Download Ophiolites: Figments of Oceanic Lithosphere? Geological Society

Geological Society, London, Special Publications
Ophiolites: Figments of Oceanic Lithosphere?
R. Hall
Geological Society, London, Special Publications 1984; v. 13; p. 393-403
doi:10.1144/GSL.SP.1984.013.01.32
Email alerting
service
click here to receive free email alerts when new articles cite this
article
Permission
request
click here to seek permission to re-use all or part of this article
Subscribe
click here to subscribe to Geological Society, London, Special
Publications or the Lyell Collection
Notes
Downloaded by
Robert Hall on 26 November 2007
© 1984 Geological Society of
London
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 discontinuous zone of ophiolites, referred to by
Ricou (1971a) as the 'croissant ophiolitique'
extends t h r o u g h southern T u r k e y to O m a n
(Fig. 1). This is the s o u t h e r n m o s t zone of
ophiolites in the Middle-Eastern sector of Tethys.
The ophiolites vary in character along the belt
from ophiolitic olistostromes and m61anges to the
almost intact stratiform complex of O m a n .
Despite their discontinuity and differences in
their style of occurrence and d e f o r m a t i o n these
ophiolites share a n u m b e r of features which can
be considered as 'rules' in the ophiolite-emplacement 'game'. All were emplaced on the Arabian
passive continental margin (rule 1), where evidence exists it appears that they were emplaced
soon after their formation (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 e m p l a c e m e n t of
all of the ophiolites, simply because ophiolite
e m p l a c e m e n t is a unique and unusual event in the
history of the Arabian margin, and occurs in such
a limited interval of time. F u r t h e r m o r e , emplacement models must also take into account the
subsequent history of the region: continent-continent collision did not occur in southern Turkey
and Iran until the Miocene, and in O m a n 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 e m p l a c e m e n t of other ophiolites,
e.g. those involving active continental margins,
and those related to c o n t i n e n t - c o n t i n e n t collision.
The Neyriz complex of southern Iran is one of
the ophiolites of the 'croissant' and several
authors have drawn attention to its similarities to
O m a n (e.g. Ricou 1971a; H a l l a m 1976; C o l e m a n
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, although some
alkali basalts similar to those of the Haybi
complex of O m a n 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, although they are c o m m o n l y
associated with volcanic m o u n t a i n belts on
continental crust (e.g. the Andes) or continental
rifts such as the East African rift (Williams &
McBirney 1979). Unusual skarns are f o u n d at the
contact between the ophiolite peridotite and
folded marbles and indicate high-temperaturecontact m e t a m o r p h i s m (Ricou 1971b; Hall 1980).
A general ophiolite model for the A r a b i a n margin
should also be able to incorporate these anomalies.
That the Arabian continental margin was a
passive margin is undisputed; it has a relatively
continuous Mesozoic record of marine sedimentation without active margin-type volcanism.
M a n y of the T e t h y a n passive margins are
considered to have been f o r m e d by rifting in the
Triassic and the Arabian margin conforms to this
pattern; until the Triassic, central Iran and
northern Arabia have a similar history but after
the Triassic their stratigraphy is quite different.
The Southern Tethys ocean formed between
Arabia and central Iran-Lut should therefore
have been floored by oceanic crust of Triassic age
in the region adjacent to the A r a b i a n passive
393
394
R. Hall
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 necessarily marks the exact margin between continental and new oceanic crust. Scandone (1975)
observed that the Triassic rift zones in the western
part of Tethys were not the zones of later oceancrust 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. Continentcontinent 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 emplacement 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 spreading 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. Hall
TRENCH-BACK ARC AXIS,-,,520 km
TRENCH- ARC ".' 200 km
PASSIVE MARGIN
TRENCH
I
FORE-ARC
ACTIVE ARC
I
BACK-ARC BASIN AXIS
I
Thick sed[menl cover: volcaniclostics,
vitric muds, ash,petogics and turbidites.
Abundant reworked sediment
O
NO accretionory wedqe
$ubduclion erosion ?
....
