Phanerozoic Tethys region

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Petroleum systems of the Tethyan region
Series AAPG Memoir
Chapter The evolution of the Tethys region throughout the Phanerozoic: A brief tectonic reconstruction
Author Fabrizio Berra, Lucia Angiolini
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In the Tethys region, the evolution of North Africa and the Arabian Plates are intimately involved with the occurrence of hydrocarbons in both regions. In the Early Paleozoic, paleogeography was characterized by the breakup of Rodinia and by the re-arrangement of the major continental plates in the Pangea supercontinent. During the assemblage of Pangea, a major role was played by the transformation from Pangea B to Pangea A during Permian time by means of dextral motion of Laurasia relative to Gondwana, which changed the relative position of the Paleozoic and Mesozoic domains facing the east-west oriented Tethys Gulf.[1] [2]

Since late Paleozoic time, the southern margin of the Tethys was affected by the time-transgressive opening of the Neo-Tethys, which gave origin to a complex mosaic of peri-Gondwanan terranes. They gradually collided, during Mesozoic and Cenozoic times, with the northern margin of the Tethys, as the oceanic lithosphere of the Paleo-Tethys Ocean was subducted below Laurasia. Collisions were distributed irregularly along the northern margin of the Tethys. The spreading of the Neo-Tethys balanced the subduction of the oceanic lithosphere along the northern margin of the Paleo-Tethys, preserving the Tethys Ocean until the beginning of Cenozoic time. The subduction of the Paleo-Tethys led to the accretion of microplates that today characterize the Middle East outside of Arabia. Accretion started in Triassic time with the Cimmerian orogeny and persisted up to today, with the collision of Arabia along the Zagros suture. The present day relationships among orogenic belts are further complicated by the presence of important strike-slip movements, which accommodated the different convergence rates among plates, from the Alps to the Himalayas.

The Mesozoic and Cenozoic evolution of the Tethys Oceans was also affected by the plate reorganizations caused by the breakup of Pangea. The opening of the Atlantic Ocean further complicated the geodynamic settings of the Laurasian and Gondwanan margins due to the changes in stress fields during different stages that characterized the breakup of Pangea. In particular, the movement and rotation of Africa, controlled by the opening of the central and southern Atlantic oceans, heavily controlled the relative motions among the numerous plates (which suffered alternatively both extensional and compressional tectonic regimes) in the Tethys. The present-day setting of south Mediterranean and Middle East regions is therefore the result of the global reorganization derived from the closure of the Tethys Ocean(s) and the time-transgressive opening of the Atlantic Ocean.

Different and sometimes incompatible reconstructions exist for some intervals and, as data can be either contradictory and/or scarce, it is frequently a matter of interpretation which of several alternative reconstructions should be favored, and which should be discarded. For these reasons, paleogeographic maps should always be viewed as a work in progress, and should be continually revised and reconsidered in the light of new data. Also, differences mainly increase with the detail of the maps. Therefore, the maps presented here constitute just one of the possible solutions and do not aim to represent definitive paleogeographic reconstructions. In our opinion, these maps generally honor the available data, which mainly summarize long, complex, and multidisciplinary studies from regions that are not always easily accessible (sometimes for political reasons).

The distribution of giant oil and gas fields in North Africa, Arabia, and the Middle East is the result of the interplay between the paleogeography of oceanic and continental areas, which favored the creation of source rocks, and the geodynamic evolution of Pangea and the Tethys Oceans during the Phanerozoic. There is a range of different basin types, with a dominance of rift and sag basins, passive margin basins, and collision-related basins, often evolving from one type to another. The interplay between sedimentation and tectonics controlled both basin development and post-depositional deformation, favoring the creation of a large number of structural and stratigraphic traps that store a significant percentage of the world’s oil and gas reserves.

Paleogeographic reconstructions[edit]

Paleogeographic maps have been reconstructed for selected time intervals: Cambrian, Late Ordovician, Early Devonian, Early Permian, Permian-Triassic boundary, Norian, Callovian, Aptian, Cretaceous-Cenozoic boundary, and Late Eocene. For each time interval both the general picture of the major plate tectonic configuration and a detail of the paleogeography and paleoenvironment of North Africa to the Middle East are presented. On these maps, the major paleoenvironmental settings (from continental to shallow marine and deep ocean) are shown for the area stretching from North Africa to Afghanistan in all the selected time slices. Besides the major tectonic events, the global climate evolution and their interplay are discussed, which in some cases led to significant biotic turnovers or even to mass extinctions (e.g., Late Ordovician, Permian-Triassic boundary, Cretaceous-Cenozoic boundary). Paleogeographic maps have been compiled from literature, selecting those based on sound paleomagnetic/paleobiogeographic data. Each map is accompanied by the description of the major tectonic events that characterized the considered time interval. When contrasting paleogeographic reconstructions were available, their differences have been discussed. In general, major differences concern the interpretation of the setting and positioning of the microplates and terranes between the major continental plates.

