sign in sign up
<<
Home , Geology of Iran

SIMON J. HAYNES Department of , Pahlavi University, Shiraz, HENRY MCQUILLAN Oil Service Company of Iran (Private Company), P. O. Box 1065, Tehran, Iran Evolution of the Zagros Suture Zone, Southern Iran

ABSTRACT lations of the sediments and the petrology , and gneisses. In the south- of the ultramafic and metamorphic rocks in eastern part of the high, the gneisses In the light of plate tectonic theory, a the neighborhood of Neyriz has allowed us contain large kyanite crystals. model for the evolution of the Zagros to develop a revised model for the evolution The metamorphic basement is overlain mountain system in southern Iran is of the Zagros orogen based on current by sediments north of the study proposed. A suture zone, which lies between theories of plate tectonics. area in the vicinity of and in the Arabian and Persian plates, comprises We consider that the Zagros mountain scattered localities north and west of Sirjan the Zagros mountain range. The suture zone chain and the region immediately to the (Stocklin, 1968). To the west, the basement is divisible into five segments, each of which north represent a still active suture zone is overlain by - continental reflects the various lithologic and tectonic between the Arabian and Persian plates that sediments and that passes environments associated with stages in the was formed by the interaction of an southward into clastics. These are overlain collision of the Arabian and Persian Atlantic-type margin (the ) by a fairly continuous line of continental masses. and a Pacific-type margin (the Persian plate). limestone along the southern edge of the A trench resulting from the subduction of We have chosen to define this suture as the stable block. the Arabian plate beneath the stable block of "Zagros suture zone." This zone is bounded Crush Zone. This zone was originally the Persian plate persisted until late on the south by the Persian Gulf and on the defined by geologists of the Anglo Iranian mid-Cretaceous time. This was the site of north by a line joining the Hamadan and Oil Company and is shown on the British radiolarian chert deposition and associated Sirjan basement highs parallel to the Petroleum Company maps of the area turbidites and olistoliths. Sharp fades northwest-southeast Zagros structural (1956, 1963). Falcon (1969) includes both changes in Upper Cretaceous sediments trend. The area presently under study is en- our crush zone and the southern portion of immediately southwest of the trench and closed between the line to the east the stable block in his thrust zone. The crush severe tectonism in the crush zone to the and the Qatar-Kazerun line to the west. zone is composed of Cretaceous limestone north of the trench are suggestive: of These lines are surface manifestations of that has undergone severe tectonism. movements which involved the rupture and ancient structural features that have Northward, the extreme crushing of these upward thrusting of slices of oceanic crust modified or controlled structural deforma- rocks dies out and they lie, relatively now manifest as a belt of ultramafic rocks in tion and sedimentation patterns through undisturbed, over the base- the trench zone. Subsequent northward much of the geologic column (Falcon, ment. Immediately northeast of Neyriz, they movement of the Arabian plate during 1967, 1969; McQuillan, 1973). East of the can be observed overlying the rocks of the time resulted in thrust faulting and Oman line, the tectonic picture is compli- trench zone along a marked low-angle overfolding in the imbricated belt adjoining cated by the interaction of three plates thrust. the trench zone and more gentle folding in (Nowroozi, 1972) and the intersection of Trench Zone. Previously, this zone was the simply folded belt to the southwest. Key several belts. The suture zone not classified as a separate entity. Through- words: tectonophysics, plate tectonics, Iran, undoubtedly passes west of the Qatar- out most of this zone, an assemblage of deep- , evolution, suture zone. Kazerun line, but there is a change in both water sediments, usually referred to as the tectonic style and lithology of the strati- colored series, occurs. Directly underlying INTRODUCTION graphic succession that suggests an envi- the crush zone thrust, the colored series In a recent review of the geology of Iran, ronment of greater compression. consists of red and green marls, grits, and Takin (1972) proposed a model for conglomerates. Southward, these rocks pass post-Paleozoic continental drift in the Zagros