Geomorphologic and structural assessment of active tectonics to NE Shiraz (Fars, SW
Iran)
Fariba.Taheri *
M.S of Tectonics, Department of Earth sciences, Shiraz Branch, Shiraz University, Shiraz,
Iran
Number 4- Parsa dead land, Ahmad Kabiri Allay –Emam Sajad Street- Esfahan
Tel:
00989138008273
Ahmad. Zamani
Professor Earth sciences Department, Shiraz University, Shiraz, Iran
Abstract
The zagros fold-thrust belt is an NW-SE trending, located in the middle part of the
seismic belt of Alps- Himalayas, The occurrence of destructive earthquakes in this belt
resulted in the destruction of buildings and economic institutions, cultural and historical
sites. The destruction of Arge Bam in the 26 December, 2003 drown the attention of
government official vulnerability of cultural and historical sites during the occurrence of
earthquake. Many of these sites are located in Fars province. One of the most important
sites is Takht-E-Jamshid. In this study following 2 topics have been investigated 1-
Analysis morphotectonic indices such as Drainage basin shape index (Bs), Mountain front
Sinuosity Index (SMF) and valley floor width – height ratio (Vf). These indices have been
measured with digital elevation model (DEM) 2- Determine Stress pattern with use of the
inversion method. The results of Morphotectonics indices indicate that the study area has a
higher level of seismic activity especially Rahmat Fault Zone which is in south of Rahmat
Anticline (Takht-E-Jamshid) and based on results from the contour diagrams. There are 3
stress directions NE-SW, N-S and NW-SE, which is obvious in this area. Fault slip
analysis reveals two successive late Cenozoic regional compressional trends, NE-SW then
N020. The latter is in good agreement with the present-day stress. The significance of the
NW-SE compression is discussed alternatively in terms of stress deviations or block
rotations in relation to the strike-slip fault system. Therefore in order to prevent serious
damage to these historical sites evaluation of the seismic risk in the study area plays an
important role in mitigation of earthquake damages.
Keywords: Morphotectonic Indices, Inversion Method, Rahmat Fault, Takht-E-
Jamshid, , Historical Sites,
Introduction
The zagros fold-thrust belt situated in the west- southwest of Iran and extends laterally to
Iraq and Turkey. This belt results from the alpine orogenic events in the alpine - himalaya
mountain rang [7], [1], [13]. Basically, structural systems observed in the Zagros result
from the interplay between kinematic boundary conditions (e.g. far-field collisional
stresses transmitted away from the Arabia–Central Iran plate boundary, foreland flexural
stresses, etc.) and local deformational events such as fold growth or movements along
major faults [20], [16]. (Figure-1)
The quantitative expression of the geometric effect of the earth relief in the Zagros belt
will allow the identification of zones with common geomorphometry in the Alpine-
Himalayan belt and will provide an understanding between the correlation of the regional
tectonic processes and geomorphometry. The reconstruction of the past kinematics and
tectonic history and geomorphic investigation in zagros fold–thrust belt explains the
history of the successive local/regional directions of stress and shortening and active
tectonics through time [16].
The Geomorphic indices are useful tools in the evaluation of active tectonics because
they can provide rapid insight concerning specific areas within a region which is
undergoing adjustment to relatively rapid, and even slow, rates of active tectonics [15],
[28]. The fault-slip data[18], calcite twin data [17] and anisotropy of magnetic
susceptibility (AMS) data [2] microtectonic data with existing palaeostress/strain data will
allow us to discuss competing kinematic models proposed to account for palaeostress
changes though time (e.g. clockwise block rotations related to the kinematics of the right-
lateral Kazerun–Karebass–Sabz-Pushan fault zones [9],[18] versus regional anticlockwise
rotation of the regional stress field during the Neogene [2], and eventually to propose
alternative explanations.