Figure 1 Time-position of the paleogeographic maps. Each map is displayed in the context of the evolution of the sea-water chemistry (aragonite vs. calcite sea, KCl vs. MgO4 evaporites), the global sea-level curve, the major volcanic events, the global climate, the major geodynamic events, and the ages of the five big extinctions of the Phanerozoic.

The paleogeography of the Earth during the Phanerozoic reflects the breakup of Rodinia and the formation of Pangea and its later breakup. Sedimentation was affected by the latitudinal position of the continents, the global climate conditions, and the seawater chemistry at the largest scale, and at the basin scale by the gradual changes in the local tectonic environment that accompanied the formation and destruction of the Tethyan oceans. Each global map is coupled with a more detailed map of North Africa and the Middle East. On these maps, the major depositional settings (emerged land, continental, shallow marine, and deep marine environments) are shown. These maps are a simplified view of the Western Tethys region, whereas the detailed facies distribution of the major domains is described in the chapters that follow in this book. The time-position of these paleogeographic maps is framed by the Phanerozoic history, so each map is viewed in the context of the evolution of the sea-water chemistry (aragonite vs. calcite sea, KCl vs. MgO4 evaporites), the global sea-level curve, the major volcanic events, the global climate, the major geodynamic events, and the age of the five major extinctions of the Phanerozoic (Figure 1). The interplay between these changing parameters is important as it controls and explains the distribution of climate belts (and thus the distribution of the climate-sensitive depositional environments), the depth and area of the submerged shelves, and the biogenic contribution to sediment production.

Figure 2 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Cambrian (about 500 Ma), modified after Cocks and Torsvik.[3] Proto-Tethys concept sensu Stampfli and Borel.[4] The Proto-Tethys waters encroached northern Gondwana, extending over parts of the present day African-Arabian plate margin as evidenced by facies distribution across these regions. The position of major Late Precambrian–Cambrian faults and rift zones (inactive at the time of this paleogeographic reconstruction), where salt deposition occurred, is indicated by the dashed faults.

Cambrian (late Cambrian, about 500 Ma)[edit]

During Cambrian time (Figure 2) most of the continents were gathered in the southern hemisphere.[3] [5] 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.[6] 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 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° geographically inverted), North and South China, and all the Cimmerian blocks fringed peripheral Gondwana at moderate to high southern latitudes. Torsvik and Cocks[5] show that South China was located close to the Equator. According to von Raumer and Stampfli,[7] 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[5] 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[4]), 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,[8] although some authors suggest even older ages for its opening.[5]

The scanty fossil record makes reconstruction of Cambrian paleobiogeography difficult. This fauna mostly comprises pelagic trilobites and articulated brachiopods.[3] Cocks and Torsvik[3] 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.

Along North Africa and North Arabia, clastic continental deposits fringed a shallow water platform comprising both shales and mixed siliciclastic and carbonate facies.[9] [10]

Figure 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[3]

Late Ordovician (about 440 Ma)[edit]

Late Ordovician paleogeography (Figure 3) is represented by Cocks and Torsvick,[3] Robardet,[11] and Ruban et al.,[12] 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.

The collision of Avalonia and Baltica occurred during Late Ordovician time, as documented by paleomagnetic, tectonic, isotope, and faunal data.[3] [11] 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,[7] 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 Torsvik[3] 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[13] 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.[11] Nysæther et al.[8] suggested that by the Late Ordovician, it is possible that part of Armorica had rifted off the NW Gondwanan margin; however, Robardet[11] 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[11] (Figure 9).

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[14] 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[15]), 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.[6] 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.[14] If the pre-Hirnantian benthos was widespread in epicontinental seas and inland basins, the Hirnantian shelly fauna[16] [17] 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.[14] 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.,[13] the accumulation of great volumes of carbonates in the pre-Hirnantian late Ordovician served as the sink of the atmospheric CO2. 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.[18] [19] 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.[20]

A very important event at the end of the Ordovician was the first of the Big Five Mass Extinctions[21] of the Phanerozoic, with disappearance of 85% of species, 61% of genera, and 12–24% of families.[22] 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.[23] A sea-level fall and rise, changes in oceanic structure,[24] nutrient fluxes,[23] and development of anoxia[25] [26] were all ultimately related to climatic change and may have contributed to the crisis.[27]

Figure 4 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Early Devonian time (about 400 Ma).

