Remote Sensing and GIS, Faculty of Engineering
Islamic Azad University of Dezful, Dezful Iran
Remote Sensing and GIS, Faculty of Engineering,
Islamic Azad University of Dezful, Dezful Iran
Ph:+98 641 6060601
Fax: +98 641 6260890
A digital elevation model (DEM) is used for hydrological study or watershed management, topography, geology, tectonic geomorphology and morphometric analysis. Geographical information system (GIS) provides a specialized set of tools for the analysis of topography, watersheds and drainage networks that enables to interpret the tectonic activities of an area. The drainage system maps of Zagros Mountains in south west Iran have been produced for two different dates (1950 and 2001) to establish the changes between geomorphic signatures and geomorphic aspect during time and to correlate them with recent neo-tectonics. This paper discusses the role of drainage for interpreting the tectonic processes as one of important signatures. The study shows variation in drainage network derived from topography maps. Thus, changes in drainage pattern, stream length, stream gradient and the number of segment drainage order from 1950 to 2001 indicate that the Zagros Mountain is subjected to recent neo-tectonic processes and it is a newly active zone.
The Zagros Mountains are parts of Alpine-Himalayan orogenic system. The Zagros Fold Belt (ZFB) is one the best exposed fold-thrust belts in structural and tectonic perspective. Geology of ZFB was studied by Stocklin (1977). Falcon (1974) has divided the orogen into three structural zones, such as (1) An inner crystalline Zone of overthrusting, (2) An Imbricate Belt, and (3) Zone of Folding often referred to the Simply Folded Belt. Later the geology of the Zagros Structural Belt (ZSB) was studied by many other workers such as Berberian (1976), Colman-Sadd (1978), Darvishzadeh (1992), Ali and Pirasteh (2004), Pirasteh (2004). They divided the ZSB into four units which are as follows:
- Sanandaj-Sirjan Zone
- Imbricate Zone
- Zagros Fold Belt
- Molasse Cover Sequence which are characterized by distinct geological and geophysical signatures.
Mountain topography is the result of highly scale-dependent interaction involving tectonic and surface processes. In ZFB, the bulk of the relief in mountainous region has been formed by uplift along thrust faults striking sub parallel to trace of the thrust zones. Therefore, there is an intimate link between uplift rates, material distribution rates, erosion (Ali and Pirasteh, 2003) and topography due to geomorphic processes and the morphology of the area. By quantifying stream length, stream gradient, the number of drainage segment order, drainage texture and pattern of two different dates (1950 and 2001) tectonic processes are inferred.
The evidence of neo-tectonic activities is commonly available through various geomorphic signatures (stream length, stream gradient, number of the stream segment order) and geomorphic aspects (such as stream drainage texture and pattern). Thus, drainage networks can be used as variable to interpret the structural and tectonic behavior of the area. Numerous studies have been made to develop relationships between tectonics and morphology in ZSB ( Mohr,1976; Ahnert, 1976; Almaz, 1988; Arnett, 1971; Bromhead, 1986; Ali and Pirasteh, 2004; Pirasteh 2004). In order to study the tectonic behavior of the ZFB morphometric parameters of the drainage systems such as stream length, stream gradient, number of drainage segment orders and pattern has been taken in consideration. The term of tectonic geomorphology is used to study the variation and changes in drainages during 1950 to 2001. Topography maps of the study area in scale 1:50000 dated 1950 are scanned and then introduced to GIS environment for digitization process. Digital topography maps in scale 1:25000 dated 2001 of the area are also provided by Iranian Survey Organization (ISO). The use of a digital terrain model of the ZFB for both the dates made it possible to calculate a synthetic drainage system in the terrain and drainage network through which the water runs. A gird-based terrain model of the ZFB represents the continuous surface of the terrain. Drainages are in the direction with the lowest elevation. Each cell has eight adjoining cells and eight possible drainage directions. The direction of the drainages based on the cell’s elevation value with the values of the adjoining cells. The drainage networks in the ZFB could be automatically vectorized in GIS environment. The digital terrain model made for the area has become a useful tool for the study of drainage morphometry.
