Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 836

The Effect of MechanicalCharacteristics of Basal

Decollement and BasementStructures on Deformation of the

Zagros Basin

BY

ABBAS BAHROUDI

ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2003

1

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

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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)

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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 Alpine-Himalayan 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

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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 Paleozoic-Mesozoic 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

6

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

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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.

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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

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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.

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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

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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.

3 cmHRTLMDL-1

3 cmHRTLMDL-1

Deformat ion Width

Deformat ion Width

b

a

c

FD

DD

RCR

profile b

profile c

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.

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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.

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,.

b

a

c3 cmExtension MDL-2

3 cmMDL-2Extension

profile b

profile c

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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).

0

5

10

15

20

25

MDL-2 MDL-3 MDL-4 MDL-5 MDL-6 MDL-8 MDL-10

Silicon thickness,mm

Number of faults

Thic

knes

s of s

ilico

n la

yer (

mm

)or

num

ber o

f fau

lts in

cov

er

Models Fig.3. Plot showing relation between number of faults and thickness of the viscous substrate in different models extended above the rubber sheet.

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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.

3 cma

3 cm

c

d

1234

3 cm

3 cm

123456

5

b

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.

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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 extension-induced 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

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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.

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

3 cma

b

3 cmc

3 cmd

3 cm

17

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.

Viscous substrate

Brittle τ

δfδb=

δf

δb

Fault

a. b.

Fault

τ

δf δb

δfδb>

Strain ellipse

Brittle

Viscous

θ

σ1

σ1

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.

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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. Sub-circular 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

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Fig.

7. D

istr

ibut

ion

of st

ruct

ures

of H

orm

uz sa

lt an

d hy

droc

arbo

n po

ols i

n th

e Za

gros

bas

in, s

how

ing

thei

r re

latio

nshi

p w

ith N

-S tr

endi

ng b

asem

ent f

aults

.

20

features and their variations in shape with time can be attributed to the evolution of salt structures.

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).

N0150 Km

34N

46E

44E

28N30N

32N

48E 50E

52E

Fars plat form

Qata

Persian Gulf

Salt structures

salt pillow

salt ridge

salt diapirs

on pre-Zagros

ridge

blind salt diapir

salt mushroom

salt fountain

salt droplet

degraded salt

droplet

breccia chimney

Cuspate salt mullion

(or welt) in core of

Zagros anticlines

21

Qatar

0 150 Km

Basr a

Paleocene-Eoceneg

N

30N

32N34N

46E

48E

50E

52E

54E

56E

58E

44E

Possible

Probable

Very likely

Certain

No Sign

Dezf u l En b aym en t

Main Zagros Reverse Fault

Sh i r az

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 Paleogene-Eocene 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.

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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

23

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.

24

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

25

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.

26

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.

27

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.

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

Ductile Substrate (SGM36)

Moving wall

Sand

La

tera

l B

ou

nd

ary

Frontal Boundary

Sid

e w

all

Fixed wall

50 cm

60

cm

Compression Direction

A B C D E

b

Mangarak Province

Laristan province

Pusht Kuh province

Dezful Enbayment

Dis t r ibut ion of Hormuz sa l t

28N

30N

32N

34N

46E

48E44E

A r a b i a n P l a t f o r m

M a i n Z a g r o s R e v e r s e F a u l t

0 150 Km

BR KZ

Qatar

B'

B(Fig. 14b)

A'

(Fig. 15b)

Pusht Kuh domain Izeh DomainZ a g r o s

M o u n t a i nF r o n t

N

A

a

50E

Kazerun-Mangarak Domain

NZ

RK Fars

plat form

Laristan Domain

Om

an

Fa

ul t

P e r s i a n G u l f

28

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).

Thrust faultAntiformal foldsPassive marker gridCollapse thrust front

3 cmHRTL

Fig

. 14

a

Fig

. 15

a

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

29

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.

