doi:10.1144/GSL.SP.1984.013.01.32 1984; v. 13; p. 393-403 Geological Society, London, Special Publications

 R. Hall  

Ophiolites: Figments of Oceanic Lithosphere? 

Geological Society, London, Special Publications

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London © 1984 Geological Society of

Ophiolites: Figments of Oceanic Lithosphere?

R. Hall

SUMMARY: To resolve the problem of what a particular ophiolite represents it is necessary to look both at the ophiolite and at the rocks surrounding it: those immediately adjacent to the ophiolite and those of the region. Detailed geochemical and petrological studies are unlikely, on their own, to contribute to an understanding of the tectonic setting of ophiolite formation, nor of emplacement. Evidence from ophiolite complexes of the Mesozoic Arabian margin is inconsistent with the view that they originated at the spreading centre of a major ocean, or in back-arc basins. Instead, it is argued that these ophiolites represent discontinuous 'oceanic' rifts formed within the Arabian passive margin. Metamorphism of rocks beneath the ophiolite is considered to mark extension of the margin associated with the formation of these oceanic rifts in the late Cretaceous. These ophiolites do not mark a major suture and the true suture lies to the north of the ophiolite zone.

'Rules' for the Arabian m a r g i n

A discont inuous zone of ophiolites, referred to by Ricou (1971a) as the 'croissant ophioli t ique ' extends through southern Turkey to Oman (Fig. 1). This is the southernmost zone of ophiolites in the Middle-Eastern sector of Tethys. The ophiolites vary in character along the belt f rom ophiolitic olistostromes and m61anges to the almost intact stratiform complex of Oman. Despite their discontinuity and differences in their style of occurrence and deformat ion these ophiolites share a number of features which can be considered as 'rules ' in the ophioli te-emplace- ment 'game' . All were emplaced on the Arabian passive continental margin (rule 1), where evi- dence exists it appears that they were emplaced soon after their format ion (rule 2), and all were emplaced in a rather short period in the late Cretaceous (rule 3). I consider that any model should be able to account for the emplacement of all of the ophiolites, simply because ophiolite emplacement is a unique and unusual event in the history of the Arabian margin, and occurs in such a limited interval of time. Fur the rmore , emplace- ment models must also take into account the subsequent history of the region: cont inent-cont i - nent collision did not occur in southern Turkey and I ran until the Miocene, and in Oman has not yet occurred (rule 4). These rules immediately exclude a whole range of models which have been

This paper was selected for conference presentation as it is controversial and would stimulate discussion; this indeed proved to be the case. However, in accepting the paper for publication we do so against the advice of the referees who, whilst accepting that some of the authors' criticisms of the concensus ophiolite model are valid, considered the author's own model to be unrealistic. Dr Hall rejected their recommendation that the alternative model be excluded, thus in order to present ~he critically acceptable part of the paper, we are obliged, albeit reluctantly, to publish the paper unmodified.

proposed for emplacement of other ophiolites, e.g. those involving active cont inental margins, and those related to con t inen t -con t inen t collision.

The Neyriz complex of southern Iran is one of the ophiolites of the 'croissant ' and several authors have drawn at tent ion to its similarities to Oman (e.g. Ricou 1971a; Hal lam 1976; Coleman 1981). However , Neyriz has some unusual features: no Triassic or Jurassic volcanic rocks have so far been found, while sedimentary rocks associated with the volcanics are of Senonian age, and include shallow marine limestones. Most of the volcanic rocks are tholeiitic, a l though some alkali basalts similar to those of the Haybi complex of Oman have been reported (Arvin 1982). Vesicularity of these rocks suggests relatively shallow eruptive depths, Rhyolitic ignimbrites occur locally at the top of the volcanic sequence (Arvin 1982). Such rocks are rare, or even entirely absent, in island arcs built on oceanic crust, a l though they are commonly associated with volcanic mounta in belts on continental crust (e.g. the Andes) or cont inental rifts such as the East African rift (Williams & McBirney 1979). Unusual skarns are found at the contact between the ophioli te peridoti te and folded marbles and indicate high-temperature- contact me tamorph i sm (Ricou 1971b; Hall 1980). A general ophiolite model for the Arabian margin should also be able to incorporate these anomalies.