Variable but often thick sediment cover
dominantly volconiclaslics, v i t r i r muds,
volcanic ash and pelagic oozes
SEA LEVEL, 0
Boninites, arc tholeiites
and calc-(llkaline lavos:
older than back-arc crust
~ r I ~ I I ~ T o c E ACRUS
NIC
BASE0~ L'rHOSPHE.E
t
O
0
~
"'~
.
~'~
- " "~"-...
km
'%''k
~ 1 0 0
~
"-300
200
I00
0
I00
km
200
300
4 00
,km
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 presumably 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 summarized 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 extension 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
TRIASSIC
A
Arabian Continental M a r g i n
km
Southern T e t h y s
continenlal crust
0
oceaniccrust
"
l-so ~ i - ~
~
1100
~
~
~
km ,0
~
-
'
- ~-~6~
~
a
u~45E OF L ~
IO0,
200~
.
.
300
B CENOMANiAN
i n t r u s i v e f e a t u r e s and
contact metamorphism
km
~ X / / f t _\_ . \. . . X~ I1~- ~ , ~ # /w/
95 0
MO 0 // / "
~."
.,/
Southern Tethys
"v,
ASTHENOSPHERE
L I00
.._,--_--
~.. - ~
~4.~'t-'_. ~
O#" " " ..~
RISE
j/
0
km i
\ /f__r,-,--~-
/1 \ \
--,..
MOHO
__._.--~-'-'-
~
L'7sosp~
I00
200
300
I
i
~L
C CAMPANIAN
400
500
I
'
Ophiolite
k~
Southern
/~%~a~o,phic r o c k ~ : : X _ .
50
MOHO
1 i - --" ~"
L~ T ~ O S P H E R ~ "
I00
~
.......
Tethys
" \
~
./
i
---
_ . . . - .-.--"
.......
km
0
L
I00
~
200
,
300
L
400
,
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).)
n o r t h e r n margin. S u b d u c t i o n - r e l a t e d volcanism
at the n o r t h e r n margin should therefore have
begun later than ophiolite e m p l a c e m e n t , the time
lag depending on subduction rates and dip of the
Benioff zone. In fact, in central Iran and n o r t h o f
the Djaz M u r i a n depression volcanic activity is
recorded f r o m the late C r e t a c e o u s - P a l e o c e n e
(Berberian et al. 1982).
Testing the models
Ophiolites and geochemicaI evidence
Most of the evidence f r o m ophiolites themselves
does not tell us very m u c h a b o u t their environment of f o r m a t i o n or means of e m p l a c e m e n t .
Since so m a n y ophiolites seem to have been
empiaced soon after their f o r m a t i o n and are
398
R. Hall
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' elements, have been proposed to resolve the
problem of tectonic setting. Clearly, if geochemical 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 possibilities of magma mixing and crustal contamination, 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 emplacement 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 discriminant rules? Figure 5 emphasizes the need for
caution. Using the same discriminant methods
chosen by Pearce et al. (1981) to deduce a backarc 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 'alpinetype' ultramafics was their lack of high-temperature 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 subophiolite 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 offaxis 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, structurally 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 metamorphosed 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 association now found in a similar tectonic situation
beneath the Bay of Islands ophiolite where
Jamieson & Talkington (1980) interpret them as
Ophiolites: Figments of oceanic lithosphere ?
399
.--.-~, . ' U - - " ~ " - - .
/within-plate
"
/
/
lavas
(.
10000
Ti
ppm
100OO
Ti
ppm
~
//"
\i\.\
[
arc tavasI
I
k
I
1000
,
,
t ~l
I
1000
t0
Zr
i
i
L
I
i
I
J
J
L
n I I J
Cr ppm
looo
I I
lOO
ppm
1000
i
loo
I
1.0
I
/
c a.b
y /
-i./~.....
rb
ppm
//. -
/ / ,
I~ ------~
9
.o
/
._../
"/ /..'/1
//////"
0.1
100
/
#,~
/
iat . . . . . . . . . . . . . .
Cr
/
/ //
,/
"
lwpO,
i
f
vab
~
j
I/
i
,
,
*
t
10
I
I
Y
i
Li
I
ppm
,,
,
1
, ~ !
d
100
0.01
I
d
I
I
I
I
J
I
0.1
t
~
~
._j_.
