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Volume 38, Issue 3 (2023)
GeoRes 2023, 38(3): 275-286 |
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Received: 2023/02/9 | Accepted: 2023/06/20 | Published: 2023/10/2
Received: 2023/02/9 | Accepted: 2023/06/20 | Published: 2023/10/2
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Khalaj M. Evaluation and analysis of Tectonic Activities in Sivand Basin area using Morphometric Indices. GeoRes 2023; 38 (3) :275-286
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M. Khalaj *
Department of Geology, Payam-e-Noor University, Tehran, Iran
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Introduction
The purpose of tectonic geomorphology is to utilize landform features to infer the processes responsible for their formation. These landforms may have been shaped by diverse climatic and tectonic processes, themselves influenced by lithological conditions [Volker et al., 2007]. Virtually no region of the world is unaffected by tectonic changes. The Earth is a dynamic system, in which transformation is one of its defining characteristics. Many of the present-day landscapes were formed during recent tectonic events [Fossen, 2016]. Tectonic geomorphology is the science that studies landforms and landscapes shaped by tectonic mechanisms. Examining the geomorphology of drainage basins provides valuable information regarding the tectonic conditions of an area and the degree of its recent activity [Burbank & Anderson, 2012].
One of the most important advantages of employing morphometric indices lies in the relative simplicity of their calculation and the efficiency of their application for assessing tectonic activity across large regions. Their required data are typically obtained from digital elevation models (DEMs) and satellite imagery, which can reveal active tectonic structures [Keller & Pinter, 2002]. Geomorphic evidence can be evaluated both qualitatively and quantitatively; the latter, commonly referred to as morphometry, allows for quantitative comparison of different landforms and the identification of specific characteristics, such as the level of tectonic activity within a given area [Keller, 1986]. Active tectonic processes influence the morphology and behavior of rivers. Analysis of rivers, their incision, and their deviations provides significant insight into the development and evolution of regional faults [Walker, 2006]. Riverbed incision, which alters river topography, is primarily induced by tectonic processes [Snyder et al., 2000].
The use of morphometric indices for studying neotectonic activity was first introduced by Horton [Horton, 1945] and later advanced by researchers such as Strahler and Bull & McFadden [Strahler, 1952; Bull & McFadden, 1980], continuing to this day. Among related studies, Yamani et al. examined qualitative geomorphic evidence alongside quantitative morphometric indices to assess active tectonics on the alluvial fans north of Damghan, confirming Quaternary fault activity [Yamani et al., 2012]. Similarly, Abdideh et al. have studied the Dez River basin and demonstrate that continental collision between the Arabian Plate and the Iranian block has resulted in extensive tectonic activity in this region [Abdideh et al., 2011]. Further noteworthy contributions include those by Wells et al., Khodabakhshnezhad et al., Babaei et al. and Rabeti et al. [Wells et al., 1988;Rabeti et al., 2018; Khodabakhshnezhad et al., 2015; Babaei et al., 2017].
Since the relationship between active tectonics and morphometry in the Sivand Basin has not yet been investigated, examining these indices to determine the role of active tectonics in the geomorphic evolution of its drainage basins and rivers is necessary. In general, geomorphic studies of tectonic activity are conducted through qualitative and quantitative approaches. Some previous investigations in areas similar to the Sivand Basin have focused on qualitative geomorphic analyses. In such studies, the presence of geological structures alone is used to infer tectonic activity and seismic potential. While this enables the identification of regions with the capacity to store energy and the likelihood of future earthquakes, it does not allow for the quantitative estimation of seismic hazards.
Conversely, quantitative morphometric studies involve precise measurements of tectonic and geomorphic structures. The results are then analyzed using mathematical equations and specialized software, enabling the numerical estimation of seismic hazards, the level of tectonic activity, and the relationship between geomorphic phenomena and geological structures. Consequently, such studies can provide seismic hazard zonation of the area. In related studies based on morphometric analysis, various indices have been applied depending on research objectives.
The objective of this study was to quantify the extent of recent tectonic activity in the Sivand drainage basin by applying quantitative indices such as drainage basin asymmetry, relative relief, form factor, hypsometric integral and curve, and river longitudinal gradient. Ultimately, by employing the relative active tectonics index (Iat), the overall tectonic activity of the study area was quantified, and the region was zoned in terms of seismic hazard.
Methodology
The Sivand drainage basin, with an area of 685.119 km², is located in southern Iran within Fars Province and represents one of the sub-basins of the Bakhtegan–Maharlu watershed. The study area is situated between 53°15′04″ and 52°44′35″ E longitude and 29°55′41″ and 30°15′17″ N latitude. This survey-based research was conducted in 2022 in Fars Province and is of an applied nature.
