Although the salt domes in Southern Iran, especially those around Bandar-Abbas
which could be called geologically salt dome province, have been a matter
of investigation resulting in numerous publications since 1908, this study
has following innovations:
||There have been vague ideas that salt domes are outcropped
along basement faults. Most of basement faults that have not been
mapped before, is mapped in this study for the first time
||This study uses the accurate location of all earthquake epicenters
of the complete last century to find their relation to the basement
Moreover, this study explains the relation between some basement faults
and the bending of the folds and develops criteria for recognizing some
faults in the basement according to the fold and salt morphology. Photogeological
and satellite imagery studies of the ZMR have led to the recognition of
lineament pattern. These linear features persist with constant alignments
throughout the region. In common with findings from other areas worldwide,
the pattern of surface lineaments is considered to be related to basement
structure. The present surface manifestations of basement discontinuities
can also be related to structural and stratigraphic patterns and anomalies.
The emplacement pattern of emergent salt domes in the ZMR is related to
The NW-SE trending Zagros Mountains, part of the Alpine-Himalaya chain,
extend for 2,000 km through Iran (Fig. 1). Together
with its equivalent in the Arabian foreland, the Neoproterozoic to Neogene
cover sequence of the Zagros fold-thrust belt hosts two-thirds of the
World`s proven oil reserves and one-third of the World`s reserves of gas
(Beydoun, 1991). The Zagros Basin is defined here as lying between the
central Iranian plateau in the NE, the Arabian Shield to the SW and the
Taurides of Turkey to the NW and Makran Subduction to the East (Alsharhan
and Nairn, 1997; Bahroudi and Talbot, 2003; Regard et al., 2005).
|| Location of Zagros and plate tectonic setting of the
area (modified from Derakhshani and Farhoudi, 2005)
The Main Zagros reverse fault at the northeastern limit of the ZMR is
the suture between the colliding plates of central Iran and the Arabian
passive continental margin (Berberian, 1995) where, the fastest shortening
seems to be occurred (Hessami et al., 2006). This Alpine orogenic
grain masks and modifies earlier patterns of structural deformation (McQuillan,
1991). Three structural belts with intensity of deformation increasing
in a northeasterly direction have been defined (Falcon, 1969).
The Zagros basin is defined by a 7-14 km thick succession of cover sediments
deposited over an extraordinary wide and long region along the north-northeast
edge of the Arabian plate (Fig. 1), since the end of
Precambrian (Bahroudi and Koyi, 2004; Sepehr and Cosgrove, 2004).
Paleozoic sedimentation was mainly epicontinental; from Permian to Miocene
time it consisted chiefly of carbonates. The Tethys Ocean began to subduct
under the Iranian Plate in the Late Cretaceous. Cessation or slow subduction
resulted in carbonate sedimentation in Early Cenozoic time as well as
the deposition of 400 m evaporatic materials in the Miocene. Orogenic
movements began in middle or upper Miocene when sedimentation became clastic.
The opening of the Red Sea intensified folding and uplifting of ZMR which
are still active in a roughly N-S direction at a rate of approximately
25-30 mm year-1 at the Eastern edge of the Arabian plate (Sella
et al., 2002). This direction is oblique o the NW-SE trend of the
orogenic belt. Earthquake focal mechanisms and the GPS velocity field
(Talebian and Jackson, 2002) suggest partitioning of this oblique shortening
along the faults in the ZMR (Sherkati and Letouzey, 2004). Most of the
salt diapers are located in Hormozgan and the Southeastern part of Fars
Provinces. The diapir field, as well as Hormoz salt basin (Fig.
2) is bounded by the Oman Line to the East, the Kazerun Fault to the
West and the Main Zagros Thrust to the North (Derakhshani and Farhoudi,
2005; Sepehr and Cosgrove, 2005). Only a few salt diapirs reach the surface
Northwest of the Kazerun Fault near the Main Zagros Thrust. They occur
in great number also in the form of small islands in Persian Gulf, South
of the boundary of the Zagros Folded Belt.
