Comprehensive Summaries of Uppsala Dissertations
from the Faculty of Science and Technology 836
The Effect of Mechanical
Characteristics of Basal
Decollement and Basement
Structures on Deformation of the
Zagros Basin
BY
ABBAS BAHROUDI
ACTA UNIVERSITATIS UPSALIENSIS
UPPSALA 2003
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Contents
Introduction……….……………………………………….………….……4
Summary of papers………………………….………………….…….……8
I. Extension above frictional and viscous decollements………..………...10
II. Modes of normal faulting in two-layer stretching models…...…….….14
III. Shapes and timing of structures of Hormuz salt….………………..…18
IV. The basement configuration of the Zagros basin.……………………22
V. Effect of Hormuz salt on contractional deformation …...…………….26
VI. Tectono-Sedimentary framework of the Gachsaran formation………31
Conclusions………………………………………………..………………36
Acknowledgements ……………….………………….………………...…38
References……………………………………………………………….…40
1
2
For in an out, above, about, below
Tis nothing but a magic shadow – show,
Play’d in a box whose candle is the sun,
Round which we phantom figures come and go.
(Omar Khayyam Nyshabouri; Persian poet)
3
Introduction
This thesis argues that two structural elements, basement grain and basal
decollement, play significant roles in governing the style of deformation in
cover sequence of Fold-thrust belts like that in the Zagros. The essential
clues to understanding the structural evolution of such regions depends on
our knowledge of how they can be affected by reactivation of basement
fabrics and the spatial distribution of decollement types.
Despite, the common use of “basement” in the geological literature, there
is no unique definition for it among Earth scientists. The meaning of
basement depends on who is speaking. For geophyicists, basement may be
the depth where their data disappears, for pedologists, the basement is
whatever the soil lies up on, and for orogenic geologists, the basement
consists of assemblage of old crystalline rocks deformed by earlier orogenies
unconformably overlain by initially undeformed a cover sequence. The
“basement” to which this dissertation refers, consists of an undifferentiated
complex of Precambrian rocks, with complicated fabrics (foliation, sutures
and faults) overlain by the mainly Phanerozoic cover sequence.
In the geological literature, many studies have addressed the interplay
between cover and basement fabrics in the tectonic evolution of orogenic
belts. There are two-end members of cover/basement interfaces, high
frictional, and a viscous or ductile decollement of, for example rock salt or
over-pressured shales. In thin-skinned tectonics, a cover is decoupled from
its basement by basal decollement which separates different deformation
styles above and below.
The Zagros fold-thrust belt is part of orogen interpolating the AlpineHimalayan chain for 2000 km through Iran and Iraq. It is an attractive case
to study the interaction between the two structural elements mentioned
above on the evolution of an orogen. The tectonic history of the Zagros belt
has generally experienced three different tectonic episodes: stable platform
in early Palaeozoic, Permo-Triassic extension and Cenozoic contraction. The
reactivation of old tectonic grains in the basement and a thick basal viscous
decollement play important roles in both the deformation phases.
Comparison of the Paleozoic stratigraphies of Arabia, Iran and the
surroundings on either side of the Main Zagros reverse fault (suture line)
reveals comprehensive correlation. This similarity is explained by many
paleomagnetic and paleogeographic studies which indicate that Central Iran,
Pakistan, Central Afghanistan, Arabia, and Turkey are all characterised by
similar epicontinental platform sediments deposited on the same passive
margin along the northern shores of Gondwana from late Precambrian to
upper Paleozoic times (e.g. Stöcklin, 1968a, 1974, 1977; Adamia et al.,
1981; Berberian and King, 1981; Koop and Stoneley, 1982; Scotese and
4
McKerrow, 1990; Beydoun, 1991; Golonka, 2000). The onset of rifting
through northern to northeastern Arabia and Iran is demonstrated by
differentiation in sedimentary facies and rift-related magmatic activity on
either side of the Main Zagros Reverse Fault. A phase of Permo-Triassic
extension along the active margin of Neo- (or Permo-Triassic)-Tethys along
the Sanandj-Sirjan zone is marked by volcanic activity associated with
shallow lagoonal deposits with rapid facies variation due to block faulting
(Stöcklin, 1974; Berberian and King, 1981; Cherven, 1986; Stampfli et al.,
1991). By contrast, isopach and facies maps of the Upper PaleozoicMesozoic carbonates and shales in the Zagros (Koop and Stoneley, 1982)
simply parallel a likely continental margin along the Main Zagros Reverse
Fault (Stöcklin, 1974; Morris, 1980; Berberian and King, 1981).
The Permo-Triassic opening and Cretaceous closure of the southern
passive margin of Neo-Tethys in Arabia was compared to a zip fastener with
basement keys defined by N-S and NW-striking Pan-African faults in the
basement (Talbot and Alavi, 1996). By contrast, the active northern margin
of Neo-Tethys was the straight NW-SE trending Main Zagros Reverse Fault.
Ophiolitic complexes associated with deep-water radiolarites exposed along
the Main Zagros Reverse Fault are attributed to Mesozoic sea-floor
spreading which was followed by subsequent subduction and ophiolite
obduction in the upper Cretaceous-Paleocene (Berberian and King, 1981;
Cherven, 1986). As Neo-Tethys closed in Iran, the same N-S and NW-SE
trending Pan-African tectonic grains in the Precambrian basement that
controlled the opening and closure of Neo-Tethys also appear to have
influenced how and where the lithosphere pinched and rifted between Africa
and Arabia on the way to opening the Red Sea in Tertiary times (Dixon et
al., 1987; Ghebreab and Talbot 2000).
The convergence between Arabia and Central Iran has resulted in
mountains with a deformation front that migrated southward and drove the
foreland basin in front of it. The Zagros fold-thrust belt widened at different
rates either side of the Kazerun fault zone which divides areas with different
basal decollements. Recent GPS measurements (Hessami, 2002) show that
the Zagros fold-thrust belt is being shortened at a lower rate (9-11 mm/y).
Earthquakes of 5.5-6 Mb are common in a seismic zone ≈200-300 Km wide
along the Zagros fold-thrust belt (Jackson, 1980; Baker et al., 1993;
Chandra, 1994; Berberian, 1995). All this seismic activity is confined to
depths of less than 40 Km and mainly attributed to reactivation of faults in
the basement (e.g. Jackson, 1980; Jackson et al., 1981; Berberian, 1995).
Nowhere is the basement exposed within the Zagros orogen or the
Arabian platform. However, samples of basement rocks have surfaced as
exotic blocks in diapirs of Hormuz salt within the orogen and have been
compared to the Pan-African basement exposed in the Arabian shield. The
5
basement of the Zagros fold-thrust belt has been inferred to have in-built
structural grains which have been suggested to reactivate episodically since
the late Palaeozoic. Lateral variations in the stratigraphic sequence of the
cover throughout the region record syn-depositional reactivation of the
basement structures. Broad gentle anticlines and narrower synclinal basins
(Beydoun, 1991) or graben with a variety of trends affect most of the
thickness of the Phanerozoic cover deposited on the Arabian shield (Al
Laboun, 1986; Alsharhan and Nairn, 1997). Such structures are commonly
referred to as lineaments, blind or hidden basement faults (Berberian, 1995),
geo-flexures (Falcon, 1974), geowarps (Ameen, 1991a, 1991b, 1992)
periclines (Alsharhan, 1989), drape folds (Edgell, 1991) or forced folds
(Cosgrove and Ameen, 2000; Sattarzadeh et al., 2000). Many of these
lineaments were recognised on satellite images, air photos or seismicity (e.g.
Barzegar, 1994; Berberian, 1995; Hessami et al., 2001a). Previously, 10 to
14 main basement faults have been assumed to underlay the Zagros belt
using mainly surface evidence (e.g. Berberian, 1995; Hessami et al., 2001a).