That the Arabian continental margin was a passive margin is undisputed; it has a relatively cont inuous Mesozoic record of marine sedimen- tat ion without active margin-type volcanism. Many of the Te thyan passive margins are considered to have been formed by rifting in the Triassic and the Arabian margin conforms to this pattern; until the Triassic, central Iran and nor thern Arabia have a similar history but after the Triassic their s trat igraphy is quite different. The Southern Tethys ocean formed between Arabia and central I ran-Lut should therefore have been floored by oceanic crust of Triassic age in the region adjacent to the Arabian passive

393

394 R. H a l l

FIG. 1. Late Cretaceous ophiolite complexes of the Arabian Margin.

margin. Rifting is considered to be marked by late Triassic, largely alkaline, basic volcanism. However, although this evidence is consistent with what is known and expected during the early stages of continental rifting we need not assume that the occurrence of Triassic volcanics nece- ssarily marks the exact margin between conti- nental and new oceanic crust. Scandone (1975) observed that the Triassic rift zones in the western part of Tethys were not the zones of later ocean- crust formation, and the present Atlantic and Red Sea margins are marked by discontinuous zones of basic igneous rocks up to about 200 km from the present continent-ocean crust boundary which were formed at the same time as ocean crust in the main ocean. These zones are often referred to as 'failed rifts', an unfortunate term since it implies that rifting has ceased and is unlikely to be renewed. Comparison with the present margins of the Red Sea and Atlantic (Hall 1982) suggests that the Mesozoic Arabian passive margin was likely to have been up to several hundred kilometres wide, underlain by thin extended continental crust, and included large volumes of basic igneous rocks of Triassic age in "failed rifts'.

Emplacement models Those of us that have worked on ophiolites along this 'croissant' have tended to look in particular to the Oman ophiolite for a model of ophiolite emplacement. It is probably the best-studied ophiolite complex in the belt, and it is certainly the most complete and least deformed. Continent- continent collision has not yet occurred in Oman, and thus the Oman ophiolite should be able to provide some insights into the pre-Miocene

history of the other ophiolites. Three types of models have been proposed that attempt to obey the rules specified above. The first of these, that of Coleman (198i), is a modification of the original proposal of Glennie et al. (1973), and is the simplest in suggesting detachment of the ophiolite from the northern side of a newly inactive spreading ridge in the Southern Tethys between Oman and Iran (Fig. 2). The ophiolite must be derived from close to a recently spreading centre to satisfy rule 2, and therefore must have crossed the older parts of the ocean during its emplace- ment to arrive on the continental margin. Glennie et al. (1973) suggest that the original width of the sub-ophiolite nappes was at least 400 kin, although they do not estimate the width of oceanic crust in the ocean itself, while Coleman (1981) provides no figure beyond the minimum of 165 km indicated by the exposure width of the ophiolite. However, unless the pattern of spread- ing was unusual and highly asymmetrical the minimum half-width of the ocean can be estimated to satisfy rule 4 from the width of the ophiolite and the amount of oceanic lithosphere currently beneath the Makran active margin. In addition to the 165 km of ophiolite on land, about 500 km of oceanic crust is required beneath the Makran accretionary wedge (Farhoudi & Karig 1977) to the active volcanic arc of Djaz Murian (Fig. 2c). This is an absolute minimum since arc volcanism began at least as early as late Cretaceous-Paleocene (Berberian et al. 1982). Thus a half-width of at least 665 km is indicated. Since the ophiolite is now underlain by 165 km of continental crust it must have travelled at least 830 km to its present position as a slab about 12 km in thickness without significant disruption. Although this might be mechanically feasible