DEM of the area is prepared for two different dates. Spatial analysis in GIS environment is approached to extract the length of the drainages, stream gradient, number of the segment streams order and further interpretation used to define changes in patterns of the drainages from 1950 to 2001. With significant improvement in resolution of available DEM and computing drainage network in ZFB the evaluation of morphotectonic in a GIS environment tends to be quantitative and more precise. Therefore the study could evaluate the changes in drainage network from 1950 to 2001 and correlate it to tectonic evolution.
2. Study area
The study area (Fig 1) extends from Dezful in north Khuzestan province to Khorramabad and Doorud-Brojerd in north Lorestan (Lurestan) province. sThese two provinces are most important from geology and geomorphology point of view. It is located in such a way that is belonging to ZSB. Both the provinces have different lithostratigraphy and geomorphological setting. The study area varies in elevation from 142 meters to 4200 meters above Mean Sea Level (MSL). The study area is bounded by Longitude 47 ° 58′ 1.42” – 49° 18′ 50.63” E and Latitude 32° 21′ 40.94” – 34° 00 00”. It is covered by digital toposheets of Dezful and Khorramabad blocks from Iranian Surveying Organization (ISO) numbers (5654, 5655, 5656, 5657, 5755, 5756, 5757, 5855, 5856) on scale 1:25000.
Fig.1 Sowing study area Zagros mountains south west Iran
3. Data source and methodology
Topography maps of the study area in scale 1:50000 and 1:25000 of two dates (1950 and 2001 respectively) were used for the study. In order to generate DEM, x,y and z attributes in text format are introduced to Rivertools software. The pixel size of proposed DEM was given on the basis of contour interval as 20 meters to increase accuracy. The methodology consists of two steps: 1- extraction and analysis of Dez basin drainage system based on scale 1:50000 and 1:25000 dated 1950 and 2001 respectively. In the present study drainage maps have been prepared from DEM using toposheets with the help of GIS software. Drainage networks of the study area of 1950 and 2001 are superimposed to emphasis the changes in stream length, number of drainage segment orders and drainages pattern. 2- Dez river profile is also digitally drawn on DEM dated 2001 to calculate the stream gradient. This is done in Rivertools software across to the field observations. This technique allows analysts to point out structural features such as faults and folds. Faults and folds on the DEM are recognized (Fig.5) on the basis of tone, slope, linearity, changes in topography and drainages direction. However, geomorphic signatures and aspects were keys to interpret the tectonic activity in ZFB. Broadly the methodology is defined as follows:
- The topography map of 1950 is scanned and digitally drawn in the form of vectors and points to define x, y and z attributes using ER-mapper 6.1, Arcview 3.2 and Arcinfo softwares.
- Vectors are introduced to Microstation J to export into text format
- Delineation of drainage networks for toposheet dated 1950. Simultaneously, the text formats were introduced to other GIS software called Rivertools 2.4 to create DEM (Fig 2) for analyzing the drainage networks (Fig 3). Morphometry analysis helps analysts to analyze and interpret tectonic geomorphology of the ZFB. Extraction of drainage networks involves three conditioning processes. They are:
- The digital topography map dated 2001 provided by the ISO in scale 1:25000 were converted to text format in the form of x,y and z using Microstation J. X, y and z attributes were introduced to Rivertools software to generate the DEM.
- Delineation of drainage networks for toposheet dated 2001- the same procedure ( as for toposheet dated 1950) was applied for morphometric analysis and to generate the drainage networks map
- Visual interpretation of the topography map. Field observations using global positioning system (GPS) to obtain elevation, faults, folds and drainage length. Dez river profile is interpreted (Burbank and Andrson, 2001) to evaluate the structural and tectonic behaviors in ZSB.
- Superimposing two drainage networks to highlight the changes of the drainages from 1950 to 2001.
- Finally, the difference between drainage networks dated 1950 and 2001 is interpreted to evaluate tectonic processes in ZFB.
*Flow grid generation- the objective of the first step in the conditioning phase is to create an adjusted “depression-less” raise to the lowest elevation value on the rim of the depression. Each cell in the depression-less digital elevation data set will then be part of cells leading to an age of the data set. A path is composed of cells that are adjacent horizontally, vertically, or diagonally in the raster (eight-way connectedness) and that steady decrease in value. The purpose of this routine is to extract a flow grid from a DEM grid.
