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During Cambrian time ([[:file:M106Ch01Fig02.jpg|Figure 2]]) most of the continents were gathered in the southern hemisphere.<ref name=Cocksandtorsvik_2002 /> <ref name=Torsvikandcocks_2009>Torsvik, T. H., and Cocks, L. R. M., 2009, The Lower Palaeozoic palaeogeographical evolution of the northeastern and eastern peri-Gondwanan margin from Turkey to New Zealand: GSL Special Publications, v. 325, p. 3–21.</ref> Gondwana stretched from the equator (Australia) to the South Pole (North Africa). Tectonic movement was active mainly as a consequence of the relative rotation of the different cratons that built Gondwana.<ref name=Veevers_2004>Veevers, J. J., 2004, Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangea to 185-100 Ma breakup: Supercontinental tectonics via stratigraphy and radiometric dating: Earth-Science Reviews, v. 68, p. 1–132.</ref> Extensional tectonics that controlled the deposition of major evaporitic successions in the Arabic peninsula (Hormuz Salt Basin) and surroundings (e.g., Punjab Salt Basin) close to the Precambrian–Cambrian boundary (see dashed faults in [[:file:M106Ch01Fig02.jpg|Figure 2]]) came to an end, and no important tectonic activity is observed in the late Cambrian (500 Ma). Laurentia lay astride the equator in both hemispheres and was separated from Gondwana by the Iapetus Ocean. Avalonia, Armorica, Perunica, Baltica (180&deg; geographically inverted), North and South China, and all the Cimmerian blocks fringed peripheral Gondwana at moderate to high southern latitudes. Torsvik and Cocks<ref name=Torsvikandcocks_2009 /> show that South China was located close to the Equator. According to von Raumer and Stampfli,<ref name=Vonraumerandstampfli_2008>von Raumer, J. F., and Stampfli, G. M., 2008, The birth of the Rheic Ocean - Early Palaeozoic subsidence patterns and subsequent tectonic plate scenarios: Tectonophysics, v. 461, p. 9–20.</ref>d the Proto-Tethys ocean, which separated North China (to the north) from the other blocks along the peripheral Gondwana, was subducting toward the south and backarc extension, which is suggested by oceanic Cambrian seaways between the peripheral Gondwanan blocks. Torsvik and Cocks<ref name=Torsvikandcocks_2009 /> recognized that their concept of the Ran Ocean used for the Cambrian-Ordovician ocean existing between Baltica and Gondwana is comparable to that of the Proto-Tethys (sensu Stampfli and Borel<ref name=Stampfliandborel_2002 />), and herein we prefer to use the latter term. Siberia was positioned at low latitudes and was separated from Laurentia and Baltica by oceanic crust. Avalonia rifted off Gondwana in the Early Ordovician with the opening of the Rheic Ocean,<ref name=Nys&aelig;theretal_2002>Nys&aelig;ther, E., Torvik, T. H., Feist, R., Walderhaug, H. J., and Eide, E.A., 2002, Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (Southern France): Earth and Planetary Science Letters, v. 203, p. 329–341.</ref> although some authors suggest even older ages for its opening.<ref name=Torsvikandcocks_2009 />
 
During Cambrian time ([[:file:M106Ch01Fig02.jpg|Figure 2]]) most of the continents were gathered in the southern hemisphere.<ref name=Cocksandtorsvik_2002 /> <ref name=Torsvikandcocks_2009>Torsvik, T. H., and Cocks, L. R. M., 2009, The Lower Palaeozoic palaeogeographical evolution of the northeastern and eastern peri-Gondwanan margin from Turkey to New Zealand: GSL Special Publications, v. 325, p. 3–21.</ref> Gondwana stretched from the equator (Australia) to the South Pole (North Africa). Tectonic movement was active mainly as a consequence of the relative rotation of the different cratons that built Gondwana.<ref name=Veevers_2004>Veevers, J. J., 2004, Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangea to 185-100 Ma breakup: Supercontinental tectonics via stratigraphy and radiometric dating: Earth-Science Reviews, v. 68, p. 1–132.