Early Devonian (about 400 Ma)[edit]

During Early Devonian time Gondwana was centered on the Southern Pole, with northern Africa and Antarctica located toward the equator (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.[28] 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,[29] 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.,[30] the detachment of Armorica from Gondwana is not older than Late Devonian, and Robardet[11] considers a fiction even the concept of an Armorica microplate. Other reconstructions[4] [29] [7] suggest that the opening of the Paleo-Tethys occurred before Early Devonian, and this is more certain for the eastward extension of this ocean.[31] Besides the strongly debated concepts of Armorica,[3] [8] [11] [5] also the relative position of the microplates detached from Gondwana with the opening of the Paleo-Tethys differs in several reconstructions: Stampfli and Borel[4] and von Raumer and Stampfli[7] identify a major continental block (Hun superterrane), whereas Torsvik and Cocks[29] suggest the presence of different independent microplates. In fact, the separation of these various microplates may have been diachronous.

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.[4] [29] [12] [7]

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.[12]. 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[32] mainly in Sudan and in the southern area of the Eastern Desert of Egypt.[15]

Figure 5 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Early Permian time (about 290 Ma).

Early Permian (about 290 Ma)[edit]

The late Paleozoic was a period of major plate tectonic reconfiguration (Figure 5). The Variscan orogeny led to the assembly of Gondwana and Laurasia into one supercontinent, Pangea. Adria and Apulia, previously separated, are from here onward assembled as a microplate that is referred to as Adria in the Early Permian and subsequent maps. The opening of the Neo-Tethys Ocean along the eastern margin of Gondwana, from Arabia to Australia, created the Cimmerian terranes (Iran, Central Afghanistan, Karakorum, Qiangtang). These migrated northward across the Tethys Ocean from southern Gondwanan paleolatitudes in Early Permian time to subequatorial paleolatitudes by the ~Middle Permian–Early Triassic times.[33] [34] [35] [36] [1] According to Muttoni et al.,[37] [38] [2] the Neotethyan opening is in part coeval to a major dextral motion of Laurasia relative to Gondwana that takes place essentially during Permian time. This relative motion causes the transformation of Pangea from an Early Permian configuration of the B-type, where Africa is placed south of Asia and South America is placed south of Europe,[39] [40] [41] [42] [43] [44] [45] to a Late Permian configuration of the Wegenerian A-type, where Africa is placed immediately south of Europe and South America is placed south of North America. The presence of a E-W trending trans-Pangean seaway (connecting the Paleo-Tethys to the Panthalassa oceans) persisting until the Late Permian is proposed by Vai[46] based on his interpretation of facies analyses and paleobiogeographic distribution of floral, reptile, and marine benthic organisms.

The proposed Early Permian reconstruction is from Muttoni et al.,[2] which is based on Early Permian poles that support a Pangea B configuration essentially similar to that originally proposed by Irving[39] and confirmed by subsequent analyses.[40] [41] [42] [43] [37] [38] [45] The Cimmerian terranes (alternatively named Cimmeria Superterrane) are placed close to the Gondwanan margin in Early Permian time on the basis of geological, paleontological, and paleomagnetic evidences.[47] [48] [49] [50] [2]

Neotethyan rifting along the eastern Gondwana margin from India[51] to Oman[52] started in Carboniferous times and was followed by continental breakup and formation of oceanic crust in Early Permian time (mid-Sakmarian[53] [54] [36]). In the same time interval, a major zone of northward subduction of Paleotethyan oceanic crust was active along the Eurasian margin and persisted through most of Permian–Triassic times.[34] [55] [35][36] Transpressive strike-slip tectonics was responsible for basin inversion in Lybia and Egypt.[15]

In Pennsylvanian–Early Permian times, an extensive glaciation affected much of Gondwana, [56] [57] leaving widespread glacial deposits at high to intermediate southern latitudes. The tropical belt was thus restricted to very low latitudes, which benefited from a warm westward-flowing equatorial current, which, upon reaching the continental shelves of the western Tethys Gulf, deflected southeastward, bringing warm surface waters toward the northern corner of Arabia.[45] This compressed tropical current gyre was bounded to the south by the thermal effects of the Gondwanan glacial climate, which directly controlled the distribution of cold biota in most other peri-Gondwanan terranes.

Figure 6 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) at the time of the Permian-Triassic boundary (about 251 Ma).