The presence of numerous salt domes, mainly of Hormuz Formation, indicates
tectonic activity of the ZMR. Although salt domes have been reported in
many places of the world, e.g., to the North of the Gulf of Mexico in
the USA and in the Northern part of Germany, most of them are not outcropped
and have been detected by geophysical methods. In contrast to them salt
domes in the ZMR and in the Persian Gulf, are not only high elevated domes,
but also in many cases the salt is flowing down the flanks as Salt Glacier.
|| Hormuz salt basins. The eastern boundary of Southern
Gulf Salt Basin seems to have been controlled by a lineament crossing
them northeastward along its boundaries (Derakhshani and Farhoudi,
Some of the salt domes are more than 1000 m higher than the surrounding
area, e.g., Kangan salt dome. Some of them have been used as open cast
mines. It`s more soluble than limestone and in contrast to the latter;
it has plastic behavior and impedes the flow of fluids. The Tertiary Gachsaran
Formation, consisting mainly of salt, anhydrite and marl layers, acts
as one of the best cap rocks of hydrocarbon in the Middle East.
ZMR structurally consist of numerous, mostly northwest trending, synclines
and anticlines. Steep flanks on most individual structures face southwest
(Falcon, 1969). Dextral displacements have been reported on faults parallel
to the Main Zagros Thrust between 33° -35°N (Tchalenko and Braud,
1974) and also on Kazerun Fault (Falcon, 1974; Baker et al., 1993).
The near-basement salts play major role in the morphology and structure
of the Zagros Thrust and Fold Belt. These salts cause disharmonic folding
and do not permit sub-salt structures to continue upward.
A variety of structures with different trends in the surface geology
have been attributed to repeated reactivation of basement faults (Falcon,
1974; Kent, 1979; McQuillan, 1991; Berberian, 1995; Alsharhan and Nairn,
1997; Hessami et al., 2001; Bahroudi and Talbot, 2003). Nevertheless,
the number, distribution and interrelation of basement faults are still
poorly constrained. Many active faults in the basement are clearly recognisable
by their seismic activity. However, GPS measurements emphasize that most
Zagros deformation is aseismic (Hessami, 2002) or relatively low level
of seismic activity (Regard et al., 2004) and other faults have
been attributed to the basement on the basis of surface evidence alone.
About two hundred emergent Hormuz salt plugs are known in southwest of
Iran. Characteristic dome-shaped swellings on elongate surface anticlines
indicate the presence of many more intrusive salt masses at shallow depths.
The majority of these unusual and spectacular diapirs are located in Fars
Province. Kent (1958) in commenting that the Zagros fold belt owes its
Jura-type folding mode to a basal plastic or incompetent layer, implies
a thick and widespread Hormuz evaporate presence. Regional location of
the plugs shows no relation to upper tertiary folding though it is interesting
to note that not all, but most, plugs are associated with Zagros fold
axis. In some cases active plugs in some positions appear to have previously
inflated the anticlines. Following on the extrusion of salt from the fold,
collapse folds have resulted. Such structures possibly associated with
axial fold displacements.
No basement rocks are exposed anywhere within the Zagros orogen and it
is generally assumed that the Zagros basement is a NE ward continuation
of the Precambrian shield exposed in Arabia (Falcon, 1969; Berberian and
King, 1981; Husseini, 1988; Alsharhan and Nairn, 1997) which is similar
to the basement exposed West of the Tabas block in central Iran (Alavi,
1991). However, recent maps of Gondwana show that the basement beneath
Iran comprises not only Pan-African (900-600 Ma) rocks extending northwards
from East Africa (Haffman, 1999); but also older rocks (>1,000 Ma)
extending westward from India (De Wit et al., 1999). The occurrence
of a Pan-African suture between Arabia and these older basement rocks
along the SWward projection of the Oman line could account for structural
trends to the east being oriented NE-SW, a trend not exposed in the Arabian
Shield but characteristic of the older basement before the dispersal of
Gondwana (de Wit et al., 1999; Derakhshani and Farhoudi, 2005).