Geology of the Study Area
According to the tectono-sedimentary classification of Iran, the study area lies within the High Zagros zone, characterized by the presence of several faults, including the Sivand River Fault, the Sivand Fault, the Mousikhani Fault, and the Avanjan Fault [Stocklin, 1968]. The relative movement and subsequent collision between the Arabian and Iranian plates following the closure of the Neo-Tethys Ocean resulted in the formation of the Zagros orogeny. This mountain belt extends from northwestern to south-central Iran, forming part of the Alpine–Himalayan orogenic system. According to Berberian [1995], it is bounded in the northeast by Central Iran and in the southwest by the Arabian Shield.
The High Zagros includes the highest peaks of the Zagros Mountains, such as Dena, Oshtoran Kuh, and Zard Kuh. Although relatively narrow (10–70 km wide), it exhibits prominent relief with steep escarpments that strongly influence surrounding landscapes. Numerous longitudinal faults within the High Zagros have displaced the limestone strata both vertically and horizontally, forming high mountain cliffs. In sections between Oshtoran Kuh and Dena, fault scarps have been uplifted along fractures, producing some of the largest mountains in the region. Therefore, studying morphometric indices in the Sivand Basin significantly contributes to the identification of tectonically active areas. The study area lies within the 1:250,000-scale Eqlid and Saadatshahr geological maps. Lithologically, the region includes the Amiran, Fahliyan, Gadvan, Dariyan, Kazhdomi, and Sarvak formations.
To extract precise fluvial features and morphometric indices and to analyze drainage basins, a digital elevation model (DEM) was employed [Maathuis & Wang, 2006]. In this study, the assessment of active tectonics using morphometric indices was carried out as follows:
The asymmetry index reflects the degree of tectonic tilting in drainage basins and is useful for understanding tilting in large-scale basins [Morisawa & Hack, 1985; Keller & Pinter, 2002]. It is calculated using Equation 1:
Af=100(Ar/At)
where Ar is the area of the basin to the right of the main stream (looking downstream), and At is the total basin area. This index is sensitive to tilting perpendicular to the main stream. In this study, Af values are expressed as |Af-50|, representing the absolute difference between the observed value and the neutral value of 50 [El Hamdouni et al., 2008].
Relative Relief Index (Bh)
Relative relief represents the elevation difference within a basin, calculated from the highest and lowest elevations. These values were extracted from the DEM in ArcGIS for each sub-basin and calculated using Equation 2 [Keller & Pinter, 2002]:
Bh=Hmax_Hmin
where Hmax is the maximum basin elevation and Hmin the minimum.
Form Factor Index (Ff)
The form factor index was calculated using Equation 3 [Horton, 1945]:
Ff=A/L2
where A is the basin area and L the basin length, measured from the basin outlet to the highest point. Values closer to 1 indicate a square-shaped basin, while smaller values (<1) indicate elongation. Elongated basins are typically associated with tectonically active regions [Singh et al., 2014].
Hypsometric Integral and Curve (Hi)
The hypsometric integral describes the proportion of un-eroded volume in a basin and characterizes the relative distribution of elevations, particularly in drainage basins. It is calculated using Equation 4 [Strahler, 1952]:
Hi=(average elevation-min elevation)/(max elevation-min elevation)
High values indicate tectonically active regions, while low values reflect older landscapes less affected by recent tectonics [Font et al., 2010]. Higher values correspond to rugged topography and significant relief, whereas medium to low values suggest a smoother drainage network [Singh, 2009].
Stream Length-Gradient Index (SL)
The SL index is a useful tool for assessing relative tectonic activity, reflecting the impact of environmental changes on river longitudinal profiles. It is computed using Equation 5 [Hack, 1973]:
SL=(∆H/∆L)L
where ΔH/ΔL is the channel slope, ΔH the elevation difference across a river segment, ΔL the horizontal distance of that segment, and L the total channel length from the measurement point to the river source. High SL values are typically observed where streams intersect faults, marking tectonically active zones [Font et al., 2010]. SL increases in regions of high uplift rates, while it decreases when streams flow parallel to strike-slip faults or structural valleys [Keller & Pinter, 2002].
Relative Tectonic Activity Index (Iat)
To evaluate tectonic activity in the Sivand Basin, the indices Af, Bh, Ff, Hi, and SL were employed. Each index was classified according to its measured values, and finally, following the method of El Hamdouni et al. [2008], the mean class values (S/n) for each sub-basin were calculated. The final output, termed the relative tectonic activity index (Iat), was obtained, enabling zonation of the study area in terms of tectonic activity levels.