</ref> Extensional tectonics that controlled the deposition of major evaporitic successions in the Arabic peninsula (Hormuz Salt Basin) and surroundings (e.g., Punjab Salt Basin) close to the Precambrian–Cambrian boundary (see dashed faults in [[:file:M106Ch01Fig02.jpg|Figure 2]]) came to an end, and no important tectonic activity is observed in the late Cambrian (500 Ma). Laurentia lay astride the equator in both hemispheres and was separated from Gondwana by the Iapetus Ocean. Avalonia, Armorica, Perunica, Baltica (180&deg; geographically inverted), North and South China, and all the Cimmerian blocks fringed peripheral Gondwana at moderate to high southern latitudes. Torsvik and Cocks<ref name=Torsvikandcocks_2009 /> show that South China was located close to the Equator. According to von Raumer and Stampfli,<ref name=Vonraumerandstampfli_2008>von Raumer, J. F., and Stampfli, G. M., 2008, The birth of the Rheic Ocean - Early Palaeozoic subsidence patterns and subsequent tectonic plate scenarios: Tectonophysics, v. 461, p. 9–20.</ref>d the Proto-Tethys ocean, which separated North China (to the north) from the other blocks along the peripheral Gondwana, was subducting toward the south and backarc extension, which is suggested by oceanic Cambrian seaways between the peripheral Gondwanan blocks. Torsvik and Cocks<ref name=Torsvikandcocks_2009 /> recognized that their concept of the Ran Ocean used for the Cambrian-Ordovician ocean existing between Baltica and Gondwana is comparable to that of the Proto-Tethys (sensu Stampfli and Borel<ref name=Stampfliandborel_2002 />), and herein we prefer to use the latter term. Siberia was positioned at low latitudes and was separated from Laurentia and Baltica by oceanic crust. Avalonia rifted off Gondwana in the Early Ordovician with the opening of the Rheic Ocean,<ref name=Nys&aelig;theretal_2002>Nys&aelig;ther, E., Torvik, T. H., Feist, R., Walderhaug, H. J., and Eide, E.A., 2002, Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (Southern France): Earth and Planetary Science Letters, v. 203, p. 329–341.</ref> although some authors suggest even older ages for its opening.<ref name=Torsvikandcocks_2009 />
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The scanty fossil record makes reconstruction of Cambrian paleobiogeography difficult. This fauna mostly comprises pelagic trilobites and articulated brachiopods.<ref name=Cocksandtorsvik_2002 /> Cocks and Torsvik<ref name=Cocksandtorsvik_2002 /> recognized Laurentia, Siberia, and peri-Gondwana as distinct faunal provinces. Few data are available on climate, which based on the general character of the sedimentary section was probably temperate warm to temperate cool, but arid at low latitudes. No ice seems to have been present at the poles.
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Along North Africa and North Arabia, clastic continental deposits fringed a shallow water platform comprising both shales and mixed siliciclastic and carbonate facies.<ref name=Guiraudandbosworth_1999>Guiraud, R., and Bosworth, W., 1999, Phanerozoic geodynamic evolution of northeastern Africa and the northwestern Arabian platform: Tectonophysics, v. 315, p. 73–108.</ref> <ref name=Konertetal_2001>Konert, G., Abdulkader, M. A., Al-Hajri, A. A., and Droste, H. J., 2001, Paleozoic stratigraphy and hydrocarbon habitat of the Arabian Plate: GeoArabia, v. 6, p. 407–442.</ref>
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[[file:m106Ch01Fig03.jpg|thumb|300px|{{figure number|3}}Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Late Ordovician time (about 440 Ma), modified after Cocks and Torsvik<ref name=Cocksandtorsvik_2002 />]]
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===Late Ordovician (about 440 Ma)===
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Late Ordovician paleogeography ([[:file:M106Ch01Fig03.jpg|Figure 3]]) is represented by Cocks and Torsvick,<ref name=Cocksandtorsvik_2002 /> Robardet,<ref name=Robardet_2003>Robardet, M., 2003, The Armorica “microplate": Fact or fiction? Critical review of the concept and contradictory palaeobiogeographical data: Palaeoeography, Palaeoclimatology, Palaeoecology, v. 195, p. 125–148.</ref> and Ruban et al.,<ref name=Rubanetal_2007>Ruban, D. A., Al-Husseini, M. I., and Yumiko, I., 2007, Review of Middle East Palaeozoic plate tectonics: GeoArabia, v. 12, no. 3, p. 35–56.</ref> and shows that most continental blocks/terranes were located in the southern hemisphere except for Siberia and Tarim, which were entirely within the northern hemisphere. South China lay across the equator. The major oceans were not too large to prevent biotic exchange; thus the biota is quite cosmopolitan.