,
I
I
l'a/Yb
a
!
1,0
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).
are perfectly consistent with an origin by
part of an ocean-island sequence although
conceding that 'the overwhelming majority of metamorphism of igneous and sedimentary rocks
formed at the margin of an ocean basin. On the
similar alkaline rock suites has been described
other hand, the high-temperature metamorphism
from continental environments where they are
generally associated with rifting'. (Jamieson & t h a t they have suffered seems inconsistent with
the subduction-zone origin suggested by Searle &
Talkington 1980, p. 474). In the Neyriz area the
Malpas (1980, 1982). Subduction-zone metamorhigh-temperature skarns at the contact of the
phism is normally considered to be typically of a
ophiolite peridotite are interpreted as melts
high-pressure-low-temperature type, as suggested
produced at the contact of hot intrusive peridotite
by the low heat flow of these regions at the
(Hall 1980). These rocks are not easily explained
present, and thermal modelling ofsubducted cold
by models postulating formation of these ophioslabs. It is odd that no high-pressure metamorphic
lites at a mid-ocean ridge or somewhere in an
rocks have been reported from the m61anges
island arc, although they fit well into the rifted
beneath the ophiolite if the continent-arc collision
continental-margin model.
Lithologies of the Oman sub-ophiolite sequence 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 temperatures. 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 jacupirangites 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 metamorphism, and younger metamorphic ages as the
rocks moved upwards. The metamorphic rocks
would therefore have a polyphase history of
deformation and mineral growth recording extension, 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
Continental
vv vv
vv
Crust
T r i assir
"failed rift"
subsidence o f O u t e r S h e l f
st .... hin 9
(by l i s t r i c f a u l t i n g ?)
~
~-~vv~]~
z o n e o f i s o t h e r m a l rise in c r u s t
new "oceanic'crust
.....
- ~
~
~
, d e e p . . . . ,a, M e , . . . .
~ ~
~
~
~
~
.hi . . . . .
~ ~ ~ ~
~
--
TEMPERATURE
margjh
J
frictional
" b e a t l?~g?, ~
f
: ilfiii!iliNii! i!!!!!iiiiiiiN
metamorphic
history
of ophiolite
btocking
re
gradual stretching
before b r e a k . u p
&
BREA K- UP
after break.up
TIME
FtG. 6. Schematic thermal evolution of Triassic '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 temperature reached. This is also the zone in
which intrusive features and contact metamorphism should be observed. Note that
the thermal maximum occurs at the time of
break-up but metamorphic ages are
younger, depending on blocking temperatures.
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 ophiolites, both above and below, should also.be able to
provide some tests for models of ophiolite
Ophiolites: Fifments of oceanic lithosphere ?
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 sedimentary 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 earlymid 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 interpretations 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 interpretation 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.
401
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 similar 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 discontinuous 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 ophiolites 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-continent collision in SE Turkey and Iran. Continentcontinent 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 discontinuous 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 presentday analogue of the Arabian margin ophiolites is
the Red Sea-Dead Sea system which appears to
be underlain discontinuously by oceanic lithosphere 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
References
ARVIN, M. 1982. Petrology and geochemistry of
ophiolites and associated rocks from the Zagros
suture, Neyriz, Iran. Ph.D. Thesis, London University.
BERBERIAN, F., MUIR, I. D., PANKHURST, R. J. &
BERBER1AN, M. 1982. Late Cretaceous and early
Miocene Andean-type plutonic activity in northern
Makran and Central Iran. J. Geol. Soc. London
139, 605-14.
COLEMAN~R. G. 1981. Tectonic setting for ophiolite
obduction in Oman. J. geophys. Res. 86, 2497-508.
FALVEY, D. A. & MIDDLETON, M. F. 1981. Passive
continental margins: evidence for prebreakup deep
crustal metamorphic subsidence mechanism.
OceanoL Act. Proc. 26th Int. geoL Congr. Paris 1980
103-14.
FARHOUDI, G. & KARIG, D. E. 1977. Makran of Iran
and Pakistan as an active arc system. Geology 5,
664-8.
GEALEY, W. K. 1977. Ophiolite obduction and geologic
evolution of the Oman Mountains and adjacent
areas. Bull. geoL Soc. Am. 88, 1183-91.
