Download The Makran, Southeastern Iran: the anatomy of a convergent plate

Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
The Makran, Southeastern Iran: the anatomy of a convergent
plate margin active from Cretaceous to Present
G. J. H. McCall & R. G. W. Kidd
SUMMARY: The inland geology of the Iranian Makran, long neglected by geologists
because of its lack of hydrocarbon potential, is now reasonably well known following a
regional mapping programme carried out on behalf of the Geological and Mineral Survey
of Iran. The mountain range can be divided into seven geotectonic provinces forming an
arc round the late Pliocene epeirogenic Jaz Murian Depression at the southern end of the
Lut block. From the edge of the Jaz Murian outwards to the S these provinces are: (1) a
marginal basin in which well-preserved ophiolites formed and deep-water pelagic sediments were deposited from Jurassic to Palaeocene; (2) a narrow zone of continental crust
of Palaeozoic metamorphics capped by shelf limestones of mainly Cretaceous age but
including Carboniferous, Permian, Jurassic and Palaeocene developments; (3) a zone of
ophiolitic m61ange, the renowned Coloured M61ange; (4) a zone of immensely thick
Eocene-Oligocene flysch; (5) a similar zone of Oligocene-Miocene flysch; (6) a southern
zone of Miocene neritic to molassic sediments and (7) a Miocene-early Pliocene neritic
zone W of the Zendan fault. The structure is dominated by steep inward-dipping reverse
faults forming schuppen. These developed in a series of events which climaxed in the late
middle Miocene with some faults continuing to be active to the present day with
continuing uplift. Intense periclinal folds of dominantly chevron style are developed in the
flysch, synchronous with the faulting. Over substantial areas in the flysch deformation has
been so intense that a m61ange has developed, resembling some Franciscan m61anges
more closely than the Coloured M61ange. It contains exotic blocks which are tectonically
intruded from the Coloured M61ange, which forms the basement to the flysch zones.
The most significant discovery of this programme is the continuation into the Makran of
the Sanandaj-Sirjan zone in the form of the Bajgan-Dur-Kan complexes (province 2
above), thus forming a sliver of continental crust that stretches from the Bitlis Massif in
Turkey to the Makran. This separates the two ophiolitic developments of the Makran: to
the south the Coloured M~lange, the trench margin sequence of a north-dipping
subduction zone formed mainly in Late Cretaceous-Palaeocene time, and to the north a
belt of well-preserved Cretaceous-Palaeocene ophiolites formed in a marginal basin. The
latter ophiolites may be the equivalent of the island arc of this subduction system. The
Makran was not involved with the third group of ophiolites in this region, those of the
Oman, Neyriz and Kermanshah, which were emplaced as a result of collision of the
Arabian continental margin with an intra-oceanic NE-dipping subduction zone in the
Campanian. Any association of these ophiolites with the Coloured Mdlange along the
Zagros is due to the Pliocene collision of Arabia and Iran.
Following uplift of the Inner Makran in the late Palaeocene northward subduction has
continued to the present day with related andesitic volcanism to the north and migration of
the trench to the south following tectonic events in the Oligocene and middle Miocene, so
that the trench is now 300 km from the present andesitic volcanism. The products of
Eocene to Present subduction, including the immense deformed flysch deposits, are thus
superimposed on the condensed Cretaceous-Palaeocene subduction system.
Introduction
T h e I r a n i a n M a k r a n comprises the 150 k m wide
area of SE I r a n b e t w e e n the Jaz M u r i a n D e pression and t h e coast (Fig. 1). T h e east-west
t r e n d i n g m o u n t a i n r a n g e rises from the coast to
a r o u n d 6000 feet and in a few places to over
8000 feet b e f o r e d r o p p i n g d o w n again into the
Jaz M u r i a n . W i t h i n I r a n it is 600 k m long f r o m
the coast at M i n a b in the west to the Pakistan
b o r d e r in the east.
A p a r t from the excellent p i o n e e r w o r k of
H a r r i s o n & F a l c o n (1936) and H a r r i s o n et al.
(1935-36) and s o m e u n p u b l i s h e d oil c o m p a n y
studies, the g e o l o g y of the M a k r a n was long
n e g l e c t e d b e c a u s e of its lack of h y d r o c a r b o n
potential. H o w e v e r , most of the I r a n i a n M a k ran m o u n t a i n r a n g e has n o w b e e n m a p p e d by
P a r a g o n - C o n t e c h (an I r a n i a n - A u s t r a l i a n joint
v e n t u r e ) on c o n t r a c t for the G e o l o g i c a l a n d
M i n e r a l S u r v e y of Iran. T h e m a p p i n g c o v e r e d
four full and two half 1:250,000 q u a d r a n g l e s (1 °
387
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
G. J. H. McCall & R. G. W. Kidd
388
1 I '1
&rot/[
z:
j'
~£_'_1
¢"
I~",O~lllllll['~-------\'/')
~
s
t
.J'~lllllll(lll)lll~ . . . .
~
"
(
f
"%
•
~
-I
~ x ; A ~ 'V
' ~-~I'r~F.._~-~---I-
--
-
Kuh-e-Birk
\
mi[~_
IRANSHAHR ~
~
"~-
~
--
.
%-
~.~
~ , _ .
--¥
\) , . L - ~
='-r._ -I
~:~5~ ~ - - r . / / / / ~
~
~ t J H , / l ~
ble connection
_~
IIDr~IIIIX-1--/-----_~-,,# -
N
,
. . ~
)
~
_ ~
-
26"
.... ~
......