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

3 cmHRTL

Shortening direction

Frictional decollement

Precambrian Crystaline basement

PaleogeneNeogeneQuartenary

Opiolitic complex

SW NE

A A'

20

10

0

Crystalline Basement

Paleozoic sediments

Triassic-JurassicCretaceous

Depth in K

ilometres

a

b

30

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).

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.

3 cmHRTL

Shortening direction

SWNE

B

Depth K

m

Zagros foredeep fault

Precambrian Crystaline basement

PaleogeneNeogeneQuartenary

Hormuz salt

0

10

20

B'

Crystalline Basement

Paleozoic sediments

Triassic-JurassicCretaceous

MFF

Dezful embaymentIzeh zone

a

b

31

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).

32

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; Colman-Sadd, 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 time-equivalents 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.

33

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 N-trending 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.

34

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 re-precipitation 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

35

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.

36

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 Pan-African 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

37

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 NE-trending 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.

38

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

39

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

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References Adamia, Sh. A., Chkhotua, T., Kekelia, M., Lordkipanidze, M.I., and Shavishvili, I.,

1981. Tectonics of the Caucasus and adjoining regions: implications for evolution of the Tethys ocean. Journal of Structural Geology 3, 437-447.

Alavi, M., 1994. Tectonics of the Zagros Orogenic belt of Iran: new data and interpretations. Tectonophysics, 229, 211-238.

Al Laboun, A. A., 1986. Stratigraphy and hydrocarbon potiential of the Paleozoic succession in both the Tabuk and Widyan basins, Arabia. In: Halbouty, M. T. (ed.), Future Petroleum provinces of the world. American Association of Petroleum Geologist Memoir 40, 399-425.

Alsharhan, A. S., 1989. Petroleum Geology of the United Arab Emirates. Journal of Petroleum Geology 12, 253-288.

Alsharhan, A.S., and Nairn, A.E.M., 1997. Sedimentary Basins and Petroleum Geology of the Middle East. Elsevier Science, 843p.

Ameen, M.S., 1991a. Alpine geowarpings in the Zagros-Taurus range: Influence on hydrocarbon migration and accumulations: Journal of Petroleum Geology 14, 417-428.

Ameen, M.S., 1991b. Possible forced folding in the Tourus-Zagros belt of northern Iraq. Geological Magzine 128, 561-584.

Ameen, M. S., 1992. Effect of basement tectonics on hydrocarbon generation, migration and accumulation in northern Iraq. American Association of Petroleum Geologists Bulletin 76, 356-370.

Bahroudi, A., Koyi, H. A., (in press). 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 of London.

Baker, C., Jackson, J., Priestly, K., 1993. Earthquakes on the Kazerun line in the Zagros mountains of Iran, Strike-slip faulting within a fold and thrust belt. Geophysical Journal International 115, 41-61.

Barzegar, F., 1994. Basement fault mapping of E Zagros folded belt (S.W. Iran) based on space-born remotely sensed data. Proceedings of the 10th Thematic conference on geologic remote sensing: exploration, environment, and engineering. 10, 455-466, San Antonio, Texas, USA.

Berberian, M., 1995. Master ”blind” thrust faults hidden under the Zagros folds: Active tectonics and surface morphotectonics. Tectonophysics 241, 193-224.

Berberian, M., King, G. C. P., 1981. Towards a Paleogeography and Tectonic evolution of Iran. Canadian Journal of Earth Sciences 18, 210-265.

Beydoun, Z.R., 1991. Arabian plate hydrocarbon geology and potential- a plate tectonic approach. Studies in Geology. American Association of Petroleum Geologists Bulletin 33, 77p.

Beydoun, Z.R., Hughes, M.W., Stoneley, R., 1992. Petroleum in the Zagros basin: A late Tertiary foreland basin overprinted onto the outer edge of the vast hydrocarbon-rich Paleozoic-Mesozoic passive margin shelf. American Association of Petroleum Geologists Bulletin Memoir 55, 307-336.

Brun, J. P., Sokoutis, D., Van Den Driessche, J., 1994. Analogue modelling of detachment fault systems and core complexes. Geology 22, 319-322.