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The collision of Avalonia and Baltica occurred during Late Ordovician time, as documented by paleomagnetic, tectonic, isotope, and faunal data.<ref name=Cocksandtorsvik_2002 /> <ref name=Robardet_2003 /> Baltica, after the accretion of Avalonia, was positioned at intermediate latitudes NW of the Northern Gondwana margin and could have deflected the South Equatorial current southward. The Iapetus Oceanic lithosphere was subducting beneath the Laurentian active margin and its width was decreasing. The Rheic Ocean was several thousands of km wide with Perunica in the northern part, having probably detached from NW Gondwana in mid-Ordovician time. According to von Raumer and Stampfli,<ref name=Vonraumerandstampfli_2008 /> the Rheic Ocean was subducting beneath the Peri-Gondwanan blocks placed along the northern margin of Gondwana. The Panthalassic Ocean was very large and mostly covered the northern hemisphere. Cocks and Torsvick (2002) suggest this ocean was comparable in size to today’s Pacific. Peri-Gondwanan blocks were located along the Gondwanan margin at high to intermediate southern latitudes. However, some faunal data (e.g., Villas et al.<ref name=Villasetal_2002>Villas, E., Hammann, W., and Harper, D. A. T., 2002, Foliomena fauna (Brachiopoda) from the Upper Ordovician of Sardinia: Palaeontology, v. 45, p. 267–295.</ref>) suggest lower latitudes, as well as the existence of peripheral blocks detaching from North Gondwana. The position and the architecture of the Armorica composite plate is still discussed (e.g., Robardet<ref name=Robardet_2003 />). Nysæther et al.<ref name=Nys&aelig;theretal_2002 /> suggested that by the Late Ordovician, it is possible that part of Armorica had rifted off the NW Gondwanan margin; however, Robardet (2003) casted doubts on the reliability of the paleomagnetic data on which the evolution of the Armorica was based and proposed a different scenario in which the southern European blocks remained attached to the northern Gondwanan margin from Ordovician to Devonian<ref name=Robardet_2003 /> ([[:file:M106Ch01Fig09.jpg|Figure 9]]).
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Global climate deteriorated at the end of Ordovician time, resulting in the Hirnantian glacial episode. The glaciation is documented by sedimentary evidence and isotopic data<ref name=Brenchleyetal_1994>Brenchley, P. J., Marshall, J. D., Carden, G. A. C., et al., 1994. Bathymetric and isotopic evidence for a short-lived Ordovician glaciation in a greenhouse period: Geology, v. 22, p. 295–298.</ref> and lasted about 0.5–1 million years. Peri-Gondwanan and Gondwanan glacial deposits occur in North Africa (where a N-S high was present in Egypt<ref name=Schandelmeierandreynolds_1997>Schandelmeier, H. and Reynolds, P. O., eds., 1997, Palaeogeographic-palaeotectonic atlas of north-eastern Africa, Arabia, and adjacent areas: Balkema, Rotterdam, 160 p. 17 pls.</ref>), South America, Arabia, and South Africa, and periglacial features are known also from Armorica and Avalonia. Several interpretations have been offered on the distribution of the ice caps during the Hirnantian glaciation (a single large ice cap vs. a number of smaller ice caps), as summarized in Veevers.<ref name=Veevers_2004 /> This glaciation followed a period of climatic amelioration along the Northern Gondwana margin, evidenced by deposition of temperate bioclastic limestones and pelmatozoan-bryozoan mud-mounds, which overlie a very thick terrigenous succession of Early-Middle Ordovician age. The change from pre-Hirnantian “greenhouse” climates to Hirnantian “icehouse” conditions was rapid and was not preceded by any climatic fluctuations, which might have helped acclimatize the biota to the climate change.<ref name=Brenchleyetal_1994 /> If the pre-Hirnantian benthos was widespread in epicontinental seas and inland basins, the Hirnantian shelly fauna (e.g., Sutcliffe et al.;<ref name=Sutcliffeetal_2001>Schandelmeier, H. and Reynolds, P. O., eds., 1997, Palaeogeographic-palaeotectonic atlas of north-eastern Africa, Arabia, and adjacent areas: Balkema, Rotterdam, 160 p. 17 pls.