Their geometry, number, and the times they reactivated, are still poorly
constrained. One reason for this uncertainty is the decoupling of cover
sequence from the basement by the Hormuz salt.
Mechanical tests on rock salt indicate that rock salt is very weak and
highly ductile at shallow levels of the crust, 3-5 km (Davis and Engelder,
1987; Carter and Hensen, 1983; Urai et al., 1986). A viscous decollement of
salt along the unconformity between the basement and its cover reduces
basal friction of the cover sequence. This leads to horizontal decoupling of
the cover from the basement. This decoupling has significant effects on
deformation style not only in extension but also in compression. In addition
to the Zagros belt, there are at least 12 other fold-thrust belts world wide that
have been shortened above an evaporitic substrate acting as a viscous
decollement (e.g. Davis and Engelder, 1985, 1987). The presence of a
viscous decollement allows the deformation front to propagate faster with a
lower taper as compared to frictional decollements during compression (e.g.
Davis and Engelder, 1987; Letouzey et al., 1995; Talbot and Alavi, 1996;
Cotton and Koyi, 2000). The Neo-Proterozoic-Cambrian Hormuz salt buried
beneath a thick (10-14 km) sedimentary cover has acted as a viscous
decollement between parts of the cover and its Precambrian crystalline
basement throughout the Phanerozoic. The distribution of the Hormuz salt is
confined towards the east of the N-trending Kazerun fault zone where there
are over 200 buried and emergent structures of Hormuz salt. Nowhere in the
Zagros basin is the sequence of Hormuz salt exposed complete and
undistorted. However, the distribution of salt structures implies an uneven
distribution of the Hormuz salt resulting in two different types of basal
decollements in different areas. The structural significance of this uneven
6
distribution of these decollements during N-S lateral extension and
compression regimes of the Zagros belt has not been given the attention it
deserves. This subject of how Zagros structures evolved is very important to
understand because they are associated with major hydrocarbon reserves.
The Paleozoic-Middle Miocene sedimentary sequence of Arabia and the
Zagros is one of the richest hydrocarbon habitats known and contains 66.7%
of the proven recoverable oil and 31.5 % of proven gas reserves of the world
(Murris, 1980; Beydoun, 1991). The Zagros fold-thrust belt and its foreland
(referred together as the Zagros basin) house 98.7% of this oil and 97.2% of
the gas (Beydoun, 1991; Beydoun et al., 1992; 1991 Edgell, 1996; Alsharhan
and Nairn, 1997).
This thesis uses analogue models, (i.e. physical models of natural
processes) to study the style of deformation in the cover sequence having
frictional and viscous decollements in different areas (with or without
reactivation of basement faults) during lateral extension and compression.
The late Paleozoic-Mesozoic extension across what is now the Zagros belt is
poorly known. The results of extended models can be indirectly applied to a
region like the Zagros belt where different parts were extended over
frictional and viscous basal decollements. This thesis also attempts to
provide more structural evidence for the extension using new method to
constrain the timing of movement of salt structures in the region. In contrast,
the results of shortened models can be applied directly to the much better
known structural evolution of the Zagros fold-thrust belt during lateral
contraction. To understand and distinguish the effect of basement
reactivation of basement faults beneath the Zagros belt, especially during
shortening, this thesis combines all available data, including isopachs,
aeromagnetic, seismicity etc., which might indicate the influence of such
basement faults beneath the Zagros orogen to constrain a basement
configuration for Arabia and the Zagros. Integrated with the model results,
this configuration can help to distinguish the effects of reactivation of
basement faults from those of decollements on the cover sediments. This
integration is finally presented as a new tectono-sedimentary model for the
evolution of the Zagros foreland basin.
7
Summary of the papers
This thesis is based on the following papers, which are referred to in the text
by their Roman numerals:
I. Bahroudi, A., Koyi, H. A., Talbot, C. J., Effect of ductile and frictional
decollements on style of extension. Journal of Structural Geology, in press.
Scaled analogue models are used to illustrate how contrasting decollements
in thin- and thick-skinned extension can result in structural differences in the
cover. These model results can be applied indirectly to lateral extension on
either side of the Kazerun fault in the Zagros basin during the Mesozoic. As
in the models, the Zagros basin is divided into two halves by the Kazerun
fault where one half is deformed on a frictional decollement and the other on
a viscous decollement.
I initiated the main idea and made all models in the Hans Ramberg
Tectonic laboratory and also prepared the original manuscript. HAK
schooled me in techniques of sandbox analogue modelling and introduced
the idea of step-wise extension with useful discussion during preparation of
the manuscript. CJT contributed with an idea about low-frictional
decollements and fruitful discussions during the preparation of the
manuscript.
II. Mulugeta, G., Bahroudi, A., Modes of normal faulting in two-layer
stretching models: implications for salt tectonics. Manuscript submitted to
Journal of Structural Geology.
This study uses two-layer models to show that the mode of thin-skinned
extension above a viscous decollement is controlled by boundary conditions
and the brittle/viscous thickness ratio. The model results can be used
indirectly for understanding the structural evolution of the east Zagros of the
Kazerun fault where Hormuz salt formed a thick viscous substrate beneath
the sedimentary cover.
GM developed the idea in this manuscript and wrote it. I made all models
and participated in preparation of the original manuscript.
III. Bahroudi, A., Talbot, C. J., Shapes and timing of structures in Hormuz
salt in the Zagros basin, Manuscript submitted to Tectonophysics.
This study uses field data and the literature for a new approach to constrain
the timing of movement of structures of Hormuz salt. The results indicate
8
that much of the Hormuz salt began moving during the extensional regime in
the Zagros basin.
I had the idea in this manuscript and wrote the original text. CJT
contributed with useful discussions about different aspects of diapirism in
the Zagros basin during preparation of the manuscript.
IV. Bahroudi, A., Talbot, C. J., The Configuration of the basement beneath
the Zagros basin, Manuscript submitted to Journal of Petroleum Geology.
This study uses all available data sets in an approach to distinguish the old
basement faults that reactivated from those new faults formed in the cover
by the Zagros shortening. Here, a new tectonic framework is suggested for
the Arabian plate.
I developed the idea and wrote the manuscript. CJT contributed with
useful discussions during preparation of the manuscript.
V. Bahroudi, A., Koyi, H. A., Effect of spatial distribution of Hormuz salt on
deformation style in the Zagros fold and thrust belt: an analogue modelling
approach, Journal of the Geological Society, London, in press.
This study illustrates the results of analogue models scaled to simulate the
Cenozoic thin-skinned shortening in the Zagros fold and thrust belt. The
model results show how the uneven distribution of the Hormuz salt has
given rise to a complicated pattern of two contrasting decollements which
result in some significant kinematic and geometric consequences during the
shortening of the cover sequence in the Zagros fold and thrust belt.
I had the initial idea in this manuscript and developed it with HK. I
carried out all models and interpreted them with HK and wrote the
manuscript.
VI. Bahroudi, A., Tectono-Sedimentary framework of the Gachsaran
formation in the Zagros foreland basin, in preparation.
This paper illustrate a consequence of the interaction between reactivated
basement faults and the spatial distribution of the Hormuz salt during the
Cenozoic shortening of one of the most important hydrocarbon seals, along
the Zagros fold and thrust belt: the Gachsaran formation.
I initiated the idea in this manuscript and also wrote it. CJT and HAK also
contributed with useful discussions and comments during the preparation of
the manuscript.
9
I. Extension above frictional and viscous decollements
Scaled sandbox models are used here to investigate the effect of basement
faults beneath frictional and viscous decollements on the style of extension
in the upper crust. The base of all models was planar and horizontal. Each
model was divided into two equal halves either parallel or oblique to the
extension direction. One half had a layer of loose sand resting on a viscous
decollement, consisting of a layer of silicone putty (SGM-36). The other half
had either a frictional decollement, made of a layer of loose sand or a low
frictional decollement consisting of glass beads resting directly on a rigid
substrate.
