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Abstract

Plate-tectonic movements made the Cretaceous a time of major change in the area of the modern Oman and Zagros Mountains. Neo-Tethys 1 had been created in the Late Permian by the calving of a microcontinent (Anatolia, Sanandaj–Sirjan/Central Iran) along the NE margin of Arabia. In the Late Triassic, a second spreading axis, Neo-Tethys 2 (more readily recognizable in Iran than in Oman) replaced that of Neo-Tethys 1 by the separation of the Central Iran and Sanandaj–Sirjan–Kawr microcontinents.

Neo-Tethys 1 had a passive continental margin during the Triassic and Jurassic as the Afro–Arabian portion of Gondwana moved westward away from the actively spreading oceanic ridge of Neo-Tethys 2. Shallow-marine sediments along the continental margin were the source of carbonate turbidity currents that flowed basinward to the abyssal plain of Neo-Tethys 1 until the early Late Cretaceous, whereas the floor of Neo-Tethys 2 seems to have been starved of coarse sediment in its Oman sector.

Early in the Cretaceous, the South American and Afro–Arabian portions of Gondwana began to separate to create the South Atlantic Ocean. South America continued to move to the west, but Afro-Arabia reversed its sense of motion. The ensuing buildup of horizontal compressional stresses led to an eastward-dipping subduction zone within the Oman sector of Neo-Tethys 2, leading to obduction of the Late Permian to mid-Cretaceous Hawasina Series (deposited in Neo-Tethys 1) and the Semail Nappe, which was generated by back-arc spreading. North of the Dibba Line, subduction also took place within Neo-Tethys 1.

The latest Cretaceous was a time of tectonic adjustment and shallow-marine carbonate sedimentation across the area of the present Oman Mountains and southern Zagros, but the effects of late Maastrichtian subduction in Neo-Tethys 2 are visible in the Inner Makran. Evidence of subduction beneath the northern half of the Gulf of Oman suggests that this process has been more or less continuous over the Makran area until today. Uplift of the Oman Mountains began in the Mio–Pliocene, about the same time as the Zagros Mountains began to form.

Introduction: Geological Setting

The tectonic evolution of the northeastern margin of Arabia (Oman and Zagros mountains) involved the successive development of two belts of spreading oceanic crust, referred to as Neo-Tethys 1 and Neo-Tethys 2 (Glennie, 1995). The first formed in the mid Permian when a large microcontinent (Anatolia?–Sanandaj–Sirjan/Central Iran) was calved from the northeastern margin of Arabia; the sedimentary fill of that basin in Oman is known as the Hawasina Series. The second oceanic belt formed in the mid to late Triassic when the axis of spreading “jumped” to the east, where it split the former microcontinent, separating Central Iran/Lut from the Sanandaj–Sirjan Zone (Figs. 1, 2A). The ensuing history of these two oceanic areas had a profound effect not only on the structural development of the Arabian continental margin as seen in the Oman and Zagros mountain belts but also over a much wider area of the Middle East.

Fig. 1.

—Plate-tectonic distribution of microcontinents (e.g., Anatolia/Taurus, Sanandaj–Sirjan, Central Iran/Lut, Helmand Block, Central Afghanistan) north of a line from the Arabian Sea to the Mediterranean, which are thought to have existed between the Arabian and Eurasian platforms during much of the Mesozoic. The Indian microcontinent finally separated from Gondwana only in the Late Cretaceous. The intervening areas (dark stipple) are interpreted to contain relics of the oceanic crust that formerly separated the microcontinents: Crush Zone = Neo-Tethys 1; Zanjan-Taftan Zone = Neo-Tethys 2. The Lake Van area between Anatolia and Sanandaj–Sirjan may represent early separation between the two microcontinents. Modified from figure 15 in Glennie (1995).

Fig. 1.

—Plate-tectonic distribution of microcontinents (e.g., Anatolia/Taurus, Sanandaj–Sirjan, Central Iran/Lut, Helmand Block, Central Afghanistan) north of a line from the Arabian Sea to the Mediterranean, which are thought to have existed between the Arabian and Eurasian platforms during much of the Mesozoic. The Indian microcontinent finally separated from Gondwana only in the Late Cretaceous. The intervening areas (dark stipple) are interpreted to contain relics of the oceanic crust that formerly separated the microcontinents: Crush Zone = Neo-Tethys 1; Zanjan-Taftan Zone = Neo-Tethys 2. The Lake Van area between Anatolia and Sanandaj–Sirjan may represent early separation between the two microcontinents. Modified from figure 15 in Glennie (1995).

Fig. 2.

—Three tectonic sketches. (A) Distribution of microcontinents of Iran and intervening areas of oceanic sediments and ophiolites. (B) Local detail in the western Inner Makran. The Tiab Zone is a klippe comprising Cretaceous limestones and Eocene flysch overlying Upper Jurassic pillow lavas dated by interpillow calpionelìids. The ophiolites to its north, which have been similarly dated, tectonically overlie metamorphic rocks of the Sanandaj–Sirjan Zone. (C) Autochthonous rocks of the Oman Mountains overlain by the Hawasina and Semail Nappes. Note extent of Miocene to Recent flysch of Neo-Tethys 2 over northern floor of Gulf of Oman. A, Jebel Akhdar; Ke, Kermanshah; N, Neyriz; Na, Nain; M, Musandam Peninsula; Ma, Mardin Metamorphics; S, Saih Hatat.

Fig. 2.