In present paper, Takht-E- Jamshid and its surroundings have been investigated to
evaluate the state of active tectonic using structural and Morphotectonic analysis and
subsurface data. The Study area is located in Fars province (East longitude 52 o 50' to 53 o
10' and North latitude 29 o 45' to 29o 55'). This is situated in 70 Km northeast of Shiraz. On
the other hand, this site is located in Rahmat Anticline (Zagros Simply Folded Belt, one of
the active tectonic regions in the world). This area included faults with combined normal,
reverse and strike–slip movements. We aim to decipher: 1- Analysis morphotectonic
indices such as Drainage basin shape index (Bs), Mountain front Sinuosity Index (SMF)
and valley floor width – height ratio (Vf) 2- Determine Stress pattern in the area (Figure-2)
Geological Setting
A- Structural Setting
The Zagros is a folded belt with an overall NW-SE strike evolving to ENE -WSW in its
SE part results of the continent– continent collision between the Arabian plate and the
Central Iran microcontinents following the closure of the Neo - Tethys ocean during the
Tertiary [23];[12]. The shortening rate increases from NW to SE and with the present-day
rates of 0.7 cm yr−1 in the Zagros as suggested by GPS studies [31] [18],[19] (Figure1).
This orogenic belt consists of four NW-SE trending parallel zones: (1) Urumieh-Dokhtar
Magmatic Assemblage (UDMA); (2) Sanandaj–Sirjan Zone (SSZ); (3) High Zagros; and
(4) Zagros Simply Folded Belt (ZSFB) (Figure-3). This belt is segmented into several
zones that differ according to their structural and depositional history [23]; [12]; [7]. The
zones are, from east to west, the Fars salient the Dezful recess and Lorestan salient. The
Fars province (In central parts of the Zagros) is limited to the west from the Kazerun–
Boradjan Fault (KBF), a seismically active major right lateral strike-slip fault [5]; [27]
[10]; [3] and the Minab–Zendan fault system is limited to the east [22].
In the Fars involves shortening nearly parallel to convergence, which is perpendicular to
the nearly E–W trend of the belt While deformation in the Western-Central Zagros. The
Arabia–Eurasia convergence is partitioned into NE–SW shortening perpendicular to the
belt and right lateral strike-slip faulting along the Main Recent Fault [30]. The N–S to
N150◦ right-lateral strike-slip fault system (The Kazerun–Borazjan fault, Karebass fault,
Sabz Pushan fault and Sarvestan fault in the western part) are in the western part of the
Fars;( Figure 2). These faults have been interpreted as an orogen-scale horse-tail strike-slip
fault termination, which transfers and distributes orogen-parallel dextral slip achieved
along the Main Recent Fault in to the thrusts and folds of the Fars [3].
The low taper angle, the Main Recent Fault is the geological boundary between the
Zagros belt to the south and Mesozoic metamorphic and magmatic belts exposed on the
Iranian plateau to the north. It thrust belt bounded to the SW by the High Zagros Fault and
up thrusted onto the Simply Folded Belt.
The folds pattern in the Simply Folded Belt is open anticlines with wavelengths
typically of 15–20 km and lengths of 100 km and more. These folds probably mainly result
from buckling and subsequent detachment folding of the 10–12 km thick sedimentary
cover above a single master décollement lying within the Hormuz salt [11], [19]. Second-
order intermediate décollement levels are, however, required to account for shorter fold
wavelengths [27].
The major earthquakes occur along the Zagros belt. These have been thought for a long
time to be responsible to High-angle thrusts in the basement belt [14]; [13]; [8]. These
basement structures caused deformation in the cover by localizing some topographic steps
and major thrust faults [8]; [19]. Critical wedge modelling [19] supports the theory that
both the deformation and the topography in the Fars are mainly controlled by basement-
involved shortening. In the western most Fars [26] estimated a cover shortening of 34 km.
In the southeastern Fars, [21] estimated a cover shortening of 15–20 km.
b- Sedimentary setting
After collision between the Arabian plate and the Central Iran microcontinents in the
Permo-Triassic setting of Jurassic–Middle Cretaceous times, the tectonic evolution during
Late Cretaceous time was governed by the obduction of the Tethyan ophiolites on to the
Arabian margin. The basinal Gurpi Formation covered nearly the entire Zagros basin in
response to the flexure of the Arabian plate during the late Cretaceous [18].
The Mountain Front Fault (MFF) isolated two sub-basins in the foreland basin, a shallow
one to the NE in which clastic rocks and carbonates accumulated and a deeper basin to the
SW where shales of the Pabdeh Formation were deposited during Paleocene–Eocene times,
[28]. The shallow marine platform limestones of the Asmari Fm deposited unconformably
above the Pabdeh Fm in the SW Zagros basin and also covered the Jahrom Fm in the NE
Fars region in Oligocene time.
