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65-803: It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Laurussia, with metamorphic events in southwestern Texas and northern Mexico , and with the Antler orogeny of the Great Basin . During the time of the Acadian orogeny, Middle Devonian (385 Ma), the paleolatitude of the Laurentia was in the southern hemisphere near the equator, between 0° to 30°S latitude. Laurentia did not change much with respect to paleolatitude during

130-400: A rising plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons. A third model suggests that successive slabs of subducting oceanic lithosphere became lodged beneath a proto-craton, underplating the craton with chemically depleted rock. A fourth theory presented in a 2015 publication suggests that the origin of

195-548: A coastal alluvial plain that was hundreds of miles long. The Middle Devonian to Lower Mississippian siliciclastic strata , deposited by the Catskill Delta, includes black shale , gray shale, sandstone , red beds , and minor argillaceous limestone. The strata was deposited in a four-stage pattern that is observed in each tectophase. The formation of the foreland basin through rapid subsidence initiated transgressive sequences that deposited basinal black shales. After

260-578: A land mass, which has been named Gondwana (present day South America, Africa, Antarctica, Arabia , the Indian subcontinent , Zealandia and Australia), straddled the space between the South Pole and the Equator on one side of the globe. Off to the west were three other masses: Laurentia , Siberia and Baltica , located as if on the vertices of a triangle. To the south of them was a large archipelago,

325-510: A large area of eastern Laurentia. During the course of the orogeny , new faults formed, while older faults were reactivated. Acadian deformation and metamorphism were asymmetric across the strike of the orogen. The Acadian plutons intruded every belt, unlike the deformation/metamorphism, of Avalonia which did not undergo much of the alteration shown in other localities. During the Middle Devonian, centers for volcanoes and uplift formed in

390-812: A new ocean opened up, the Rheic Ocean , during the Middle to Late Devonian, and subsequently its closure resulted in the formation of the Alleghanian orogeny . Laurentia is the North American paleocontinent , which also includes present day northwest Ireland , Scotland , Greenland , the north slope of Alaska , and the Chukotsk Peninsula of northeastern Russia . During the Ordovician-Devonian time, Laurentia remained at

455-457: A solid residue very close in composition to Archean lithospheric mantle, but continental shields do not contain enough komatiite to match the expected depletion. Either much of the komatiite never reached the surface, or other processes aided craton root formation. There are many competing hypotheses of how cratons have been formed. Jordan's model suggests that further cratonization was a result of repeated continental collisions. The thickening of

520-1310: A term for mountain or orogenic belts . Later Hans Stille shortened the former term to Kraton , from which craton derives. Examples of cratons are the Dharwar Craton in India, North China Craton , the East European Craton , the Amazonian Craton in South America, the Kaapvaal Craton in South Africa, the North American Craton (also called the Laurentia Craton), and the Gawler Craton in South Australia. Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice

585-584: Is assumed they originated from areas near promontories , areas along the continental margin where deformation is concentrated. The earliest tectophase was located at the St. Lawrence promontory in northern New England and in the Canadian Maritime Provinces. The St. Lawrence tectophase was active during the Early to Middle Devonian with intense transpressional deformation which formed a basin in

650-718: Is believed to come from the margin of Gondwana, sometime in the Early Ordovician. Avalonia rifted from Gondwana during the onset of igneous activity in the Ardennes, Wales, and southeast Ireland that consumed the Tornquist Sea oceanic crust. It drifted in a northerly direction and probably collided with Baltica in the Late Ordovician, and then with Laurentia in the Late Devonian. Evidence for this

715-532: Is consistent with paleomagnetic data which place Avalonia at a temperate latitude during the Ordovician and in a subtropical latitude during the Late Ordovician through the Devonian. The Acadian orogeny resulted from oblique convergence or major transcurrent movement along a large strike-slip fault which represents the zone of convergence between Laurussia/Laurentia and Avalon terranes. One or more of

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780-477: Is coupled to the four tectophases of the Acadian orogeny, both in terms of provenance and depositional settings. The relief resulting from the orogeny was the fundamental source of the delta sediments. The Catskill Delta complex consists of a coarsening upward sequence of rocks. Its thickness is greatest in eastern Pennsylvania and thins westward into Ohio. The Catskill paleogeography appears to consist of many small streams, which deposited their sedimentary load along