** Basin outlet- This is a graphic routine that allows one of specify the basin that one wants to analyze by providing Rivertools with the precise location of the basin’s outlet. In the present study the complete DEM was used. Therefore an outlet was specified in the GIS environment on DEM on the basis of filed observations.
***RT Treefile- This routine creates a Rivertools “treefile” for one or more of the basins in the DEM, from a Rivertools flow grid. The treefile is a vector format file, which can store data for many disjoint basins. Every pixel in the particular basin is the outlet pixel for a sub-basin that is contained in that basin.
Fig. 2 DEM of the study area
SSZ- Sanandaj Sirjan Zone
MZT-Main Zagros Thrust
HZ- High Zagros
Fig 3. Stream gradient map of the study area derived from topography map dated 2001-ZFB south west Iran
4. Geomorphology and Morphometric Analysis
Tectonic force is an impulsive event that occurs at the beginning of the geomorphic cycle. Subsequently geomorphic process attack and degrade the topography. Since tectonic activities in the area started during Triassic to Late Cretaceous ( Pirasteh and Ali, 2005) then the oldest geomorphic features have formed in the ZFB and resulted the rugged topography. The tectonic activity in ZFB is more than the rate of erosion therefore erosion may change the topography and uplifting ( Ali and Pirasteh 2003 as stream length, stream orders (Tables.1) and drainage patterns are carried out. Drainages in the study area also often shift the direction of flow due to folding, thrusting, and rock type variations as the orogen is exhumed (Fig.4).
Fig.4. Structural Zone and transverse drainage of the Zagros Mountains.
The influence of topography on drainage morphometry has been well known for decades or even centuries. The idea of behavior on drainage is given in this paper by trying to correlate individual topography morphological attributes and the processes of tectonics and rock faulting. Among morphological attributes, the most important for the study of tectonics are probably stream length, stream gradient and other geomorphic features as pointed out by Burbank and Anderson (2001). Currently, advantages of GIS have allowed many investigators to contrast digital elevation model and extract drainage morphometry attributes in order to study the correlation between topography and drainage morphometric. In this study, we relied on this approach as there are advantages of GIS on how topographic attributes and comparison of two topography maps in different dates can be linked to interpret tectonic processes and tectonic evolution in ZFB. A quantitative, morphometric analysis of a drainage basin of two different dates is considered to be the most satisfactory method to tectonic processes interpretation. It enables us (i) to understand the relationships among different aspects of the drainage pattern of the basin, (ii) to make a comparative evaluation of drainage derived from different topography maps and DEMs (dated 1950 and 2001). The morphometric parameters computed include number of stream segment order, stream gradient, stream length (Table 1) and drainage patterns. The morphological attributes displayed by channel length, stream gradient, etc. at each segment order for morphological evolution.
Geomorphologically the overall drainage pattern of the basin is sub-dendritic indicating the heterogeneity of rocks and gradual slope towards the main stream. At places parallel to sub-parallel, trellis and dendritic type of network is seen.
In order to determine the relationships between tectonic activity and drainage systems or to find the present day morphometric behavior of the study area, the geomorphology parameters are studied.
4.1. Drainage networks
The fluvial dissection of the landscape consists of valleys and their included channel ways organized into a system of connection known as a drainage network. Drainage networks in ZSB display many types of quantitative regularity that are useful in analyzing tectonics evolution. One very useful property is the pattern of dissection. Basically it is summarized the geological significance of various drainage patterns. Dendritic patterns occur in the presence of structural control, as on the ZFB. Areas of trellis and sub parallel drainage include central ZFB and the high Zagros, respectively. An example of drainage pattern is shown in Figure 3.
4.2. Stream Length
The stream length of the drainages in ZSB is varying in each segment order. It is depending on slope, lithology and structural features. Generally stream length of the first segment order is greater than steam length of the second segment order and so on ( Table 1). But it is seen that the length of seventh order stream is lessthan eight order for both the drainage systems ( 1950 and 2001). It is probably because of faulting and folding as field observation suggested. However, the comparison of stream length for two drainage systems depicted from DEMs suggests that drainage length is increased.
