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

ttectonics@yahoo.com

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