Two different basal configurations were used to extend the models. A
folded banded sheet which resulted in stepwise extension, was used to
simulate small displacements along basement faults. A rubber sheet was
used to simulate homogeneous thin-skinned extension.
Step-wise extension
Comparison of the final extended length of models with their initial lengths
above the frictional decollement shows that most of the extension occurs
along normal faults formed in the sand layer (Fig.1). By contrast, extension
propagated above the viscous decollement in a wider zone consisting of
horsts and grabens associated with a substantial amount of layer-parallel
penetrative strain.
Comparison of profiles recovered from the viscous decollement and the
frictional decollement halves show that:
• Extension is accommodated in a wider zone above the frictional
decollement.
• Normal faults are steeper ≅60° above the frictional decollement.
• Normal faults do not intersect above the viscous decollement.
• Block-rotation is more common above the viscous decollement.
• Normal faults are clearly more numerous above the frictional
decollement.
• Penetrative strain occurs above the viscous decollement.
In profiles, normal faults extrapolate down to the edges of rigid strips
with a one to one correlation above the frictional decollement halves and
conjugate sets intersected each other at shallow level within the sand layer
(Fig1b). By contrast, there is no such correlation between active strips and
faults formed in the sand layer extended above the viscous decollement
halves (Fig. 1c).
In plan view, normal faults deflect, overlap and terminate at a boundary
zone between the frictional and viscous decollement halves (Fig.1a). This
10
boundary represents a transfer or accommodation zone with relay ramps
providing weak links between faults.
Homogenous extension
The development of normal faults in some models is almost restricted to the
viscous decollement halves (Fig.2a). Apart from a few faults generated close
to the end walls, no faults formed in the sand layer above the frictional
decollement halves (Fig. 2c). Measurements of initially square markers on
the surface of the deformed models indicate some differences in strain
distribution in the viscous decollement and frictional decollement halves.
MDL-1 HRTL
3 cm
CR
R
b
FD
Deformat ion Widt h
a
profile b
profile c
Deformat ion Widt h
MDL-1 HRTL
c
3 cm
DD
Striped sheet
Fig. 1. Line drawings of profiles and planview of extended model 1, a) model plan
view showing development above the FD and DD halves b) profile of the FD half
shows closely spaced hors-graben structures above active stripes, dark and small
rectangulars, c) the DD half with wider horst and graben in a longer deformation
zone.
11
Square markers above the viscous decollement show between 0-10%
penetrative extension of strain. By comparison, markers were extended
homogeneously and uniformly above the frictional decollement half (i.e.,
about 90% of the bulk extension was by penetrative strain). The frictional
coupling between the rubber sheet and the overlying sand layer was
sufficient to produce uniform and homogeneous layer-parallel extension.
The boundary between the viscous decollement and the frictional
decollement halves of the models is indicated in plan view by the
development of small fractures and fissures where the normal faults which
developed above the viscous half decollement die out when passing into the
frictional half decollement.
Extension
MDL-2
3 cm
b
a
profile b
profile c
Extension
MDL-2
3 cm
c
Fig. 2. a) Line drawing of plan view of model 2, b) profile shows horst-graben
structures with a reactive diapir after 20% bulk extension, c) profile across the FD
half showing homogeneous extension of sand layers,.
12
In profiles, the halves of models above the viscous decollement extended
by well-defined normal faults each of which began to form near the moving
wall of the box. Successive faults developed sequentially toward the fixed
wall. In models where the viscous substrate was thick, a reactive diapiric
structure rose where the sand layer was thinned by ~ 20% of bulk extension.
As the brittle/viscous thickness ratio increased, symmetric horsts and
grabens developed over most of the length of the models.
Thus our analogue models confirm recent numerical models by Harper et
al., (2001) who also found that frictional cover sequences uniformly
extended above frictional decollements thin uniformly without significant
faulting.
Model results show that there is an inverse relationship between the
thickness of the viscous substrate and the number of faults formed in the
overlying layers. As the thickness of the viscous substrate decreases the
number of faults formed in the overlying layers increase and vice versa (Fig.
3).
25
Number of faults
Thickness of silicon layer (mm)
or number of faults in cover
20
15
10
5
Silicon thickness,mm
0
MDL-2
MDL-3
MDL-4
MDL-5
MDL-6
MDL-8
MDL-10
Models
Fig.3. Plot showing relation between number of faults and thickness of the viscous
substrate in different models extended above the rubber sheet.
13
II. Modes of normal faulting in two-layer stretching models
This study uses a series of two-layer models (consisting of loose sand above
silicone putty) with different brittle/viscous thickness ratios to investigate
modes of normal faulting in the brittle layer during extension. Two types of
models are considered: 1) Unconstrained models in which the front
collapses and extends freely under gravity (Fig. 4); and 2) constrained
models in which the rate of displacement of an end boundary controls the
rate of gravitational collapse (c.f. Figs. 4 and 5). These two sets of
experiments resulted not only in different geometries of normal faults but
also produced different cross-sectional topographies as a result of different
distributions of strain.
a
3 cm
5
4
b
6
5
3
1
2
3 cm
4
3
c
2
1
3 cm
3 cm
d
Fig.4. Final geometry of unconstrained extended models with different brittle/
viscous thickness ratios, a, b) models with high brittle/ viscous thickness ratios
showing synthetic normal faults accommodating antithetic faults between the blocks.
c) models with low initial brittle/ viscous thickness ratios showing sequential normal
faults above the viscous layer, d) showing conjugate faults. Arrows indicate
extension direction.
14
In contrast to previous models (e.g. Vendeville and Jackson, 1992) which
suggested extension at a steady rate, this study shows that the collapse rate
varies along the viscous layer with time in both types of models. This has
great implications for the coupling between a brittle layer and its viscous
substratum and hence for the kinematics of normal faulting and extensioninduced diapirism.
Unconstrained gravitational collapse of models with high brittle/viscous
thickness ratios produced domino style normal faults and a wedge-shaped
topography in cross-section. A sequential array of synthetic normal faults
propagated rearwards from the initial deformation front (Figs. 4a-d). With
time, steeper antithetic normal faults also developed as accommodation
structures within the blocks bounded by the synthetic faults in response to
the rotation and locking of the synthetic faults (Fig.6a). The interface
between the frictional and viscous material developed a saw-tooth
perturbation owing to rotation of the fault blocks during extension. Because
the buoyancy effect of the viscous substratum is subdued by the strength of
the thick brittle overburden, there is no local rise of the viscous material as
diapiric intrusions. In nature, the structural characteristics of unconstrained
extension are salt rollers beneath synthetic normal faults (Brun et al., 1993).
In nature, there are numerous examples of synthetic normal fault arrays
which develop above allochtonous salt (e.g. Diegel et al 1995; Mohriak et al
1995; Morley and Guerin 1996). By comparison, unconstrained models with
low to moderate brittle/viscous thickness ratios lack a strongly preferred
vergence of normal faults; and thus exhibit faults with mixed fault facing
directions (Figs. 4c, d). In agreement with the experiments, natural examples
of thin-skinned fault systems with low to intermediate brittle/viscous
thickness ratios often show faults with mixed polarity (e.g. Trudgill and
Cartwright, 1994).
In constrained models with high initial brittle/viscous thickness ratios (e.g.