4.3. Stream Gradient
The zone of rapid rock uplift had a steeper gradient, higher relief, and higher gradient indices (Burbank and Anderson 2001). In order to calculate the stream gradient the Dez river profile was digitally drawn on DEM dated 2001 from Chalanchoolan police station near Brojerd city. The total length of the stream is approximated to 165 km (Fig 5). The profile was divided into three sectors such as 1- high Zagros (Imbricate Zone), 2- folded Zagros and 3- folded Zagros towards Dezful embayment in south of Shahbazan station (Fig 6) and ( Fig 7.-A,B,C ).
Steplike river profile of the study area is predicted to approach a graded profile which indicates that area has been tectonically disturbed. Stream gradient indices deduced in each part of the profile, shows variation from 806m to about 142m per kilometer during the geological time scale which means that the river profile is experiencing a regarding stages.
Fig 5. Dez river and structural features on DEM of the study area
Fig 6. Dez river profile with about165km
Fig 7.A Stream-Gradient indices, topography profile High Zagros
Fig 7.B Stream-Gradient indices, topography profile for Folded Zagros
Fig 7.C Stream-Gradient indices, topography profile for ZSB-folded Zagros towards Dezful embayment
The high index shows the steeper gradient and high tectonic activities are mainly with thrusting and faulting like Main Zagros Thrust, Hoor thrust, Chamsangar fault, Shahbazan strike slip fault (Fig 8) and Baraftab fault. The Dez river profile indicated that the most active tectonic zone falls in folded Zagros where the stream gradient indices vary from 806m to 165m per kilometer (Fig7.B). It also evaluated that the reduction in gradient towards the Dezful embayment may point towards lesser tectonic activities.
Fig 8. Field photograph showing Shahbazan fault, Dez river follow the fault
The different formations dominating various type of rocks like limestone and evaporates in Gachsaran formation, shale of Aghajari formation, marls of Kashkan formation, Cretaceous calcareous in contact of the Imbricate Zone and ZFB with Sanandaj-Sirjan Zone may also approaching graded profile of Dez river during the geological time scale in the study area.
For the disturbed rivers profile the high SL values or stream gradient may indicate high tectonic activities. The systematic stream gradient map (Fig.3) of the Zagros Structural Belt in GIS environment was made to interpret tectonic correlation. Hence, it is resulted that the Sanandaj-Sirjan Zone exhibits 0 to 0.441 and has low tectonic activity. It also reveals that the ZFB exhibits 0.441 to 4.268 and has higher tectonic activity than other lithotectonic units in the ZSB. These activities in the Zagros mountains generate terraces.
When a river that is flanked by flight of fluvial terraces is also oriented at a high angle to strike-slip fault (Burbank and Anderson 2001), the terraces displayed by the fault provide an excellent record of progressive offsets. These terraces have been seen in the study area by changing in river course on the basis of two DEMs dated 1950 and 2001. Because of changes in river course of the Dez river through time in the ZSB the height of steps between terraces along strike-slip faults is a general guide to correlate terraces. The vertical (dip-slip) displacement along the faults is responsible to river height and variation as well.
The Dez river in the ZSB cross an active folds, so fluvial terraces is recorded as progressive displacement. The presence of fault and continuation in tectonic activities, rupture the surface. The growth of the structure itself was tectonically pulsed, such that terraces formed during intervals of reduced deformation rates ( Medwedeff et al. 1992). It is expected that the age and height of the terrace generally correlate with the magnitude of displacement. The field checks show that in the ZSB where the folding and faulting are closely spaced, a single terrace is displaced by several faults.
5. Geological settings
Geologically the study area consists of various lithological units, ranging in age from Cretaceous units of in contact with the SSZ in the northeast, dominated by calcareous strata, and sub Recent and Recent units in the ZSB in the southern part of the study area (Pirsateh and Ali, 2005). A generalized stratigraphic column for the Zagros Simple Folded Belt, showing Cretaceous through Miocene strata grouped into four units according to relative resistance to erosion.
The area is mostly dominated by calcareous Cretaceous, dolomite, limestone, shale, and marls in north, and evaporates (such as gypsum) and highly cemented conglomerate of the Pliocene Bakhtiari formation in the south. The closure of the Zagros basin during Cretaceous-Miocene time generated diverse styles of folding and faulting. These structures, especially in the ZFB exhibit tight, NW-SE-trending folds with closely spaced fracture systems (e.g., see Fig. 6). These types of geological setting have facilitated severe erosion and the formation of rugged and immature topography and a closed drainage system.