</ref> Jin and Copper<ref name=Jinandcopper_2008>Jin, J., and Copper P., 2008, Response of brachiopod communities to environmental change during the Late Ordovian mass extinction interval, Anticosti Island, eastern Canada: Fossils and Strata, v. 54, p. 41–52.</ref>) was mostly restricted to the continental margins, due to the sea-level drop caused by the glaciations in the latest Ordovician. The Hirnantian glaciation seems to have occurred during times of very high levels of the greenhouse gas CO2 (14–18 times the present atmospheric value). Brenchley et al.<ref name=Brenchleyetal_1994 /> considered that the onset of glaciation was the result of an early Hirnantian increment in burial rates of organic carbon acting as a major sink for the atmospheric CO2. However, according to Villas et al. (2002), the accumulation of great volumes of carbonates in the pre-Hirnantian late Ordovician served as the sink of the atmospheric CO<sup>2</sup>. At the end of the Hirnantian, the ice cap melting caused a rapid, eustatic sea-level rise and the development of low-oxygen conditions on the shelves.<ref name=Rongandharper_1988>Rong, J.-Y., and Harper, D. A. T., 1988, A global synthesis of the latest Ordovician Hirnantian brachiopod faunas: Transactions of the Royal Society of Edinburgh Earth v. 79, p. 383–402.</ref> <ref name=Owenandrobertson_1995>Owen, A. W., and Robertson, D. B. R., 1995, Ecological changes during the end-Ordovician extinction: Modern Geology, v. 20, p. 21–39.</ref> The end of the glaciation was followed by the deposition of organic-rich shales (Lower Silurian “hot shales”) which represent the most important source rocks in North Africa and one of the major in the Arabian peninsula.<ref name=Luningetal_2000>Luning, S., Craig, J., Loydell, D. K., Storch, P. B., and Fitches B., 2000, Lower Silurian ‘hot shales’ in North Africa and Arabia: Regional distribution and depositional model: Earth Science Review, v. 49, p. 121–200.</ref>
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A very important event at the end of the Ordovician was the first of the Big Five Mass Extinctions<ref name=Raupandsepkoski_1982>Raup, D. M., and Sepkoski, J. J., 1982, Mass extinctions in the marine fossil record: Science, v. 215, p. 1501–1503.</ref> of the Phanerozoic, with disappearance of 85% of species, 61% of genera, and 12–24% of families.<ref name=Sepkoski_1997>Sepkoski, J. J., 1997, Biodiversity: Past, present, and future: Journal of Paleontology, v. 71, p. 533–539.</ref> The close correlation between the Ordovician extinction and the glaciation suggests climatic change as the proximate cause. However, the extinction was probably a complex event.<ref name=Brenchleyetal_1995>Brenchley, P. J., Carden, G. A. F., and Marshall, J. D., 1995, Environmental changes associated with the “first strike” of the late Ordovician mass extinction: Modern Geology, v. 20, p. 69–82.</ref> A sea-level fall and rise, changes in oceanic structure,<ref name=Wildeandberry_1984>Wilde, P., and Berry, W. B. N., 1984, Destabilization of the oceanic density structure and its significance to marine ‘extinction’ events: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 48, p. 143–162.</ref> nutrient fluxes,<ref name=Brenchleyetal_1995 /> and development of anoxia<ref name=Fortey_1989>
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Fortey, R. A., 1989, There are extinctions and extinctions: Examples from the lower Palaeozoic. Philosophical Transactions of the Royal Society of London. Series B, v. 352, p.327–355.</ref> <ref name=Briggsetal_1998>Briggs, D. E. G., Evershed, R. P., and Stankiewicz, B. A., 1998, The molecular preservation of fossil arthropod cuticles: Ancient Biomolecules, v. 2, p. 135–146.</ref> were all ultimately related to climatic change and may have contributed to the crisis.<ref name=Brenchleyetal_2001>Brenchley, P. J., Marshall, J. D., and Underwood, C. J., 2001, Do all mass extinctions represent an ecological crisis? Evidence from the late Ordovician: Geological Journal, v. 36, p. 329–340.</ref>
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[[file:M106Ch01Fig04.jpg|thumb|300px|{{figure number|4}}Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Early Devonian time (about 400 Ma).]]