2
~
o_0_
0.. . . .
A
H
B
A
H
A
R
~
,oo
R~ft hke spreading zone (loner
Makran marginal basin)
3 ~
Carbonate Fore-Arc zone (Bajgan-Our Kan zone)
6 ~
4 r~
Coloured M~lange
7 ['~
Miocene Neritic molasslc zone
8 [~
(Makran Unit) Mio-Pli . . . .
]~]
E. . . . . -Olig . . . . .
flysch . . . .
Oligocene-Miocene flysch zone
,t,c tool. . . . . . . . .
Fro. 1. Sketch map of the Iranian Makran showing the eight geotectonic provinces and the lines of
the sketch sections (Figs 2 & 3).
by 11o~
). In addition, nine maps (~o by ~o) of
special interest were mapped on a 1:100,000
scale and a 1:500,000 tectonic map accompanied the overall report. In all an area of over
80,000 km 2 (about the size of Scotland) was
mapped in 2 years using helicopters and more
than 30 geologists. The maps and reports with
full details of the geology will be published
shortly by the Geological and Mineral Survey of
Iran (McCall 1981).
This paper is intended to relate the broad
results of this programme to the global tectonics
of this area. The mapping was a group project
but the conclusions in this paper are those of
the two authors alone. G. J. H. McCall was
senior consultant to this project and compiled
the reports (McCall 1981) and R. G. W. Kidd
contributed to it in the second field season, and
wrote the section on the plate tectonics and
evolution of the region in the final report.
Geology
of the Iranian
Makran
There are eight geotectonic provinces (including the Jaz Murian) in the Iranian Makran
(McCall 1981)--these are shown in Fig. 1.
Sketch sections across the Makran mountain
range are shown in Fig. 2 (Minab sector--NW)
and Fig. 3 (Fannuj sector--centre). The lines of
these sections are on Fig. 1. The structure is
dominated by steep inward dipping (toward the
Jaz Murian) faults forming schuppen. Thus the
relationships between the geotectonic provinces
(apart from the Jaz Murian) are largely tectonic. From the Inner Makran out towards the
coast these provinces are:
(1) J a z M u r i a n D e p r e s s i o n
This is a late Pliocene epeirogenic depression
300 km in length in an east-west direction and
over 100 km across north-south. The only rocks
are superficial: alluvial fans of gravel, silt
plains, playa lakes, dunes and salt pans.
(2) I n n e r M a k r a n s p r e a d i n g z o n e
This is a zone of rifting occupied by largely
undeformed ophiolites (as defined by the 1972
Penrose Conference). There are three distinct
ophiolites. One is tholeiitic and resembles the
Troodos Complex, Cyprus. It is Early Cretaceous to early Palaeocene in age and consists
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
An active convergent plate margin
SW
16
[
13
ii
8
1,O
4
9
8
/
4
3
6
/
5
it
4
3
~
1,
"'E
it
3
2
1[~
13
IL
5,6,7
7
12
14
Pl()
310 Km
1
VERTICAL SCALE EXAGGERATED
TOPOGRAPHy GENERALISED
FIG. 2. Sketch section across the Makran in
the Minab sector along the line shown in
Fig. 1. Geotectonic province numbers are
shown in heavy type beneath the section.
Other numbers indicate geology as follows:
(1) gravel and silt fans of the Jaz Murian
depression; (2) Lower-Upper Cretaceous
calc-alkaline ophiolite: mainly sheeted
dykes of intermediate to acid composition,
screens of pillow lavas, pelagic sediments,
gabbro and trondhjemite; (3) lowerupper Eocene proximal turbidites, acid
welded tufts, diabase and diorite sills and
shelly limestone, all overlying ophiolite
sequence; (4) Lower Cretaceous-lower
Palaeocene pillow lavas of outer tholeiitic
ophiolite with pelagic limestones; (5)
serpentinite; (6) low-level cumulate gabbro
of outer ophiolite; (7) high-level noncumulate gabbro; (8) trondhjemite at top
of 7; (9) sheeted diabase dykes; (10) DurKan Complex: mainly Cretaceous but including Permian and Jurassic shelf carbonates structurally overlying 11" (11)Bajgan
Complex: greenschist and amphibolite
facies Palaeozoic metamorphics; (12) tectonic enclaves of ultrabasics with
chromite; (13) Coloured M61ange: Lower
Cretaceous-lower Palaeocene block-toblock m61ange; (14) ultrabasic tectonites,
partly layered with chromitite cumulates;
(15) Eocene-Oligocene flysch; (16) upper
Oligocene-lower Miocene flysch plus
some neritic, gypsiferous Miocene
sediments;
(17)
Burdigalian reefal
limestone" (18) Makran unit: upper
Miocene-lower Pliocene neritic to molassic sediments; (19) coastal mudflats.
of cumulate gabbros overlain by high-level gabbro, trondhjemite, d/abase sheeted dykes, pillow lavas with copper shows and pelagic sediments. The other two ophiolites are distinctly
calc-alkaline and may represent separate parts
of the same ophiolite sequence. One of these,
of Early to Late Cretaceous age, has only the
upper parts exposed and consists mainly of
389
sheeted dykes which are largely dacitic-rhyolitic
and include micro-trondhjemite. It also contains screens of pillow lavas and of gabbro and
trondhjemite. The third ophiolite, of Early Cretaceous to early Palaeocene age, consists of
fragmented plutonics (ultrabasics, troctolite,
cumulate gabbro, high-level gabbro and trondhjemite) folded into immense flexures, and
diabase dykes above (some sheeted), grading
up into pillow lavas overlain by pelagic sediments.