Figs. 5a-d), the viscous substratum both stretches and increasingly decouples
the brittle overburden by migrating towards the front to rise as a diapir in the
gap opened by the extension. This model illustrates the diapir-generating
effect of an edge-discontinuity as a consequence of decoupling of the brittle
layer from the viscous substratum (Fig. 5a). In nature, such a damming effect
may be provided by lateral facies changes which result in basin-edge
diapirism (Jenyon, 1986). In constrained models, decreasing the
brittle/viscous thickness ratios induces pronounced growth of secondary
perturbations and hence reactive diapirism beneath conjugate normal faults
(Figs. 5b-d). Constrained gravitational collapse of the two-layer systems
with low to moderate brittle/viscous ratios, results in the formation of
conjugate normal faults in the brittle layer (Fig. 5b). As these evolve into
grabens, the viscous substratum thickens beneath the grabens and thins in the
15
intervening regions. This work also suggests that extensional diapirs rise at
the sites of normal faulting in response to the higher differential stress
arising from variable loading of a viscous substrate by the brittle overburden,
as compared to the differential stress arising from gravity collapse of the
viscous substratum.
a
b
c
d
3 cm
3 cm
3 cm
3 cm
Fig.5. Final geometry of constrained models with different brittle/viscous thickness
ratios, a) models with high brittle/viscous thickness ratios and normal faults, note
decoupling and front-edge diapirism of the viscous substratum. b-d) models with
moderate to low initial brittle/viscous thickness ratios showing extrusive diapirs
developed where conjugate normal faults thinned and weakened the overburden.
Arrow indicates extension direction.
It has been suggested that “because salt is so weak at geologic strain rates,
viscous forces in flowing salt cannot drag or stretch the overburden (Jackson
16
et al., 1994). Here, it is contended that the degree of coupling between the
brittle and viscous materials controls the style of normal faulting in the
brittle layer.
The relative magnitudes of these stresses determine the coupling and
hence sense of shear at the interface between the brittle layer and the viscous
substratum. The orientation of normal faults is determined by the local
orientation and magnitude of the principal axes of finite strain, during
extension.
a.
σ1
Brittle
σ1
b.
δ >δ
b
f
δ =δ
b
f
Fault
Fault
δ
f
δ
Brittle
δ
δ
f
b
b
θ
τ
τ
Viscous
Viscous substrate
Strain ellipse
Fig. 6. Effects of stress distribution and shear coupling at the interface between the
viscous and brittle material on modes of normal faulting: a) single polarity normal
faulting, b) conjugate normal faulting.
17
III. Shapes and timing of structures of Hormuz salt
Despite more than 200 salt structures of the Hormuz series being known in
the Zagros basin (Fig.7), and despite many published studies, the
stratigraphic sequence of the Hormuz series, the times of the onset of
movement, and subsequent episodes of growth of salt structures are not well
constrained.
In the absence of any complete undisturbed sequence of the Hormuz
series, its sequence has been reconstructed on the basis of indirect
observations (Player, 1969; Kent, 1979; Edgell, 1991). The Hormuz series is
distinguished into two sequences of salt (1-2.5 km thick) separated by a few
hundred metres of carbonates and red beds. The lithofacies boundaries of
both these sequences occur above old faults in the Precambrian basement
(Talbot and Alavi 1996).
The evolution of Zagros basin is generally divided into three major phases
of tectonic activity: 1) stable continent to the Permian, 2) lateral extension
that opened Neo-Tethys in early Permo-Triassic times and 3) the Cretaceous
closure of Neo-Tethys and subsequent continental convergence. During the
first of these phases, about 3 km of clastic sediments accumulated and
consolidated on the Hormuz salt (Setudehnia, 1972, 1975; Husseini, 1990,
1991) without any indication of any salt movement.
The Hormuz salt appears to have first moved into elongate pillows forced
above pre-existing faults in the Precambrian basement reactivated by late
Palaeozoic-early Mesozoic tectonics. Such early structures still exist as
gentle drape-folds in the cover above N-S to NE-SW trending faults in the
basement beneath the oil fields of Arabia still not affected by Zagros
deformation (Figs.7 and 8). Later reactivations of the same basement faults
by lateral extension thinned and weakened the cover in linear zones through
which reactive salt diapirs could be driven by differential loading (Jackson
and Vendeville, 1994). This lateral extension was likely regional along
normal faults during the opening of Neo-Tethys (Falcon, 1969; Jackson
1980).
As suggested by Kent (1979) isopach maps were used as a tool to study
salt movement in the Zagros basin. These maps (Koop and Stoneley, 1982)
allow inferences about the bathymetry of the Zagros basin through time. The
inferred bathymetry provides a useful background against which the effects
of basement fault reactivation and salt movement can be detected. Subcircular isopach patterns only several tens of kilometres across are
considered as local features. As many emergent plugs of Hormuz salt are
surrounded by local highs in the current deposition surface, such local
18
19
Fig. 7. Distribution of structures of Hormuz salt and hydrocarbon pools in the Zagros basin, showing their
relationship with N-S trending basement faults.
features and their variations in shape with time can be attributed to the
evolution of salt structures.
N
34
N
32
N
30
N
28
Fa
p l a t f orrsm
0
N
44E
150 Km
46E
Persian G
ulf
48E
50E
52E
Qata
Salt structures
salt pillow
blind salt diapir
salt ridge
salt mushroom
salt droplet
degraded salt
droplet
salt diapirs
on pre-Zagros
Cuspate salt mullion
(or welt) in core of
Zagros anticlines
salt fountain
breccia chimney
ridge
Fig. 8. Block diagram of the Zagros fold-thrust belt and Gulf showing distribution of
Hormuz salt (in white) and the shapes of structures in it.
The effect of salt movements on local changes in thickness of
contemporary sedimentary units is visible as many individual isopach
closures in Laristan province. This approach extends methods used by earlier
field geologists to unexposed and older country rock units around each salt
plug (Fig.9; Kent, 1958; Player, 1969). The dating of salt activity by field
observations is inevitably restricted by the age of the oldest strata exposed
around each salt plug. However, until seismic data for the region becomes
available, isopach maps offer the only approach to timing earlier activity in
the Hormuz salt. Isopach maps indicate that movement of Hormuz salt began
as early as the Triassic and have generally increased since then.
Individual isopach closures around them indicate that nearly all the
structures increased in the area, number and amplitude. The structures of
Hormuz salt along Kazerun-Mangarak zone and the shoreline of Laristan
province reactivated repeatedly after they emerged at the surface (Fig.9).
20
Main
30
N
34
32
N
N
g
Zagros
Reverse
Paleocene-Eocene
Fault
58
E
Sh i r az
D ezf u l En b a y m en t
56
E
Basr a
No Sign
Possible
Probable
Qatar
N
E
54
Very likely
Certain
150 Km
0
E
E
52
50
E
48
E
46
E
44
Fig.9. Paleocene-Eocene isopach map of the Zagros basin showing depocentres and
homoclines (interpreted for different periods) and likelihood of activity of structures
of Hormuz salt in different periods.
However, the clearest activity of Hormuz salt appears on the PaleogeneEocene isopach map with its relatively high resolution (Fig.9). The evidence
in this paper indicates that most of structures of Hormuz salt in the Zagros
basin were active before Zagros folds began propagating to the SW from the
Main Zagros reverse Fault in the Eocene.
21
IV. The configuration of the basement beneath the Zagros basin
The number, distribution, space-time activity and interrelation of faults in
the crystalline basement to the Zagros basin are still poorly understood. No
seismic profiles across the region down to the basement are in the public
domain, and there are no other criteria for distinguishing basement faults
from faults imposed on the sedimentary cover by Zagros shortening. This
study develops criteria for recognising old faults in the basement and their
reactivation and applies them to the 14 cases previously suggested so as to
clarify the basement configuration of the region through time (Fig.10).
Although, the Precambrian basement is not exposed anywhere within the
Zagros basin, it is usually assumed to be a north-eastward continuation of the
Precambrian Nubian-Arabian shield exposed in Arabia (Falcon, 1967, 1969;
Al Laboun, 1986; Alsharhan and Nairn, 1997). Probable samples of the
Zagros basement that have surfaced as inclusions in diapirs of Hormuz salt
indicate that it consists of migmatites, gneissose granites, garnetiferous
limestones, schists, and phyllitic mudstones with some amphibolites, mafic
mylonites and serpentinites intruded by granite, gabbro and basalt (Harrison,
1930, 1931; Kent, 1970, 1979; Gansser, 1992).