—Three tectonic sketches. (A) Distribution of microcontinents of Iran and intervening areas of oceanic sediments and ophiolites. (B) Local detail in the western Inner Makran. The Tiab Zone is a klippe comprising Cretaceous limestones and Eocene flysch overlying Upper Jurassic pillow lavas dated by interpillow calpionelìids. The ophiolites to its north, which have been similarly dated, tectonically overlie metamorphic rocks of the Sanandaj–Sirjan Zone. (C) Autochthonous rocks of the Oman Mountains overlain by the Hawasina and Semail Nappes. Note extent of Miocene to Recent flysch of Neo-Tethys 2 over northern floor of Gulf of Oman. A, Jebel Akhdar; Ke, Kermanshah; N, Neyriz; Na, Nain; M, Musandam Peninsula; Ma, Mardin Metamorphics; S, Saih Hatat.

The northeast continental margin of Arabia extends from the Arabian Sea/Gulf of Oman to the Mediterranean Sea. The Cretaceous was a period of profound change along this margin and in the two adjacent marine basins (Neo-Tethys 1 and 2). A reversal in the sense of plate-tectonic movement converted a long-lasting passive-margin relationship between continent and ocean into one involving successive ocean–ocean subduction in the two oceanic basins, leading eventually to continent–continent collision in Iran. The history of events during and preceding the Cretaceous in the first of these basins is more clearly discerned in the Oman Mountains, whereas the second is better defined in Iran, the two areas with which this paper is mostly concerned. But how did all this come about? The scene needs to be explained.

Paleozoic History

Throughout the Paleozoic Era, the site of modern Arabia was located within the Southern Hemisphere (Scotese and McKerrow, 1990; Scotese and Barrett, 1990; Schandelmeier and Reynolds, 1997) near the northeastern edge of the supercontinent Gondwana, where it was flanked by the ocean Paleo-Tethys. For most of that long time span, Arabia lay about midway between the South Pole and the Equator, with siliciclastics dominating sedimentation (Schandelmeier and Reynolds, 1997). Towards the end of the Paleozoic Era, however, there was a relatively rapid change in Oman from the climatic conditions needed for a Late Carboniferous to Early Permian glaciation (Braakman et al., 1982; Love, 1994) to the warm waters of Late Permian coral reefs and other carbonate environments (Blendinger et al., 1990).

The glaciation in SE Oman occurred while the area was probably no farther from the Equator than about 45° (Scotese and Langford, 1995) and possibly closer. Al-Beloushi et al. (1996) suggest that the glaciation was of mountain type, and connected with uplift prior to the creation of an Early Permian triple junction adjacent to the SE end of the present Oman Mountains; Le Métour et al. (1994) reported associated erosion of up to 4000 m at this time in northern Saih Hatat. The NW–SE trending arms were associated with the calving of microcontinents (Anatolia, Iran, Tibet) from the northeast margin of Arabia (present north) and the development of Neo-Tethys 1. The other arm involved the coeval separation of SE Arabia and the Helmand block (now southern Afghanistan) and creation of an intervening Batin–Helmand seaway (Al-Beloushi et al., 1996), which, on the basis of mid-Permian marine faunas, possibly extended as far as Madagascar (Flores, 1970); this separation developed fully only during the final Late Jurassic breakup of eastern Gondwana (see Immenhauser, 1996; Gnos et al., 1997).

The early introduction of warm-water faunas is attributed to the rapid collapse of the glaciated mountains and the development of nearby newly formed oceanic areas (Al-Beloushi et al., 1996). This event was coupled with a postglacial rise in sea level and the lowering and perhaps partial collapse of the flanking land areas as they moved away from the new axes of spreading, resulting in a mid to late Permian transgression over much of eastern Arabia (fig. 6 in Murris, 1980; Blendinger, 1990; Le Métour et al., 1994; Glennie, 1995). A slow northward drift of Arabia towards and across the Equator added to the climatic change and ensured that carbonates were the dominant marine sediment throughout the Mesozoic Era except where deposited below the carbonate compensation depth (e.g., figs. 1 and 8 in Bernoulli et al., 1990).

The Varying Character of Neo-Tethys 1 and 2

The former existence of Neo-Tethys (Stöcklin, 1974), or the same ocean under other names (e.g., Hawasina Ocean of Glennie et al., 1973, 1974; Fig. 4A; or the Southern Tethys of Stoneley, 1990) has been inferred by many workers, whereas two oceanic areas (Fig. 1) subparallel to the continental margin of Arabia have been proposed by Glennie et al., 1990) and Glennie (1995), and are indicated by the ophiolite distribution in figure 2 of Sengör (1990). Evidence for inferring their existence is given in these and other publications, the main points only being summarized here. Because the supporting evidence differs in Oman and Iran, each will be treated separately.

Fig. 4.

—A series of cartoons illustrating similarities and differences between the history of Neo-Tethys 1 and 2 in Iran and Oman. (A) Late Permian. (B) Jurassic–Cretaceous boundary. (C) Aptian–Albian. (D) Campanian. (E) Plio-Pleistocene. No fixed scale is implied.

Fig. 4.

—A series of cartoons illustrating similarities and differences between the history of Neo-Tethys 1 and 2 in Iran and Oman. (A) Late Permian. (B) Jurassic–Cretaceous boundary. (C) Aptian–Albian. (D) Campanian. (E) Plio-Pleistocene. No fixed scale is implied.