The Miocene Fars Group ( Gachsaran, Mishan, Agha Jari formations;) represents a first-
order regressive sequence (up to 3000 m) that reflects the progressive infilling of the
Zagros foreland basin. The initiation of foreland siliciclastic deposition is delayed toward
the foreland: the onset of clastic deposition is Chattian (∼ 28 Ma) in the northern Fars
whereas it is Burdigalian (20–16 Ma) near the Persian Gulf [19] (Figure-4).
Materials and Method
A- Determining morphotectonic indices
Morphometric analysis basis for relative adjustments between local base-level processes
(tectonic uplift, stream downcutting, basin sedimentation and erosion) and the fluvial
systems which cross structurally controlled topographic mountain fronts [6]. This type of
morphometric analysis has not previously been used morphometric study on the dangers of
historical sites. Therefore, manual sampling of drainage network was again adopted from
topographic maps (1:25000) in combination with computerized tools, certainly the
Geographic Information System (GIS), which were of great significant in this application.
The study area, comprising mountain fronts and drainage basin associated with the systems
of faults constituting the Rahmat Anticline were selected for morphometric analysis.
Mountain fronts were selected for this study on the basis of topographic, lithological,
geomorphological and structural continuity. Drainage basin was given to each stream by
following Strahler [25] stream ordering technique. The attributes were assigned to create
the digital data base for drainage layer of the river basin. The map showing drainage
pattern in the study area (Figure 6). Sample selection was determined according to
particular geomorphological criteria that provided high reliability and confidence in the
representativeness of the morphometric data produced. (Table1)
b- Determining Paleostress Regimes from Inversion of Fault Slip Data
The kinematics of a fault population is defined using striations observed on mesoscale
fault planes at many sites. For each fault, strike, dip, slickenside, rake and polarity of
movement are measured and determined in the field. The main objective is to define the
successive Cenozoic states of stress and the related faulting events and their probable
significance in relation to regional tectonic events. The methodology of fault kinematic
studies to determine paleostress fields and identify temporal and spatial changes in stress
states has been used in many areas worldwide over the past 30 years [21], [19]
To determine the stress fields responsible for Cenozoic deformation in the investigated
area, we have carried out a quantitative inversion of distinct families of slip data
determined at each individual site, using the method proposed by Angelier [6].Fault slip
inversion method assumes that:
(1) The analyzed body of rock is physically homogeneous and isotropic and if
prefractured, is also mechanically isotropic, i.e., the orientation of fault planes is random,
(2) The rock behaves as a rheologically linear material [28], (3) Displacements on the fault
planes are small with respect to their lengths and there is no ductile deformation of the
material and thus no rotation of fault planes. Moreover, the computation assumes that (4) A
tectonic event is characterized by a single homogeneous stress tensor, (5) The slip
responsible for the striation occurs on each fault plane in the direction and the sense of the
maximum resolved shear stress on each fault plane (Wallace-Bott principle), the fault
plane being the preexisting fractures, and (6) The slip on each of the fault planes is
independent of each other. Because in the cover of the SFB of the Fars region the train of
folds is regular and almost devoid of major thrust zones at least reaching the surface, In the
study area, along the major faults, geological features such as slickensides along fault
planes, crushed zones, and offsets of rock units including strike separations, were analyzed
(Figure5).
Results and Discussion
A- Morphotectonic Indices
The stream order is the first step in drainage basin analysis for classifying relative
location of a reach (a stream segment) within the river basin. The stream order method
followed the procedure method that modified by Stahler. Stream order 1 has one connected
edge, and then at the confluence of two1st order streams assigns the downstream reach of
order 2, and so on for the rest orders. Study basin system has 4-stream orders, and thus a
map was obtained using GIS system (Figure. 6).