As a template for the Zagros basement, the Nubian-Arabian shield
contains two main tectonic trends. There are the left-lateral strike-slip faults
of the Najd fault system that offset the older N-S ophiolitic belts by up to
300 km (Fig.10). These structures not only show trends similar to those in
the ZFTB, but also several structures can be extrapolated northward into the
Zagros basin before they were reactivated and/or distorted by the arrival of
the Zagros front during the Cenozoic (Berberian, 1995; Talbot and Alavi,
1996).The latter effect is particularly obvious in the pattern of hydrocarbon
fields. (Fig.10).
Using air photos (McQuillan, 1991) or satellite images (Furst, 1990;
Barzegar, 1994), morphological, sedimentary, and seismic evidence,
previous workers have interpreted 14 of the major fault zones in the Zagros
belt as due to the reactivation of old faults in the underlying basement (Fig.
10; Murris, 1980; Koop and Stoneley, 1982; Motiei, 1995; Berberian, 1995).
In the Zagros fold-thrust belt, the N-S trending faults of the 14 include the
Kazerun, Kareh Bas, Hendijan, Khanaqin, and Sarvestan faults and the
NNE-SSW trending Razak fault. These are all steep to vertical with
significant strike-slip displacements (Barzegar, 1994; Berberian, 1995;
Hessami et al., 2001a). The remainder trend NW-SE, like the Najd fault
system and include the Main Zagros Reverse Fault (MZRF), the High
Zagros thrust fault (HZTF) and the Zagros Mountain Front Fault (ZMFF).
Fault plane solutions for earthquakes indicate that all the NW-SE faults are
22
Fig. 10. Main structures in the Arabian plate: (1) Contract ional Fault, (2)
Intercontinental basin, (3) Strike-slip Faults: DI, Dibba fault; BSF, Bostaneh fault;
BAF, Bastak fault; RZ, Razak fault; NZ, Nezamabad fault; SF, Savastan Fault Zone;
KBF, Kar Bas Fault Zone; KF, Kazeron Fault Zone; HNF, Hendijan fault; BR,
BalaRud fault; and KHF, Khanqin fault;EAF, East Anatolian Fault; (4) Transform
Fault, (5) Normal Fault, (6) Spreading Axis, (7) Master Fracture, (8-10) Basement
lineaments in Arabia (Anticline/Arch, Syncline/Basin, and Flexure) including TSB,
Tabuk Basin; MH, Mardin High; EG, Euphrates Anah Graben; HRA, Hail Ga’ara
Rutbah Arch; WSB, Widyan Basin; TAG, Trans Arabian lineament; CAG, Central
Arabian Graben; KA, Kuwait Arch; SP, Summan Platform; EN, En Nala Safaniya
Trend; QA, Qatar Arch; RKB, Rub Al Khali Basin; MLA, Mender Lekhwair
High;OA, Oman basin lineament; HQA, Huqf Arch; WD, Wadi Al-Batin lineament;
HA, Hadhramout Arch; MA, Mukalla Arch; NA, Nisah-sahba lineament; SG, Sadah
Graben, (11) Oil and gas fields.
23
reverse faults and dip about 60º NE. East of the Kazerun fault, the Bastak,
Nezamabad and Bostaneh faults are all left-lateral strike slip faults with NE
trend that have been also attributed to reactivation of basement structures.
Even the Bala Rud fault with its unique E-W trend has been nominated as a
basement fault (Motiei, 1995; Hessami et al., 2001a).
In this study, a variety of data sets were used to develop some criteria to
decide which of all 14 faults previously proposed as basement structures
show evidence actually having been so. These included magnetic intensity,
depth to basement, thermal gradient and isopach maps for different times. In
this approach, any significant linear anomaly on a data set that coincides
with some of proposed fault zones is considered as a possible indication of
activity of a deep-seated basement lineament. The examination of data sets
revealed linear anomalies in data sets which coincide with the ZMRF, HZF,
ZMFF and the Kazerun, Hendijan, Khanaqin, Bastak, and Bostaneh faults to
which can thus be accepted as old structures in the basement. However, the
Bala Rud, Nezamabad, and Razak fault, and possibly the Sarvestan fault, are
not considered as lying in the Zagros basement. Until the Bala Rud fault first
appears in maps of Palaeocene-Eocene or Oligo-Miocene isopachs or
lithofacies, there is no definite evidence to support even its temporary earlier
reactivation. Therefore, the above 4 faults are interpreted as syn-Zagros
faults in the cover sequence rather than having been imposed by the
reactivation of old faults in the basement. The result of the data study is a
new model for the configuration of the Zagros basement. This comprises
only those faults confirmed by the various data sets. The kinematics of these
faults relate to their trends in the Zagros strain field with a N40E trending
shortening direction. These kinematics are also supported by surface offsets,
earthquake fault-plane solutions and/or different velocity vectors for groups
of GPS stations on each block (Hessami, 2002).
On the regional scale, the new model for the Zagros basement is
compatible to the East Arabian Block (EAB) suggested by previous workers
for the Arabian platform (Figs. 10, 11; Hancock and Al-Kadhi, 1978;
Weijermars, 1998). This block is bound to the SW by the Central Arabian
Graben and to the E by the 450km long left-lateral Nisah-Sahba strike-slip
fault (Weijermars, 1998; Fig. 10). The Trans-Arabian Gulf fault, suggested
as a gulfward extrapolation of the Nisah-Sahba fault (Weijermars, 1998),
passes north of Qeshm Island to join the left-lateral strike-slip Bastak and
Bostaneh faults in Iran (Figs.10, 11). The lateral boundaries of the EAB are
therefore extrapolated across the Zagros all the way to the Zagros main
reverse fault. Integration of the Zagros basement configuration with the EAB
gives rise a new tectonic framework named here the East Arabian-Zagros
block (EAZB). Based on the age of the lineaments within the EAZB, this
block was probably first defined by lateral extension when Neo-Tethys
24
opened in late Palaeozoic times and inverted to lateral shortening in
Cenozoic times.
Fig.11. Cartoon block diagram of the proposed configuration of the basement
beneath the Zagros Mountains integrated with the East Arabian block on the Zagros
foreland.
The new suggested regional tectonic framework implies that some Zagros
deformation has propagated forward of the present Zagros front into the
EAZB. This raises two possibilities: a) an aseismic sub-horizontal
decollement in the basement beneath the EAZB with or without b) cryptic
shortening in the EAZB cover. The EAZB model also implies that Bahrain
and the SE Zagros may be both moving southwest relative to Arabia on the
same basement block. Convergence between Eurasia and Arabia may be
accommodated not only by shortening northeast of the Zagros suture, but
might have propagated into the EAZB to the leading edge of the EAZB.
The oil-gas fields in the EAZB are generally larger than those in its
surroundings. This characteristic is attributed to activity of the basement
faults in the EAZB for much longer than those in other parts of Arabia.
Repeated reactivation of these structural elements since the late Palaeozoic
resulted in deposition of different source-rocks and seals as well as
formation of long gentle drape folds in which hydrocarbons could
accumulate.
25
V. Effect of Hormuz salt on contractional deformation
The significance of Hormuz salt as a viscous basal decollement on
controlling the deformation style in the Zagros fold and thrust belt (ZFTB)
has been already pointed out by previous workers (Davis and Engelder,
1985; Koyi, 1988; Talbot and Alavi, 1996). The styles of deformation in the
Zagros indicates contrasting mechanical signatures of the viscous and
frictional basal decollements above which the fold and thrust belt has
formed. In previous studies of the ZFTB, the interaction of these two
different types of contrasting decollements on the style of deformation has
not been given the attention it deserves. Scaled analogue models were
therefore used to investigate the effect of spatial variation in the distribution
of Hormuz salt alone on deformation styles in different parts of the Zagros
fold and thrust belt.