Suture Zone into a series of radiolarian chert enclosing Middle East. However, his geologic model The Zagros suture zone can be divided blocks of limestone and metamorphic for the interaction of the Arabian plate with into five parallel belts of varying widths (Fig. rocks. The exact ages of all the limestone the Persian plate is based primarily on 1). blocks are not known, but the metamorphic recently resurrected theories of large-scale Stable Block. This area has previously rocks bear a striking resemblance to the nappe movement (Ricou, 1968b, 1970) been called the -Sirjan Ranges Precambrian of the Sirjan high. along the contact zone between the (Stocklin, 1968), or the Rezaiye- Cutting the colored series is a belt of continents, an idea earlier proposed by Esfandagheh orogenic belt (Takin, 1972). ultramafic rocks that can be traced over a Gray (1950). In particular, Takin's model Since this region is now generally regarded distance" of about 100 km northwest of does not take into account the alternate as the southern margin of the Persian Neyriz. They form lenticular pods of hypothesis (advocated by several of Gray's continent, we feel that geographic names harzburgites, pyroxenites, and dunites that contemporaries in the then Anglo Iranian serve no useful purpose. are locally serpentinized to varying degrees. Oil Company) that the rocks of this zone are In the Neyriz region, the stable block In general, these pods are aligned parallel to essentially an in situ assemblage modified by comprises a metamorphic basement com- the strike of the Zagros orogen. Associated small thrust faults. This alternate idea has plex (the Sirjan basement high) thought to be with these ultramafic bodies are small been maintained up to the present by a of Precambrian age (Stocklin, 1968). The deposits of chromite. Although Wells (1969) number of geologists (Falcon, 1967, 1969; rocks represent an assemblage of regionally has placed emphasis on the restriction of the Stocklin, 1968; Wells, 1969). metamorphosed igneous and sedimentary chromite to the north of this belt, we have Although we agree in principle with the rocks, ranging from low-grade phyllites and observed numerous chromite showings in hypothesis of an in situ assemblage, a con- marbles through talc-tremolite—bearing the neighborhood of Neyriz, and we tinuing geologic investigation of the field re- carbonates, mica schists, and epidote consider chromite to be an integral part of

Geological Society of America Bulletin, v. 85, p. 739-744, 2 figs., May 1974

739

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/739/3418311/i0016-7606-85-5-739.pdf by guest on 25 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/739/3418311/i0016-7606-85-5-739.pdf by guest on 25 September 2021 EVOLUTION OF ZAGROS SUTURE ZONE, SOUTHERN IRAN 741

the petrology of the whole ultramafic belt. In thrust from the north along a shallow fault Imbricated Zone. The imbricated zone, general, the ultramafic bodies are separated plane. Wells (1969), however, considers that as described by Falcon (1969), embraces from each other by septa of colored series the pre-Cretaceous ages obtained by Ricou both our imbricated and trench zones. The rocks. Although is only subordinate, are due to reworking of fossils, and that the imbricated zone is developed along the we consider that these ultramafic rocks colored series is a normal Cretaceous leading edge of a thick sequence of represent an ophiolite sequence. succession. Much of his evidence is based on sediments deposited from to About 30 km northwest of Neyriz, a large unpublished work by geologists of the time (James and Wynd, 1965). body of diopside-bearing marble (several Iranian Oil Operating Companies who have Minor disconformities are present through- kilometers wide) is surrounded by apparently assigned a out the succession, and facies changes ultramafic rocks. This marble has been (Senonian) age to the colored series. reflecting epeirogenic movements are com- interpreted by Ricou (1971) as the result of However, due to the problem of accurately mon, but the area was not affected by major the contact by peridotitic dating Radiolaria, we think that the lower orogenic processes until the Pliocene. Then, magma. However, this same stratigraphie age may range into the Jurassic the sediments were folded and thrust can be seen cutting blocks of limestone and possibly even Triassic. The large parallel to the present northwest-southeast contained within the colored series. Al- number of olistoliths present in the cherts Zagros trend. though the limestone