Permian-Triassic Boundary (251 Ma)[edit]

The beginning of the Mesozoic was marked by the end of the transitional stage from the Pangea B to Pangea A configurations.[39] [40] [41] [42] [43] [44] [45] The change in the configuration of Pangea required a west to east translation of Laurasia of some 3000 km (1864 mi) with respect to Gondwana (Figure 6). The dextral movement was accommodated along a lithospheric shear zone which runs from the subduction zone of Panthalassa eastward to a triple junction joining this shear zone, the Neo-Tethys ridge, and the subduction of Paleo-Tethys below the southern margin of Laurasia. This triple junction was located along the northern margin of the Tethys, close to southern Europe.[2]

Northern Gondwana was characterized by the development of a mature passive margin along the Neo-Tethys Ocean, which started to open with a time-transgressive trend beginning in Carboniferous time.[52] [51] Oceanic crust began forming as early as Early Permian time.[53] [54] [36] The opening of the Neo-Tethys was responsible for the northward flight of the peri-Gondwanan blocks: in Early Triassic time these continental blocks were moving northward toward the southern margin of Asia. These blocks include a number of minor, semi-independent blocks such as Apulia, Taurides, Iran (NW and Central), Sanandaj Sirjian, Heland, and Northern Tibet.[12] In detail, Iran was crossing the equator[2] before its docking to Eurasia, which gave rise to the Cimmerian orogeny. In contrast, to the east most of the blocks were still located south of the equator. The different velocities of the peri-Gondwanan blocks were probably controlled by the presence of roughly N-S trending transform faults, which defined portions of the Neo-Tethys oceanic ridge characterized by different rates of spreading. The existence of a complex network of oceanic branches during the opening of the Neo-Tethys has been suggested by Sengör,[58] who interpreted the peri-Gondwanan blocks as three independent ribbons migrating northward.

The present-day North Africa was located along the southern passive margin of Neo-Tethys and was characterized by the development of extensional basins in Lybia and Levant (probably transtensional basins[15]). Paleogeographic reconstructions are more complex in the western part of the Tethys, as the narrowing of the oceanic basin along with the closer proximity of the Neo-Tethys ridge and the Paleo-Tethys subduction zone complicate the definition and precise position of the numerous blocks which can be identified in this zone. Furthermore, the position and significance of blocks such as the Taurides and Apulia are still a matter of debate. Paleogeographic reconstructions in the western part of the Tethys Ocean are often contrasting.[59] [4] Differences can be ascribed to the puzzle of small plates and to the complex history of aperture and closure of small oceanic branches which characterize, for most of the Mesozoic, the Alpine and Dinaric domains. There are also different interpretations of the significance of the multiple extensional events along northern Gondwana, which is the most likely to represent breakup and formation of the passive margin at any particular location.

The Permian-Triassic boundary was characterized at a global scale by low sea level, reflected by widespread continental or shallow marine facies. The beginning of Triassic time was characterized by a sea-level rise that can be traced worldwide.

The end Permian crisis is generally considered the most dramatic extinction of the last 600 million years[60] and led to the extinction of 75–96% of species. Some of the invoked mechanisms included rapid sea-level change and/or anoxia or euxinia; [61] [62] extraterrestrial impact;[63] enormous volcanic eruptions and/or an extreme global warming;[64] [65] [66]); or ozone layer collapse.[67] The pattern of the latest Permian extinction evaluated statistically[68] [69] [70] [71] indicates that the extinction was a sudden event occurring during a sea-level rise. The evidence that the extinction was abrupt in different latest Permian paleogeographic settings is consistent with scenarios in which mass extinction resulted from climatic and environmental deterioration triggered by the Siberian Traps volcanism, which also increased greenhouse gas emissions into the atmosphere and thus global warming and ozone depletion. This is also supported by the pronounced negative δ13C excursion recorded worldwide near the latest Permian extinction event.[72] [73] [74]

Figure 7 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Norian time (about 205 Ma).