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===Early Devonian (about 400Ma)===
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During Early Devonian time Gondwana was centered on the Southern Pole, with northern Africa and Antarctica located toward the equator ([[:file:M106Ch01Fig04.jpg|Figure 4]]). The continental plates that would form future Laurasia were located immediately to the south of the equator, so that most of the continental masses were in the southern hemisphere. The Iapetus Ocean had been closed by the collision between North America and Baltica, giving rise to the Caledonian orogeny. The Avalonia Ocean between southern England and Scotland was still present, and other minor oceanic seaways separated different blocks. These blocks would later form the components of Europe, assembled during the Carboniferous Variscan orogeny, with the closure of the Rheic Ocean. The width of the Rheic Ocean is still questioned, but the paleobiogeographic distribution of different groups of fossil organisms between Laurasia and Gondwana suggests that this ocean was relatively narrow during Early Devonian.<ref name=Wehrmannetal_2010>Wehrmann, A. et al., 2010, Devonian shallow-water sequences from the North Gondwana coastal margin (central and eastern Taurides, Turkey): Sedimentology, facies and global events: Gondwana Research, v. 17, p. 546–560.</ref> The Paleo-Tethys Ocean was opening along northern Gondwana, generating a fringe of microplates (e.g., Armorica, Adria, Pontides, Hellenic, and Moesia terranes). We follow the interpretation of Torsvik and Cocks,<ref name=Torsvikandcocks_2004>Torsvik, T. H., and Cocks, L. R. M., 2004, Earth geography from 400 to 250 Ma: A palaeomagnetic, faunal and facies review: Journal of the GSL v. 161, p. 555–572.</ref> who considered Adria and Apulia as separate microplates split apart by the opening of Paleo-Tethys. The width of the Paleo-Tethys Ocean at this time is still a matter of discussion. According to Robardet et al.,<ref name=Robardetetal_1990>Robardet, M., Paris, F., and Racheboeuf, P. R., 1990, Palaeogeographic evolution of southwestern Europe during Early Palaeozoic times, in W. S. McKerrow and C. R. Scotese, eds., Palaeozoic palaeogeography and biogeography: GSL Memoirs 12, p. 411–419.</ref> the detachment of Armorica from Gondwana is not older than Late Devonian, and Robardet<ref name=Robardet_2003 /> considers a fiction even the concept of an Armorica microplate. Other reconstructions<ref name=Stampfliandborel_2002 /> <ref name=Torsvikandcocks_2004 /> <ref name=Vonraumerandstampfli_2008 /> suggest that the opening of the Paleo-Tethys occurred before Early Devonian, and this is more certain for the eastward extension of this ocean.<ref name=Metcalfe_2002>Metcalfe, I., 2002, Tectonic history of the SE Asian-Australian region: Advances in Geoecology, v. 34, p. 29–48.</ref> Besides the strongly debated concepts of Armorica (e.g., Cocks and Torsvik,<ref name=Cocksandtorsvik_2002 /> Nys&aelig;ther et al.,<ref name=Nys&aelig;theretal_2002 /> Robardet,<ref name=Robardet_2003 /> Torsvik and Cocks<ref name=Torsvikandcocks_2009 />), also the relative position of the microplates detached from Gondwana with the opening of the Paleo-Tethys differs in several reconstructions: Stampfli and Borel<ref name=Stampfliandborel_2002 /> and von Raumer and Stampfli<ref name=Vonraumerandstampfli_2008 /> identify a major continental block (Hun superterrane), whereas Torsvik and Cocks<ref name=Torsvikandcocks_2004 /> suggest the presence of different independent microplates. In fact, the separation of these various microplates may have been diachronous.
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Most of the Asian terranes, mainly located immediately north of the equator, were still separated by seaways before their collision with and incorporation into Pangea. The position and vergence of subduction among these blocks is not clear, and different models have been proposed (e.g., Stampfli and Borel,<ref name=Stampfliandborel_2002 /> Torsvik and Cocks,<ref name=Torsvikandcocks_2004 /> Ruban et al.,<ref name=Rubanetal_2007 /> von Raumer and Stampfli<ref name=Vonraumerandstampfli_2008 />).
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During Early Devonian time, a global sea-level fall was responsible for the reduction of the neritic belts. The occurrence of a wide depositional hiatus close to the Early Devonian in most of the Middle East is ascribed to this sea-level low-stand, probably enhanced by a tectonic uplift.<ref name=Rubanetal_2007 />. The middle latitude position of North Africa and part of Arabia favored the development of alluvial deposits. Tectonic activity was weak and limited volcanic flows are documented<ref name=Meneissy_1990>Metcalfe, I., 2002, Tectonic history of the SE Asian-Australian region: Advances in Geoecology, v. 34, p. 29–48.</ref> mainly in Sudan and in the southern area of the Eastern Desert of Egypt.<ref name=Schandelmeierandreynolds_1997 />
     

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