Carter, N. J., Hansen, F. D., 1983. Creep of rock salt. Tectonophysics 92, 275-333. Chandra, U., 1984. Focal Mechanism solution for earthquakes in Iran. Physics and

Earth Planetary International 34, 9-16.

41

Cherven, V. B., 1986. Tethys-Marginal sedimentary basins in western Iran. Geological Society of American Bulletin 97, 516-622.

Colman-Sadd, S. P., 1978. Fold development in Zagros simply Folded Belts, Southwest Iran. American Association of Petroleum Geologists Bulletin 62, 984-1003.

Cotton, J. T., Koyi, H. A., 2000. Modelling of thrust fronts above ductile and frictional dećollements: Application to structures in the Salt Range and Potwar Plateau, Pakistan. Geological Society of American Bulletin 112, 351-363.

Davis, D. M., Engelder, T., 1985. The role of salt in Fold-and-Thrust belts. Tectonophysics 119, 67-88.

Davis, M.G., Engelder, T., 1987. Thin-skinned deformation over salt. In: I. LERCHE, J. J. O’BRIEN, (eds.): Dynamical Geology of Salt and related structures. Academic Press, 301-337.

Diegel, F. A., Karlo, J. F., Schuster, D.C., Shoup, R.C., Tauvers, P.R., 1995. Cenozoic structural evolution and tectono-stratigraphic framework of the northern Gulf coast margin. In: Jackson, M.P.A., Roberts, D.G., Snelson, S. (Eds), Salt tectonics: A global perspective. American Association of Petroleum Geologists Memoir 65, 109-151.

Dixon, T.H., Stern, R.J., and Hussein, I.H., 1987. Control of Red Sea Rift Geometry by Precambrian Structures. Tectonophysics 6, 551-571.

Dunnington, H. V., 1968. Salt-Tectonic Features of Northern Iraq. Geological Society of America Special Paper 88, 183-227.

Edgell, H. S., 1991. Ptoterozoic salt basins of the Persian Gulf area and their role in hydrocarbon generation. Precambrian Research 54, 1-14.

Edgell, H. S., 1996. Salt tectonics in the Persian Gulf basin. In: Alsop, G.L, Blundell, D.L., Davison, I. (Eds.), Salt tectonics. Geological Society of London Special Publication 100, 129-151.

Falcon, N.L., 1967. The Geology of the North-East Margin of the Arabian Basement Shield. Advancement Science 24, 1-12.

Falcon, N. L., 1969. Problems of the relationship between surface structures and deep displacements illustrated by the Zagros Range. In: Kent, G.E., Satterhwaite and Spencer, A.M. (Eds.), Time and Place Orogeny. Geological Society of London, Special Publication 3, 9-22.

Falcon, N.l., 1974. South Iran: Zagros mountains. In: Spencer, A. M. (Ed.), Mesozoic-Cenozoic Orogenic belts-Data for Orogenic studies. Geological society of London, Special Publication 4, 199-211.

Fürst, N.I., 1990. Strike-slip faulting and Diapirism of the south-eastern Zagros ranges. Proceeding of Symposium on Diapirism (Iran), 149-181.

Ganssar, A., 1992. The enigma of the Persian salt dome inclusions. Eclogae Geologica Helveticae 85, 825-846.

Ghebreab, W., Talbot, C. J. 2000. Red Sea extension influenced by Pan-African tectonics grain in East Eritrea. Journal Structural Geology 22, 931-946.

Gill, W. D., Ala, M. A., 1972. Sedimentology of Gachsaran Formation (Lower Fars Series), Southwest Iran. American Association of Petroleum Geologists Bulletin 56, 1965-1974.

Glonoka, J. 2000. Camberian-Neogene Plate tectonic maps. Wydawnictwo Uniwersytetu Jagiellonskiego, 125p.

Hancock, P.L., Al kadhi, A., and Sha’at, N.A., 1984. Regional joint sets in the Arabian platform as indicators of intraplate. Tectonics 3, 27-43.