Neo-Tethys 1 in Oman

The best evidence for the existence of Neo-Tethys 1 is found in the Oman Mountains. Le Métour et al. (1994) cited considerable data to support an early Late Permian origin for Neo-Tethys 1 (e.g., in northern Saih Hatat, continental separation is indicated by sedimentary sequences dated by Permian faunas that are interbedded with basaltic pillow lavas and are associated with sills and dikes, and even Early Permian volcanics (Le Métour et al., 1994).

Palinspastic reconstructions of the Hawasina sequences indicate that northeast of the Arabian platform margin, a continental slope developed, down which carbonate-rich turbidity currents flowed towards the abyssal plain from the mid or late Permian until the mid Cretaceous. Paleocurrent data show the source of turbidity currents to have been in the west and southwest (Glennie et al., 1974; Watts, 1987; Blendinger, 1988; Blendinger et al., 1990; Cooper, 1990; Glennie, 1995); many of the carbonate grains found in the resulting turbidites consist of microfossils that confirm the shallow-marine platform-margin origin of the sediments that form them. For example, Late Triassic fore-reef talus of the Sumeini Group (Glennie et al., 1974; Watts, 1990) was later removed tectonically from proximity to its original reef; its faunas have been described in some detail by Bernecker (1996). In the deepest parts of the depositional realm, middle to Late Permian radiolarians occur near the base of the allochthonous Hawasina (De Wever et al., 1990).

From the order of superposition of Hawasina nappes in the Oman Mountains, a fairly regular depositional pattern can be deduced across Neo-Tethys 1 (Hamrat Duru Basin) from southwest to northeast: up to 1600 m of continental-slope reef talus, slumps, slides, and minor turbidites (Sumeini Group); base-of-slope turbidites up to 1000 m thick (Hamrat Duru Group) grading into about 300 m of finer-grained turbidites of a more siliceous nature (e.g., Wahrah Formation) beneath the deep waters of the abyssal plain (Glennie et al., 1974).

Neither continental nor confirmed oceanic crust has been identified from below the oldest basinal units of the Hawasina. There is commonly an association with mostly alkaline volcanic rocks, which have a MORB tholeiitic character in the Hawasina Window. Béchennec et al. (1990) interpret the alkaline volcanics as the product of Late Permian extensional thinning of a continental crust; this crust possibly became fully disrupted over northeastern parts of the Hamrat Duru Basin, leading to the emplacement of pseudo-oceanic crust. A submarine volcanic component of such crust might be present in submarine volcanics within the basal Zulla Formation (see Bernoulli et al., 1990). In the most distal depositional area of the Hawasina, shallow-marine limestones (Oman Exotics; Glennie et al., 1974) are underlain by the Haybi Volcanics, which include pillow lavas that also have a MORB character. Lippard et al. (1986) suggest that the Haybi Volcanics indicate an origin in a small ocean basin like the Red Sea. This interpretation is supported by El-Shazly et al. (1994), who note that greenschists, blueschists, and eclogites in the Saih Hatat area were originally enriched MORB erupted axially in a small oceanic basin in the Late Permian. The pressure required for blueschist–eclogite metamorphism was provided within the subduction trench, which Searle et al. (l994) suggest was achieved at depths of up to 80 km during Late Cretaceous subduction.

Glennie et al. (1974), Glennie et al. (1990), and Glennie (1995) considered that the sub-Hawasina volcanics and the lack of evidence of underlying continental crust indicate the ease of basal Hawasina décollement during later nappe emplacement; this, they believed, also favored an interpretation of Hawasina deposition over an oceanic crust (Neo-Tethys 1 or Hawasina Ocean) that developed from about the mid Permian to the Middle Triassic.

Obduction of the Hawasina and Semail nappes has been shown to have taken place predominantly within the Campanian to Maastrichtian time span. While the Hawasina sediments all predate the time of obduction, the age of the overlying Semail ophiolite is approximately coeval with the time of obduction (i.e., Late Cretaceous) (Glennie et al., 1974; Lippard et al., 1986; Béchennec et al., 1990).

Neo-Tethys 1 in Iran

The arcuate but discontinuous distribution of ophiolites and associated “radiolarites” (see below) northwest of Oman, along the Crush Zone of Iran, and south of Anatolia to Syria and Cyprus (see Fig. 1 and, e.g., fig. 2 in Sengör, 1990) is taken as evidence that a relatively narrow seaway formerly connected the Hawasina Basin to the present Mediterranean area. In Iran, the former basin fill and overlying ophiolites are displayed only in two widely separated areas, near Kermanshah in the northwest (Fig. 2A) and near Neyriz in the southeast (Fig. 2A, 3). As in Oman, the oceanic crust of the Crush Zone is also of Late Cretaceous age, and similarities in basin fill suggest a similar history of development. This distribution was named by Ricou (1971) “Le Croissant ophiolitique peri-Arabe”, an interpretation supported by Biju-Duval et al. (1974) and Coleman and Irwin (1974).

Fig. 3.

—Map showing how the Oman and Zagros mountains on the eastern edge of Arabia relate to the Sanandaj–Sirjan and Central Iran/Lut microcontinents and the intervening areas of ophiolites and ocean-floor sediments. Cenozoic volcanics mask much of the ophiolite–radiolarite sequence surrounding Central Iran/Lut. The southern Makran was the site of late Cenozoic subduction of Neo-Tethys 2, and subduction of its floor is still occurring beneath the northern half of the Gulf of Oman. The Musandam Peninsula, north of the Dibba Line, is probably more closely related to the Zagros than the rest of the Oman Mountains; this highlights the origin of the Dibba Line as a probable major transform fault separating the two areas. A, Jebel Akhdar; B, Nappes of Batain Coast; K, Kushmandar; Kw, Jebel Kawr; M, Musandam Peninsula; Ma, Mardin Metamorphics; N, Nain; Q, Jebel Qamar; S, Saih Hatat; SA, Salakh Arch.