Norian (about 205 Ma)[edit]

The Norian represents a key interval in the evolution of the Paleo-Tethys Ocean (Figure 7). During this stage the first evidence for the closure of the Paleo-Tethys Ocean was recorded by the onset of the Cimmerian orogeny along the southern margin of Asia, due to the collision of the Iran blocks with the active margin of Turan. This collisional episode is known as the “Eocimmerian” event, which was followed during Jurassic time by additional collisions of other microplates.[75]

All the microplates that detached from Gondwana were approaching Southern Asia, which was an active margin situated above a north-dipping subduction zone. The Neo-Tethys Ocean was widely open. The paleogeographic situation is clear in the central part of the Tethyan Gulf, but the geodynamic setting was extremely complex close to the triple junction located in the western part of the Tethys. Intense extensional to strike slip tectonics (likely transtension) was recorded along the southern margin of Europe, close to the future axis of the Alpine Tethys. This tectonic activity was connected westward with the Central Atlantic, where rift basins were forming during Norian time (Newark Basins). Among the different small plates in the Mediterranean region (Apulia, Greece, Turkey), minor deep-water seaways, partly floored with oceanic crust, have been recognized. Northern Africa and Arabia acted as the southern passive margin of the Neo-Tethys, with extensional basins in Lybia and Egypt (Schandelmeier and Reynolds, 1997). Extensional tectonics (Palmiryde Basin) was recorded along the Lebanon-Northern Israel shelf. Emplacement of basalts, probably related to local rifting, is documented in Israel. Intraplate alkaline magmatic activity (intrusive and subvolcanic) occurred in Sudan and S-E Egypt.[15]

Frequent unconformities associated with extension along the Neotethyan margin (from Syria to Libya through Arabia) can be interpreted as far-field effects of the Cimmerian orogeny. Alternatively, they can be interpreted as local extensional events that preceded late Triassic-Liassic breakup between Apulia (sensu lato) and northern Gondwana.[76]

During Norian time, climate was generally arid at Tethyan latitudes: Carbonate facies that were deposited along the Tethys margin were bordered by evaporitic facies and coastal and continental fluvial to playa environments. Arid climate conditions probably favored the early, widespread, and pervasive dolomitization observed on most of the Norian carbonate platforms.[77] [78] The arid belt probably extended beyond 40° latitude north and south, as reflected by the distribution of climatically sensitive facies. Climate reconstructions[79] [80] indicate that, during most of Triassic time, the arid belt extended to the equator, and a humid equatorial belt was probably absent. Pervasive dolomitization of carbonate platforms ended close to the Norian-Rhaetian boundary in the Western Tethys due to a shift to humid conditions,[81] [82] reflected also by the increase of siliciclastic deposits, which can be traced along part of the northern margin of the Tethys.

Figure 8 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Callovian time (about 164 Ma).

Callovian (about 164 Ma)[edit]

The breakup of Pangea continued in Callovian time with spreading in the Central Atlantic and the complete detachment of Africa from North America (Figure 8). A rifting event along the future axis of the North Atlantic Ocean caused the development of a network of roughly parallel rift basins along the Iberian margin and Newfoundland. Oceanic crust was also forming between India and Arabia, following an Early Jurassic rifting stage. In the Tethys area the situation was more complex,[83] due to the existence of an extensional regime in the west (spreading in the Alpine Tethys, also known as the Penninic Ocean) and to the south (spreading in the Mesogea Ocean) and to the onset of the Neo-Tethys subduction along the southern margin of Laurasia, which followed the docking of the peri-Gondwanan blocks with Laurasia in early Middle Jurassic time (Middle Cimmerian orogeny). The northward subduction of the Neo-Tethys was responsible for the development of a volcanic belt bordering to the north the subduction zone, whereas retro-arc basins formed between the margin of Laurasia and the volcanic arc. Sedimentation in the backarc basins ranged from deep to shallow marine carbonates and clastics. The opening of the Mesogea Ocean separated the margin of Arabia and North Africa from a major continental block, composed of a large carbonate platform and related peri-platform basins, which would later form part of the Turkish jigsaw and the Dinaride-Pelagonian blocks. The southern coast of the Mesogea Ocean was marked by coastal deposits along present-day northern Africa,[9] whereas shelf carbonates were deposited on most of the eastern to southern Arabia. The latter, bordered to the north by the Mesogea Ocean, faced to the south the new oceanic seaway, which began to separate Arabia from India. This geodynamic position, between two extensional basins, probably favored the development of deep seaways (at least partially controlled by extensional tectonics, as the Arabian Basin) on the Arabian shelf.

Figure 9 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) during Aptian time (about 120 Ma).