A.1 Drainage basin shape: The basin in the tectonically active mountain range is
elongate, and basin shapes become progressively more circular with time after cessation of
mountain uplift Thus, the planimetric shape of a basin may be described by an elongation
ratio of the diameter of a circle with the same area as the basin to the distance between the
two most distant points in the basin [6]. The elongation ratio Bs defined as: where BI is the
length of the basin, measured from its mouth to the most distant drainage divide, and BW is
the width of the basin measured across the short axis (Table I). The index reflects
differences between elongated basins with high values of B (high tectonic activity) and
more circular basins with low values (low tectonic activity). The drainage basin shape was
calculated for the 40 drainage basins of streams that cross the main faults of the Rahmat
Anticline. The purpose of calculating the drainage basin shape (B) index was to identify
elongated basins which reveal primarily downcutting in areas of continuing rapid uplifts.
Results indicate high values of dissection and elongated drainage basins characteristically
occur in the N part of the Rahmat Anticline. (Figure- 7)
( Bs=BlBw
) (1)
A.2 Mountain front sinuosity. The mountain front sinuosity index (SMF) is defined as
the ratio length of mountain front along the mountain–piedmont junction (Lmf) and the
straight-line length of the front. (L) [6]. The SMF index reflects a balance between the
tendency of stream and slope processes to produce an irregular (sinuous) mountain front
and vertical active tectonics to produce a prominent straight front [15]. Values of Smf
approach 1· on the most tectonically active fronts, whereas SMF increases if the rate of
uplift is reduced and erosional processes begin to form a sinuous front that becomes more
irregular over time (Table I). However, values of mountain front sinuosity index depend on
image scale, and small topographic maps produce only a rough estimate of mountain front
sinuosity. Therefore, mountain front sinuosity and all morphometric variables were
measured on small-scale topographic maps (1:25000, with 10m contour intervals). Results
indicate low values of SMF, characteristically occur in the N part of the Rahmat Anticline.
(Figure- 8)
(Smf = LmfLs
) (2)
A.3 Valley floor width – height ratio: The Valley floor width – height ratio (Vf) is the
width of valley floor, Eld and Erd are the respective elevations of the left and right valley
divides and E is the elevation of the valley floor [6]. The index reflects in this way
differences between broad-floored canyons (U shape) with relatively high values of Vf, and
V-shaped canyons with relatively low values [15]. (Table I). Thus, in this study transverse
valley profiles were located 0·5 km upstream from the mountain front in smaller drainage
basins, and in large drainage basins transverse valley profiles were located 0.5 and 1km
upstream from the mountain front. The reason for working with different ranges for the
location of the cross valley transects is that valley floors tend to become progressively
narrower upstream from the mountain front in larger drainage basins for a given mountain
range. Values of Vf may also vary widely among streams with different drainage basin
areas, discharges and underlying bedrock lithologies. Consequently Vf ratios were not used
directly in this study to estimate the relative levels of tectonic activity of specific fronts, as
this would require comparison of Vf values among streams of variable size and lithology.
Instead, several Vf values were determined along the length of streams in each subarea
with similar geological and morphological characteristics [28]. The data were combined
with the longitudinal profile and valley morphology to indicate changes in valley and
profile morphologies suggesting localized uplift in channel reaches upstream from
mountain fronts crossed by a given stream. Results indicate low values of dissection and
elongated drainage basins; characteristically occur in the N part of the Rahmat Anticline
(Figure- 9).
(V f =2V fw
(Eld−E sc)+(E rd−E sc) ) (3)
B - Paleostress
To analyze the stress in the area, fault planes, the associated slickenside is measured.
Numerous shear data are determined from historical locations in the study area and
categorized into 26 fault plane sites. According to the inversion method (proposed by
Angelier [1990][4].) which includes determination of the mean stress tensor orientation
and sense of slip on numerous faults and joints. Faults and joints data are classified based
the principal stress axes and corresponding compressional and extensional directions are
calculated (with the use of tectonicsFp software). Based on the derived results from the
diagram, suggested 3 stress directions NE-SW, N-S and NW-SE compressional stress
directions, which is obvious in study area. Fault slip analysis reveals two successive late
Cenozoic regional compressional trends, NE-SW then N020. The latter is in good
agreement with the present-day stress. The significance of the NW-SE compression is
discussed alternatively in terms of stress deviations or block rotations in relation to the
strike-slip fault system. (Figure-10).
CONCLUSION
The region under study is located in Fars province (SW Fold Simple Zagros).
