisional orogens, Tectonophysics 365, pp. 7–22.; 2003: Erdgeschichte – Die Entstehung der Kontinente und Ozeane, 325 pp., Walter de Gruyter, Berlin (5e druk), ISBN 3110176971.; 1990: Geological Atlas of Western and Central Europe, Shell Internationale Petroleum Maatschappij BV
Royal Dutch Shell
Royal Dutch Shell plc , commonly known as Shell, is a global oil and gas company headquartered in The Hague, Netherlands and with its registered office in London, United Kingdom. It is the fifth-largest company in the world according to a composite measure by Forbes magazine and one of the six...
(2nd ed.), ISBN 90-6644-125-9.