Hypsometric Properties of South Zagros Fold-Thrust Belt Basins : A Case Study in Namdan Basin in SW Iran

Study area is located in Southwest Iran with relatively flat topography. Outcropped lithological units in the region belong from Triassic and Jurassic till recent era. The study area of the Namdan basin is related to the tectonic zone of the high Zagros region and the compressional-extensional regime. Geomorphic indices of active tectonics are useful tools to analyze the influence of activity. One of them is hypsometric integral which has generally been used to reveal the stages of geomorphic development. It is estimated by the graphical plot of the measured contour elevation and encompassed area by using empirical formulae. In constructing the hypsometric integral curve, a Digital Elevation Model (DEM) with 30 m spatial resolution has been used. This index is calculated in the study area. Then, based on index of its values, the hypsometric properties of drainage basins are analyzed in Namdan basin. Three different approaches were used for estimation of hypsometric integrals. The hypsometric integral values (HI) range between 0.18 and 0.31 for all the basins of study basin. In the study area, one stage of erosion cycle development, namely old stage is distinguished. Our results indicate that there is anomaly in Hi value which is located on faulted area. The results indicate the Northwest of Namdan basin and a small part of its Southeast are more active than other ones.


Introduction
Tectonic geomorphology is inflexible competition between tectonic and surface processes [1].In fact, evaluation of structures and landforms during history genesis of them is a subject for tectonics and geomorphology [2].Investigation of the geomorphic record provides the basic data necessary to understand the role of active tectonics in the development of a site or an area.
Morphometry is becoming a unique part of structurally geomorphologic studies after W. B. Bull and L. D. McFadden (1977) and it develops with the introduction of DEM and GIS technologies [3].
Geographic information system (GIS) is used in different branches of earth science such as providing geomorphology, hazard, zoning and mineral potential maps.The most important objective of geographic information system is the integration of spatial data and their final assessment.The GIS has display facilities and analysis of data concurrently which make it possible for geologist to work with many geological data in more speed and accuracy.It should be mentioned that it is impossible in analog and tradition methods [4].
Each one of geomorphology indices represents a relative classification for quantity of tectonically activities.There is a huge progress in development of quantitative hypsometry.Vast research is carried out in studying the hypsometry index [5]- [8].
In this study, we tried to measure the required parameters using Digital Elevation Model with 30 meter resolution at ARC GIS software.In order to identify the hypsometry anomalies, we use climate, rock strength level, geomagnetic and structural maps.In fact, the purpose of this study is to use the procedure and the information of geology and climatology to understand the hypsometry properties of Namdan basin.
Iran is a part of the Alpine-Himalayan orogenic belt that represents the great Tethys Sea once located between two large continents, Gondwana and Laurasia, during Paleozoic-Mesozoic eras.Many geologists have studied structural history and tectonics for Iran [9] [10].They show that there are many different structural units, for example Berberian [9] who divides Iran into four major structural-geological units separable on the basis of regional difference in structural-geological characteristics which are included: Zagros active folded belt [11] [12], Central Iran, KopehDagh ranges and Alborz Mountains (from Bandar Pahlavi to Gorgan).Last zoning map of Iran based on Physiographic-tectonic of sedimentary basins has been prepared by Arian [13] (Figure 1).Iran has divided into four continental unites consist of, Cimmerian miniplate at least can be divided to the smaller part, East-Central Iran and North-Central Iran microcontinents, Eurasian and Arabian continents.East Alborzhinterland is the oldest orogenic belt and Zagros hinterland is the newest orogenic beltof Iran [13].Zagros zone extends from Bandar Abbas in the south to Kermanshah in the northwest and continues through to Iraq.It is in fact the northeastern edge of the Arabian plate.
The study area is about 304,000 hectare in the terminal zone of the high Zagros belt in Fars Province, southwest Iran.Major rock groups are Bakhtiari, Asmari, Jahrom, Fahliyan, Daryan, Khanehkat, and Neyriz (Figure 2).Central point of Namdan basin is located at longitudes 52˚42'E and latitude 30˚55'N.Average altitude of it is various from 3370 meter at Mountains to 2200 meter at low terrains from open seas level.
The annual temperature average of this region is 7.5 varying from 37˚C to -22˚C from winter to summer.The annual average precipitation of this region is 32% to 43%.The regional climate according to Do martin method is semi-aired cold [14].The average rainfall of Namdanbasin is 300 to 600 millimeter .The climate of the region is characterized by a hot summer and well distributed seasonal rainfall.The Shadkam River is the most important river of this watershed which takes its origin in Shadkam spring at Almaijeh Mountain.
The Kaftar Lake is the one of fresh water lakes in Iranian plateau which has been located in the easternmost part of Namdanbasin.This lake (with area approximately 7500 acres) is 2300 meters high above sea level and is 24 km long and 6 km wide [15].