The dyke trends in these ophiolites are
roughly parallel to the strike of the ophiolites as
a whole--following the curvature of this Inner
Makran spreading zone round from a northsouth trend in the NW of the area to east-west
near Fannuj. This suggests that these ophiolites
were formed in approximately their present
positions and have not been tectonically emplaced from elsewhere. Indeed their present
width of about 50 km in the NW to as little as
15 km near Remeshk (see Fig. 1) may not
necessarily be a great deal less than their original width, as apart from steep bounding reverse
faults they are relatively undeformed. Near
Fannuj, however, there is a local development
of metamorphics including blueschists in a
m61ange within the Spreading Zone. This includes Cretaceous Globotruncana limestone
and radiolarites, and metamorphics which may
be as old as Palaeozoic.
(3) BajgannDur-Kan zone
This is a narrow but continuous zone of
continental crust. In the NW of the area where
it is up to 40 km wide (see Figs 1 & 2) it consists
of Palaeozoic metamorphics (the Bajgan Complex). Eastward the Bajgan Complex is overlain
by a sequence of shelf limestones of the DurKan Complex (see Figs 1 & 3), which are
predominantly of Early Cretaceous to early
Palaeocene age, but have tectonic inclusions of
Carboniferous, Permian and Jurassic shelf
limestones. This continental sliver thins eastward and appears to pass into the Kuh-e-Birk
range through a sigmoid flexure and thence into
Pakistan. To the NW of the mapped area it can
be traced up through the Sanandaj-Sirjan zone
(see Fig. 4) as far as the Bitlis Massif in Turkey
(Stocklin 1977). It has been suggested that it
may be an Alpine-type nappe (Gansser pers.
comm.) but the structure of this region is one of
steep schuppen not of low-angle thrusts and a
nappe thousands of kilometres long seems improbable. It must be considered a global
geotectonic entity and the continuation of the
Sanandaj-Sirjan zone into the Makran is impor-
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
G. J. H. McCall & R. G. W. Kidd
390
SOUTH
7
9
8
6
6
8
\
II
5
6
3 4
5
3
NORTH
4
3
5
..It~
4 & 3
7
imbricated
'i'
2
:'I.....
5
3
2
I L _ _
1
VERTICAL SCALE EXAGGERATED
TOPOGRAPHY GENERALISED
Ft6. 3. Sketch section across the Makran in the Fannuj sector along the line shown in Fig. 1.
Geotectonic province numbers are shown in heavy type beneath the section. Other numbers indicate
geology as follows: (1) superficial gravel fans and dunes of Jaz Murian depression; (2) mainly
greenschists and blueschists with Lower-Upper Cretaceous pelagic limestones and radiolarites; (3)
pillow lavas and pelagic sediments; (4) diabase dykes, some sheeted; (5) fragmented and folded
ophiolitic plutonics: layered cumulates at base (ultrabasics, troctolite, gabbro), non-cumulates above
(gabbro, trondhjemite); (6) Dur-Kan complex: mainly Cretaceous, but including Permian and
Jurassic, shelf carbonates; (7) Bajgan Complex of Palaeozoic metamorphics structurally overlain by
6; (8) Coloured M61ange; (9) lower-upper Eocene distal flysch and minor pelagic and shelf
limestones dislocated throughout with exotic blocks; (10) tectonic dislocation and protrusion
m61ange, 80% flysch, 20% exotics; (11) lower-middle Miocene proximal flysch, deep water facies;
(12) upper Oiigocene-iower Miocene mainly distal flysch; (13) lower-middle Miocene neritic
sediments, mainly fine gypsiferous mudstones; (14) exotic raft to harzburgite; (15) middle-upper
Miocene neritic sediments; conglomerates and sandstone in the north, sandstone, siltstone and
mudstone in the south, lateral facies changes to deltaic and estuarine facies with some evaporites.
tant in the consideration of any plate tectonic
model for the region.
(4) Coloured M61ange Zone
This is a tectonic, ophiolitic, block-to-block
m61ange (Gansser 1974) consisting of serpentinite, other ultrabasic and basic ophiolitic rocks,
pillow lavas, pelagic limestones, radiolarites
and distal turbidites. It also contains minor
andesite, rhyolite, rhyolitic welded tuff,
trachyte and exotic components (metamorphics
and Lower Cretaceous reefal limestone). There
is no true matrix: block boundaries are sheared
and often serpentinite acts as a 'lubricant'. The
m61ange is not all chaotic: much of it consists of
stacked slabs that are the right way up and
consistently dip steeply inward toward the NE.
The fossil ages are enigmatic in that the radiolarites are Jurassic-Coniacian while the pelagic
limestone (GIobotruncana) is CenomanianMaastrichtian, yet the radiolarites and pelagic
limestones sometimes appear to form an interbedded sequence. The youngest rocks are early
Palaeocene biomicrites. The pillow lavas and
sediments are commonly interbedded.
We suggest that the Coloured M61ange was
formed in the trench of a NE- to north-dipping
subduction zone by scraping off fragments of
the downgoing plate, possibly mixing these
fragments with small bits of the overriding
plate. Whether there is a contribution to the
m61ange from the overriding plate depends
upon establishing the derivation of the exotic
components of the m61ange (shallow water
limestones and metamorphics). This question
has not yet been resolved.