Ophiolites : Figments of oceanic lithosphere ? 395

FIG. 2. A and B show detachment and emplacement of the Oman ophiolite according to Coleman (1981). C shows the present configuration of the Oman-Makran region along 59~ based on Manghnani & Coleman (1981) and Farhoudi & Karig (1977).

under the most favourable conditions (see Hubbert & Rubey 1959, but cf. Hs~i 1969), it seems unlikely. The dimensions of the Oman ophiolite are now about 450 • 90 • km (Searle & Malpas 1980) and one of the conditions would have to be the virtual absence of friction at the base of the sheet which would preclude significant frictional heating. Furthermore, the problem of the missing southern half of the ocean remains: a minimum of 665km of oceanic lithosphere has disappeared without trace. I therefore conclude that this model is implausible.

Gealey (1977) neatly ~voids some of these problems by proposing zollision between the Arabian passive margin and an intra-oceanic island arc. He suggests that the Oman ophiolite represents the forearc limb of the island arc. This model can satisfy all the rules, but only if rather special circumstances are invoked. To satisfy rules 2 and 3 subduction must be initiated at, or very near to, a spreading centre active immediately before the initiation of subduction, and the subduction zone must have been parallel to the Arabian margin for a distance of about 3000 km

396 R . H a l l

TRENCH-BACK ARC AXIS,-,,520 km TRENCH- ARC ".' 200 km

PASSIVE MARGIN TRENCH FORE-ARC ACTIVE ARC BACK-ARC BASIN AXIS I I I

Thick sed[menl cover: volcaniclostics, vitric muds, ash,petogics and turbidites. Abundant reworked sediment

Variable but often thick sediment cover Boninites, arc tholeiites dominantly volconiclaslics, v i t r i r muds,

NO accretionory wedqe and calc- ( l lkal ine lavos: volcanic ash and pelagic oozes $ubduclion erosion ? older than back-arc crust SEA LEVEL, 0

O . . . .

~ r I ~ I I ~ T o c E A N I C CRUS

BASE 0~ L'rHOSPHE.E . ~'~ '%''k t O 0 ~ " ' ~ - " "~" -... ~ ~ 1 0 0

km "-- km

3 0 0 2 0 0 I 0 0 0 I 0 0 2 0 0 3 0 0 4 0 0 , k m

FIG. 3. Scaled cross-section across an island-arc-passive continental-margin convergence zone shortly before collision. This was the situation of the Oman ophiolite in the late Cretaceous according to the models of Gealey (1977) and Pearce et al. (1981). The arc is based on information from the Marianas from Sager (1980) and Hussong et at. ( 1981), and the passive margin is based on the Atlantic margin from information in Sheridan et al. (1979). The area of the back-arc basin enclosed by the dotted line corresponds approximatefy to the dimensions of the Semail ophiolite.

so that collision between the continental margin and the arc occurred synchronously along the belt. As Gealey himself observes (1977, p. 1190), the evidence for the island arc is weak. In the Oman region it must be located in the present Gulf of Oman, whereas elsewhere it has presum- ably been lost during Miocene continent-conti* nent collision, Gealey's model has been modified by Pearce et al, (1981 ) who derive the ophiolite on geochemical grounds from a back-arc basin rather than the forearc limb. Comparison with modern arcs suggests that the whole of the forearc region and the island arc itself have been lost during collision, although the back-arc basin has survived remarkably well. Fig. 3 shows the scale of the loss; one feels that Pearce et al. are in a somewhat similar position to Mr Worthing-- losing the forearc may be regarded as misfortune, but to lose both the forearc and the arc . . . . The sedimentary cover of the ophiolite should indicate whether either of these island-arc models is plausible or not. The sediments drilled in the Marianas arc (Hussong et al, 1981) in both forearc and back-arc regions, even quite close to the active spreading centre, include a considerable volcanic component (Fig. 3). No such sediments have yet been found in the Oman region, nor associated with other ophiolites of the belt. On the contrary, the sediments preserved are entirely pelagic and lack any volcanogenic material (e.g. Tippit et al. 1981).