Materials and Methods
In the field of tectonics geomorphology and landscape evolution, the use of GIS is relatively recent.The availability of the DEM has produced a great revolution in this field.It has replaced old topographic maps, allowing for better and faster analysis of topographic parameters.One of the most important features of DEM is the possibility of extracting river networks with stream gradients and catchments areas [16].
Geographical information system has been used for data preparation, data manipulation and analysis of data.ARCGIS 9.3 has been used for the present study.The Digital Elevation Model (DEM) with 30 m spatial resolu- tion has been used as a base map.The drainage basin's boundary has been identified through an extension called arc hydro tools 9 (fill, flow direction, flow accumulation, stream definition, stream segmentation) in ARCGIS software using DEM model as input.We have been controlled drainages obtained by DEM with drainages of the survey of Iran topographical map in 1:25,000 scales.So that, 9 basins have been chosen for studying (Figure 3).Stream ordering method as suggested by Strahler has been employed [5].
Hypsometric describes area distribution at different elevations [5] and can be estimated using the hypsometric curve or the hypsometric integral (HI).The index is defined as the relative area below the hypsometric curve and thus expresses the volume of a basin that has not been eroded [17].The hypsometric integral can be approximated by means of the following equation (Keller and Pinter, 2002   The elevation value of DEM (Figure 4) has been used to find out the hypsometric integral for each basin in the Namdan basin.Programming in excel has been used to determine the hypsometric curve values.In order to generate the map of hi value, at the first, polygon shape file converted to point features by xtools pro and then the spatial analyst has been used.Finally, we classified Namdan basin based on three classifies called Strahler [5], El Hamdouni et al. [18] and Ramu and Mahalingam [6].
Strahler [5] interpreted the shapes of the hypsometric curves by analyzing numerous drainage basins and classified them as youth (convex upward curves), mature (s-shaped hypsometric curves which is concave upwards at high elevations and convex downwards at low elevations) and peneplain or distorted (concave upward curves) [19].HI values were grouped into three classes with respect to the convexity or concavity of the hypsometric curve by El Hamdouni et al. [18]: class 1 with convex hypsometric curves (HI ≥ 0.5); class2 with concaveconvex hypsometric curves (0.4 ≤ HI < 0.5); and class3 with concave hypsometric curves (HI < 0.4).Ramu and Mahalingam [6] have been classified the HI values as following.If the result value was between 0.6 and 1; it indicates the youthful state of dissection; if the result value was between 0.3 and 0.60, it indicates a maturely dissected landform; and if the result was less than 0.35, then it indicates an equilibrium or old state of dissection.