42

Harper, T., Fossen, H., Hesthammer, J., 2001. Influence of uniform basement extension on faulting in cover sediments. Journal of Structural Geology 23, 593-600.

Harrison, J.V., 1930. The geology of some salt plugs in Laristan (Southern Persia). Geological Society of London Quarterly Journal 86, 463-522.

Harrison, J.V., 1931. Salt domes in Persia. Journal of Institute of Petroleum Technology 17, 300-320.

Henson, F. R. S., 1950.The stratigraphy of the main producing limestone of the Kirkuk oil-field (Iraq). International Geological Congress part 6, 68-73.

Hessami, K., 2002. Tectonic History and Present-Day Deformation in the Zagros Fold-Thrust Belt. Uppsala University, Ph.D thesis.

Hessami, K., Koyi, H. A., and Talbot, C. J. 2001a. The significance of strike-slip faulting in the basement of the Zagros fold-thrust belt. Journal of Petroleum Geology 24, 5-28.

Hessami, K., Koyi, H. A., Talbot, C. J., Tabasi, H., and Shabanian, E., 2001b. Progressive unconformities within an evolving foreland fold-thrust belt, Zagros Mountains. Journal of the Geological Society of London 158, 969-981.

Hussieni, M.I., 1990. The Cambro-Ordovician Arabian and adjoining plates: A Glacio-Eustatic Model. Journal of Petroleum Geology 13, 267-288.

Hussieni, M.I., 1991. Tectonic and depositional model of the Silurian-Devonian Arabian and adjoining plates. American Association of Petroleum Geologists Bulletin 75, 108-120.

Jackson, J. A., 1980. Reactivation of basement faults and crustal shortening in Orogenic belts. Nature 283, 343-346.

Jackson, J. A., McKenzie, D. P., 1984. Active Tectonics of Alpine-Himalayan belt between western Turkey and Pakistan. Geophysical Journal of Royal Astronomy Society 77, 185-264.

Jackson, J. A., Fitch, T. J., McKenzie, D. P., 1981. Active thrusting and evolution of the Zagros fold belt. In: McClay, K., Price, N. J. (Eds.), Thrust and nappe tectonics. Geological Society of London, Special Publication 9, 371-379.

Jackson, M. P. A., Vendeville, B. C., 1994. Regional extension as a geologic trigger for diapirism. Geological Society of American Bulletin 106, 57-73.

Jackson, M. P. A, Vendeville, B.C., Schultz- Ela, D., 1994. Structural dynamics of salt systems. Annual Review Earth and Planetary Sciences 22, 93-117.

Jamali, F. H., 1990. Review of Seismo-tectonics of the Zagros belt, Southwest Iran. Geological Survey of Iran, Unpublished report (in Farsi), Tehran, Iran.

James, G.A., Wynd, J.G., 1965. Stratigraphical nomenclature of Iranian Oil Consortium Agreement Area. American Association of Petroleum Geologists Bulletin 49, 2182-2245.

Jenyon, M. K. 1986. Salt Tectonics. Elsevier applied science publishers, London. Kashfi, M. S., 1980. Stratigraphy and Enviromenta Sedimentology of Lower Fars

Group (Miocene), South-Southwest Iran. American Association of Petroleum Geologists Bulletin 64, 2095-2107.

Kent, P., 1958. Recent studies of South Persian salt plugs. American Association of Petroleum Geologists Bulletin 42, 2951-2979.

Kent, P.E., 1970. The salt Plugs of the Persian Gulf Region.Leicester Literary and Philosophical Society Transactions 64, 55-58.

Kent, P. E., 1979. The Emergent Hormuz Salt Plugs of Southern Iran. Journal of Petroleum Geology 2, 117-144.

43

Koop, W. J., Stoneley, R., 1982. Subsidence history of the Middle East Zagros basin, Permian to recent. In: Kent, P.E., Bott, M.P., McKenzie, D.P., and William, C.A.(Eds.), Philosophical Transactions Royal Society London 305, 149-168.