Stoneley (1974, 1975) has suggested that the Southern Tethyan ophiolites originated within the Arabian passive margin during the late

Cretaceous. This suggestion avoids some of the problems of the previous two models and the anomalous features of Neyriz can be incorporated in a general model (Hall 1982) which is sum- marized in Fig. 4. The form of the Arabian margin in the Triassic is based on cross-sections of the Red Sea margins constructed from geological and geophysical data by Lowell & Genik (1972). Continental lithosphere becomes thinner in an irregular way as the continent-ocean crust boundary is approached and crustal exten- sion is assumed to occur by listric normal faulting. Parallel to the Southern Tethys is a 'failed rift' (cf. the Danakil rift) which is underlain by thinned lithosphere. This 'failed rift' is reactivated in the Cenomanian to form a narrow discontinuous ocean which may have been up to 2-300 km wide in the Oman region, but rather less to the north. I suggest that in about the late Santonian there was a change to a convergent regime and an attempt to begin subduction at the passive margins of the Southern Tethys. McKenzie (1977) has argued that the creation of new subduction zones is not an easy process and considerable work is required to produce a sinking slab about 180 km long before subduction becomes self-sustaining. I suggest that during the change from an extensional to a convergent regime in the Southern Tethys less work was required to compress the over-extended hot and weak Arabian passive margin than to initiate subduction. It was therefore not until after compression of the continental margins that subduction began in the Southern Tethys, at its

Ophiolites: Figments of oceanic lithosphere ? 397

A TRIASSIC

Arabian Cont inenta l Marg in Southern Te thys

km con t i nen la l c rus t o c e a n i c c r u s t 0 " - '

l - s o ~ i - ~ ~ ~ ~ ~ ~ - ~ - ~ 6 ~ a

1100 ~ u~45E OF L ~

km ,0 IO0, 200~ . . 3 0 0

B C E N O M A N i A N in t rus ive f ea tu res and

km con tac t me tamorph ism Sou the rn Te thys

~ f t \\XI~~,~#// /1 \ \ \ /f__r,-,--~- X / / _ _ . . . . ~ 1 - w "v, --,.. .._,--_--

MO 0 / / / " ~.. - ~ MOHO ~." A S T H E N O S P H E R E RISE ~4.~'t-'_. ~ __-

�9 5 0 . , / O#" " " ..~ . _ . - - ~ - ' - ' - ~

j / L'7sosp~

L I 0 0 0 I00 200 300 400 500

km i I i ~L I '

C CAMPANIAN Oph io l i t e Southern Te thys

k~ /~%~a~o,phic r o c k ~ : : X _ . " \

5 0 MOHO 1 i - --" ~"

L~ T~OSPHER~" ~ . / i

I00 ~ --- . . . . . . . _ . . . - .-.--"

.......

0 I 0 0 2 0 0 3 0 0 4 0 0 km L ~ , L ,

FIG. 4. Suggested Mesozoic evolution of the Arabian margin. No sedimentary cover is shown and the sections are drawn with no vertical exaggeration. Lithosphere thicknesses are speculative but note in particular the thick lithosphere under the Southern Tethys, consistent with its age, and very thin lithosphere beneath the 'tailed rift' and ophiolite. Extension is assumed to occur by listric normal faults reactivated as [istric thrust faults in the Campanian, (The diagram is discussed in the text and more fully in Hall (1982).)

nor thern margin. Subduct ion-rela ted volcanism at the nor thern margin should therefore have begun later than ophiolite emplacement , the time lag depending on subduct ion rates and dip of the Benioff zone. In fact, in central Iran and north of the Djaz Murian depression volcanic activity is recorded f rom the late Cre taceous-Pa leocene (Berberian et al. 1982).

