The Rhenohercynian Zone is in structural geology a fold belt of west and central Europe, formed during the Hercynian orogeny (about 350 to 280 million years ago). 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 existing paleocontinent Laurussia.
The Rhenohercynian Zone presently forms a narrow zone through western and central Europe, from Cornwall in the west to the Harz mountains of central Germany in the east, including the Rhenish Massif (Ardennes, Taunus, Eifel and Hunsrück). 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. The sedimentary rocks are often weakly metamorphic (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 or the Rhenisch basin in Belgium and Germany.
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The Rhenohercynian Zone is a part of the northern foreland of the Hercynian orogen. It has a lower grade of metamorphism than the Saxothuringian Zone 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 Avalonian terranes). From the south it was overthrusted by the Mid-German Crystalline High, 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 Rhenohercynian basin was situated north of the Rheic Ocean, the ocean that spread between the continents of Laurussia (north) and Gondwana (south) from the Silurian onwards. The southern margin of Laurussia was formed during the Caledonian orogeny of the Silurian 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. 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.[2]
In the Middle Devonian (from 390 million years ago) a subduction zone existed south of Laurussia, where oceanic lithosphere of the Rheic Ocean subducted beneath the Mid-German/Normannian highs. Volcanism above the subduction zone created a cordillera-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 in a system of horsts and grabens together with basaltic 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 thrusted over the sedimentary basin fill of the Rhenohercynian basin.
The Rhenohercynian basin disappeared when the continent Gondwana collided 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 sequences of Cornwall, the Ardennes, the Eifel and the Harz.
From the Frasnian age (380 million years ago) the mafic volcanism ended and the basin came locally under compressional 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.[3] This was the short Bretonnic phase of the Hercynian orogeny. It was followed, from the Tournaisian (early Carboniferous, 355 million years ago) till the end of the Visean by a new period of extension.[4]
During the Sudetic (main) phase of the Hercynian orogeny (330-320 million years ago, Late-Visean and Namurian/Serpukhovian) 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 and Stephanian) the Rhenohercynian zone formed the foreland of a relatively fast developing Hercynian mountainbelt to the south. Isostatic subsidence of the foreland resulted in the development of a deep foreland basin. This filled with the products of erosion 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.[5]
The Rhenohercynian basin was filled with Devonian and Carboniferous sediments. Sedimentation was often disrupted by tectonic phases, 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) flysch and molasse sediments. The Namurian is characterized by flysch, in the Westphalian this gradually grades into molasse and other continental deposits, among which the thick coal layers of the Belgian coal measures.