GINZBURG, A., MAKRlS, J., FUCHS, K. & PRODEHL, C.
1981. The structure of the crust and upper mantle
in the Dead Sea rift. Tectonophysics 80, 109-19.
GLENNIE, K. W., BOEUF, M. G. A., HUGHES-CLARKE,
M. W., MOODY-STUART, M., PILAAR, W. F. H. &
REINHARDT, B. M. t973. Late Cretaceous nappes
in Oman Mountains and their geologic evolution.
Bull. Am. Assoc. Petrol. Geol. 57, 5-27.
GRAHAM, G. 1980. Evolution of a passive margin and
nappe emplacement in the Oman Mountains. In:
PANAYIOTOU, A. (ed.) Ophiolites: Proc. International Ophiolite Symposium Cyprus 1979,
pp. 414-23.
HALL, R. 1976. Ophiolite emplacement and the
evolution of the Taurus suture zone, southeastern
Turkey. Bull. geol. Soc. Am. 87, 1978-88.
1980. Contact metamorphism by an ophiolite
peridotite from Neyriz, Iran. Science208, 1259-62.
1982. Ophiolites and passive continental margins.
Ofioliti 7, 279-98.
HALLAM, A. 1976. Geology and plate tectonics
interpretation of the sediments of the Mesozoic
radiolarite-ophiolite complex in the Neyriz region,
southern Iran. Bull. geol. Soc. Am. 87, 47-52.
HsO, K. J. 1969. Role of cohesive strength in the
mechanics ofoverthrust faultingand of landsliding.
Bull. geol. Soc. Am. 80, 927-52.
HUBBERT, M. K. & RUBEY, W. W. 1959. Role of fluid
pressure in mechanics of overthrust faulting. Bull.
geol. Soc. Am. 70, 115-66.
HUSSONG, D. M., UYEDA, S. et al. 198I. InitialReports
DSDP Leg 60. Washington (U.S. Government
Printing Office).
JAMIESON, R. J. & TALKINGTON, R. W. 1980. A
jacupirangite-syenite assemblage beneath the
White Hills peridotite, northwestern Newfoundland. Am. J. Sci. 280, 459-77.
LANPHERE, M. A. 1981. K-Ar ages of metamorphic
rocks at the base of the Samail ophiolite, Oman. J.
geophys. Res. 86, 2777-82.
LOWELL, J. D. & GENIK, G. J. 1972. Sea-floor spreading
and structural evolution of southern Red Sea. Butt.
Am. Assoc. Petrol. Geol. 56, 247-59.
MATTEY, D. P. 1980. The petrology of high-calcium, lowalkali thoteiite dykes from the Isle of Skye regional
swarm, Ph.D. Thesis, London University.
MANGHNANI,M. L. & COLEMAN,R. G. (1981) Gravity
profiles across the Samail ophiolite. J. geophys.
Res. 86, 2509-25.
MCKENZlE, D. P. 1977. The initiation of trenches: a
finite amplitude instability. In: TALWANI, M. &
PITMAN, W. C. (eds) Island Arcs, Deep Sea Trenches
and Back Arc Basins. American Geophysical
Union, Washington D.C. pp. 57-61.
PEARCE, J. A., ALABASTER, T., SHELTON, A. W. &
SEARLE, M. P. 1981. The Oman ophiolite as a
Cretaceous arc-basin complex: evidence and implications. Philos. Trans. R. Soc. London A300,
299-317.
RIcou, L.-E. 1968. Sur la mise en place au CrGtac6
supGrieur d'importantes nappes h radiolarites et
ophiolites dans les Monts Zagros (Iran). C. R.
Acad. Sci. Paris 267, 2272-5.
1971a. Le croissant ophiolitique peri-arabe: une
ceinture de nappes mises en place au CrGtac6
supGrieur. Rev. G~ogr. phys. Gkol. dyn. Paris 13,
327-49.
1971b. Le mGtamorphisme au contact des
pGridotites de Neyriz (Zagros interne, Iran):
d~veloppement de skarns h pyrox~ne. Bull. Soc.
g~oL Fr. 13, 146-55.