845-543: Is known as the Coastal Lithotectonic Block. The collision between Laurentia and Avalonian terranes is actually more complex than described above. The collision is broken into three or possibly four tectophases which represent a successive collision of the Avalonian terranes with eastern Laurentia. Because major clastic wedges and basinal deposits are distributed in a southwestward progression, it

910-522: Is marked by a classic strike-slip suture zone between very distinct suspect terranes, and clear evidence can be seen of ductile shearing between high-grade metamorphic rocks and lower grade sedimentary rocks in a wide belt north of the Algarve and extending into the northernmost part the autonomous region of Andalusia and southern Extremadura . In the Czech Republic and southwestern Poland

975-553: Is poor and is found primarily in the plutonism of the Blue Ridge and metamorphism of the Cat Square terrane. The Acadian orogeny experienced at least three major phases of deformation, and in places, unconformities are recognized. These phases are called tectophases and represent the sequence of collisions that occurred from the Avalonian terranes accreting to Laurentia. As a result of these tectophases, deltas developed on

1040-521: Is represented by regional uplift, which accompanied the collision of an Avalon terrane with a promontory, and subsequently, developed a regional disconformity . The fourth and final stage is represented by tectonic quiescence with a widespread carbonate deposition in a slowly transgressing sea. Variscan orogeny The Variscan orogeny , or Hercynian orogeny , was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form

1105-452: Is strongly influenced by the inclusion of moisture. Craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium and iron. Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent

1170-553: Is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons are composed of two layers: a continental shield , in which the basement rock crops out at the surface, and a platform which overlays the shield in some areas with sedimentary rock . The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen , referring to stable continental platforms, and orogen as

1235-625: The Bohemian Massif is the eastern end of the unmodified Variscan belt of crustal deformation in Europe. Further Variscan developments to the southeast are partly hidden and overprinted by the Alpine orogeny . In the Alps a Variscan core is built by Mercantour , Pelvoux , Belledonne , Montblanc and Aar Massif . Dinaric , Greek and Turkish mountain chains are the southeastern termination of

1300-540: The Gaspé Peninsula , northern New Brunswick, and northern New England. Clastic wedges were present in this area, but the evidence for them has been mostly destroyed by succeeding tectonism. The second tectophase, during the Middle Devonian, represents the collision with the New York promontory. The southward migration of deformation reflected the third tectophase, which marks the collision of Avalon terranes with

1365-1058: The Vale of Glamorgan . Its effects are present in France from Brittany , below the Paris Basin to the Ardennes , the Massif Central , the Vosges and Corsica . The Variscan Belt reappears in Sardinia in Italy and in Germany where the Rhine Massif (Ardennes, Eifel , Hunsrück , Taunus and other regions on both sides of Middle Rhine Valley), the Black Forest and Harz Mountains remain as testimony. In southern Iberia it

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1430-688: The supercontinent of Pangaea . The name Variscan comes from the Medieval Latin name for the district Variscia , the home of a Germanic tribe, the Varisci ; Eduard Suess , professor of geology at the University of Vienna , coined the term in 1880. ( Variscite , a rare green mineral first discovered in the Vogtland district of Saxony in Germany, which is in the Variscan belt, has

1495-764: The terrane Avalonia , rifted off the north Gondwana margin in early Ordovician . By the end of the Silurian and in Early Devonian times, Baltica and Laurentia drifted towards each other, closing the Iapetus Ocean between them. They collided in the Caledonian orogeny and formed the Caledonide mountains of North America, Greenland , the British Isles and Norway . Seafloor spreading to

1560-406: The "cratonic regime". It involves processes of pediplanation and etchplanation that lead to the formation of flattish surfaces known as peneplains . While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over geological time leads to the formation of so-called polygenetic peneplains of mixed origin. Another result of

1625-669: The Acadian foreland basin as a response to the Acadian orogeny. These deposits extend from central New York and Pennsylvania westward to Ohio, and south along the Appalachian Mountains through Virginia and Tennessee to Alabama. The Acadian delta complex is categorized into two deltas, the Catskill Delta of Middle and Upper Devonian age, and the Price-Rockwell in the Pocono Mountains delta of Late Devonian and Early Mississippian age. The Acadian delta complex

1690-473: The Acadian orogeny, since it demonstrated an unusually long duration (Mississippian-Early Pennsylvanian). Subsequently, the Pocono and equivalent clastic wedges essentially filled the epicontinental sea . The deposition of Middle Mississippian carbonates marks the end of the Acadian orogeny and Catskill Delta complex. Foreland basins are a product of tectonic deformational loading, or crustal thickening along