The outcrop of Coloured M61ange is about
30 km wide in the NW of the area but thins
rapidly to a few kilometres and pinches out to
the south of Remeshk (see Fig. 1), again reappearing in the area not mapped in this project
to the south of Iranshahr. However, exotics
within the flysch of the sediments of provinces
5, 6 and 7 indicate that the Coloured M61ange
extends 70 km of the south of its area of outcrop
beneath these sediments (see below).
The age of onset of m61ange formation cannot be determined but the abundance of
Cenomanian-Maastrichtian rocks suggests that
much of it did not start forming a m61ange until
Maastrichtian time. The involvement of early
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
An active convergent plate margin
391
N
?
/ cAsP'A" I
,
V
'x
IRAN
%
+
t
'X,
v
qN~+
%,\
q,,'• +
~
Z 'g
N ~ n g/
+
+
+
+
+",v k+ +" +".
~"~.o.
• \ \ VX"+
"%.'2~+\
,
+
I
5ooKm
1-
I
[ f " 4-" %
"+
V ~ /
B L0 C K
-:- \
AFGHANISTAN
V
+/,
+
'
Birjand e
".-+/+
5~./,.,,,Tx~
,
• Sabzevar
V
f ~ "
+
.",
+
,
+\
,
+
~\
*/jAz \
4/
+
(i)
+
+
+
+
+
+
+
+
_ s, sTA .+
\+
,
+
+
,
,
-t-
+
v
DUR-KAN
UUCTION
ARABIA
ZONE
GULF
OF
OMAN
OMA
N
(iJ)
/
C'}H! if~(~rllH I MHrCJlH
Colouted
VV
ill(~ I~l i H~u
/iotlh Sid(ff o~ .~(~(l[ll{3ltl
("'+'-'~ M~t:ro(ontHlentHf
,,_.,, .--"
[~?lhy~
blol:k
F~6.4. The three distinct ophiolite developments in the Makran and surrounding regions with the
associated microcontinent of Southern Tethys; (i) and (ii) are section lines shown in Fig. 5.
Palaeocene rocks indicates that some m61ange
formation continued into the Palaeocene. This
event ended by the late Palaeocene when there
was an abrupt change in palaeography mainly
involving uplift of geotectonic provinces 2, 3
and 4. These provinces are patchily obscured by
minor younger shelf limestone and flysch but
for the most part marine deposition ceased in
the Inner Makran in the Palaeocene.
(5) Eocene-Oligocene flysch
To the south of the Coloured M61ange Zone
is an immensely thick flysch sequence. It is
mainly distal, calcareous, turbiditic flysch, with
thousands of repetitions of classic, complete or
partial, Bouma sequences. It has beautifully
preserved trace fossils and classic flute and tool
marks. Continuous sequences may be more
than 10,000 m thick and certainly occasional
unfaulted sections several thousand metres
thick may be traversed in the field. However,
for the most part the flysch is highly folded and
faulted. There are box, kink, isoclinal and open
folds but by far the predominant type are
periclines of chevron style. There is a slight
south-vergence so that synclines have long
south limbs and short north limbs, thus there is
an overall northward dip of the strata as a result
of the folding. Folds are developed on all scalds
up to a few kilometres across but frequently
both limbs and axial regions are cut out by
north-dipping, steep, reverse faults. There is
dominant faulting out of the anticlines and
preservation of the synclines. This contrasts
with the Zagros style of dominant preservation
of anticlines that occurs west of the Z e n d a n
fault south of Minab in geotectonic province 8
(Fig. 1).
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
392
G. J. H. McCall & R. G. W. Kidd
Commonly the flysch has been so tectonized
that it has become dislocated and often has
developed into a m61ange. This m61ange is
distinct from the Coloured M61ange being very
similar to some of the Franciscan m61anges
(Cowan 1978, 1981). It contains exotic blocks
derived from the underlying Coloured M61ange
basement and also blocks of Eocene pelagic and
shelf limestones. It has been referred to as
wildflysch but it is clearly not. It is very similar
to the exotic flysch of the Arakan, Burma
(Brunnscheiler 1966). The exotic blocks were
all emplaced tectonically, being squeezed up
from the basement like pips. Some large blocks
are composed of Coloured M61ange itself, not
just of blocks that were once constituents of
Coloured M61ange.
In the east of the project area the EoceneOligocene Flysch Zone widens out to over
100 km in width. In this region the flysch is less
commonly tectonized to a m61ange and contains
less exotic components. In the north of the
eastern part of the project area the flysch
appears to grade up into a series of shallowwater basins.
The style of folding described above is similar
to that described by White (1977, 1981) off the
coast of the Makran. In the inland Makran the
folding has been many times more intense and
has developed on much smaller scales as well as
on a large scale. As a result of the intense
deformation it is not possible to demonstrate
that there was syn-sedimentary deformation of
basins as White has described off the coast.
(6) Oligocene-Miocene flysch
Further uplift occurred in this region in the
mid-Oligocene. The trough of flysch deposition
shifted to the south (and to the SW in the
extreme west of the area). The Oligocene to
earliest Miocene flysch is mainly distal, while
the early to middle Miocene flysch is proximal.
There is no evidence of any co-existing shelf
limestone during this time, unlike in the
Eocene.