Testing the models Ophiolites and geochemicaI evidence

Most of the evidence f rom ophiolites themselves does not tell us very much about their environ- ment of format ion or means of emplacement . Since so many ophiolites seem to have been empiaced soon after their format ion and are

398 R. H a l l

considered to have been hot during their emplacement it appears that they were not formed in major ocean basins like the Atlantic or Pacific, and most authors appeal to narrow Red Sea-type basins or marginal basins to explain their existence. Their structure, metamorphism and sedimentary cover seem only to require marine conditions with extrusion and intrusion of basic magma in relatively narrow zones, and such conditions might be found in a variety of tectonic settings. However, geochemical evidence, in particular arguments based on 'immobile' ele- ments, have been proposed to resolve the problem of tectonic setting. Clearly, if geo- chemical methods are to be used to determine tectonic setting they must work in all modern environments and be capable of distinguishing tectonic settings that have no analogues at present. Geochemical differences can presumably be used as diagnostic of tectonic setting insofar as they result from processes that are restricted to those settings. These processes, which include melting behaviour in a heterogeneous mantle under a wide range of P-T conditions, variations in the amount of melting, fractional crystallization and differentiation at different levels of the lithosphere during magma ascent, and possi- bilities of magma mixing and crustal contami- nation, are still incompletely understood. To complicate matters, 'immobile" elements not infrequently become mobile during alteration and metamorphism. Furthermore, each of the three models of ophiolite genesis and emplace- ment appeals at one stage or another to some non-actualistic variant of present-day processes. Can we be sure that currently unknown tectonic environments obey empirically derived discrimi- nant rules? Figure 5 emphasizes the need for caution. Using the same discriminant methods chosen by Pearce et al. (1981) to deduce a back- arc setting for the Oman ophiolite, two of three groups of Tertiary basaltic rocks recognized from Skye, whose tectonic setting is presumably well known, fail to be correctly classified. The Fairy Bridge basalts are clearly classified as ocean-floor tholeiites while the Preshal Mhor basalts are arguably ocean-floor or island-arc tholeiites. I suggest that the Skye basalts have a comparable tectonic setting to many of the ophiolites of the Tethyan belt, i.e. within a passive continental margin.

Metamorphic rocks associated with ophiolites

Until a few years ago, one of the major problems associated with emplacement of 'alpine- type' ultramafics was their lack of high-tempera- ture metamorphic aureoles. Now that many of

these ultramafic bodies are recognized as obducted ophiolites their high-temperature basal metamorphic rocks have also been discovered. There now seems to be a great enthusiasm for identifying metamorphic rocks associated with ophiolites as part of their sub-ophiolite sole despite the observations that many of them are too thick, compositionally unlikely, structurally in the wrong position or the wrong age to be produced by ophiolite obduction (e.g. Crete, south-east Turkey, Neyriz). Even in Oman where an inverted metamorphic sequence is well preserved at the base of the ophiolite their interpretation as due to obduction has a number of problems.

Radiometric dating is showing that many sub- ophiolite metamorphic rocks are considerably older than ophiolite emplacement (e.g. Spray & Roddick 1980; Thuizat et al. 1981). Therefore, they are considered to pre-date the obduction event and instead date the initiation of thrusting within the ocean basin. In Oman, ages of the amphibolites immediately beneath the ophiolite are only a few Ma younger than ages of igneous rocks at the spreading centre. Since the igneous rocks are considered to have moved into an off- axis position, indicating continued spreading after their formation, and metamorphism of ocean crust to amphibolite facies requires thrusting, heating to approximately 750~ and cooling to a temperature at which the K-Ar system closes (7500~ spreading and thrusting must have been almost contemporaneous. Lanphere (1981) considers that overthrusting occurred 3-7 Ma after formation of the ophiolite at the spreading centre, but modifying the assumptions used in his argument (e.g. increasing temperature of hornblende formation, lowering blocking temperature, changing conduction rates), all of which are poorly constrained, could lead to the conclusion that metamorphism is as old, or possibly older, than spreading ages. There is then a gap of about 10 Ma between the oldest and highest-grade rocks and the younger, struc- turally lower rocks in the metamorphic sole (Lanphere 1981).