contains chert suggests that olistostromes may also have Structurally, the imbricated zone consists nodules, no calc-silicate minerals have slumped into the deep-water trench; thus the of a series of tight folds and southwest- formed, and the only evidence of succession observed by Ricou could well be directed overthrusts, the displacement metamorphism is minor recrystallization of a series of such displaced blocks. This amounts of which decrease toward the calcite and brecciation within 4 m of the succession could also be explained by simply folded belt. The southwest limit of contact. Since the diopside-bearing marble movement along a number of thrust slices the zone in the area concerned is marked by is petrologically consistent over several upthrown on their northern sides. Even the recumbent isoclinal fold of Kuh-e Bamu kilometers and is quite different from the Ricou admits to the existence of several just north of Shiraz. It is interesting to note limestone blocks, we believe the marble is a reverse faults in this area and cites these as that northwest along strike across the large slumped block or olistolith of I're- imbrications at the leading edge of his Qatar-Kazerun line, this fold passes into the cambrian basement that slid into the deep nappe. We have found, however, that the large displacement fault scarp coincident trench before the introduction of the intensity of the thrusting and folding with the culmination of the high Zagros. ultramafic bodies. increases toward the belt of ultramafics, Simply Folded Belt. Rocks within this The colored series adjacent to the which is what would be expected if the belt exhibit lithologies similar to those of ultramafic bodies is intensely deformed and ultramafics were uplifted along high-angled the imbricated zone, except that rapid has previously been termed "colored thrust planes. Unfortunately, the amount of facies changes occur in Late Cretaceous and mélange" by oil company geologists. At this uplift is impossible to determine due to early Tertiary times. These are manifest in Neyriz, southwest of the ultramafic belt, we the difficulty of tracing individual beds shallow-water grading south- have recognized blocks of widely varying within the radiolarian chert rocks. We have ward into deeper water pelagic marls. The ages. These include Fahliyan (Neocomian) been unable to discover any plane of simply folded belt was originally defined by and Wynd, 1965), black probable Cam- décollement in the area, and we therefore Falcon (1969) and is characterized by large brian limestone, and metamorphic rocks ir- believe that there is no evidence for nappes open folds separated by narrow, often regularly jumbled together with cherts over but rather abundant evidence for large-scale squeezed synclines. The mode of folding an area of 4 sq km. The absence of any vertical movements. has been modified by structural anomalies marked shearing associated with the blocks Recent work in the Oman Mountains that have been attributed both to renewed at this locality precludes their tectonic em- (Glennie and others, 1973) covers an area in movements along basement-controlled placement. We therefore interpret these which a very similar rock sequence and lineaments and the plastic mobility of sub- blocks and those immediately adjacent to structural pattern is evident. They interpret surface salt that commonly forms pierce- the ultramafic bodies as olistoliths. Also, we the emplacement of older sedimentary rock ment domes (McQuillan, 1973). consider that the ultramafic rocks were em- sequences (Hawasina), together with as- placed as a series of cold intrusions in the sociated (Semail), as nappe sheets Plate Tectonic Model form of thrust slices along serpentine- during Late Cretaceous time, a mechanism Our model for the development of the su- lubricated, high-angled thrust planes. similar to that proposed by Gray (1950) and ture zone is shown in Figure 2. Recent Further evidence for this includes a zone of Ricou (1968b, 1970) for the similar geophysical work by Nowroozi (1972) in- intense serpentinization, which displays a sequence in Iran. Though only a cursory dicates the existence of a plate about 60 km marked fracture cleavage parallel to :he examination of part of the Oman sequence thick (probably composed of lithosphere), contact and which is peripheral to the has been made by one of us (H. M.), we feel which dips roughly 20° from the Persian ultramafic pods; a zone of brecciation that a more detailed study of gross structural Gulf toward the stable Persian block. At within the marble