The deformation of the Oligocene-Miocene
flysch is similar to that of the Eocene-Oligocene
flysch. There is similar development of m61ange
with exotic blocks some of which are of Eocene
flysch. The pronounced spatial realtionship of
dislocation and m61ange formation to Miocene
faults suggests that this extreme tectonism may
be predominantly a Miocene event.
(7) Miocene Neritic Zone
In the south of the area rapid shallowing to
platform sea conditions commenced in the
Aquitanian producing a trough to the north in
which flysch deposition persisted in the late
middle Miocene between the emerging southern area and emergent provinces 2 to 5. Shallow
shelf evaporites and reefal limestones were
formed in the south. The latter have yielded
important and diverse coral collections and
accompanying foraminiferal faunas. In the late
middle Miocene when the flysch trough finally
filled up, coarse detritus spread over the entire
southern area. These middle to upper Miocene
estuarine, deltaic and shallow shelf sediments
range from conglomerate to fine gypsiferous
mudstones. There are local developments of
fluviatile fanglomerate (true molasse) at the top
of this sequence. This sequence forms the chain
of immense open synclines (see Fig. 3) which
are so conspicuous on the Landsat imagery.
These were incorrectly interpreted as uplifted
deepwater sequences of the trench slope by
Farhoudi & Karig (1977). In general the neritic
sediments are contorted where they were thinly
bedded and incompetent and are folded into
large open folds where they are thickly bedded
and competent.
(8) Mio-Pliocene Neritic Zone
In the project area rocks of this zone (the
Makran unit) (Huber 1952; Stocklin 1952) are
only exposed west of the Zendan Fault (see Fig.
1). This sequence is the youngest marine sequence of the Makran and includes gypsiferous
mudstones, deltaic sandstones and estuarine
conglomerates with minor fluviatile conglomerates at the top. The term Makran (or Mekran)
has been loosely applied by palaeontologists to
virtually any neritic sediments encountered
near the Makran coast in Iran and Pakistan
which range in age through the entire Miocene
and Pliocene. Here the name has no stratigraphic significance outside the limited zone
described. Equivalent sediments probably also
occur along the south coast of the Makran
(Stocklin 1952; Anon 1962) outside the area
mapped in this project.
West of the Zendan fault these sediments are
highly folded and even Pliocene beds may be
vertically disposed. The synclines are frequently faulted out so that the anticlines are preferentially preserved. This is the opposite of the
dominant process within tectonic provinces 5, 6
and 7. The different styles of folding may be
due to the different nature of the basement: to
the west of the Zendan fault the basement is
probably continental whereas to the east of the
Zendan fault it is oceanic or comprises oceanic
lithologies broken up into a m61ange.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
393
An active convergent plate margin
Plate tectonics and the evolution of the
Makran
Figure 4 shows the continents, microcontinents, ophiolites and ophiolitic m61anges that
make up the essential tectonic units and sutures
of the Makran and surrounding region. The
plate tectonic development of this region will be
demonstrated by means of diagrams (Fig. 5)
ST
PT
...f]JHH!Hlil]I]I]iilHII~I[~]I/IIII]I]I~[E]HIH]I]ILINIIiJI
b. M i d - J u r a s s i c -
Early
Cretaceous
~:[],H[]I[t]H]':I~I:]
iL
OS z
c
J
~?
~
ST
IMR
Cenomanian - CamDanian
i
that show the relationships of the different
tectonic units across two sections (see Fig. 4):
(1) from Arabia NW across the Persian Gulf,
the Zagros mountains including the Neyriz
ophiolite, the Zagros Crush Zone, the Sanandaj-Sirjan belt, the Baft-Nain belt, the Lut
block, the Birjand-Zahedan belt into the
Afghanistan-Sistan block, and (2) from Arabia
including the Oman ophiolite north across the
Oman Sea, the Makran including the flysch
belt, Coloured M61ange zone, Bajgan-Dur-Kan
zone and Inner Makran ophiolite zone into the
Lut block.
The exact positions in the Palaeozoic of the
microcontinental fragments that now form Iran
are not likely ever to be established but the
similarities of the Palaeozoic sequences of
India, Iran and Arabia (Stocklin 1977) suggest
that along with India, most of lran was once
part of Gondwanaland and separated from
Gondwanaland during the Triassic (Fig. 5a).
By middle Jurassic time a substantial Southern Tethys had developed in this region. Subduction appears to have commenced along the
Sanandaj-Sirjan zone, indicated by andesites of
g. Present
Maastrtchtian
d,
¢~'~N
O
BNMB
.,7
BZMB
SO
.
ii.
SO
ii. HA
CM
NO
~'~
~' AZ~/llZiIHllrL![[iiliNjrljiEilr~:lli:li,l]Hi;i[ir~
~J'__~lllllll!ilillll;l!!lmMl:lil!llllllllllltllNTl~
f.
Late Miocene
~
A. A
....
~
~[llliIlllllllllllllllllllllllllllllllllllll[IHl~ LUT
OMF
AO
AO
~U
MPNS OM.F
EF
AS
~
IMSZ
e. Eocene
i.