Unusual alkali peridotites and jacupirangites occur in the Haybi complex immediately beneath the metamorphic sole in Oman, and metamor- phosed jacupirangites occur with amphibolites in the metamorphic sole. These rocks are late Cretaceous in age (92.5 +__ 4 Ma, Searle et al. 1980) and occur as dykes and sills indicating intrusion in an extensional environment. Searle et al. compare them to a jacupirangite-syenite asso- ciation now found in a similar tectonic situation beneath the Bay of Islands ophiolite where Jamieson & Talkington (1980) interpret them as

Ophiolites: Figments o f oceanic lithosphere ? 399

100OO

Ti ppm

1000 t0

.--.-~, . ' U - - " ~ " - - . / w i t h i n - p l a t e "

/ / l avas

(.

//" ~ \i\.\ [ arc tavas I I k I

I

I i i L I i I I

lOO Zr ppm

10000

Ti ppm

1 0 0 0 , , t ~ l i I J J L n I I J

loo Cr ppm looo

1000

C r

ppm

1 0 0

vab

I

rb

�9 I~ ------~

1.0

lwpO, i f

~ j

I / Li I , , , 1 , ~ !

0.1

/ /

/ #,~

c a.b / y / iat . . . . . . . . . . . . . . -i./~..... /

/ / . - ._../ / / , . o

"/ /.. ' /1 / / / / / / "

/ / / , / "

i , , * t I I i d I d I I I I J I t ~ ~ ._ j_ . , I I a !

1 0 1 0 0 0 . 0 1 0 . 1 1 , 0 Y ppm l'a/Yb

FIG. 5. Discriminant plots used by Pearce et al. (1981) to deduce a back-arc setting for the Oman ophiolite applied to Tertiary basalts from Skye: dots are Preshal Mhor basalts, squares are Fairy Bridge basalts. Data from Thompson (1982), Thompson et al. (1972, 1980) and Mattey (1980).

part of an ocean-island sequence although conceding that ' the overwhelming majority of similar alkaline rock suites has been described from continental environments where they are generally associated with rifting'. (Jamieson & Talkington 1980, p. 474). In the Neyriz area the high-temperature skarns at the contact of the ophiolite peridotite are interpreted as melts produced at the contact of hot intrusive peridotite (Hall 1980). These rocks are not easily explained by models postulating formation of these ophio- lites at a mid-ocean ridge or somewhere in an island arc, although they fit well into the rifted continental-margin model.

Lithologies of the Oman sub-ophiolite sequence

are perfectly consistent with an origin by metamorphism of igneous and sedimentary rocks formed at the margin of an ocean basin. On the other hand, the high-temperature metamorphism

t h a t they have suffered seems inconsistent with the subduction-zone origin suggested by Searle & Malpas (1980, 1982). Subduction-zone metamor- phism is normally considered to be typically of a high-pressure-low-temperature type, as suggested by the low heat flow of these regions at the present, and thermal modelling ofsubducted cold slabs. It is odd that no high-pressure metamorphic rocks have been reported from the m61anges beneath the ophiolite if the continent-arc collision model is correct, since the model involves

400 R. Hall

subducting the margin for a distance of at least 165 km to emplace the ophiolite.