at the -contact; and com- field relations there may make an alternative present, it is unclear whether continental or plex crushing and folding of the colored interpretation of an in situ assemblage along oceanic crust comprises the basement un- series at the contact. the lines outlined in this paper feasible. derlying southern Iran. Our observations of Toward the southern margin of the trench At Neyriz, the ultramafic bodies and the blocks of basalt and in the emergent zone, turbidites are prevalent and a sub- colored series are unconformably overlain salt plugs of southern Iran suggest a proba- marine canyon can be observed just east of by the Maestrichtian rudist limestone of the ble basement of oceanic crust. However, we Arsinjan as a U-shaped channel filled with Tarbur formation. It is interesting to note realize that further geophysical work is carbonate breccia cutting obliquely across that one of us (H. M.) has observed a similar necessary to resolve this problem. Also, the the strike of the turbidite. Farther to the relation between the Hawasina chert and age of the basement is unknown, but it is southeast, Ricou (1968a) recognized Trias- Maestrichtian limestone at Wadi Jizi in presumably older than the overlying sic and Jurassic limestone interbedded with Oman. The Tarbur is conformably overlain and Paleozoic sediments. We are the chert overlying the mid-Cretaceous by red beds of the Sachun formation not concerned in this paper with the origin Sarvak formation. He postulated that a large (formerly red beds) followed by of this oceanic crust, but it was probably nappe of colored series and ultramafics was limestones of the Jahrum formation. formed at an ancient oceanic ridge system.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/739/3418311/i0016-7606-85-5-739.pdf by guest on 25 September 2021 Figure 2. Schematic model for development of the Zagros suture zone. Relative movements between plates assume a fixed Persian plate.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/739/3418311/i0016-7606-85-5-739.pdf by guest on 25 September 2021 EVOLUTION OF ZAGROS SUTURE ZONE, SOUTHERN IRAN 743

Up until late mid-Cretaceous time, subduc- ably by the Maestrichtian Tarbur forma- Dalneshin (Gray, 1950), which occur on the tion of the Arabian lithospheric plate be- tion at Neyriz. Elsewhere, the Tarbur lime- southern margin of the ultramafic belt, are neath the Persian plate had resulted in the stone appears to grade laterally into the the result of plastic salt movement at depth. formation of a well-developed trench chert rocks (James and Wynd, 1965; Wells, After the main Pliocene folding, conglom- situated above the zone of subduction. On 1969). This suggests that uplift and erosion erates of the Bakhtyari formation were the northern side of the trench, Jurassic may have started and finished within the deposited in the synclinal depressions. More clastic rocks had infilled basins on the nar- Maestrichtian. Thus, at the time of colli- recent movements have tilted and folded row continental shelf. This detritus had sion, the Tarbur formation marked the line these deposits, and the present seismic built up a platform where the Cretaceous of a ridge separating an elongated basin activity in southern Iran indicates that limestone of the crush zone and the stable from the trench. compression is still taking place (Nowroozi, block were deposited. During subduction, After uplift and erosion of the trench 1972; McQuillan, 1973). the Persian continent was subjected to ten- succession, the Tarbur sedimentation trans- sion due to the frictional drag of the Ara- gressed northeastward across the trench Implications of Our Model bian lithospheric plate moving under the zone. Subsequently, this region was uplifted Our model for this type of continent- Persian plate. At the same time, blocks of and the Sachun formation evaporites and continent collision may give a clue to the metamorphic basement and overlying sed- red beds were deposited. Sedimentation driving mechanism of plate motions. If iments slumped into the trench. Along the continued in the Eocene and with lithospheric plates are pulled down into southern flank of the trench, turbidity cur- the deposition of shallow marine limestones subduction zones by density changes at rents laid down clastic deposits, but of the Jahrum and Asmari formations. depth (or by other factors), it is difficult to sedimentation within the trench itself ap- Marls continued to be dominant in the explain why the ultramafic belt is now pears to have been restricted to the biogenic southwest