RIGO DE RIGHI, M. & CORTESINI, A. 1964. Gravity
tectonics in Foothills Structure Belt of southeast
Turkey. Bull. Am. Assoc. Petrol. Geol, 48, 1911-37.
ROYDEN, L., SCLATER,J. G. & YON HERZEN, R. P. I980.
Continental margin subsidence and heat flow:
important parameters in formation of petroleum
hydrocarbons. Bull. Am. Assoc. Petrol. Geol 64,
173-87.
SAGER, W. I980. Structure of the Mariana Arc inferred
from seismic and gravity data. J. geophys. Res. 85,
5382-8.
SCANDONE, P. 1975. Triassic seaways and the Jurassic
Tethys ocean in the central Mediterranean area.
Nature, Lond. 256, 117-9.
SEARLE, M. P. & MALPAS, J. 1980. Structure and
metamorphism of rocks beneath the Semail
ophiolite of Oman and their significance in
ophiolite obduction. Trans. R. Soc. Edinburgh 71,
247-262.
t~; MALPAS, J. 1982. Petrochemistry and origin of
sub-ophiolitic metamorphic and related rocks in
the Oman Mountains. J. geol. Soc. London 139,
235-48.
~,
LIPPARD, S. J., SMEWlNG, J. D. & REX, D. C. 1980.
Volcanic rocks beneath the Semail ophiolite nappe
in the northern Oman mountains and their
significance in the Mesozoic evolution ofTethys. J.
geol. Soc. London 137, 589-604.
SHERIDAN, R. E., GROW, J. A., BEHRENDT, J. C. &
BAYER, K. C. 1979. Seismic refraction study of the
continental edge off the eastern United States.
Tectonophysics 59, 1-26.
Ophiolites: Figments o f oceanic lithosphere ?
SIBSON,R. H. i977. Fault rocks and fault mechanisms.
J. geoL Soc. London. 133, 191-213.
SPRAY, J. G. 1982. Mafic segregations in ophiolite
mantle sequences. Nature, Lond. 299, 524-528.
& RODDICK, J. C. 1980. Petrology and 4~
geochronology of some Hellenic sub-ophiolite
metamorphic rocks. Contrib. Mineral Petrol 72,
43-55.
STONELEV, R. 1974. Evolution of continental margins
bounding a former Southern Tethys. ln: BURK, C.
A. & DRAKE, C. L. (eds) Thegeology of Continental
Margins. Springer-Verlag, New York, pp. 889-903.
1975. On the origin of ophiolite complexes in the
Southern Tethys region. Tectonophysics25,303-22.
1981. The geology of the Kuh-e Dalneshin area of
southern Iran, and its bearing on the evolution of
Southern Tethys. J. geol. Soc. Lond 138, 509-26.
THOMPSON, R. N. 1982. Magmatism of the British
Tertiary volcanic province. Scott. J. Geol, 18,
49-107.
-
-
-
-
--,
403
ESSON, J. & DUNHAM,A. C. 1972. Major element
chemical variation in the Eocene lavas of the Isle of
Skye, Scotland. J. Petrol. 13, 219-53.
- - , GIBSON, I. L., MARRINER, G. F., MATTEY,D. P. &
MORRISON, M. A. I980. Trace element evidence of
multistage mantle fusion and polybaric fractional
crystallisation in the Paleocene lavas of Skye, NW
Scotland. J. Petrol 21,265-93.
THUIZAT, R., WHITECHURCH, H., MONTIGNY, R. &
JUTEAU, T. 1981. K-Ar dating of some infraophiolitic metamorphic soles from the eastern
Mediterranean: new evidence for oceanic thrustings before obduction. Earth planet. ScL Lett, 52,
302-10.
TIPPIT, P. R., PESSAGNO,E. A. & SMEWING,J. D. 1981.
The biostratigraphy of sediments in the volcanic
unit of the Samail ophiolite. J. geophys. Res. 86,
2756-62.
WILLIAMS,H. & MCBIRNEY, A. R. 1979. Volcanology.
Freeman, Cooper & Co., San Francisco.
R. HALL, Department of Geology, University College, London WC1E 6BT.