Koyi, H., 1988. Experimental modeling of role of gravity and lateral shortening in Zagros mountain belt. American Association of Petroleum Geologists Bulletin 72, 1381-1394.

Letouzy, J., Colletta, B., Vailly, R., Chermette, J.C., 1995. Evaluation of Salt Related Structures in Compressional Setting. In: M.A.P. Jackson, D. G. Roberts, S. and Snelson, (Eds.), Salt Tectonics: a global perspective. AAPG Bull. Mem., 65, 41-60.

McQuillan, H., 1991. The role of basement tectonics in the control of sedimentary facies, structural pattern and salt plug emplacements in the Zagros fold belt of southwest Iran. Southeast Asian Earth Science 5, 453-463.

Mohriak, W. U., Macedo, J. M., Castellani, R. T., Rangel, H. D., Barros, A. Z. N., Latge, R. J. A., Mizusaki, A. M. P., Szatmari, P., Demercian , L. S., Rizzo, J. G., Aires, J. R., 1995. Salt tectonics and structural styles in the deep water province of cabo Frio region. In: Jackson, M.P.A., Roberts, D. G., Snelson, S. (Eds). Salt tectonics: A global perspective, American Association of Petroleum Geologists Memoir 65, 273-304.

Morley, C. K., Guerin, G., 1996. Comparison of gravity driven deformation styles and behaviour associated with mobile shales and salt. Tectonics 15, 1154-1170.

Motiei, H. 1993. Stratigraphy of Zagros. Geological Survey of Iran (in Farsi), 536p. Motiei, H. 1995. Petroleum Geology of Zagros. Geological Survey of Iran (in Farsi),

589p. Murris, R.J., 1980. Middle East: Stratigraphic Evolution and oil habitat. American

Association of Petroleum Geologists Bulletin 64, 597-618. O’Brien, C. A. E., 1957. Salt diapirism in South Persia. Geologie en Mijnbouw, 19,

337-376. Player, R. A., 1969. The Hormuz salt plugs of southern Iran. Ph.D. Thesis, Reading

University, p. 300. Scotese, C. R., and McKerrow, W. S. 1990. Revised world maps and introduction.

In: McKerrow, W. S., Scotese, C. R. (eds.) Palaeozoic Palaeogeography and Biogeography. Geological Society Memoir 12, 1-21.

Setudehnia, A.O., 1972. Iran du Sud-Ouest. In: Iran, Lexique stratigraphique International, Volume 3(Asia), CNRS, Paris, 285-376.

Setudenia, A.O., 1975. The Paleozoic sequence of Zard Kuh and Kuh-e-Dinar. Iranian Petroleum Institute Bulletin 60, 16-33.

Stampfli, G., Marcoux, J., Baud, A. 1991. Tethyan margins in space and time. Palaeogeography, Palaeoclimatology, Palaeoecology 87, 373-409.

Stöcklin, J., 1968a. Structural history and tectonics of Iran: a review. American Association of Petroleum Geologists bulletin 52, 1229-1258.

Stöcklin, J., 1968b. Salt deposits of the Middle East. Geological Society of America Special Paper 88, p. 157-181.

Stöcklin, J., 1974. Possible ancient continental margin in Iran. In: Burk, C.A., Drake, C.L.(Eds), Geology of the continental margin. Springer, New York, 873-887.

Talbot, C. J., Alavi, M., 1996. The past of a future syntaxis across the Zagros. In: Alsop, G.I., Blunderll, D. J., and Davison, I. (Eds.), Salt Tectonics. Geological Society of London Special Publication 100, 89-109.

44

Trudgill, B., Cartwright, J. A., 1994. Relay ramp forms and normal fault linkages, Canyonlands National Park, Utah. Geological Society of America Bulletin 106, 1143-1157.

Urai, J. L., Spiers, C. J., Zwart, H. J., and Lister, G. S., 1986. Weakening of rock salt by water during long-term creep. Nature, 324, 554-557.

Weijermars, R., 1998. Plio-Quaternary movement of the East Arabian block. GeoArabia 3, 509-540.