6. Structural and Tectonics
Drainage may adjust passively to varying resistance of geologic materials, or it may be actively induced to follow a particular course by tectonism. Examples of the latter include faulting, as in the Shahbazan fault. Growing folds and lineaments have affected drainage in the ZSB.
Streams that emerge from mountain fronts onto surrounding plains display a fascinating array of structural and tectonic controls. Where mountain fronts are erosional because of a complex interplay of geomorphic variables, they may develop flanking surfaces of plantation called pediments. Deposition at the mountain front produces alluvial fans and knickpoints ( Pirsasteh, 2004) because of the tremendous increase in width as a stream emerges from a mountain canyon through the geological time scale.
Passive adjustment to structure is a quality of nearly all the study areas. Perhaps the most interesting situations, however, are drainage anomalies, where streams cut across structural zones. Some streams appear to take the most difficult routes possible through fold belts.
7. Results and discussion
This study begins with topography maps of the area in two dates (1950 and 2001). The drainage networks of the study area of different dates are extracted using DEM in GIS environment. The study reflects changes in geomorphology parameters that lead the analyst to define the tectonic processes in the ZFB.
Spatial analysis of DEMs in conjunction with the field observations by GPS and numerical geomorphology in GIS environment provides a mean for characterizing tectonic activity of ZFB in a quantitative way. Using DEMs data (1950 and 2001) and software tools have made the study easy and accessible every where. Under the frame work of GIS analysis of geomorphology spatial data (stream length, stream gradient, number of stream segment) and drainage pattern to derive relationships between tectonics and geomorphology parameters becomes increasingly important.
Superposition of the drainage networks ( Fig.9)of the area derived from DEMs shows that the drainages form straight line for 1950 (dated 1950), but in case of drainage network (dated 2001), it is seen that almost changes have been tried to become non-straight line and has lost the previous straight direction line. The analysis of two topography maps in GIS environment suggests that drainage networks are following the slope and structural features in ZFB. Present drainages pattern in the study area is dendritic to sub dendritic and parallel to sub parallel. The comparative study of topography and drainage system dated 1950 with topography and drainage system dated 2001 represents that the drainage patterns has been also subjected to some changes due to tectonic activities. For example the trellis drainage pattern changes to sub-parallel and sub-dendritic pattern. It is seen in Fig 9.
The total length of the drainage is 25001.386 km. The spatial analysis in GIS environment calculated the drainage basin to be 8109.680 km2. It is seen that the number of the first order streams segment are increased. Morphometric analysis is also suggested that the second order streams segment derived from DEM (dated 2001) are increased to 7940 ( Table 1). It also shows that length of the first order, second order, third order , forth order, fifth order and sixth order streams obtained from DEM ( dated 2001) is increased as compare to length of the streams in 1950. An interesting result is seen for the length of seventh and eight’s drainage segment order. Morphometrically the eight’s drainage order should be less than seventh stream order while in the study area it is not seen. This reason indicates that ZFB has also been subjected to tectonic activity from 1950 to 2001. Thus it is called Recent neo-tectonic activities. The pattern of the drainages is also suggests that the area is new and tectonically active.
The systematic stream gradient map (Fig 3) of the ZSB in GIS environment was made to interpret tectonic correlation. From the map (Fig 3), it is resulted that the Sanandaj-Sirjan Zone that exhibits 0 to 0.441 has low tectonic activity. High stream gradient values have generally been associated with high rates of tectonic activity. Stream gradient map derived from topography map dated 2001 and 1950 shows variation and changes in stream gradient values. It is seen that stream gradient values derived from DEM (dated 2001) has higher stream gradient values (dated 1950). It also reveals that the ZFB that exhibits 0.441 to 4.268, has higher tectonic activity than other lithotectonic units in the ZFB. These activities in the Zagros mountains have generated terraces. However, increasing in length of the stream, number of the drainage segment order, stream gradient reveals that the area has subjected to tectonic activity during past 51 years.