Results and Discussion
The hypsometric integral value ranges from 0.18 (sub-basin Lahsheykhi) to 0.31 (sub-basin KuheSefid).The hypsometric curve and the hypsometric integral are valuable tools in characterizing topography because they are correlated with the stages of geomorphic development of the landscape [16].The values of elevation necessary for the calculation are obtained from a Digital Elevation Model.The average elevation is from 50 points of elevation taken at random from the drainage basin.The hypsometric curve represents the relative proportion of area below (or above) a given height (Figure 5).
The total of Namdan basin is located in mature in the peneplain or distorted based on Strahler [5] (Table 1).HI values were grouped into three classes with respect to the convexity or concavity of the hypsometric curve by El Hamdouni et al. [18] as mentioned the previous section.On based it, Namdan basin was located into class 3 (Table 1).Our area indicates old state of dissection based Ramu and Mahalingam classification [6] (Table 1).
Hypsometric integral data were derived for each of the nine drainage basins from 30 m DEM has been indicated in Table 1.The result of the hypsometric integral shows all drainage basins come under the class 3 in all of classifications.The result of hypsometric integral values has been mapped (Figure 6).The average value of  HI value is 0.258.Then, HI value (Figure 7) was contoured (Figure 6) by spatial analyst extension and the hypsometric integral value map of Namdan basin has been used as a base map.It shows Hi changes are equal in all of parts of Namdan basin.We compare the results in three mentioned classification and provide HI distributions maps based on them to see the visual situation of basin according them (Figure 8).In Strahler [5], El Hamdouni [18] and Ramu and Mahalingam [6] classification shows all of sub-basins located in class 3.  Consider to hypsometric is affected principally by tectonics, lithology, and climatic factors [1] [16] so that these factors considered in interpretation.The hypsometric curves not only have been used to infer the stage of development of the drainage network but also it is a powerful tool to identify different between tectonically active and inactive areas [2].
In this study, spatial variations of tectonic activity at Namdan basin were investigated by hypsometric integral analysis.The changes of hypsometric integral have point to a general trend of increasing tectonics activity towards the northwest.
We matched hypsometric integral value, tectonics map, lithology and climatic data (Figure 9) for distinguishing their effects.The results show that basins located at a part of northeast and southwest of Namdan basin have high values of HI.The part of northeast basin has been covered by limestone and other high strength rocks so that it has low erosion rather than other portions whereas the other portions have high erosion because of having low strength rock even southwest part except, a small part of Kaftar sub-basin.It is essential to notice Namdan basin has a same climate in all of itself why lithology less vast.So that, the role of tectonics is more than other factors in northeast portion and small part of Kaftar sub-basin (Figure 10).HI value distribution in lithology data is same and it doesn't differ significantly from one lithology to other one but in the strike of tectonics structures such as faults and folds variation of HI value has been happened.In fact, high hypsometric integral values indicated more tectonics activity.
In the other hand, based on the hypsometric integral values, there are in one stage of erosion cycle development, (old stage).This situation is not consist with its location on high Zagros area, because based on previous work on the salt and mud diapirism [20]- [31] and neotectonic regime in Iran [32]- [39], Zagros in south Iran is the most active zone [40]- [61].Then, Alborz [62]- [101] and Central Iran [102]- [117] have been situated in the next orders.It seems that hydro-climatic conditions and old glacier evidence in the study area have got considerable role in this position.

Conclusions
Hypsometric integrals for the all 9 basins have been computed using GIS following Strahler [5], El Hamdouni et al. [18] and Ramu and Mahalingam [6] and plotted.It is considered to be suitable for evaluating these basins.The following conclusions have emerged from this study: The study of hypsometric integral and curve has been retrieved in which the integral values vary from 0.18 to 0.31.
The maximum hypsometric integral belongs to Kuhe Sefid sub basin.Among the nine drainage basins, all of them are in the old state.No drainage basin comes in two other states in the study area.
The resultant hypsometric curve graphs drowned by excel has shown that s-shaped less rather than concave curve.
The value of HI was found to be high along major faults and folds.The emphasis of the hypsometric integral on the active tectonic region in the northwest and a part of southeast of basin is completely in agreement with structures in these parts.So, these parts of Namdan basin are more active than other parts.

Figure 1 .
Figure 1.Physiographic-tectonic zoning map of Iran's sedimentary basins Iran modified from [1].The study area is shown in the black rectangle.

Figure 3 .
Figure 3. Determination of sub-basins in Namdan basin based on Digital Elevation Model (DEM).

Figure 5 .
Figure 5. Hypsometric curves of basins (a) is the total surface area within the basin above a given line of elevation (h), (h) is the highest elevation of basin.

Figure 6 .
Figure 6.It shows the distribution of obtained hypsometric integral value and its contoured map at Namdan basin.Contour interval was selected 0.01 according to HI value.

Figure 7 .
Figure 7.It shows the changes of HI value at different Namdan sub-basins.

Figure 8 .
Figure 8.It shows HI combined classification map.This map is the combination of three classifications Strahler, El Hamdouni, and Ramu and Mahalingam.

Figure 9 .
Figure 9.It shows rock strength map.

Figure 10 .
Figure 10.It shows HI changes map for the study area on DEM.