Fig. 3.

—Map showing how the Oman and Zagros mountains on the eastern edge of Arabia relate to the Sanandaj–Sirjan and Central Iran/Lut microcontinents and the intervening areas of ophiolites and ocean-floor sediments. Cenozoic volcanics mask much of the ophiolite–radiolarite sequence surrounding Central Iran/Lut. The southern Makran was the site of late Cenozoic subduction of Neo-Tethys 2, and subduction of its floor is still occurring beneath the northern half of the Gulf of Oman. The Musandam Peninsula, north of the Dibba Line, is probably more closely related to the Zagros than the rest of the Oman Mountains; this highlights the origin of the Dibba Line as a probable major transform fault separating the two areas. A, Jebel Akhdar; B, Nappes of Batain Coast; K, Kushmandar; Kw, Jebel Kawr; M, Musandam Peninsula; Ma, Mardin Metamorphics; N, Nain; Q, Jebel Qamar; S, Saih Hatat; SA, Salakh Arch.

The term “radiolarites” is used as an abbreviation in the sense of Stöcklin (1974); it includes a range of sediments from the characteristic reddish radiolarian chert to carbonate turbidites and masses of white marble. Contained fossils indicate a time span of deposition from Permian to mid Cretaceous, although the fauna in many sequences has been destroyed by recrystallization. Because of late Cenozoic transpression, some large slabs of the ophiolites and radiolarites are now both tectonically and sedimentologically interleaved with younger sedimentary sequences ranging in age from late Maastrichtian to Miocene, and much additional evidence is now presumed to lie beneath the overthrust Sanandaj–Sirjan zone.

As in Oman, the imbricated sedimentary fill of radiolarites was thrusted over shallow-marine carbonates of the continental margin during the Late Cretaceous (Fig. 3, NW of Neyriz). This sequence, in turn, is overlain tectonically by slabs of ophiolitic material rich in clearly recognizable peridotite as well as extrusive rocks. A schematic cross section of these relationships near Neyriz is given in figure 4 of Stöcklin (1974).

The broad similarity with the Hawasina and Semail of the Oman Mountains is striking. The ophiolites, like those in Oman, are considered by Hooper et al. (l994) to result from a Late Cretaceous “supra-subduction zone (i.e., back arc) spreading”. These authors believe that the Arabian plate margin comprised a series of promontories and embayments, which may explain the presence of only two areas where ophiolites and radiolarites are exposed (K and N in Figure 2A). In the Neyriz area, Ricou (1971) considered the sedimentary basin to have had a minimum width of 50 to 100 km, with another 100 km for the ophiolite nappe. This compares with a minimurn deduced width for the Hawasina Basin of the order of 400 km (Glennie et al., 1974).

Neo-Tethys 2 in Oman

The northeast side of the Hawasina Basin was the site of deposition of the Oman Exotics (Kawr Group of Béchennec et al., 1990) on the Kawr (or Misfar) Ridge (Fig. 4B). The mainly shallow-marine carbonates of the Exotics were deposited over two distinct time spans, early Late to latest Permian and Late Triassic to Early Jurassic; many smaller Exotics have been fully recrystallized to marble. The Liassic top to the 1000 m-thick Late Triassic limestone of Jebel Kawr (Figs. 3, 4) is truncated and capped by thin Cretaceous argillaceous and radiolaria-rich deep-water limestones that imply sediment starvation. The Late Permian and especially Late Triassic Exotics are commonly underlain by pillow lavas and other volcanics, which, as mentioned above, are suggestive of an oceanic location. In the case of Jebel Qamar (just south of the Dibba Line; Fig. 3), however, which is underlain by the Ordovician Rann quartzites, a continental substrate is indicated.

The sub-Exotic alkaline to transitional tholeiitic lavas of Triassic age (Haybi Volcanics, Lippard et al., 1986) are believed by Searle and Graham (1982) to have been erupted during the early stages of continental rifting. As already implied, evidence from Iran indicates that there were probably two events of continental separation, which in Oman may have been recorded by the distinct Permian and Triassic ages of the Oman Exotics. The Exotics were flanked by deep water into which flows and slides of derived coarser shallow-marine carbonate material were deposited (Fig. 4B).

The sediments deposited in the Umar Basin (Fig. 4B) (Béchennec et al., 1990), northeast of the Kawr Ridge, are very fine grained, with only limited coarser detritus thought to have been derived from the Triassic Exotics of the ridge itself. Like Searle and Graham (1982), Béchennec et al. (1990) associated the Haybi Volcanics with continental separation. Separation in this case involved the creation of Neo-Tethys 2. Because the narrow Kawr Ridge intervened between the deposirional areas of Neo-Tethys 1 and 2, the Umar Group had no sedimentary input from the continental margin of Arabia.