Aptian (about 120 Ma)[edit]

The major oceanic seaways that characterize the present-day configuration are readily recognized in the Aptian map (Figure 9). The North Atlantic Ocean was opening between Iberia and Newfoundland, whereas the northernmost part of the future Atlantic Ocean was characterized by the development of a network of rift basins seen between Canada, Greenland, and Scandinavia. Africa was separated from India and Antarctica by narrow seaways. The South Atlantic Ocean was opening between southern Africa and the southernmost part of South America. Continental South America and Africa were still connected along the future Equatorial Atlantic. The Alpine Tethys connected the Central Atlantic with the Neo-Tethys Ocean. The passive margin of North Africa and northernmost Arabia faced the Mesogea Ocean.[83] The active southern margin of Laurasia was still characterized by the subduction of the Neo-Tethys Ocean beneath it. Along this margin it was still possible to recognize a volcanic arc with backarc basins. The central part of the Neo-Tethys was characterized by the slow northward motion of a complex puzzle of blocks, including parts of future Turkey, Greece, Dinarids, and Adria microplates. Within this block, bordered by passive margins facing to the north the Neo-Tethys Ocean and to the south the Mesogea Ocean, extensional tectonics were responsible for the development of a major basin (Pindos-Olonos basin). Albian time probably marked the beginning of the compressional regime in the Alpine Tethys, with the possible onset of its subduction. During Aptian time sea level was close to its Phanerozoic maximum, so the size of the continental shelves are very large. Along the uplifted shoulder of the southern coast of Mesogea the coastal deposits were represented by a narrow belt, whereas most of the eastern and southern part of the Arabian block were characterized by shallow to deep carbonate deposits. The future Red Sea area was uplifted and emergent, whereas in its surroundings (mainly in the present-day Libya and Egypt, where extensional basins were formed[15] fluvio-lacustrine deposits passed seaward to deltaic clastics and evaporites (Mesogea Ocean coast). Volcanic activity is recorded in Israel (alkali-basaltic lavas), whereas syenitic intrusions are reported from NE and S Sudan.[15]

During Albian time the paleoceanographic setting was characterized by a major anoxic event that can be traced all across the Albian seas.

Figure 10 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) at the time of the Cretaceous-Paleocene boundary (about 65.5 Ma).

K-T Boundary (65.5 Ma)[edit]

At the end of Cretaceous times the present-day continents were completely defined (Figure 10). Only the northernmost Atlantic Ocean was not completely opened (rifting was still active between Greenland and Canada). Africa detached from Antarctica and India, which began its northern flight that would eventually lead to the Himalayan orogeny. The Alpine Tethys and related basins, which linked the Central Atlantic Ocean with Neo-Tethys, were in a convergent plate regime. The large and complex puzzle of blocks of Adria, Greece, and Turkey were approaching the southern margin of Eurasia, after the almost complete subduction of the Neo-Tethys Ocean. The collision between these complex assemblages of different microplates would produce the Alpine-Dinaric and Turkish orogenic belts. In the Alpine area the lower plate was represented by Eurasia, whereas east of the Alps Laurasia represented the upper plate. This change can be ascribed to the different age and origin of the subducting oceanic crust (Alpine Tethys in the Alps, Neo-Tethys in the east). The possible occurrence of minor oceanic basins (Vardar, Pindos, and Lycian oceans[4]) north of the Mesogea Ocean between the Alpine Tethys and Neo-Tethys accounted for the presence of multiple verging subduction zones. To the north of the subduction-collision belt it was still possible to recognize the occurrence of backarc basins, from the Black Sea to the Caspian Sea. The progressive closure of the Neo-Tethys also affected the evolution of the passive margin of Arabia, where the Peri-Arabian Massif high delivered sediments both northward (toward the Neo-Tethys) and southward. The origin of this high was related to the approach of the lower plate (Arabia) to the southern margin of Laurasia (represented by the Sirjan blocks of central Iran) or, alternatively, to an intra-oceanic subduction zone.[4] The southern margin of Arabia was probably represented by a transform separating this plate from India. South of the Peri-Arabian Massif, on the Arabian plate, sedimentation was represented by prevailing deep-sea clastics and shallow-water carbonates passing to a large coastal plain with deposition of alluvial sediments. Extensional basins with deep-sea carbonates (Sirt Basin) developed along the northern passive margin of Africa and into the Sirt gulf. Rift basins (filled by continental clastic deposits) were also present across the interior of central-eastern Africa to the south.

End-Cretaceous time recorded the last of the Big Five mass extinctions,[84] [85] so drastic and so close in time to leave a biogeographic imprint even on modern biota.[86] Extinctions happened both in the sea (marine reptile, cephalopods, foraminifers, brachiopods, sharks) and on land (dinosaurs, pterosaurs, some bird groups, marsupial mammals). However, the pattern of this extinction is still disputed, with some groups interpreting gradual decline before the K-Pg boundary, and others catastrophic die-off.[84] [87] [88] One of the first proposed causal mechanisms was a major asteroid impact[89] [90] with proposals of craters, such as Chicxulub Crater, Yucatan.[91] Among other suggested triggering mechanisms are global warming and flood basalts (Deccan Traps).[92]

Figure 11 Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) at the time of the Eocene-Oligocene boundary (about 34 Ma).