~,
---___%
AZ
i,
NO ZCZ
EF
FIG. 5. Sketch sections along lines (i) and
(ii) in Fig. 4 showing reconstructions of the
probable relationships of continents,
micro-continents, ridges, subduction zones
and back-arc basins from Triassic to the
present:
AO Arabia (Oman), AS Afghanistan-Sistan Block, AZ Arabia (Zagros), BDKZ
Bajgan-Dur-Kan zone, BNMB Baft-Nain
marginal basin, BNR Baft-Nain rift,
BZMB Birjand-Zahedan marginal basin,
CM Coloured M61ange, EF Eocene flysch,
HA Hawasina, I Iran, IMR Inner Makran
rift, IMSZ Inner Makran Spreading Zone,
LLIT Lut Block, MPNS Miocene-Pliocene
neritic and molassic sediments, NO Neyriz
ophiolite, NSZ Neyriz subduction zone,
OMF Oligo-Miocene flysch, OSZ Oman
subduction zone, PT Palaeozoic Tethys,
SO Semail ophiolite, SSZ Sanandaj-Sirjan
Zone, ST Southern Tethys, ZCZ Zagros
Crush Zone.
T
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
394
G. J. H. M c C a l l & R. G. W. K i d d
this age (Fig. 5b(i)). There is no direct evidence
of any subduction in the Makran at this time
although Jurassic pelagics in the Inner Makran
(geotectonic province 2) indicate the development of a rift (Fig 5b(ii)). During Early Cretaceous time or possibly a little later, an
intra-oceanic NE-dipping subduction zone developed. It was as a result of collision of the
Arabian continental margin with this subduction zone that the Kermanshah, Neyriz and
Oman ophiolites were emplaced in the Campanian.
Fig. 5(c) shows the situation in Cenomanian
to Campanian time. The continental margin of
Arabia was about to collide with the NEdipping subduction zone. The Hawasina thrust
slices of the Oman were being stacked up in this
subduction zone--the distal uppermost sheet
first followed by successively lower and more
proximal sheets. Finally, in Campanian time
the collision was completed with the Semail
ophiolite being emplaced on top of the stacked
Hawasina continental margin sequence. The
oceanic crust that was subducted would have
been mainly Jurassic and Triassic, yet the
Semail ophiolite is Cenomanian to Coniacian
(Glennie et al. 1973) and hence came from the
ocean to the NE of the subduction zone-where it was part of a forearc, backarc or
even represents the island arc itself. The origin
of the Neyriz and Kermanshah ophiolites (Fig.
5c(i)) is identical to that of the Oman ophiolite.
The only difference is that the Zagros (regarded
as part of the Arabian continental margin to the
SW of the Zagros Crush Zone) collided with
lran along the Zagros Crush Zone during the
Pliocene.
Fig. 5(d) shows the situation in the Maastrichtian when the ophiolites have been emplaced. There was ocean to the NE of both the
Oman and Zagros; this ocean may have still
been spreading at this time. There was certainly
no collision of Arabia with the Makran region
during the Campanian.
In the Makran the Inner Makran spreading
zone was actively spreading from Early Cretaceous until the early Palaeocene. If this was a
subduction-related process then it follows that
there was some subduction of oceanic crust in a
NE or northward direction throughout this
period as indicated in Fig. 5(b(ii), c(ii) and
d(ii)). The Baft-Nain, Birjand-Zahedan and
Sabzevar belts, were also spreading during the
Cretaceous after initiation in the Jurassic. In
the Makran the age of formation of the Coloured M61ange as opposed to the age of its
constituent rocks cannot be precisely dated.
M61ange formation may have commenced as
early as Early Cretaceous or even Jurassic with
late addition of younger rocks. However, the
great abundance of Campanian-Maastrichtian
rocks suggests that most of it was not formed
until Maastrichtian or even Palaeocene. Thus
the Coloured M61ange was mainly formed after
completion of the emplacement of the Oman
ophiolites. Two possible causes for the production of the Coloured M~lange are: (1) collision
of the intra-oceanic subduction zone with the
Arabian continental margin in the Campanian
would cause a major rearrangement of plate
boundaries in this region probably leading to an
increased rate of subduction in the Makran, and
(2) complete subduction of the older oceanic
crust would result in attempted subduction of
young oceanic crust that would be standing
much higher than the old crust and so would be
more easily scraped off the downgoing plate.
The emplacement of the Oman, Neyriz and
Kermanshah ophiolites appears to coincide
with changes in the relative motion between the
African and Eurasian plates (Dewey et al. 1973)
from predominantly compressional with a slight
sinistral component in the region of Arabia to
dextral strike slip. However, the existence of
several plates and more than one subduction
system without preservation of any oceanic
crust between Africa/Arabia and Eurasia
makes it impossible to reconstruct the motion
along a single plate boundary from the motions
of the major continental plates. The probable
age of Coloured M61ange formation is coincident with the breaking away of India from
Gondwanaland and its rapid acceleration northward.
There are no typical andesitic rocks in the
Iranian Makran and it is suggested that the
Inner Makran spreading zone may be the
equivalent of the arc. As described above the
orientation of the sheeted dykes along the
length of the zone and the distinctly calcalkaline
nature of the inner (more northward or northeastward) ophiolites support this suggestion. It
is possible that 'arc' rocks may lie beneath the
Jaz-Murian depression (see Fig. 1) or have been
tectonically removed, but in the former case the
ophiolites would be in front of the arc (rather
than being back-arc) and in the latter it is
surprising that no trace should remain when the
ophiolites are not severely deformed. There are
small amounts of possible 'arc' rocks in the
Coloured M61ange but these could equally well
originate from oceanic island volcanoes or
simply be unusual ocean-floor rocks. It is possible to construct a more complicated plate tectonic model involving many microplates.
However, in this paper we wish to put forward
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
An active convergent plate margin
the simplest model compatible with the known
geology although accepting that whole sections
might have completely vanished.