Neo-Tethys 2 in Iran

The foregoing geometrical evidence related to the Oman Exotics and overlying Umar Group supports the hypothesis that Neo-Tethys had two parts to it. The older Neo-Tethys 1 is presumed to have stopped spreading in about mid Triassic time, when the axis of crustal spreading “jumped” eastward to separate the Sanandaj–Kawr Ridge from Central Iran/Lut. Neo-Tethys 2 possibly continued to spread from about the mid Triassic until the mid or late Cenozoic (Glennie, 1995), but at least a part of this basin was involved in subduction in the latest Cretaceous. In Oman, sedimentary evidence for the existence of the Umar Basin (Neo-Tethys 2) is poorly displayed in small outcrops between the Oman Exotics and the sheared base of the flanking Semail Nappe. In Iran, the margins of Neo-Tethys 2 are sharply defined by its flanking continental areas, but its sedimentary basin fill and associated ophiolites are, in general, not nearly so well exposed as in Oman, and are further complicated by intense tectono-sedimentary reworking from the Maastrichtian to late in the Cenozoic (Figs. 2B, 3).

In the NW Makran, the exposed metamorphic rocks of the Mardin and Kushmandar areas (Ma and K in Figure 3) at the SE end of the Sanandaj–Sirjan Zone plunge to the southeast beneath the ophiolites of the Inner Makran and adjacent Tiab klippe (Figs. 2B, 3; see Glennie et al., 1990; Glennie, 1995). The metamorphic rocks are capped by mountain-size blocks of white marble that are reminiscent of the Oman Exotics. These mountains of marble are tentatively correlated with the Oman Exotics of the Kawr Ridge, recrystallization having possibly occurred during a Carnian metamorphic event that affected this area (see Hooper et al., 1994); if so, metamorphism would have coincided with the initiation of Neo-Tethys 2. If this correlation is correct, then the Kawr Ridge plays an important role in relating the geology of Oman to the Inner Makran, and for reconstructing the Mesozoic history of the Arabian continental margin in these areas of the Middle East.

The Sanandaj–Sirjan Zone is separated from the continental crust of Central Iran by a low-lying belt that is typified by Tertiary extrusive rocks (Fig. 3) but through which an ophiolitic suite of rocks is locally exposed (e.g., near Nain; Na in Figure 2A). Using the Tertiary volcanics as a guide, this second zone of ophiolites and radiolarites can be traced from between the Lesser Caucasus and the NE corner of Anatolia (Fig. 1), southeastward via the Zanjan area of NW Iran (Fig. 2A) to the Makran and on to the vicinity of the Quaternary Bazman and Taftan volcanoes south and southeast of the Lut Block (Fig. 3; see also distribution of “Colored Mélange” (Gansser, 1955) in figure 1 of Stöcklin, 1974). The Colored Mélange can be separated into its constituent sedimentary and ophiolitic parts. As individual blocks within a wildflysch, some of the sedimentary sequences are similar to those found in the Crush Zone (grainstone rurbidites, radiolarian cherts, and blocks of marble or shallow-marine limestone that are associated with basic igneous rocks). The basic and ultrabasic ophiolite components are dominantly of peridotite but include dike swarms overlain by pillow lavas.

The Mardin and Kushmandar metamorphics (Fig. 3) are overlain by Alpine-type peridotite masses that are highly sheared and serpentinized along their basal contacts. To the east and southeast, less metamorphosed peridotites grade up into gabbros similar to those mapped in the Oman Mountains (e.g., Reinhardt, 1969, Lippard et al., 1986). Dike swarms are overlain by, and imbricated with, mudstones bearing Tithonian to Berriasian calpionellids and radiolaria. These ages for ophiolite generation are in strong contrast to the Late Cretaceous age recorded by inter-pillow microfossils of the Semail Nappe in Oman. Along the margin of the Inner Makran, the gabbros grade up into dike swarms that form the feeders to massive lava flows, tuffs, and pillow lavas, which are overlain by Eocene lithic sandstones. This ophiolite slab is in high-angle thrust contact with the Tiab Klippe (Figs. 2B, 3; see also figure 3B of Glennie et al., 1990). Beneath a cover of Eocene conglomerates, sandstones, and shales, the klippe comprises Maastrichtian to Paleocene shallow-marine limestones unconformably overlying Lower Cretaceous limestones and Upper Jurassic to Lower Cretaceous sandstones and shales; these, in turn, can grade down into sheared serpentinite or pillow lavas that again enclose Tithonian to Berriasian calpionellid-bearing limestone and shale. The Late Jurassic age of the ophiolites in the Inner Makran contrasts strongly with the Late Cretaceous age (also on interpillow fossils) of the Semail ophiolite in Oman.

Events Leading to Cretaceous Nappe Emplacement along the Arabian Margin

To summarize the foregoing events, it is clear that crustal separation took place along the northeastern margin of Arabia in two distinct phases: (1) From the early Late Permian until the middle or Late Triassic (Carnian), Neo-Tethys 1 was actively spreading in the Hawasina–Crush Zone–Cyprus ocean. (2) Spreading then “jumped” to Neo-Tethys 2, which began to spread during the Middle to Late Triassic, and in the Sabsavar Zone (Fig. 2A) continued to spread until possibly the mid Cenozoic.

Passive-margin conditions of sedimentation prevailed within Neo-Tethys 1 and along the SW margin of Neo-Tethys 2 as Gondwana moved southwestward away from the axis of spreading. The early Barremian start of spreading in the South Atlantic Ocean (Milner et al., 1995) is thought to have been responsible for the Aptian (?) onset of subduction in Neo-Tethys. As Afro-Arabia moved away from the Atlantic axis of spreading, the resulting increase in crustal compression in Neo-Tethys 1 and 2, intensified by continued spreading in Neo-Tethys 2, is thought to have been relieved by the initiation of subduction. By the late Maastrichtian, the sedimentary fill of Neo-Tethys 1 had been obducted onto the continental margin of Arabia.