Eocene-Oligocene Boundary (about 34 Ma)[edit]

The opening of the Atlantic and Indian Oceans was coupled with the movement of Africa toward the southern boundary of Eurasia and with the gradual closure of the Neo-Tethys Ocean (Figure 11). The rapid northward flight of India was responsible for the continental collision and the development of the Himalayas, following the complete closure of the eastern Neo-Tethys. The former complex puzzle of microplates that was present north of the Mesogea and south of the Neo-Tethys was sandwiched in the collision zone along an area stretching from the Alps to India.[34] [83] [93] This time-transgressive collision gave rise to the orogenic belts from the Alps to Himalaya, including the Serbo-Pelagonian area, the Pontides, and the Taurus. North of the collision belt, basins such as the Carpathian Flysch Basin, the Black Sea, and the Caspian developed. After the collisions, recorded by this complex assemblage of microplates, the continued compressional regime related to the counterclockwise rotation of Africa produced the development of the northward subduction of the Mesogea Ocean (evolving into the Eastern Mediterranean Basin) below the newly accreted terranes on the southern border of Eurasia. The Peri-Arabian Massif was approaching Laurasia, which initiated development of the Zagros deformation front.[83] The emerged area of the north-eastern side of the Arabian plate can be interpreted as the peripheral bulge of the lower plate. The docking of Arabia to Eurasia led to partial separation between the Indian Ocean to the east and the Eastern Mediterranean Basin to the west. The Arabian plate was significantly uplifted, so that the former shelf area was almost entirely exposed. Sedimentation (shallow marine carbonate passing to deep-water clastics, Kirkuk Basin) was reduced to a narrow belt along the future Mesopotamia and Persian Gulf. Northern Africa was still characterized by a passive margin facing north toward the Eastern Mediterranean Basin. Deep marine clastics were deposited in the Sirt Gulf, whereas continental deposits accumulated in present-day Egypt, Libya, and Sudan. Close to the time of the Eocene-Oligocene boundary, intense magmatic activity was recorded in the Afar area (Afar Traps). Volcanics were also deposited along the western margin of the Arabian Plate, where a rift valley, in which alluvial-lacustrine sediments were deposited, marked the beginning of the opening of the future Red Sea. A complex network of rift basins developed along the future Aden Gulf, and volcanic activity was recorded within the orogenic belts of southern Eurasia (mainly Lut Block, Central Iran, and Armenia[83]).

Paleogeography and petroleum plays[edit]

The paleogeographic and tectonic evolution of the southern Tethys area during the Phanerozoic plays an important role in determining the distribution of the source rocks and reservoirs as well as the origin of stratigraphic and tectonic traps, both strictly related to the geodynamic evolution of the area. This recognition of these controls on hydrocarbon resources and accumulation of oil and gas along the Tethyn margin has been the major thrust of the publications of many geologists, including Murris,[94] Beydoun,[95] [96] [97] May,[98] and Sharland et al.[99]

The giant fields of North Africa, Arabia, and the Middle East reflect episodes of enhanced primary productivity with high export production and storage of this organic matter in the sedimentary successions of different types of sedimentary basins.

The factors that control primary productivity are light intensity, nutrient inputs (nitrate and phosphate), and climate.[100] Today maximum productivity in the oceans is recorded on the inner shelf of the continental platforms and in ocean upwellings because of high nutrient concentration and relatively clear water.[101] The paleogeographic configurations of the late Paleozoic-Mesozoic time interval is dominated by E-W oceans, particularly in North Africa, Arabia, and the Middle East, where they extended mainly from the equator to the tropics; indeed, for most of this interval the Tethyan Seaway was present at very low latitudes north of Africa and Arabia, indenting the Pangea supercontinent. Paleocurrent models for a general Pangea configuration[102] [103] [104] envisage a westward-flowing equatorial surface current which, upon reaching the continental shelves of the western Tethys Seaway, deflected southeastward and northeastward; in the meanwhile, a deep water circulation brought cold waters from high latitudes to the equator. Ocean upwellings of these cold and nutrient-rich bottom waters were created by monsoonal wind circulation[105] [106] [107] along the Gondwanan margin and in the lee of continental blocks scattered in the Paleo- and Neo-Tethys, as well as at the equatorial divergence zone.