The structural style of the ZFTB is generally divided into several domains
which match the litho-facies in the cover sequences and. These are: the
Lorestan domain, the Izeh, the Dezful embayment, the Fars Platform, the
Mangarak-Kazerun, and Laristan domains (Fig. 12a; Motiei, 1995). The
boundaries between these domains have been usually defined solely in terms
of the reactivation of basement faults questioned by this work.
In part of the model the sand layer was separated from the rigid basement
by a layer of silicone (SGM-36) which simulated the inferred distribution of
Hormuz salt in the region. Where Hormuz salt is inferred to be absent, the
sand layer rested directly on the rigid substrate in domains (Fig. 12b).
This differential distribution of the viscous layer in the model resulted in two
contrasting decollements which led to the formation of five domains labelled
A to E during shortening of the model. These model domains are compared
with their natural equivalents in the ZFTB during lateral shortening (Fig.
12).
During shortening of the model, the deformation front propagated faster
and further above the viscous decollement than that above the frictional
decollement (Fig. 13). This led to the formation of different deformation
zones (equivalent to the ZFTB) with different widths in the direction of
shortening. The zone of shortening is wider above the viscous decollement
in domains C and D than above the frictional decollement in domain A
(Fig.14). However, the width of the deformation zone is less in domains B
and D which are partly shortened above a viscous decollement than in
domain A which is entirely shortened above a frictional decollement
(Fig.12b). Most of the deformation in domains C and D occurred by
thickening of the viscous layer against the frontal ramp.
26
B'
Z a g r o s
N
28
30
R e v e r s e
F a u l t
Om
M a i n
A'
N
32
N
34
N
a
an
Pusht Kuh province
Izeh Domain
Laristan Domain
Domain
Fars
platform
NZ
KZ
Dezful Enbayment
lt
Front
BR
Laristan province
Mangarak
KazerunProvince
Mangarak
Mountain
RK
Zagros
Fau
Pusht Kuh domain
A (Fig. 14b)
B
(Fig. 15b)
Persian Gulf
Arabian Platform
N
Compression
Direction
B
C
E
D
Moving wall
Frontal
Boundary
60 cm
Sand
Lateral Boundary
Ductile Substrate
(SGM36)
Side wall
E
50
E
E
E
A
48
46
44
b
Qatar
150 Km
0
Distribution of Hormuz salt
Fig. 12. a. Distribution of the Hormuz salt and
major structural domains in Iran and the gulf
and main faults are labelled: RK: the Razak
fault; NZ: Nezamabad fault; KZ: Kazerun
fault; BR: Bala Rud fault. b) Plan view of
initial arrangement of the viscous (SGM 36)
and brittle (sand) decollements in the models
showing the shortening direction and different
domains A to E across the model.
Fixed wall
50 cm
Domain B in the model simulated the Izeh domain where the boundary
between the viscous and frictional decollements in domain B is
perpendicular to the shortening direction. The viscous material thickened
against this boundary during shortening. This led to the development of a
significant morpho-tectonic feature simulating the Mountain Front Fault
(MFF), which separates the intensive deformation in the Izeh domain to the
27
Fig. 15 a
Fig. 14 a
north from lower deformation in the Dezful embayment to the south (Figs.12
and 15). This implies that even without any reactivation of faults in the
basement during lateral shortening, the distribution of Hormuz salt alone can
lead to formation of a structural feature simulating the MFF. The model thus
provides an alternative to the common concept that attributes the MFF to the
reactivation of basement fault (Falcon, 1974; Berberian, 1995).
HRTL
Collapse thrust front
Passive marker grid
3 cm
Antiformal folds
Thrust fault
Fig. 13. Top view of the model after 30% of bulk shortening, showing deflection
zone in the deformation front shortens above the contrast viscous and frictional
substrates. Arrow indicates shortening direction.
A reassessment of the isopach map of Upper Jurassic anhydrite and shape
of oil/gas field suggests that the deformation front east of the Kazerun fault
can be divided into two antitaxial arcs on either side of a syntaxial arc rather
than the simple antitaxial festoon proposed by previous workers (e.g. Talbot
and Alavi, 1996). Adding such deflections to the geometry of the Zagros
deformation front is compatible with the presence in the Fars platform
geometry (Fig.12).
Model results suggest that the significant post-Eocene subsidence
proposed by previous workers (e.g. Berberian, 1995; Motiei, 1995) and the
28
thick syn-tectonic sequence in the Dezful embayment south of the MFF need
not involve the reactivation of a basement fault. Differential uplift and
erosion of the domains underlain by Hormuz salt may be sufficient
particularly in the Izeh domain to the north.
HRTL
3 cm
a
Shortening direction
Frictional decollement
A
A'
NE
0
10
20
Crystalline Basement
b
Quartenary
Neogene
Paleozoic sediments
Paleogene
Cretaceous
Precambrian Crystaline basement
Depth in Kilometres
SW
Triassic-Jurassic
Opiolitic complex
Fig. 14.a) Line drawing of a profile parallel to the shortening direction along
domain A (see Fig. 13 for location) showing the steep taper of a stack of closely
spaced imbricate thrust faults formed above a frictional decollement. b) Geological
cross section of the Pusht Kuh domain (see Fig. 12a for location) showing an
imbricate stack formed mainly in the cover sediments (redrawn from Spaargaren,
1987).
The deformation front in the model is segmented laterally by four
deflection zones in the shortening structures. These overly initial
discontinuities between the viscous and frictional decollements (Figs.12b,
13). Some of these deflection zones involve sinistral and the others dextral
strike-slip displacements. These deflection zones have the same kinematics
as their prototypes in the ZFTB such as the Kazerun and Bala Rud fault
zones (Fig. 12b) both generally proposed reactivated basement faults
(Falcon, 1974; Berberian, 1995; Motiei, 1995; Talbot and Alavi, 1996).
Model results suggest a different origin for these zones and their structural
characteristics. For example, the observed offset of the MFF along the Bala
Rud (130 km) and Kazerun faults (160 km) may not be entirely a result of
the strike slip movement along faults in the basement (Fig. 12a; Berberian,
1995; Hessami et al., 2001a). Instead, model results suggest that these
displacements could be due merely to decoupling between the cover and the
basement along the Hormuz salt without any need for reactivation of the
basement fault (Fig. 13). Whether any Kazerun fault in the basement was
absent or inactive, a significant amount of differential displacement would
29
have occurred due solely to the different decollements. In addition, the Bala
Rud fault can also be considered as a young fault zone in the sedimentary
cover due to differential propagation of the deformation front over adjoining
frictional and viscous decollements (Fig. 12a).
a
Shortening direction
HRTL
3 cm
B
MFF
Izeh zone
Zagros
foredeep
fault
SW
b
B'
NE
Dezful embayment
Crystalline Basement
Quartenary
Paleozoic sediments
Neogene
Paleogene
Hormuz salt
20
Cretaceous
Depth Km
0
10
Triassic-Jurassic
Precambrian Crystaline basement
Fig. 15. a) Line drawing of a profile parallel to the shortening direction along
domain B (see Fig. 13 for location) showing gentle taper with some thrust faults
above a viscous substrate which changes into a distal frictional substrate. Note
pronounced thickening of the viscous substrate ramped over the boundary of the
viscous-frictional decollements and injecting along the fault planes. In this part of
the model, slumped sand was overrun by the hanging wall of the frontal fault. b)
Geological cross section of the Izeh domain and the Dezful embayment across the
Zagros belt (see Fig. 12a for location) showing a narrow imbricate stack formed
behind the Mountain front fault which separates very low deformation the Dezful
embayment from the highly deformed Izeh domain (redrawn from Spaargaren,
1987).