basin during this time, but exposed at the surface. Although it can be deposition of radiolarian chert. A modern gradually it became infilled, and marine argued that the slices of ultramafics were example of this model is the present-day sedimentation gave way to the deposition of uplifted by underthrusting of oceanic Peru-Chile trench (Scholl and others, the evaporites of the Gachsaran formation lithosphere after it split up, a pulling action 1970). Although no Mesozoic volcanic and later the continental clastics of the should require that both parts of the oceanic rocks have been reported from the southern Agha Jari formation (James and Wynd, plate be subducted equally efficiently. Persian continent, Stocklin (1968) mentions 1965). The occurrence of reworked On collision of the Persian and Arabian the occurrence of Mesozoic and radiolarian chert pebble beds in the Sachun plates, the miogeoclinal sediments overlying diorite. Thus, evidence for subduction formation to the north of Shiraz and a chert the Arabian plate would no longer be could be considered weak. However, an ex- pebble conglomerate (70-m thick) in the passively rafted but would exert a frictional trusive belt of Eocene to volcanic Razak formation to the south of Shiraz in- pull, especially at the point of contact of the and intrusive rocks lies to the north of the dicate that the trench zone was being ac- continents. The sediments would tend to suture zone (Fig. 1), and it is possible that tively eroded in the and . buckle upward at this point, and the added these overlie an older belt of igneous rocks The formation of the previously men- mass would exert a downward force on the or that calc-alkaline magma takes a long tioned ridge (the site of Tarbur rudist lime- oceanic plate. For subduction to continue, time to rise from the zone of melting to the stone deposition), separating the trench the oceanic plate would need to overcome surface. A comprehensive program of from a marine basin, may have been due to both the horizontal friction and the geologic mapping, chemical analyses, and the upthrusting of slices of oceanic plate downward push. If a pulling mechanism of isotopic dating of these igneous rocks will coincident with the collision of the Persian subduction was involved, the oceanic plate help in the solution of this problem. and Arabian plates (Fig. 2). The marine would just sink deeper into the astheno- On the Arabian side of the trench, a thick basin persisted well into Tertiary time, be- sphere below the point of collision. The sequence of Paleozoic to mid-Cretaceous cause the oceanic plate then slid beneath the overlying sediments would be dragged with sediments had been built out as a uplifted slices and tended to drag the over- it, and no part of the plate would rise to miogeoclinal wedge from the edge of the lying miogeoclinal wedge with it. form surface ultramafic masses. continent to cover the underlying oceanic During further northward movement of If a pushing mechanism was operative, crust. This wedge was passively rafted the Arabian continent during Pliocene time, the oceanic plate would have to overcome northward with the Arabian continent. the thick wedge of miogeoclinal sediments the above restraints, plus the resistance of During the mid-Cretaceous, the miogeo- was squeezed between the two continents. the mass of oceanic plate already sub- clinal sediments overlying the Arabian As the edge of the Persian continent offered a ducted. The weakest point in the plate is the lithospheric plate collided with the trench resistant barrier, the sediments at the leading local zone of tension at the top of the plate sediments. Turbidity currents flowing from edge of the Arabian plate were subjected to where the curvature is greatest. Vertically the Arabian side carried carbonate detritus the greatest compression. This resulted in above this zone, the overlying sediments into the trench, while blocks and possibly extensive thrusting, such as at Khaneh Kat, would be most affected by buckling and olistostromes of the miogeoclinal wedge and overfolding north of Shiraz, in the exert their greatest downward lithostatic slumped into the deeper water chert rocks. imbricated zone. Farther southwest, the force. Thus, it is likely that the plate would At the same time, we believe the underlying compression was less, and the rocks of the split there into a number of slices. Once oceanic plate was ruptured and slices of simply folded belt were more gently folded. rupture had occurred, that part of the plate oceanic crust and mantle were thrust