The combination of extended continental platforms in the proximity of a nutrient-delivering supercontinent and developed ocean upwellings caused the increase of primary productivity during favorable climate conditions, particularly at low latitudes where light intensity was higher and rate of mineralization (hence greater nutrient supply) more rapid. Storage of increased production as organic matter in the sediments was in turn enhanced by high sedimentation rates and availability of accommodation space.

The latitude and relative position of the Pangea and the Tethys therefore favored the deposition of source rocks, whereas the continuous and time-transgressive generation and evolution of different sedimentary basins controlled the creation of reservoirs and traps (such as those related to tectonic inversion of extensional structures), leading to the impressive concentration of oil and gas fields in this area.

The distribution of giant oil fields is related to the nature of the sedimentary basins. According to Mann et al.,[108] most of the giant oil and gas fields known until 2000 are related to continental passive margins facing the major ocean basins (34.66%), continental rifts and overlying sag basins (especially failed rifts at the edges or interiors of continents; 30.90%), and collisional margins produced by terminal collision between two continents (19.73%). These types of basins are common in the succession of North Africa and the Middle East. Due to the geodynamic evolution of this area, rift basins (mainly formed due to the opening of the Tethys oceans and to the extensional events affecting North Africa) rapidly evolved to passive margins (e.g., evolution of the peri-Gondwanan blocks) and then to active margins, with the development of collision-related basins (e.g., foredeep related to the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia).

As a consequence, different types of sedimentary basins were continuously created by the movement of continental blocks, so that at any time different basin types can be recognized (e.g., divergence on the southern side of the Tethys and convergence on the Asian margin) in North Africa and the Middle East. Therefore, favorable conditions for the development of petroleum plays were almost continually present. Though there are some differences in tectonic evolution across the margin plate scale, correlations of stratigraphy and thus petroleum systems are possible.[94] [95] [96] [97] [98] [99]

When considering the distribution of the giant oil and gas fields, two major groups of sedimentary basins can be identified: one in North Africa mainly dominated by rift, sag, and passive margins, and one in the Middle East, where oil fields are mainly preserved in sag and passive margin and collision-related basins.[109] [110] In the latter sector, oil fields are clustered in two major sets: Eastern Arabia-Persian Gulf-Zagros and Caspian Sea.

North Africa experienced several stages of alternating passive margin (e.g., Devonian, Carboniferous) and rift settings, related to different geodynamic events (e.g., effects of the Tethys opening in late Paleozoic) which recurred in this area. Rift and passive margin stages are commonly separated by local or regional compressional events (e.g., Cretaceous) with reactivation/inversion of extensional structures (Boote et al., 1998). Favorable environmental conditions controlled the deposition of major source rocks, such as the Hot Shale during the Silurian. This unit reflects the environmental changes (warming and sea-level rise) that postdated the Ordovician glaciations (Figure 3). The history of repeated rifting, sag stage, and folding favored the creation of a large number of stratigraphic and tectonic traps that store, at different stratigraphic levels, several giant fields.[111] [112] The presence of a wide, shallow-water shelf in an arid environment (Triassic-Jurassic) led to the deposition of thick and widespread salt layers, which represent an effcient seal at the regional scale.[111]

If in North Africa the basins are mainly related to rift basins followed by passive margin, the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia led to the formation of a major concentration of giant fields along the northern passive margin of the peri-Gondwanan blocks and in the overlying peripheral basins related to their collision. In the southern Caspian Sea area, the giant fields are mainly stored in collision-related basins[108] whose origin was controlled by the docking of the peri-Gondwanan along the southern margin of Eurasia (e.g., Cimmerian orogeny). A similar origin is suggested for the Northern Caucasus Basins,[108] whereas a complex history (from cratonic backarc extension and rifting followed by a sag basin stage) is recorded in the Pricaspian Basin.[113]

In the Arabian peninsula, Mann et al.[108] identified three basin types preserving giant fields: continent–continent collision for the elongate fields along the Zagros Mountain front; passive margin basins of the southern shore of the Neo-Tethys (central Arabian peninsula and Persian Gulf area); and continental rifts with overlying sag basins on the eastern Arabian Peninsula. Source rocks and reservoirs are present at different stratigraphic levels, reflecting a complex interaction of depositional and tectonics events.[114] [115]

See also[edit]

References[edit]

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  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Muttoni, G., Gaetani, M., Kent, D. V., Sciunnach, D., Angiolini, L., Berra, F., Garzanti, E., Mattei, M., and Zanchi, A., 2009, Opening of the Neo-Tethys Ocean and the Pangea B to Pangea A transformation during the Permian: GeoArabia, v. 14, no. 4, p. 17–48.
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