I suggest that an alternative to the proposal that many of the metamorphic rocks associated with ophiolites are directly related to obduction might be the following (Fig. 6). Rifting of the Arabian continental margin in the Triassic resulted in formation of failed rifts within the margin containing Triassic igneous rocks. Renewed extension of the continental margin in the mid-Cretaceous resulted in formation of new oceanic lithosphere. Prograde metamorphism of crust of the continent-ocean margin should have culminated at this stage since a thermal peak occurs at the time of break-up, after which the lithosphere cools, and this type of metamorphism is considered to contribute to the post-break-up subsidence of continental margins (e.g. Falvey & Middleton 198I; Royden et al. 1980). Extension on listric faults produced mylonites and folds in ductile shear zones at depth. Mineral growth and recrystallization occurred during shearing due to frictional heating in the shear zones and partly as a consequence of rise of isothermal surfaces during lithosphere thinning. Mineral formation would therefore be syn- or post-tectonic. Subsequently, cessation of movement on faults, and subsidence of isothermal surfaces during lithosphere cooling and thickening, caused these minerals to cool below their blocking tempera- tures. Thus, metamorphic ages would post-date break-up, although by how much would depend on the contribution made by frictional heating and rate of lithosphere cooling. Furthermore, this is the zone in which one would expect to see intrusive relationships and formation of rocks such as the Neyriz skarns and Oman jacupiran- gites and alkali peridotites associated with the early stages of continental rifting. Reactivation of listric normal faults as listric reverse faults during compression of the continental margin and emplacement of the ophiolite formed in this narrow rift would result in imbrication of these mylonites, renewed dynamothermal metamor- phism, and younger metamorphic ages as the rocks moved upwards. The metamorphic rocks would therefore have a polyphase history of deformation and mineral growth recording exten- sion, static cooling and thrusting. The suggestion that the older metamorphic ages are indicative of extension is also more consistent with sedimentary evidence indicating extension of the continental margin at the same period (see below), evidence of dykes cutting the metamorphics, and would explain the observation that spreading in Oman is almost as young as, and elsewhere much younger than, high-temperature metamorphic cooling ages. Interestingly, this model is also consistent

Outer Shelf

Me,sozo c sediments

C o n t i n e n t a l C r u s t

v v T r i ass i r v v v v " fa i led r i f t "

subsidence o f Ou te r She l f

s t . . . . h i n 9 (by l i s t r i c f au l t i ng ?) ~ ~-~vv~]~

zone o f i so the rma l r ise in c rus t

new "ocean i c ' c rus t

. . . . . , d e e p . . . . ,a, Me , . . . . . h i . . . . . -- : ilfiii!iliNii! i!!!!!iiiiiiiN

- ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ me tamorph i c h i s t o r y

o f oph io l i te

TEMPERATURE marg j h

, J f r i c t iona l " bea t l?~g ? ~

f g radua l s t r e t ch i ng

b tock ing re

before break.up & a f t e r b reak .up

BREA K - UP T IME

FtG. 6. Schematic thermal evolution of Tri- assic 'failed rift' after renewed extension leading to break-up in the late Cretaceous. Extensional faults at the margin of the ophiolite should be marked by mylonites at depth (cf. Sibson 1977) and shear heating may contribute to the maximum tem- perature reached. This is also the zone in which intrusive features and contact meta- morphism should be observed. Note that the thermal maximum occurs at the time of break-up but metamorphic ages are younger, depending on blocking tem- peratures.

with the suggestion of Spray (1982) that many ophiolites represent the edges of ocean basins adjacent to continental margins, based on the occurrence of unusual marie segregations in ophiolite-mantle sequences.

Sedimentary evidence

The sedimentary rocks associated with ophio- lites, both above and below, should also.be able to provide some tests for models of ophiolite