1755-753: The Appalachian Mountains and used to form the eastern part of the Appalachian orogeny before the opening of the Atlantic Ocean in Jurassic times. 'Variscan' mountains in a broad chronological sense include the Urals , the Pamir , the Tian Shan and other Asian foldbelts. The Variscan orogeny involved a complicated heterogeneous assembly of different microplates and heterochronous collisions, making

1820-539: The Appalachian foreland basin at the onset of the Acadian orogeny were reactivated during foreland lithospheric flexure . These structures affected the foreland basin evolution and sedimentation patterns, and the preexisting faults partitioned the basin into regions of fault-controlled uplift and depocenters . The Appalachian basin, during the Middle Devonian and Early Mississippian, is characterized by large volumes of deltaic sedimentary rocks that were deposited in

1885-562: The Appalachians. Craton A craton ( / ˈ k r eɪ t ɒ n / KRAYT -on , / ˈ k r æ t ɒ n / KRAT -on , or / ˈ k r eɪ t ən / KRAY -tən ; from ‹See Tfd› Greek : κράτος kratos "strength") is an old and stable part of the continental lithosphere , which consists of Earth's two topmost layers, the crust and the uppermost mantle . Having often survived cycles of merging and rifting of continents, cratons are generally found in

1950-400: The Avalonian terranes accreted with the eastern margin of Laurentia, most likely beginning in the late Early Devonian. The evidence for the Acadian orogeny is abundant and widespread in the northern Appalachians, recorded by the plutonism and the migration of the northern Appalachian deformation front toward the craton . In the central to southern Appalachians, evidence for the Acadian orogeny

2015-648: The British Isles, northern Germany, Scandinavia and western Russia). In late Devonian and in the Carboniferous the archipelago Armorica of southern Europe, which had rifted off Gondwana after Avalonia later in the Ordovician, was pushed into Avalonia, creating a second range, the North American/European Variscan, to the east of the Caledonide/Appalachian. The collision of Gondwana proper with Laurussia followed in

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2080-564: The Devonian. Gondwana , on the other hand, traveled a large distance, such that in the Ordovician the South Pole was located in northern Africa , where it then moved west of southern Chile during the Silurian , and moved back to central Africa during the Devonian. However, more recent research, from Scotese & McKerrow, suggests that in Late Devonian, the South Pole was in north-central Argentina rather than northern Africa, which

2145-556: The New England region and shed fine-grained clastic material into an inland seaway that covered a large part of southern and central Appalachia. Today, portions of the ancient Avalonia landmass occur in scattered outcrop belts along the eastern margin of North America. One belt occurs in Newfoundland; another forms the bedrock of much of the coastal region of New England from eastern Connecticut to northern Maine , where it

2210-637: The Variscan proper. The Variscan was contemporaneous with the Acadian and Alleghenian orogeny in the United States and Canada, responsible for forming the Ouachita and Appalachian Mountains . North American areas with Variscan foldbelts include New England , Nova Scotia and Newfoundland and Labrador . The Moroccan Meseta and the Anti-Atlas in northwestern Africa show close relations to

2275-721: The Virginia promontory in the Middle to Late Devonian time. The effects of the New York and Virginia promontories together produced the Catskill Delta complex. As the migration of deformation continued southward along the fault zone, during the Early Mississippian time, the final collision occurred with the Alabama promontory . Ettensohn refers to the fourth tectophase as the Mississippian tectophase of

2340-423: The adjacent parts of the stable craton, eastern margin of Laurentia. These deltas are described as foreland-basin, delta-complex clastic wedges, which are responsible for the large volumes of sediment input into the Appalachian basin. The collision of Avalonia with Laurentia initiated a sequence of events where the older rocks were subjected to deformation , plutonism, metamorphism , and uplift that occurred over

2405-438: The black shales were deposited, the migration of deformation continued southward, and regression dominated, particularly on the east side of the basin. As collision intensified, subsidence in the foreland basin declined, and sedimentation was replaced by an influx of calcareous silty shales and carbonates. These deposits reflect small transgressive-regressive cycles in a delta-front and delta platform environments. The third stage

2470-502: The craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years. Rock fragments ( xenoliths ) carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites . These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotite

2535-413: The cratons is similar to crustal plateaus observed on Venus, which may have been created by large asteroid impacts. In this model, large impacts on the Earth's early lithosphere penetrated deep into the mantle and created enormous lava ponds. The paper suggests these lava ponds cooled to form the craton's root. The chemistry of xenoliths and seismic tomography both favor the two accretional models over

2600-399: The crust associated with these collisions may have been balanced by craton root thickening according to the principle of isostacy . Jordan likens this model to "kneading" of the cratons, allowing low density material to move up and higher density to move down, creating stable cratonic roots as deep as 400 km (250 mi). A second model suggests that the surface crust was thickened by

2665-427: The crystalline residues after extraction of melts of compositions like basalt and komatiite . The process by which cratons were formed is called cratonization . There is much about this process that remains uncertain, with very little consensus in the scientific community. However, the first cratonic landmasses likely formed during the Archean eon. This is indicated by the age of diamonds , which originate in

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2730-426: The depleted "lid" formed by the first layer. The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either. All these proposed mechanisms rely on buoyant, viscous material separating from a denser residue due to mantle flow, and it is possible that more than one mechanism contributed to craton root formation. The long-term erosion of cratons has been labelled

2795-422: The early Carboniferous, when the Variscan belt was already in place and actively developing. By the end of the Carboniferous, Gondwana had united with Laurussia on its western end through northern South America and northwestern Africa. Siberia was approaching from the northeast, separated from Laurussia only by shallow waters. Collision with Siberia produced the Ural Mountains in the latest Paleozoic and completed

2860-430: The eastern continental margin, and the resulting foreland-basin and clastic wedges . Avalonian terranes that constitute Avalonia are the following modern-day regions: northern France, Belgium (the Ardennes ), England , Wales , southeastern Ireland , eastern Newfoundland , Nova Scotia , southern New Brunswick and some coastal parts of New England . The basement consisted of Late Precambrian age arc rocks and

2925-404: The exact reconstruction of the plate tectonic processes difficult. Plate convergence that caused the Caledonian orogeny in the Silurian continued to form the Variscan orogeny in the succeeding Devonian and Carboniferous Periods. Both orogenies resulted in the assembly of a super-continent, Pangaea , which was essentially complete by the end of the Carboniferous. In the Ordovician Period,

2990-412: The fold belt proper. One of the pioneers in research on the Variscan fold belt was the German geologist Franz Kossmat , establishing a still valid division of the European Variscides in 1927. The other direction, Hercynian , for the direction of the Harz Mountains in Germany, saw a similar shift in meaning. Today, Hercynian is often used as a synonym for Variscan but is somewhat less used than

3055-458: The formation of Pangaea. Eastern Laurussia was still divided from Gondwana by the Paleotethys Ocean. In the Triassic Period of the Mesozoic Era, animals could move without oceanic impediment from Siberia over the North Pole to Antarctica over the South Pole. In the Mesozoic Era, rifting and subsequent opening of the Atlantic split Pangaea. As a consequence, the Variscan Belt around the then periphery of Baltica ended up many hundreds of miles from

3120-443: The interiors of tectonic plates ; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock , which may be covered by younger sedimentary rock . They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle. The term craton

3185-471: The late Archean, accompanied by voluminous mafic magmatism. However, melt extraction alone cannot explain all the properties of craton roots. Jordan notes in his paper that this mechanism could be effective for constructing craton roots only down to a depth of 200 kilometers (120 mi). The great depths of craton roots required further explanation. The 30 to 40 percent partial melting of mantle rock at 4 to 10 GPa pressure produces komatiite magma and

3250-484: The latter in the English speaking world. In the United States, it is used only for European orogenies; the contemporaneous and genetically linked mountain-building phases in the Appalachian Mountains have different names. The regional term Variscan underwent a further meaning shift since the 1960s. Geologists generally began to use it to characterize late Paleozoic fold-belts and orogenic phases having an age of approximately 380 to 280 Ma. Some publications use

3315-405: The longevity of cratons is that they may alternate between periods of high and low relative sea levels . High relative sea level leads to increased oceanicity, while the opposite leads to increased inland conditions . Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn Craton of Western Australia was flattish already by Middle Proterozoic times and

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3380-470: The orogen, a consequence of overthrusting and folding . The Acadian foreland basin is categorized as a retroarc foreland basin , which occurs on the overriding continental lithosphere , adjacent to a foreland fold-thrust belt behind a continental margin arc. The initial result of loading is a bulge move out and uplift of the foreland, which generates a localized unconformity. The distributions of unconformities display an asymmetric pattern in relation to