The model emphasise that the arcs and deflection zones in the
deformation front of a young orogens such as the ZFTB can be affected
significantly by the nature of the basal decollement to the cover, and need
not necessarily involve reactivation of the basement faults during shortening.
30
VI. Tectono-Sedimentary framework of the Gachsaran Formation
Precipitation of the considerable volume of Gachsaran salt (2000 m thick)
during Miocene or earlier in the Zagros foreland basin, requires three main
contemporaneous conditions; a) evaporation exceeding seawater supply, b)
arid climate, and c) at least partial isolation of the salt basin.
Most available literature about the Gachsaran Formation deals with either
stratigraphy and or mechanical behaviour of this formation in response to
differential stresses which lead to disharmonic structures (e.g. O’Brien,
1950, 1957; Dunnington, 1968; Stöcklin, 1968b; Gill and Ala, 1972; Kashfi,
1980; Motiei, 1993). The Gachsaran Formation provides the most important
seal at higher stratigraphic level in one of the richest hydrocarbon habitats in
the world (O’Brein, 1957; James and Wynd, 1965; Dunnington, 1968;
Stöcklin, 1968b; Colman-Sadd, 1978; Murris, 1980; Beydoun, 1991;
Beydoun et al., 1992; Motiei, 1993; Edgell, 1996).
Previous workers (Stöcklin, 1968b; Gill and Ala, 1972; Kashfi, 1980;
Motiei, 1993) have pointed to the need for obstacles or barriers to control
seawater influx to the Gachsaran basin. However, the distribution, number,
and origin of such barriers are still unconstrained. This study provides an
explanation for the nature of such barriers and how they may have formed.
This explanation is based on considering the geometry of the Zagros
foreland and its Eocene-Miocene evolution in response to reactivation of
basement faults and the differential propagation of Zagros deformation. It
focuses on how these two factors affected syn-tectonic sedimentation in
Gachsaran salt sub-basins which were spatially restricted in front of the
shortening cover sequence to the north. The facies changes and thickness of
the Gachsaran Formation are reviewed within the structural context of a
foreland basin migrating southward. A new tectono-sedimentary framework
is suggested for syn-orogenic sedimentation within the Zagros foreland
basin.
Gachsaran Formation
Although, Gachsaran salt is rarely naturally exposed, it is known from wells
and quarries in the Dezful embayment (Dunnington, 1968; Gill and Ala,
1972; Kashfi, 1980; Motiei, 1993). Stratigraphic correlation between wells
drilled in different parts of the Zagros indicates that the Gachsaran
Formation becomes younger from southeast to northwest (Fig.16a).
Gachsaran litho-facies indicate a shallow basin that occasionally desiccated
at least north-west of the Kazerun fault (Gill and Ala, 1972). Detail
stratigraphic studies of this formation suggest a rhythmic character in which
each cycle was “capped” by a bed of continental eolian sediment (e.g. James
and Wynd, 1965; Dunnington, 1968; Stöcklin, 1968b; Gill and Ala, 1972).
31
The thickness and facies of the Gachsaran Formation, particularly those parts
with salt beds, vary significantly over short distances west of the Kazerun
fault (O’Brien, 1957; Dunnington, 1968; Stöcklin, 1968b). This is because
the salt flowed from above the growing anticlinal crests to the synclinal
troughs in response to pressure gradients due to development of folds in the
underlying Formations (see O’Brien, 1957; Dunnington, 1968; ColmanSadd, 1978).
Fig. 16.a) Cenozoic Formations correlated within the Zagros fold-thrust belt (after
James and Wynd, 1965), b) Distribution of Gachsaran litho-facies and its timeequivalents along the Zagros basin.
The Razak Formation, a time-equivalent of the Gachsaran deposit, is
considered to be a product of erosion of the Zagros Mountains emerging in
the north because of a north-eastward increase in size and angularity of
terrigenous grains in beds that also thicken northeast-ward (e.g. Kashfi,
1980; Hessami et al., 2001b). Along the SE-NW trending southern margin of
the Zagros foreland basin, other time-equivalents of the Gachsaran
Formation, consist of anhydrite, anhydiric limestone, siltstone, sandstone
and marls deposited in Subkha and very shallow water. This margin had a
length of 1500 km and starched from Arabian Emirate to Syria.
32
Basement faults
Precambrian basement is not exposed anywhere in the Zagros basin. Most of
the basement structures are inferred from their effects on sedimentation
and/or deformation of the cover sequence on the Arabian plate. Some of the
faults are active seismically (Jackson and McKenzie, 1984; Jackson et al.,
1993; Berberian, 1995), others are inactive but may have been active in
Gachsaran times (see Murris, 1980; Motiei, 1993). Some of these faults
(longitudinal) trend NW-SE parallel to ZFTB (e.g. Berberian, 1995). Others
trend N-S or NE-SW across the ZFTB. The NE-SW trending faults are fewer
and poorly known in the ZFTB; they include the Trans Arabian Gulf
lineament, the Dibba, Bostaneh, and Bastak faults. In contrast, the Ntrending Kazerun-Qatar, Khanqin and Hail Ga’ara lineament are relatively
well-known.
Hormuz salt distribution
The uneven distribution of over 200 emergent structures of Hormuz salt
implies an uneven initial distribution of the Neo-Proterozoic-Cambrian
Hormuz salt deposited on the Pan-African basement in the Zagros and
Arabia. Previous studies indicate that many of these structures surfaced
before and during the Zagros shortening (e.g. Player, 1969; Kent, 1958;
1979).
In the southeast of the Zagros basin, the Gachsaran Formation is
differentiated into three members with different facies (James and Wynd,
1965; Motiei, 1993). Variation in thickness of these members was attributed
to the activity of many structures of Hormuz salt in the Fars province.
Geological evidence (e.g. Jamali, 1990; Talbot and Alavi, 1996; Sattarzadeh
et al., 2001; Hessami, 2002) and modelling (Bahroudi and Koyi, in press)
emphasise that the amount of shortening has not been uniform along the
Zagros belt.
Modeling results show that uneven distribution of a basal viscous
decollement, (e.g. Hormuz salt) has led to the formation of transpression
zones, different topographic wedges, variation in strain partitioning within
the ZFTB, and differential sedimentation in its foreland basin (Bahroudi and
Koyi, in press).
Tectono-sedimentary model
A new tectono-sedimentary model is suggested here for the foreland basin
which formed during Zagros shortening in Miocene. This model takes into
account the effect of reactivating basement faults and differential
propagation of the deformation front above different decollement types
(viscous and frictional). The litho-facies map of Lower Miocene deposits
shows four sub-basins which contain Gachsaran salt. From south-east to
north-west these are the Qeshm, Dezful, Kirkuk and Sinjar sub-basins (Fig.
33
16b). The Fars and Pusht Kuh segments are two sub-basins without
Gachsaran salt. The boundaries between sub-basins with and without salt
coincide with basement faults which are inferred to have acted as barriers
across the Zagros foreland basin.
It has been suggested that the accumulation of a 2000m thick Gachsaran
salt in the very small Qeshm salt sub-basin cannot be explained by
evaporation alone (Kashfi, 1980). An obvious additional source is the reprecipitation of Hormuz salt extending on land in adjoining parts of the
ZFTB.
Modeling results show that the uneven initial distribution of the Hormuz
salt must have segmented the evolving ZFTB into domains with different
taper (Bahroudi and Koyi, in press). Domains with Hormuz salt shortened,
thickened and gained height more rapidly than those without salt. Huge
volume of Hormuz salt also extruded from these domains. The Gachsaran
salt is noticeably confined to basins in front of these domains. Brines
dissolved from extrusions drained into nearby foreland sub-basins where
they were re-deposited. The concept of the recycling of the Hormuz salt as
Gachsaran salt can be applied to the Miocene salt in the Dezful sub-basin.