Ophiolites: Fifments of oceanic lithosphere ? 401

formation and emplacement. I have pointed out above that the lack of volcanogenic material in the sedimentary cover of the ophiolites in the 'croissant' is inconsistent with an island-arc origin. If these ophiolites did originate within the Arabian continental margin we might expect to see some effects of renewed rifting and extension in the sedimentary sequences over a wide area. There are some tantalizing indications of this. In the Neyriz area, the stratigraphy of the sedi- mentary rocks in nappes beneath the ophiolite indicate a tensional regime throughout most of the late Cretaceous (Hallam 1976; Stoneley 1981). On the outer part of the shelf there was a change to deeper-water sedimentation associated with slumping and normal faulting in the Cenomanian (Stoneley 1981). To the NE, in the basin represented by the Pichakun nappes, there was a change in the direction of derivation of sediment in the Aptian: earlier detritus was derived from the SW, but later from the NE (Ricou 1968). This cannot represent the beginning of compression of the continental margin and formation of a foreland basin in front of the advancing nappes, since sediments associated with the Neyriz pillow basalts indicate that volcanic activity continued into the Senonian (Arvin 1982). I therefore prefer to interpret this as evidence of rifting within the deeper parts of the continental margin, followed by subsidence and 'oceanic' crust formation. Not until the late Santonian or early Campanian were sedimentary nappes emplaced from the east followed soon after by the ophiolite. A similar sequence of events is observed in SE Turkey where a late Turonian unconformity marks a change in sedimentation on the outer shelf to deeper-water conditions terminated by ophiolitic gravity slides in the late Campanian-early Maastrichtian (Rigo de Righi & Cortesini 1964). In Oman, the history of sedimentation in the Hawasina margin becomes less clear in the early- mid Cretaceous. Instability on the shelf edge may be indicated by the deposition of mid-Cretaceous thick debris flows and the later Cretaceous history is thought to be represented by m~langes (Graham 1980). Although previous interpre- tations have been different (e.g. Glennie et al. 1973; Graham 1980) there appears to be nothing to contradict the same interpretation as in the Neyriz area: rifting within a continental margin, perhaps beginning a little earlier (?Albian).

The biggest problem with this type of interpre- tation is separating the effects of regional tectonics and eustatic changes in sea level. Global eustatic changes in the late Cretaceous will have affected these stratigraphic sequences, and equally, rapid spreading in the late Cretaceous must have contributed to sea-level changes.

Conclusions

Emplacement of the ophiolites of the 'croissant ophiolitique' is a unique event in which young 'oceanic' lithosphere was emplaced very soon after its formation. I suggest that the almost simultaneous emplacement of ophiotites of simi- lar age along a belt about 3000 km in length is consistent with their origin by rifting within the Arabian passive continental margin in the early late Cretaceous. The present discontinuous nature of the zone of ophiolites is interpreted as a primary feature reflecting an originally discon- tinuous rift. Metamorphic rocks now associated with the ophiolites were formed as a result of intrusion in the initial stages of rifting and by dynamothermal metamorphism associated with extension of the margin. The older metamorphic rocks do not therefore date the initiation of overthrusting but break-up initiating spreading. A change from an extensional to compressional regime in the margin in the late Cretaceous caused detachment and emplacement of the ophiolites within the passive margin. The ophio- lites are therefore essentially parautochthonous; the relatively small distances of overthrusting (less than 100km on each side of the Oman ophiolite) can explain why very large but extremely thin slabs can be emplaced without major disruption. I suggest that the compression of the Arabian passive margin reflects a major change in plate motions at the end of the Cretaceous associated with the initiation of subduction of the Southern Tethys. This was followed by subduction-related volcanism at the northern margin and Miocene continent-conti- nent collision in SE Turkey and Iran. Continent- continent collision has not yet occurred in the Oman region and subduction of the last remnants of the Southern Tethys continues under the Makran. The ophiolites do not mark what is normally interpreted as a suture, but a discon- tinuous zone within the Arabian passive margin. The true suture lies to the north of the ophiolite zone and is marked by a zone of m~langes including Tertiary flysch and disrupted ophiolites (Hall 1976; Stoneley 1981). The closest present- day analogue of the Arabian margin ophiolites is the Red Sea-Dead Sea system which appears to be underlain discontinuously by oceanic litho- sphere in the Red Sea and Dead Sea (Ginzburg et al. 1981) and is close to a continental margin for at least part of its length.

ACKNOWLEDGMENTS: I thank Mohsen Arvin, Mike Audley-Charles, Curtis Cohen, Ernie Rutter and Bob Stoneley for discussion.

402 R. Hall

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