Whereas obducción in the Iranian sector of Neo-Tethys 2 may have begun only in the latest Cretaceous or Paleocene, subduction of its remaining oceanic crust seems to be continuing today beneath the northern Gulf of Oman (White and Klitgord, 1976; White, 1982).

The following is an attempt to rationalize the above developments into a coherent history of events (Fig. 4; Table 1).

Table 1.

—Generalized timing of major events in the structural history of the Oman and Iranian sectors of the Arabian continental margin, and the associated Neo-Tethys 1 and still extant Neo-Tethys 2 (Gulf of Oman).

AgeArabian Continental MarginNeo-Tethys 1 (Hawasina/Crush Zone)Neo-Tethys 2 (Makran)
Plio-Pleistocene Active folding of post-Hormuz Zagros sequences Crush-Zone transpression Northward subduction of floor of Gulf of Oman 
Late Miocene Flooding of Arabian Gulf  Closure of Neo-Tethys 2 in Iran? Deep-marine sediments in Gulf of Oman 
Oligocene Uplift of Oman Mountains  Right-lateral transpression 
Eocene Shallow-marine deposition  Flysch deposition 
Paleocene Shallow-marine & turbidite deposition  Wildflysch deposition 
Maastrichtian Shallow-marine Simsima deposition End Semail/Hawasina obduction Juweiza/Amiran deposition Closure of Hawasina/Crush-Zone ocean Start of active subduction S and W of Lut Block 
Campanian Start of Muti/Faraghan deposition & Hawasina obduction   
Cenomanian Youngest Hajar Supergroup   
Albian Second up-warping of continental edge Start of Semail back-arc spreading  
Aptian First up-warping of continental edge Start of Hawasina subduction  
Portlandian Deep-marine continental margin  Dated oceanic crust 
Triassic (late)  End of spreading in Neo-Tethys 1 Start of Zanjan-Taftan (Neo-Tethys 2) spreading (& Umar Group ot Hawasina) 
Triassic (early)  Neo-Tethys 1 actively spreading  
Permian Marine transgression by Hajar Supergroup Triple-junction ice cap Neo-Tethys 1 starts to spread  
AgeArabian Continental MarginNeo-Tethys 1 (Hawasina/Crush Zone)Neo-Tethys 2 (Makran)
Plio-Pleistocene Active folding of post-Hormuz Zagros sequences Crush-Zone transpression Northward subduction of floor of Gulf of Oman 
Late Miocene Flooding of Arabian Gulf  Closure of Neo-Tethys 2 in Iran? Deep-marine sediments in Gulf of Oman 
Oligocene Uplift of Oman Mountains  Right-lateral transpression 
Eocene Shallow-marine deposition  Flysch deposition 
Paleocene Shallow-marine & turbidite deposition  Wildflysch deposition 
Maastrichtian Shallow-marine Simsima deposition End Semail/Hawasina obduction Juweiza/Amiran deposition Closure of Hawasina/Crush-Zone ocean Start of active subduction S and W of Lut Block 
Campanian Start of Muti/Faraghan deposition & Hawasina obduction   
Cenomanian Youngest Hajar Supergroup   
Albian Second up-warping of continental edge Start of Semail back-arc spreading  
Aptian First up-warping of continental edge Start of Hawasina subduction  
Portlandian Deep-marine continental margin  Dated oceanic crust 
Triassic (late)  End of spreading in Neo-Tethys 1 Start of Zanjan-Taftan (Neo-Tethys 2) spreading (& Umar Group ot Hawasina) 
Triassic (early)  Neo-Tethys 1 actively spreading  
Permian Marine transgression by Hajar Supergroup Triple-junction ice cap Neo-Tethys 1 starts to spread  

  1. Mid to Late Permian: Creation of Neo-Tethys 1 by the calving of a major microcontinent (or linear series of them) from the northeastern margin of Arabian Gondwana (see, e.g., figure 9 in Al Beloushi et al., 1996). By the Mid to Late Triassic, Neo-Tethys 1 was some 400 km wide in the Oman sector (Fig. 4A).

  2. Approximately Middle to Late Triassic: Crustal spreading ceased to be active in Neo-Tethys 1 and the axis “jumped” to NE to initiate spreading between the Sanandaj–Sirjan Zone and Central Iran/Lut. Geometric evidence of the Oman Exotics and the Kawr Ridge suggests correlation with Sanandaj–Sirjan Zone, which could be crucial in relating Neo-Tethys 2 in Oman and Iran.

  3. The Permian to Early Cretaceous basin fill of Neo-Tethys 1 in Iran is not well enough known to reconstruct its width and sedimentary history in such detail as in Oman. In Oman, palinspastic reconstructions indicate a wedge of oceanic sediment that thins to the northeast towards the Kawr Ridge.

  4. No sediment from the Arabian continental margin reached the Umar Basin of Neo-Tethys 2, and none is known to have been derived from the more easterly microcontinents (Fig. 4B). A palinspastic reconstruction has not been attempted on the sedimentary fill of Neo-Tethys 2 in either Oman or Iran; this would be possible but difficult to assemble in the Inner Makran (see, e.g., figure 3B in Glennie et al., 1990).

  5. The Umar Group forms the uppermost part of the Hawasina Series of Oman. If sedimentation of this Group followed initiation of Neo-Tethys 2, as implied by its relationship to the Haybi Volcanics and the Kawr Ridge, then the subduction that led to obduction of the Hawasina and Semail must have begun in Neo-Tethys 2 (Fig. 4C) and then involved Neo-Tethys 1 with little change of pace because the narrow Kawr Ridge provided almost no resistance to the subduction/obduction process.