Geomorphologic and stress regime assessment in the historical site of Fars province
( Takht-e- Jamshid) is the main object of this research. This study the following topics
have been investigated
1- Analysis of active structural using of morphotectonic indices: Variations in the
morphology of the fault-defined mountain fronts that form the Rahmat Anticline provided
the basis for the morphometric assessment of relative degrees of tectonic activity. To
simplify the analysis and discussion of morphometric indices in the Rahmat Anticline,
Results indicate Low sinuosity (Smf) in the N Rahmat Anticline reflects a relatively
straight fault-bounded mountain front. A high proportion of continuous undissected
escarpments, low values of dissection and elongated drainage basins(Bs) and low (Vf )
values, characteristically occur in the N (Takht-E-Jamshid historical site) part of the front,
suggesting a relatively higher degree of tectonic uplift in this area. Therefore The Rahmat
Anticline shows the highest level of uplift. Rahmat fault zone which is in south of study
area has a higher level of activity Takhte Jamshid are located close to this fault.
2- To analyze the stress in the area, fault planes, the associated slickenside and joints are
measured. Numerous shear data are determined from historical locations in the study area
and categorized into 26 fault plane sites. Determining stress regime in the study area:
Based on the derived results from the diagrams, suggested 3 stress directions NE-SW, N-S
and NW-SE compressional stress directions, which is obvious in study area. Fault slip
analysis reveals two successive late Cenozoic regional compressional trends, NE-SW then
N020. The latter is in good agreement with the present-day stress. The significance of the
NW-SE compression is discussed alternatively in terms of stress deviations or block
rotations in relation to the strike-slip fault system.
In order to prevent serious damage to these historical sites evaluation of the seismic risk in
the study area plays an important role in mitigation of earthquake damages.
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Table 1 – Morphotectonic Indeces[28]
Morpho
metric
paramete
r
Mathe
matical
derivati
o*
Measureme
nt
Procedure
Purpose SignificanceSourc
e
Smf=
Mountai
n
Smf =LmfLs
Reflect a balance
between the tendency
of stream and slope
Smf = 1·0 – most
tectonic activity
Bull
and
frontsinu
osity
processes to produce
irregular(sinuous)mount
ain front and vertical
active tectonics that
tend to produce a
prominent straight front
(Keller, 1986)
Smf > 1·0 – less
tectonic activity
McFa
dden,
1977
Vf =
Valley
floor –
valley
height
ratio
V f=2V fw
(E ld−E sc)+(E rd−E sc)
Define the ratio of the
width of the valley floor
to the mean height of
two adjacent divides
The index reflects
differences between
broad-floored canyons
with relatively high
values of Vf, and V-
shaped canyons with
relatively low Vf values
Bull
and
McFa
dden,
1977
Bs=
Drainage
basin
shape
ratio
Define the planimetric
shape of a basin
High Bs values =
elongated basins and,.
high tectonic activity;
low Bsvalues = circular
basins, i.e. low tectonic
activity
This
study
after
Canno
n
(1976)
Fig 1. Geodynamic setting of the Arabia–Eurasia collision. Arabia–Eurasia convergence
vectors after NUVEL-1 (DeMets et al. 1990) (black) and GPS studies (Vernant et al. 2004)
(white). Velocities in cm yr-1 The frame indicates the study area. L – Lorestan; D – Dezful;
F – Fars
Fig2- Geology map of the study area
Fig-3 Iran tectonic zoning
(Stocklin, 1968.Stocklin&Nabavi, 1973.modified by Zamani&Hashemi, 2004)
Fig 4-Main characteristics of the Upper Cretaceous–Cenozoic sedimentary units of the
Fars
Fig-5 slickenside (N- Kazeron), b- fiber lineation (W-Hossin Anticline), c-cross section on
fault planes (NE-Rahmat Anticline)
Fig-6 Stream Order to study area
Fig-7 Analysis index Bs, to study area
Fig-8 Analysis index Smf to study area
Fig-9 Analysis index Vf, to study area
Fig.10-Stress regime to the study area
Fig1.[16]
Fig2
Fig3 [32]
Fig4 [18]
Fig5
Fig6
Fig7
Fig8
Fig9
Fig10
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