3445-420: The plume model. However, other geochemical evidence favors mantle plumes. Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than 150 km (93 mi) and may be Archean, while the second is found at depths from 180 to 240 km (110 to 150 mi) and may be younger. The second layer may be a less depleted thermal boundary layer that stagnated against

3510-411: The promontories. Subsidence follows bulge movement and uplift and is produced on the cratonic side of the orogen due to regional isostatic adjustment to the load by the lithosphere. Once thrust propagation declines, substantial relief and drainage nets have had time to develop, and the as a resulting coarser clastic sediment is eroded and transported to the foreland basin. The basement structures of

3575-436: The roots of cratons, and which are almost always over 2 billion years and often over 3 billion years in age. Rock of Archean age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, this suggests that only 5 to 40 percent of the present continental crust formed during the Archean. Cratonization likely was completed during the Proterozoic . Subsequent growth of continents

3640-432: The same etymology.) Hercynian , on the other hand, derives from the Hercynian Forest . Both words were descriptive terms of strike directions observed by geologists in the field, variscan for southwest to northeast, hercynian for northwest to southeast. The variscan direction reflected the direction of ancient fold belts cropping out throughout Germany and adjacent countries and the meaning shifted from direction to

3705-414: The same paleolatitude, slightly south of the equator in the southern hemisphere, with relatively the same paleolongitude. Major defining tectonic events include the Neoproterozoic rift sequence from the breakup of Grenville basement rocks, thermal subsidence related to the Early Cambrian to Middle Ordovician drift sequence during the opening of the Iapetus Ocean, the Appalachian accretionary events to

3770-399: The south of Avalonia pushed the latter into north Laurentia and thrust up the northern Appalachian Mountains in the acadian phase of the Caledonian orogeny. Contemporaneously the Tornquist Sea between Avalonia and Baltica was entirely closed. Thus Avalonia formed the southern coast of the new continent Euramerica ( Laurussia , the Old Red Sandstone continent in present-day North America,

3835-515: The surrounding hotter, but more chemically dense, mantle. In addition to cooling the craton roots and lowering their chemical density, the extraction of magma also increased the viscosity and melting temperature of the craton roots and prevented mixing with the surrounding undepleted mantle. The resulting mantle roots have remained stable for billions of years. Jordan suggests that depletion occurred primarily in subduction zones and secondarily as flood basalts . This model of melt extraction from

3900-407: The term Variscan for fold belts of even younger age, deviating from the meaning as a term for the North American and European orogeny related to the Gondwana-Laurasia collision. The North American and European Variscan Belt includes the mountains of Portugal and Spain ( Galicia , and Pyrenees ), southwestern Ireland (i.e. Munster ), Cornwall , Devon , Pembrokeshire , the Gower Peninsula and

3965-503: The typical 100 km (60 mi) thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into the asthenosphere , and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons. Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy and a low intrinsic density. This low density offsets density increases from geothermal contraction and prevents

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4030-455: The upper mantle has held up well with subsequent observations. The properties of mantle xenoliths confirm that the geothermal gradient is much lower beneath continents than oceans. The olivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity. Rhenium–osmium dating of xenoliths indicates that the oldest melting events took place in the early to middle Archean. Significant cratonization continued into

4095-422: Was by accretion at continental margins. The origin of the roots of cratons is still debated. However, the present understanding of cratonization began with the publication in 1978 of a paper by Thomas H. Jordan in Nature . Jordan proposes that cratons formed from a high degree of partial melting of the upper mantle, with 30 to 40 percent of the source rock entering the melt. Such a high degree of melting

4160-423: Was possible because of the high mantle temperatures of the Archean. The extraction of so much magma left behind a solid peridotite residue that was enriched in lightweight magnesium and thus lower in chemical density than undepleted mantle. This lower chemical density compensated for the effects of thermal contraction as the craton and its roots cooled, so that the physical density of the cratonic roots matched that of

4225-477: Was supported with paleoclimatic evidence. The paleolatitude of Gondwana during the Middle to Late Devonian resided around intermediate latitudes of about 50°S. The collision initiating the Acadian orogeny resulted in the closing of the southern Iapetus Ocean and the formation of a high mountain belt . After the Acadian collision took place, Gondwana began to retreat from Laurentia with the newly accreted Avalonian terranes left behind. As Gondwana moved away,

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