  6. In Iran, the Sanandaj–Sirjan Zone was much broader than the Kawr Ridge. Furthermore, subduction seems to have begun within the Crush Zone sector of Neo-Tethys 1 (approx. Campanian) before that of the Inner Makran area of Neo-Tethys 2 (late Maastrichtian). The difference in timing and geometry in the Oman sector can be explained if the Dibba Line (Figs. 2C, 3) is considered as a major transform fault of long standing (since the mid Triassic or even mid Permian), on either side of which both crustal spreading and later subduction acted independently of each other.

  7. In the Oman sector of Neo-Tethys 2, the Semail ophiolites were generated during the Late Cretaceous by back-arc extension associated with subduction (Lippard et al., 1986; Glennie, 1995, fig. 6). Although less well constrained, a similar origin is believed for its Iranian sector. No clear representatives of an underlying Triassic or older oceanic crust is known from Neo-Tethys 1, although the sub-Exotics Haybi Volcanics and the volcanics beneath and interbedded with the basal Zulla Formation in the Hawasina Window in Oman come close. In the Makran sector of Neo-Tethys 2, the known oceanic crust was generated in the Late Jurassic-Early Cretaceous. As field evidence indicates a time span of generation from Mid Triassic to Late Maastrichtian (150 Ma or more) before subduction began, the width of Neo-Tethys 2 could have reached 7500 km by the Late Maastrichtian if the spreading rate averaged 5 cm/year.

  8. The start of subduction during the Aprian (Fig 4C) was accompanied by gentle crustal warping of the Arabian continental platform (see later). Warping was repeated at the start of obduction over the continental platform edge during the Campanian (Fig. 4D). This latter event coincided with development of the Muri (Oman) and Faraghun (Iran) fore-arc basins, which were partly covered by the advancing nappes before obduction ceased.

  9. Subduction led to closure of Neo-Tethys 1 and obduction of the Hawasina, Semail, and Crush Zone nappes by the mid Maastrichtian. Subduction then ceased because Arabia’s thick and relatively buoyant continental margin could not be consumed into the mantle. The South Atlantic Ocean and Neo-Tethys 2 were still spreading, however, and the resulting increase in crustal compression led in Iran to the creation of a new subduction zone in Neo-Tethys 2 before the end of the Maastrichtian.

  10. The Late Maastrichtian release of crustal compression is possibly the reason why the obducted nappes remained close tosea level rather than forming a high mountain chain. Theyoungest Cretaceous sediments in the vicinity of the OmanMountains are the mainly shallow-marine post-obductioncarbonates of the Simsima Formation.

    The areal and volumetric differences in ophiolite exposure between Iran and Oman might be explained as follows:

    • In Iran, subduction resulted in continent–continent collision, the ophiolites remaining buried beneath the relatively broad but rigid Sanandaj–Sirjan Zone microcontinent.

    • In Oman, however, the collision was of continent–ocean type; subduction ceased when the second subduction zone became active (as in Iran), thereby releasing the compressive forces, and allowed isostatic uplift of the combined volume of narrow Kawr Group continental crust and a large volume of hanging-wall future Semail. With no further destruction caused by horizontal compression, the Semail Nappe remains the largest and best preserved ophiolite in the world.

  11. The start of mountain building in Oman coincided with the opening of the Red Sea and with a major plate-tectonic collision to the northeast between India and Eurasia (see Gnos et al., 1996). In Iran, the Zagros Mountains began to form in the Mio-Pliocene. There, the Eo-Cambrian Hormuz salt acted as a plane of décollement between basement and overburden, above which long anticlinal folds of younger strata are pierced by many diapirs (Fig. 4E). The Oman Mountains lack halite, and where it is present beneath interior Oman, salt piercements, under the influence of plate-tectonic movements in the Indian Ocean, occur through the overburden rather than causing major folds; the Salakh Arch may be an exception

  12. Right-lateral continent–continent collision between Arabia and the Sanandaj–Sirjan Zone makes the Crush Zone an area of strong seismicity. In the Oman sector, the Arabian margin is flanked by the remnants of Neo-Tethys 2, the underlying oceanic crust of which is being subducted south of the Makran coast (Figs. 3, 4E).

Some Cretaceous Events on the Arabian Platform

Deposition of the earliest Cretaceous Kahmah (Thamama) Group saw a change from deep-marine conditions along the continental margin to a shallow-marine environment farther west (Fig. 5A). The eastward progradation of shallow-marine reservoir rocks over potential source rocks of the Habshan Formation (Aziz and El Sattar, 1997) adds to the economic interest of SE Arabia.

Fig. 5.

—Sedimentation over eastern Arabia at three Cretaceous times. (A) Valanginian: most of the area covered by shallow-marine carbonates. Euxinic intra-shelf deposition in the Luristan Basin in the north and deeper open-marine conditions along the continental margin in NE Oman and in the Neyriz region of the Zagros. (B) Aptian: warping of the Arabian platform created both reservoir grainstones and intra-shelf euxinic basins west of the continental margin. (C) Campanian: outer edge of the Arabian platform depressed as it starts to be subducted. Erosion of advancing Hawasina led to deposition of Muti and Faraghun-rype sediments west of the nappe front in Oman and the Zagros thrust front in Iran. Later erosion of the ophiolite thrust front led to Juweiza and Amiran deposition in northern Oman and Iran. Note the effect of the Dibba Line (DL) on deposition. R, Riyadh. Simplified from Murris (1980).