An Infra-Cambrian age has been assigned to evaporates and polygenetic
assemblage of rocks which comprise the Hormuz group (Kent, 1979). These
rocks are seen only in small, scattered, emergent plugs associated with
thrust faults in the imbricated belt of Khuzestan Province and more abundantly
throughout Fars Province. These large plugs and salt glaciers contain
rafts of Cambrian sediments and a mixed assemblage of dominantly intrusive
rocks grouped under the general term of greenstones (McQuillan, 1991).
These exotics provide the only indications of possible basement composition.
The pattern of surface lineament is thought to relate to discontinuities
in the basement surface. Studying of Landsat images, aerial photographs
of the study area and distribution of earthquake epicenters (Fig.
3), combined with field checking has revealed abundance of surface
linear features. These include stream alignments, subtle topographical
patterns, tonal variations, faulting and structural modifications which
do not appear to relate to the late Tertiary folding episode.
The major lineaments of the Kazerun Fault and Oman Line form obvious
boundaries to distinct structural regimes. An examination of individual
folds exhibits the frequent occurrence of marked structural anomalies
in the form of abrupt axial bends or transcurrent disruptions. Regionally
such anomalies are seen to lie along trends which correspond to one or
more of the major lineament directions. This indicates the ongoing influence
of deep seated basement structural features and their modification of
later folding modes.
12 strike slip faults are recognized in the study area (Fig.
4). Some of these faults could have important role for salt uplifting.
They are as follows:
Fault No. 1: This dextral strike slip fault with 147 ° azimuth,
deviated the axis of Forgun, Neyzeh and Kushkuh anticlines as well as
displaced some features and caused linear outcropping of salt plugs specially
Fault No. 2: This dextral strike slip fault with 139 ° azimuth
has caused deviation of the axis of Muran, Finu, Baz, Handun and Namak
anticlines. The rotation of Muran anticline is an interesting feature
of the previous activity of this fault. Several salt plugs are outcropped
in this fault`s trend such as Handun and Namak salt plugs.
|| Earthquake epicenters and salt domes distribution in
the study area. Earthquakes are divided and shown in 4 distinct colors
according their surface magnitudes
|| Trends of postulate basement faults in the study area
Fault No. 3: This strike slip fault with 135 ° azimuth causes
deviation in the axis of Gachin, Gashu, Guniz, Muran, Bundasht, Shemilu
and Shab anticlines. There are six salt plugs such as Anguran in the trend
of this fault. Minor thrust faults that could be seen in Bundasht anticline
probably are formed by this fault.
Fault No. 4: This fault with 135 ° azimuth has caused right
lateral offset of the axis of some of anticlines such as Kuh-e Kohneh,
Pishavar, Gach, Burkh, Nakh and Champeh. There are seven salt plugs such
as Kermostaj in Gach anticline, Bam and Namakdan in Qeshm island, which
are outcropped in the direction of this fault. To the southeast, this
fault may cross Tonbe Bozorg salt plug. In Kohneh anticline, it creates
some minor faults parallel to itself and some of them to the axis of Kohneh
anticline. Because of the activity of the minor faults, recognition of
the anticlinal axis is very hard at this part of the anticline. This fault
has created some minor faults parallel to itself by crossing Nakh anticline.
Fault No. 5: This dextral strike slip fault has caused deviation
of the axis of Bostameh, Champeh, Herang, Nakh, Gavbast and Paskhand anticlines.
It also displaced the axis of Nakh syncline, which is located between
Gavbast and Nakh anticlines. This right lateral displacement is very clear
in Asmary, Jahrom, Gachsaran and Guri Formations at the northwest limb
of Nakh anticline. Exposure of salt domes at the East and Northeast of
Bastak could be related to this fault`s activity. There are several salt
plugs such as Kildun, which are outcropped on this fault in 140 °
azimuth. Kildun salt plug has a sharp fault boundary parallel to the fault
direction. Cutting of Bakhtiari and Aghajari formations and forming sharp
scraps in the salt plugs are also evidence of fault activity. To the southeast,
this fault may cross Tonbe Bozorg salt plug.