Any Hormuz salt contributing to the Dezful sub-basin would have extruded
along the Kazerun-Mangarak zone in the east and the Izeh zone to the north.
Fig.17. Schematic presentation of the tectono-sedimentary framework of the
Gachsaran formation.
However, it is unlikely that dissolved Hormuz salt was re-precipitated
along the entire 1500 km length of the Gachsaran basin. The episodic supply
and subsequent evaporation of sea water must have played a more important
role in the accumulation of Gachsaran salt in the Kirkuk and the Sinjar salt
sub-basins (which are far from domains with Hormuz salt). This cyclicity in
34
the western salt could have been due to world-wide changes in sea level or
discrete episodic movements in the Zagros fold and thrust belt (Henson,
1950; Gill and Ala, 1972; Falcon, 1974; Alavi, 1994; Talbot and Alavi,
1996; Hessami et al., 2001b). Here, the episodicity is attributed to
reactivation of successive basement faults as the Zagros deformation
propagated southward.
Fig. 18. Litho-facies map of the Aghajari formation showing distribution of 1) marl,
partly sandstone, 2) abundant gypsiferous beds, 3) sandstone, siltstone, partly marl,
4) marl and sandstone, 5) sandstone, in the Zagros basin with some main basement
lineaments.
This tectono-sedimentary model suggests that the propagation of Zagros
deformation during the convergence between Arabian, Iran and Turkey has
affected not only the growing fold-thrust belt, but also the syn-tectonic
sedimentation in its foreland basin segmented by basement faults. Hancock
et al., (1984) and Weijermars (1998) reported significant displacement in
Plio-Quaternary times along reactivated Precambrian faults that offset the
cover sediments on the Arabian platform beyond the Zagros deformation
front.
Although, the model in general focuses on variation of facies and
thickness in the Gachsaran Formation, it might also explain the segmentation
of post-lower Miocene sedimentation of the Aghajari and Bakhtyari
Formations as well. Isopach and litho-facies maps of these formations
indicate structural features similar to those described for the Gachsaran
Formation.
35
Conclusions
This thesis shows how two basic elements, the mechanical characteristics of
the basal decollement and pre-existing basement structures, individually or
together, significantly influence the sedimentary and structural evolution of
the tectonically active region called here the Zagros basin.
The presence of different basal decollements, viscous and frictional, leads
to variation in along-strike and along-dip deformation style in the cover
sequences during both lateral extension and shortening. The deformation
style in the cover above a frictional decollement is controlled by the nature
of the underlying basement. By contrast, the deformation style above a
viscous decollement is more sensitive to the boundary conditions and the
brittle/viscous thickness ratio.
This thesis shows that lateral extension or shortening above adjacent
viscous and frictional decollements results in a narrow deformation zone
above the frictional decollement and a wider deformation zone above the
viscous decollement. The narrow and wide deformation zones are separated
by deflection zones behind an irregular deformation front. Depending on the
orientation of initial viscous/frictional boundary relative to direction of the
shortening in the Zagros belt either frontal ramps or transfer
(accommodation) zones developed within the overlying cover. Variation in
the viscous/frictional boundary also controls the syn-tectonic sedimentation
beyond the deformation front.
In the active region with two different types of basal decollements, it is
important to distinguish faults in the basement from those formed only in the
cover sequences. Such distinction depends on the knowledge about either the
initial spatial distribution of the basal viscous decollement or the exact
number, orientation and location of the basement faults.
This thesis has confirmed earlier studies that basement faults of the
Zagros basin have variable trend and unevenly distributed, but adds their
probable relative ages of reactivation. As seen in Arabia, the faults in PanAfrican Arabian shield have three different orientations, NW-SE, N-S and
NE-SW. West of a NE boundary passing Bandar Abbas, two sets of these
faults, N-S (transverse) and NW-SE (longitudinal) are more common. East
of that possible Pan-African suture between Arabia and India, the basement
faults trend NE-SW.
This thesis presents a new model for the architecture of the basement in
the Zagros basin. Reactivation of the basement faults affected the Zagros
sedimentary column regionally and triggered many local salt structures. The
new model distinguishes the active East Arabian-Zagros block in the
basement of the Zagros basin. The active block contains nearly two-third of
36
oil and one-third of gas proven reserves of the world and large number of
salt structures.
The NW-trending basement faults were at high angle to the direction of
lateral extension when Neo-Tethys opened in the Mesozoic and Zagros
shortening when Neo-Tethys closed in the Tertiary. The transverse basement
faults were sub-paralleled to the directions of lateral extension and
shortening. During Zagros shortening, movement along mainly N- and NEtrending basement faults, propagated beyond the Zagros deformation front
and its foreland basin. These faults transferred part of the Zagros lateral
shortening beyond the so-called or classical Zagros deformation front.
The structural variations along the Zagros fold-thrust belt are also
reflected in the sedimentary history of the Zagros foreland basin. This study
shows that the Gachsaran Formation, which caps the hydrocarbon reserves in
the Zagros fold-thrust belt, differs in age, facies and thickness along the
Zagros foreland basin which was compartmentalized by basement faults and
the initial uneven distribution of Hormuz salt.
37
Acknowledgements
When I began my M.Sc., in 1988, one day in Library of Geological Survey
of Iran, I found a red-cover book with strange title “Gravity, deformation
and the Earth´s Crust” written by Hans Ramberg. The contents of the book
were incredibly weird. It suggested building of a giant orogen like
Caledonian orogen in a small box as large as a palm of Human hand using
the Centrifuge. This book fascinated me irresistibly and made my sweet
dream. I was living with the dream to model geological structures in such a
Tectonic laboratory in Uppsala for long time, until Chris asked me “would
you like come to Uppsala?” Unbelievable, my dream came true.
Hence, I wish to express my profuse gratitude to my first supervisor,
Professor Christopher Talbot not only for making my dream come true, but
also for his supervision, encouragements, and invaluable help during these
four years of my stay in Uppsala. Thank and respect to Rosemary Talbot for
her kindness and friendly behaviour to me and my family.
I also thank Dr. Hemin Koyi, my second supervisor for schooling me
analogue modelling at the Hans Ramberg Tectonic Laboratory, inspired
discussions, invaluable advices and help.
This study was funded by Ph.D. grant from Uppsala University which I
am greatly indebted.
Thanks are due to Dr. G. Mulugeta who also taught me analogue
modelling and also for inspired discussions and very friendly behaviour.
My appreciation goes to M. T. Koriei, Head of Geological Survey of Iran
and Dr. M. Ghoreshi, Geological Deputy for helps and encouragements.
I acknowledge the support of the administrative and technical staff,
teachers, researchers, technicians of Department of Earth Sciences
represented by Hans Annersten, Kersti Gloersen. Regards to all colleague
students and friends at the Institute, in particular Faramarz, Marcelo, Yasir,
Behrouz, David, Tomas, Katerina, Lijam, Olga of Geocentrum.
Special thanks to my dear friend Taher Mazloomian, at Geotryckeriet for
his invaluable help and friendship towards me and my family during our stay
in Uppsala. Regards and respect to his wife, Anita and children for kindness.
Regards and respect go to my dear Iranian friends for help, support,
friendship and for great enjoyable moments, particularly Ziba and Mehrdad
for warm welcome and for helping us with accommodation at the beginning.
I would like to thank Ahmadreza, Davood at the Institute for kindness and
friendship.
And last, but by no means least, my deepest gratitude and respect go to
my wife, Parisa for love, support, encouragement, listening to my geological
discussions and making me to realize that there are other important things in
life than work and Geology. But, Parisa, you know that “we did all this
38
together”. Many thanks go to our parents for all love and support in Iran and
particularly my parents-in-law for invaluable supports and helps in the home
town and particularly in those hard moments when my son, Arsham was
born in Uppsala far away from home. I am indebted to both of you for every
thing.
Abbas Bahroudi, Uppsala, Spring 2003
39
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