Table. 4-1- Comparative geomorphic parameters showing data summery for the Dez River BASIN, ZSB, SW Iran ( depicted from DEM dated 1950 and 2001)
Fig. 9 Superimposed drainage networks derived from DEM for two dates (1950-2001) south west Iran
Tectonic activities and rock faulting have great influence on stream gradient and stream length. The strong variation and changes in stream segment order and other geomorphic parameters indicates that tectonic processes and evolution in ZFB ( Table 1). The study shows the evolution of drainages in ZFB as well as its partial correlation with tectonic processes.
Utility of DEM and GIS techniques for regional scale tectonic studies has been shown in this paper. With the help of digital terrain data, important morphometric parameters are calculated for interpreting tectonic processes. Both the morphometric parameters of Dez drainage basin in 1950-2001 and stream gradient indices were investigated in detail and found to be in agreement with the existing superimposed drainage basin map. This type of analysis can be used in cross disciplinary study of landslide hazards, earthquake, and geotechnical studies in tectonically active ZFB.
Authors are thankful to authorities of Asman Sanjesh-e-Jonoob research and consulting company authorities for providing facilities. The authors are thankful to Prof. SM.Ramasamy director of remote sensing, University of Bharathidasan, tamilnau, India for his ideas.
- Ahert, F.,1976. Brief description of a comprehensive three-di-mensional process-response model of landform development, Z.Geomorphology N.F., Suppl.25,29-49
- Ali Syed Ahmad and Pirasteh Saied, 2003, Remote Sensing and GIS Study of Tectonics and Net Erosion Rates in the Zagros Structural Belt, Southwestern Iran, Vol.40, No.4, PP:258-268
- Ali Syed Ahmad, Saied Pirasteh, 2004, Geological application of Landsat Etm for mapping structural geology and interpretation: Aided by Remote sensing and GIS, International journal of remote sensing, UK(under print)-Vol.25,issue 21, pp.4715-4727
- Almaz, G.,1998. Slope instability assessment in the Blue Nile gorge, Ethiopia. In: Moore, D., Hungr, O. (Eds), Proceedings of the 8th International IAEG Congress, Vancouver, Balkema, Rotterdam, pp. 1437-1442
- Arnett, R.R., 1971. Slope form and geomorphological processes; an Australian example. In: Brunsden, D. (Ed), form and processes. Institute of British Geographers Special Publication, Vol.3, pp:1-92
- Berberian,1976, Geological survey of Iran, Tectonics and Seismotectonic section, PP.53-100
- Bromhead, E.N., 1986. The stability of slopes. Chapman&Hall, New York. 372pp
- Burbank Douglas W., and Robert S. Anderson, 2001, Tectonic Geomprphology ,Department of Geosciences, The pennsyvania State university, PP.80-198
- Colman-Sadd, S.P.(1978) Fold development in Zagros Simply Folded Belt, southwest, Iran. Bulletin American Association of Petroleum Geologist, 62,PP:984-1003
- Darvishzadeh.A, 1992, Geology of Iran, PP.191-210
- Densmore, A.L., Anderson, R.S., McAdoo, B.G., Ellis, M.A., 1977. Hill -slope evolution by bedrock landslides. Science 275, 396- 372.
- Falcon , N.L., 1974, An out line of the geology of the Iranian Makran. Geographical Journal, 140 (2), PP.284-291
- Mohr,P., 1967. Review of the geology of the Simiem Mountains. Bulletin of Geophysical Observatory, Vol. 10. Addis Ababa University, Addis Ababa, Ethiopia, pp.79-93
- Oberlander, T., 1965, The Zagros stream, syracuse geog. Series 1, syracuse university press, P. 164-168
- Pirasteh Saied, 2004, Use of remote sensing and GIS to interpret structural and tectonic of Zagros Mountains, SW Iran, Ph.D thesis, Deptt. of Geology, Aligarh Muslim University, Aligarh, India, P.206
- Pirasteh, Saied and Ali, Syed Ahmad, 2005, lithostratigraphic study of Zagros Mountains Areas SW Iran using digital topography, remote sensing and GIS, Indian petroleum geology Journal, Vol. 13, No. 1, PP:1-13
- Stocklin J., and Steudehnia, 1977, Stratigraphy Lexicon of Iran. Report No.18, 2nd edition, published by Geological Survey of Iran