In this Makran subduction system the Bajgan-Dur-Kan Zone (geotectonic province 3 in
Fig. 1) formed a forearc of shelf limestones on
continental basement. This forearc zone extends to the NW and appears to have been
emergent during the Cretaceous in the NW of
the project area and in the Sanandaj-Sirjan
belt. Since there is a continuation of the Makran to the NW in the Sanandaj-Sirjan and
Baft-Nain belts it is inferred that subduction
also took place along the line of the Zagros
Crush Zone where there are remnants of Coloured M61ange. This is supported by the presence of andesites and flysch of Cretaceous age
along the Sanandaj-Sirjan belt.
The Baft-Nain, Sabzevar and Inner Makran
ophiolite troughs closed up by the late
Palaeocene (Fig. 5e) so that the SanandajSirjan and Bajgan-Dur-Kan microcontinents
had coalesced with the Lut block to form the
Central and East Iran block of Takin (1972). In
the Inner Makran there was uplift and compression of the Coloured M61ange, BajganDur-Kan and Inner Makran spreading zones. It
is possible that there was some minor subduction of the latter zone indicated by the blueschists near Fannuj.
By the end of the Eocene the BirjandZahedan trough had also closed, joining the
Afghan-Sistan Block to the Central and East
Iran microcontinent to form a larger continental block. The resulting tectonic situation was a
simple north-dipping subduction zone in the
Makran with a single continental mass to the
north (Fig. 5e(ii)). There was substantial
Eocene andesitic volcanism to the north of the
project area, which provided the source for the
huge flysch deposits of geotectonic province 5.
This flysch was deposited on an already
accreted prism of Coloured M61ange up to 70
km wide, or possibly partly on oceanic crust,
parts of which were later detached from the
subducting plate and tectonically intruded into
the flysch. It might be possible to distinguish
between these alternative hypotheses for the
nature of the basement to the flysch by detailed
study of the exotics within the flysch. However,
since the oceanic crust subducted during and
subsequent to the Eocene may be similar to that
which formed the Coloured M61ange in the
Maastrichtian or Palaeocene this might prove
difficult.
Along the Zagros Crush Zone there also
appears to have been substantial NE subduction of oceanic crust in the Eocene (Fig. 5e(i)).
395
The main evidence for this subduction is the
great volume of Eocene volcanics 150 km NE
of the Crush Zone in a parallel belt more than
1600 km long. There are also minor accumulations of Eocene flysch indicating the presence
of a trough along the Crush Zone at this time.
These may be remnants of a large volume of
Eocene flysch that has now vanished by subduction or by overthrusting of Central Iran on to
the Zagros. However, it is possible that the
Sanandaj-Sirjan zone was mainly emergent and
blocked off the supply of flysch from the volcanics to the north.
In the Oligocene there was some adjustment
to the north-dipping subduction zone in the
Makran. This resulted in a southward shift of
the trench and uplift of the Eocene flysch,
which then provided part of the source of the
Oligocene-Miocene flysch. This event may be
related to a readjustment of plate motions at
this time which include the cessation of seafloor spreading in the Indian Ocean (McKenzie
& Sclater 1971). The further readjustment of
plate motions when Indian Ocean sea-floor
spreading recommenced in the middle Miocene
approximately coincides with renewed uplift
and yet further southward migrations of the
trench in the Makran. The andesitic volcanics
to the north, the tectonics, and the flysch
deposition, however, are all consistent with the
existence of a north-dipping subduction zone in
the Makran throughout the Tertiary. The discrete tectonic events probably relate to changes
in the rate and/or direction of subduction.
Fig. 5(f) shows the relationship between Arabia and Central Iran in the late Miocene.
Arabia/Zagros is about to collide with Central
Iran while there is still a substantial width of
ocean separating Oman and the Makran. Not
until the Pliocene collision (Stocklin 1977) did
Arabia/Zagros have any direct relationship with
Central Iran. This collision resulted in the
juxtaposition of two separate subduction systems: (1) the subduction system represented by
the Neyriz and Kermanshah ophiolites and
related rocks which had been sitting passively
on the Arabian/Zagros continental margin since
their emplacement in the Campanian, and (2)
the subduction system that had been active
along the SW edge of the Sanandaj-Sirjan belt
from the Jurassic through to the Tertiary. Due
to the deep embayment in the Arabian (plus the
Zagros) continental margin represented by the
Gulf of Oman, Arabia has not yet collided with
the Markan (Fig. 5g(ii)). In the Zagros the
continued covergence resulted in intense folding of the Arabian continental platform sediments and thickening and possible subduction
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
396
G. J. H. McCall & R. G. W. Kidd
even of the continental basement. Along the
Zagros Crush Z o n e all but a few remnants from
the trench zone of subduction system (2) above
have been obliterated by overthrusting of Central Iran from the NE. This effect can be seen in
the NW of the project area near Minab (Fig. 1)
where the i m m e n s e flysch belt of the Makran
has been thinned against the Z e n d a n fault
(which is a continuation of the south side of the
Zagros Crush Zone). Going further NW into
the Zagros (see Fig. 4) the Coloured M61ange
Z o n e also thins except for a few remnants so
that the Arabian continental margin is juxtaposed against the Sanandaj-Sirjan belt (the NW
continuation of the B a j g a n - D u r - K a n Zone).