up- At the same time, the Cambrian salt of the already subducted would become uncou- ward to form the belt of ultramafic rocks. Hormuz formation was squeezed plastically pled from the downward driving force. The The whole of the trench succession was into zones of weakness and formed the lithospheric plate still attached to the Ara- then rapidly uplifted and eroded to expose classic outcropping salt plugs of Fars bian continent would be pushed below the the thrust slices of oceanic plate at the belt province (Falcon, 1969). Although minor uncoupled plate and would aid its upward of ultramafic rocks. This uplift and erosion salt plugs occur within the belt of motion. Unless density changes occur in a must have been accomplished in a remark- ultramafics, the salt appears to have been subducting lithospheric plate, it would be ably short time, since the radiolarian chert effectively dammed to northward move- less dense than the surrounding mantle. rocks are underlain by the Cenomanian ment by this belt, and we consider that Thus, the uncoupled plate should rise by Sarvak formation and overlain unconform- anomalous structural trends, such as Kuh-e isostatic response. Although we are uncer-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/739/3418311/i0016-7606-85-5-739.pdf by guest on 25 September 2021 744 HAYNES AND McQUILLAN

tain of the relative amount of uplift pro- M. W., Moody-Stuart, M., Pilaar, W.F.H., lite nappes in the Iranian Zagros moun- duced individually by isostatic release and and Reinhardt, B. M., 1973, Late Creta- tains: Geol. Mag., v. 107, no. 5, p. underthrusting, we consider that the time ceous nappes in Oman mountains and their 479-480. geologic evolution: Am. Assoc. Petroleum interval was too short for underthrusting to 1971, Le croissant ophiolitique Peri-Arabe. Geologists Bull., v.57, no.l, p. 5-27. une ceintive de nappes mises en place au have accomplished uplift alone, especially Gray, K. W., 1950, \ tectonic window in Crétacé supérieur: Rev. Géographie Phys. et as the rate of plate subduction was proba- south-west Iran: Geol. Soc. London Quart. Géologie Dynam. (2), v. 13, no. 4, p. bly slow after collision due :o increased Jour., v. 105, p. 189-222. 327-350. frictional drag, consequent on the compres- James, G. A., and Wynd, J. G., 1965, Strati- Scholl, D. W., Christensen, M. N., von Huene, sion of the miogeoclinal sediments between graphic nomenclature of Iranian oil consor- R., and Marlow, M. S., 1970, Peru-Chile the two plates. tium agreement area: Am. Assoc. Petroleum trench sediments and sea-floor spreading: Geologists Bull., v. 49, no. 12, p. Geol. Soc. America Bull., v. 81, p. 2182-2245. 1339-1360. REFERENCES CITED McQuillan, H., 1973, A geological note on the Stocklin, J., 1968, Structural history and tec- British Petroleum Co., Ltd., 1956, Geological Qir earthquake SW Iran, April 1972: Geol. tonics of Iran: A review: Assoc. Pet- maps and sections of southwest Persia: In- Mag., v. 110, no. 3, p. 243-248. roleum Geologists Bull., v. 52, no. 7, p. ternat. Geol. Cong., 20th, Mexico 1956 Nowroozi, A. A., 1972, Focal mechanism of 1229-1258. (Printed by Edward Stanford, Ltd., Lon- earthquakes in Persia, , west Pakis- Takin, M., 1972, Iranian geology and continen- don), scale 1:1,000,000. tan, and and plate tectonics of tal drift in the Middle East: Nature, v. 235, Survey Branch, 1963, Geological maps of the Middle East: Seismol. Soc. America p. 147-150. southwest Iran: London, British Petroleum Bull., v. 62, no. 3, p. 823-850. Wells, A. J., 1969, The crush zone of the Iranian Co., Ltd., scale 1:250,000. Ricou, L-E., 1968a, Une coupe à travers les séries , and its implications: Falcon, N. L., 1967, The geology of the north- à des monts Pichakun (Zagros, Geol. Mag., v. 106, no. 5, p. 385-394. east margin of the Arabian basement shield: Iran): Bull. Soc. Géol. de France, v. 7, no. Adv. Sci., v. 24, p. 31-42. 10, p. 478—485. 1969, Problems of the relationship between 1968b, Sur la mise en place au Crétacé surface structure and deep displacements il- Supérieur d'importantes nappes à radiola- lustrated by the Zagros range, in Time and rites et ophiolites dars les Monts Zagros MANUSCRIPT RECEIVED BY THE SOCIETY JUNE place in orogeny: Geol. Soc. London, Spec. (Iran): Acad. Sci. Comptes Rendus, v. 267, 27,1973 Pub. no. 3, p. 9-22. p. 2272-2275. REVISED MANUSCRIPT RECEIVED NOVEMBER 2, Glennie, K. W., Boeuf, M.G.A., Hughes Clarke, 1970, Comments on radiolarites and ophio- 1973

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/5/739/3418311/i0016-7606-85-5-739.pdf by guest on 25 September 2021