Fault No. 6: This strike slip fault in 167 ° azimuth crosses
Chiru salt plug and deviates the axis of some anticlines such as Chiru,
Lavarestan, Gavbast (also, there is about 10 km right lateral offset in
this anticline axis) and Bavash. Also, there is some minor faults parallel
to the major fault which is formed as the result of this fault activity.
These minor faults specially could be seen in the Lavarestan, Gavbast
and Bavash anticlines. Existence of erosion and collapse structures parallel
to this fault trend, in Gavbast and Bavash anticlines could be related
to this fault activity too. Minor faults trends and the pattern of erosion
in Gavbast anticline, increase the possibility of a hidden salt plug in
Fault No. 7: This fault with 80 ° azimuth, has caused deviation
of the axis of some anticlines like Bostameh, Ashuru, Champeh, Khamir,
Shab, Genow, Guniz, western end of Namak, eastern end of Handun, Neyzeh
and eastern end of Furghun. Eight salt plugs outcropped in its trend.
Fault No. 8: This sinistral strike slip fault with 36 ° azimuth,
has a concavity in its trend, to the east. It has deviated Muran, Shab,
Iicheh, Champeh and eastern end of shemilu anticlines axis. Outcropping
of several salt plugs such as Homeiran in the trend of this fault and
creating minor faults special in Muran and Iicheh anticlines, are considerable.
Fault No. 9: This fault with 76 ° azimuth, causes laterally
displacement in the axis of Baz, Shab, Bam, Nakh, Herang, Namaki and Chiru
anticlines and crosses in its southwestern end to Hendurabi island. Seven
salt plugs are outcropped in its trend such as Bam and Shab salt plugs.
This fault has created several minor faults in Baz, Shab, Bam and namaki
anticlines. It has deviated the axis of Shab anticline and has created
some clear minor faults there. Also, outcropping of shab salt plug in
this anticline could be related to this fault activity. Five kilometers
displacement in the axis of Nakh anticline and creating several minor
faults in circular form maybe resulted to a hidden salt plug there.
Fault No. 10: This sinistral strike slip fault with 14 ° azimuth
is concave to the west. It has started in the North from the Western end
of Dasht-e Kanar and has deviated the axis of Qaleh Shur and eastern end
of Chahal anticlines. Seven salt plugs have outcropped on its path. This
fault controlled edge of a salt plug located in Herang anticline. It also
formed some minor faults in Bam salt plug. This fault may pass through
Farur Bozorg and Farur Kuchak islands.
Fault No .11 : Deviation in eastern end of Gach anticline axis
and also the axis of Burkh, eastern end of Gavbast, eastern end of Gezeh,
Kuh-e namaki and Chiru anticline, outcropping of seven salt plugs on the
trend of this fault, creating of several minor faults specially in Burkh
and Gavbast and sharp cutting of Bakhtiari and Aghajari formations between
Gavbast and Gezeh anticlines are some evidences for this sinistral fault
with 50 ° azimuth, which is ended to the northeast to Dasht-e namak
Fault No. 12: This sinistral strike slip fault with 67 ° azimuth
is started from the eastern end of Kurdeh anticline. It has deviated the
axis of some anticlines such as Kurdeh, Gach, western end of Burkh, Eastern
end of Paskhand, Gavbast and Dehnow. Sharp and linear edge of Kurdeh salt
plug, displacement of Gach anticline axis, clear deviation between Paskhand
and Burkh anticlines and outcropping of a salt plug there, forming minor
faults specially in Kurdeh, Gach and Dehnow anticlines and outcropping
of six salt plugs are some evidences for the activity of this fault.
Displacement and bending of anticlines and synclines, location of salt
domes and concentration of earthquake epicenters indicate a close relationship
between basement faults, salt plugs and epicenters. By means of all these
features, location and trend of basement faults have been mapped. These
basement faults could be strike-slip faults regarding the displacement
of landforms (anticlines, synclines and etc.) in their paths. Also there
is a rectilinear pattern of salt plug emplacements. These implied lines
of weakness are almost certainly related to basement structural trends.
However, preferred orientations which correspond to the major basement
lineament trends described earlier result when other lineament associated
surface anomalies are taken into consideration. Thus basement structure
controls plug emplacement through its continued influence of the sedimentary