Subduction in the Makran is continuing at the
present day with deformation of the sediment
of the Gulf of O m a n off the Makran coast
where the subducting plate initially bends down
only about 1° (White & Klitgord 1976; White
1977). Further inland the subducting plate
must bend down more steeply, but the initial low
dip is related to the unusually great (250 Am)
separation between the trench and the 'arc'.
From the Eocene onwards the 'trench' has
migrated southwards while the 'arc' has re-
mained approximately stationary. In the process a substantial accretionary prism of sediment has built up. Accretion of material from
the downgoing plate clearly occurred during the
Late Cretaceous and Palaeocene and may have
occurred since (see discussion above).
If subduction continues as at present, O m a n
will collide with the Makran and this accretionary prism may be overthrust from the north and
obscured as has occurred along the Zagros
Crush Z o n e (assuming such an accretionary
prism was once developed there). The result
will be a single suture from Turkey to Pakistan,
but one marking the disappearance of at least
two subduction systems.
ACKNOWLEDGMENTS:This paper was prepared at the
suggestion of the organizers of this symposium to
present an up-to-date model of the tectonics of this
region based on the recent work carried out on behalf
of the Geological and Mineral Survey of Iran. The
full descriptions of the geology, only summarized
here sufficiently to support the plate tectonic model,
will be published by the Geological and Mineral
Survey of Iran. The contributions of the survey and of
the geologists who worked on this regional mapping
programme are acknowledged.
References
ANON, 1962. Unpublished report on the Geology of
the Southern Makran, AGIP, Milan, filed at the
Geological and Mineral Survey of Iran, Tehran.
BRUNNSCHEILLER,R. O. 1966. On the geology of the
lndo-Burman ranges. J. geol. Soc. Aust. 13,
137-94.
COWAN, D. S. 1978. Origin of blue-schist bearing
chaotic rocks in the Franciscan complex, San
Simeon, California. Bull. geol. Soc. Am. 89,
1415-23.
1981. Deformation of partly dewatered and consolidated Franciscan sediments near Piedras Blancas Point, California (this volume).
DEWEY, J. F., PITMANIII, W. C., RYAN, W. B. F. &
BONNIN, J. 1973. Plate tectonics and the evolution of the Alpine system. Bull. geol. Soc. Am.
84, 3137-80.
FARHOUDI, G. & KARIG,D. E. 1977. Makran of Iran
as an active arc system. Geology, 5, 664-8.
GANSSER, A. 1974. The ophiolitic melange, a world
wide problem, and Tethyan examples. Ecolog.
geol. Heir. 67, 479-507.
GLENNIE, K. K., BOEUF, M. G. A., HUGHES-CLARKE,
M. W., MOODY-STUART,M., PILAAR,W. F. H. &
REINHARDT,B. M. 1973. Late Cretaceous nappes
in the Oman Mountains and their geologic evolution. Bull. Am. Assoc. Petrol. Geol. 57, 5-27.
HARRISON, J. V., & FALCON,N. L. 1936. Geology of
the Coastal Makran. Unpubl. maps, filed at the
Nat. Iranian Oil Co., Tehran and the Geol. Soc.
Lond.
- - ALLISON,A., HUNT, J. A., MALING,P. B.
d MCCALL, R. J. C. 1935-36. Geology of the
Landward Makran. Unpubl. maps, filed at the
Nat. Iranian Oil Co., Tehran and the Geol. Soc.
Lond.
HUBER, H. 1952. Geology of the Western Coastal
Makran area. Unpubl. report, Iranian Oil Co.,
filed at the Iranian National Oil Co., Tehran,
No. GR 9lB.
MCCALL, G. J. H. 1981. Compiler of reports of
Geological and Mineral Survey of Iran:
(1) Report on East Iran Project Area No. 1.
(2) Explanatory text of the Minab quadrangle
map, 1:250,000.
(3) Explanatory text of the Tahenii quadrangle
map 1:250,000.
(4) Explanatory text of the Fannuj quadrangle
map 1:250,000.
(5) Explanatory text of the Pishin quadrangle
map 1:250,000.
(6) Explanatory text of the Nikshahr quadrangle
(southern half), 1:250,000.
(7) Explanatory text of the Saravan quadrangle
(southern half), 1:250,000 (in press).
MCKENZIE, D. P. & SCLATER,J. G. 1971. The evolution of the Indian Ocean since the Late Cretaceous. Geophys. J. R. astron. Soc. 24, 437-528.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 12, 2016
A n active c o n v e r g e n t plate m a r g i n
397
STOCKLIN, J. 1952. Geology of the Central Coastal
Makran Area. UnpubL report, Iranian Oil Co.,
WHITE, R. S. 1977. Recent fold development in the
Gulf of Oman. Earth planet. Sci. Lett. 36, 85-91.
filed at the National Iranian Oil Co., Tehran,
No. GR 91C.
1977. Structural correlation of the Alpine
ranges between Iran and Central Asia. Mere. h.
Ser. geol. Fr. 8, 333-53.
TAKIN, M. 1972. Iranian geology and continental drift
in the MiddleEast. Nature. London,235,147- 50.
--
& KLrT~ORD, K. 1976. Sediment deformation
and plate tectonics in the Gulf of Oman. Earth
planet. Sci. Lett. 32, 199-209.
WHITE, R. S. 1981. Deformation of the Makran
accretionary sediment prism in the Gulf of Oman
(this volume).
G. J. G. McCALL, 57 Venns Lane, Hereford, U.K.
R. G. W. KIDD, RSMAS-MGG (Marine Geology and Geophysics), University of
Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, U.S.A.