Fig. 5.

—Sedimentation over eastern Arabia at three Cretaceous times. (A) Valanginian: most of the area covered by shallow-marine carbonates. Euxinic intra-shelf deposition in the Luristan Basin in the north and deeper open-marine conditions along the continental margin in NE Oman and in the Neyriz region of the Zagros. (B) Aptian: warping of the Arabian platform created both reservoir grainstones and intra-shelf euxinic basins west of the continental margin. (C) Campanian: outer edge of the Arabian platform depressed as it starts to be subducted. Erosion of advancing Hawasina led to deposition of Muti and Faraghun-rype sediments west of the nappe front in Oman and the Zagros thrust front in Iran. Later erosion of the ophiolite thrust front led to Juweiza and Amiran deposition in northern Oman and Iran. Note the effect of the Dibba Line (DL) on deposition. R, Riyadh. Simplified from Murris (1980).

The initiation of subduction caused warping of the Arabian continental margin (Figs. 4C, 5B) to produce both basins and erosional highs (e.g., Murris, 1980, Masse et al., 1997), the depositional details of which can now be deduced using high-resolution sequence stratigraphy (e.g., Van Buchem et al., 1996). The warping was sufficient for the development of grainstone reservoir rocks (e.g., Shuaiba) around erosional highs that were separated by euxinic basins in which hydrocarbon source rocks were deposited (Van Buchem et al., 1996). During the Albian-Cenomanian, the siliciclastic Wasia Sandstone prograded eastward from the Arabian Shield over the western part of the carbonate shelf (Murris, 1980). At the onset of obduction during the Turonian or earliest Campanian, this episode was accompanied by the widespread “post-Wasia break” and was succeeded by the creation of the Muti foreland basin in Oman and the less well known Faraghun Basin along the Crush-Zone edge of the Zagros Mountains. Ophiolite components occur in the younger Juweiza (Oman) and Amiran (Zagros) formations of the Late Cretaceous. In interior Oman, dextral transtension and sinistral transpression occurred along fault zones under the influence of plate-tectonic movements that were taking place in the Indian Ocean (Loosveld et al., 1996).

After subduction ceased in the Oman sector of Neo Tethys in theLate Maastrichtian, the obducted Hawasina and Semail did not form a mountain range but lay close to sea level. The erosion of exposed highs led to the deposition of local fluvial conglomerates (e.g., Qahlah Formation in the SE Oman Mountains; Glennie et al., 1974) while widespread shallow-marine grainstones of the Simsima Formation covered most of the remaining area of obducted rocks and spread far onto the continental platform, although it is rather patchy in southern Oman. A deeper-water facies was deposited in the Suneinah Trough west of the northern Oman Mountains (Hughes Clarke, 1988). In the southeastern Oman Mountains, local Paleocene to Eocene deep-water conglomerates and rurbidites (Filbrandt et al., 1990) possibly mark the true southeastern fault-scarp limit of the Oman Mountains; the adjacent Batain Coast deposits are associated with obduction of the Arabian Sea’s Masirah ophiolite at the Cretaceous-Tertiary boundary (Immenhauser, 1996).

Conclusions

The northeastern margin of the Arabian platform is deformed into the Zagros (Iran) and Oman Mountains, which achieved their current stature only during the Neogene. The forerunners to these mountain ranges comprise platform-margin rocks over which the Permian to mid Cretaceous sedimentary fill of a narrow oceanic basin (Neo-Tethys 1), together with Late Cretaceous back-arc ophiolites, were obducted between the Campanian and Early Maastrichtian.

Neo-Tethys 1 came into being when the Anatolia-Central Iran-Afghanistan microcontinent calved from the northeastern margin of Arabia (Gond wana) during the Permian; it had a short history of spreading from about mid Permian until the Mid Triassic, when the spreading axis jumped eastward to separate Central Iran/Lut from the Sanandaj–Sirjan Zone of Iran, thereby creating Neo-Tethys 2.

Neo-Tethys 1 received a basin fill of sediments (e.g., Hawasina Series of Oman) derived from the Arabian platform’s passive margin. The spreading axis of Neo-Tethys 2 was probably active until the end of the Mesozoic, although its only known oceanic crust dates as Tithonian to Berriasian.

Gondwana migrated passively westward until the Aptian, when the effects of the newly opening South Atlantic Ocean caused Afro-Arabia to move towards the spreading axis in Neo-Tethys 2. The resulting crustal compression was relieved when a subduction zone formed within Neo-Tethys 1 north of the Dibba transform fault and in Neo-Tethys 2 south of that fault, leading to obduction of the basin fill and associated back-arc oceanic crust over Arabia’s platform margin. Obduction ceased because the Arabian margin was too thick and buoyant to be consumed down the subduction trench, but because the South Atlantic was still opening and Neo-Tethys 2 was still spreading, another trench had to form in Neo-Tethys 2. Subduction has continued within and to the south of the Makran coast until today, while the Musandam Peninsula is in process of being subducted beneath the Strait of Hormuz.

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372
.

Acknowledgments

Heiko Oterdoom (PD Oman) reviewed the final draft. Reviewers Brian R. Pratt and John D. Smewing improved the readability of the text. Barry Fulton, University of Aberdeen, is thanked for drafting the figures.

Figures & Tables

Contents

GeoRef

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Trench-Forearc Geology: Sedimentation and Tectonics on Modern and Ancient Active Plate Margins
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Geological Society of London
, Special Publication 10, p.
357
372
.

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