Findings
Based on the indices outlined in the methodology section, this part presents the analysis of the results obtained for each index.
Analysis of Drainage Basin Asymmetry Index
To calculate the asymmetry index using ArcGIS software, the total basin area and the right-side area of the main stream were separately measured for 21 sub-basins within the study area. Subsequently, a map of basin tilting was prepared by recording asymmetry index values along with the main structural features of the region. From the perspective of tectonic activity, this index was classified into three categories: Class 1 with very high tectonic activity (15<|Af–50|), Class 2 with moderate tectonic activity (15≥|Af–50|≥7), and Class 3 with low tectonic activity (7>|Af–50|). Numerical results showed that the asymmetry index ranged from 0.1 to 32.03. The highest value and greatest tilting belonged to basin 13 (32.03), whereas the lowest value was observed in basin 5 (0.1).
Analysis of the Relative Relief Index
High relative relief values indicated higher uplift rates, stronger water flow, and lower infiltration, all of which reflect more intense tectonic activity. In addition to tectonic factors, higher elevations were associated with reduced erosion and consequently higher tectonic activity. The zoning map of the relative relief index was prepared for the study area, and tectonic activity was classified into two categories: Class 2 (moderate activity; 2000>Bh>1000) and Class 3 (low activity; Bh<1000). Class 1 (high activity; Bh>2000) was not observed in the region, as the uplift rate was low to moderate and no evidence of high uplift was detected.
Analysis of Form Factor Index
Using the 30 m resolution DEM and the main channel of each basin, the area and length of all basins were calculated individually, and the form factor index was measured for the 21 drainage basins. A basin-scale zoning map was then prepared. Based on tectonic activity, the index was classified into three categories: Class 1 (Ff<0.2) indicating high tectonic activity, Class 2 (0.25>Ff≥0.2) indicating moderate tectonic activity, and Class 3 (Ff>0.25) indicating low tectonic activity.
Analysis of Hypsometric Integral and Curve
Using the 30 m resolution DEM, mean, minimum, and maximum elevations were determined separately for each sub-basin, and the hypsometric integral was calculated. A tectonic activity zoning map was then prepared. Based on the hypsometric integral values and the convexity or concavity of the curve, the results were categorized into three classes: Class 1 (Hi>0.5) with convex hypsometric curves indicating high topography relative to the basin mean; Class 2 (0.5≥Hi≥0.4) with convex–concave curves representing intermediate conditions; and Class 3 (Hi<0.4) with concave curves.
Hypsometric curves of selected basins with the highest index values included basin 1 (0.51), basin 8 (0.48), and basin 5 (0.47), indicating high to moderate tectonic activity.
Analysis of Stream Length-Gradient Index
This index was calculated using the 30 m resolution DEM along each flow path, the longest channel within each basin, and topographic layers. The mean values were obtained for each basin. Based on tectonic activity, this index was classified into: Class 1 (950<SL<3000) representing high activity, Class 2 (300<SL≤950) representing moderate activity, and Class 3 (0<SL≤300) representing low activity.
Analysis of Relative Active Tectonic Index
Based on the results of morphotectonic indices including drainage basin asymmetry, relative relief, form factor, hypsometric integral, and stream length-gradient, the relative tectonic activity of the 21 sub-basins was calculated and classified into three groups: Class 2 (high activity; 1.5<S/n≤2), Class 3 (moderate activity; 2<S/n≤2.5), and Class 4 (low activity; S/n>2.5).
According to the Iat results, the Sivand basin showed no Class 1 tectonic activity, meaning that none of its sub-basins fell into the very high tectonic activity category (1<S/n≤1.5).
The morphometric analysis of the Sivand watershed in this part of the Zagros Mountains revealed that, due to the influence of active faults such as the Sivand River fault, Sivand fault, Musakhani fault, and Āvenjān fault, 37.8% of the study area showed high tectonic activity, 19.8% moderate activity, and 42.4% low activity. The northern, northwestern, central, and southern parts of the region were more active compared to other areas.
Discussion
The aim of this research was to evaluate and analyze tectonic activity in the Sivand watershed using morphometric indices. Morphotectonic hazards are among the major environmental hazards that cause serious disruption to human activities worldwide. In recent years, recognition of hazardous and risk-prone areas has increased, and morphotectonic approaches have been developed as solutions to environmental issues. Since no morphometric studies had previously been conducted in the Sivand watershed, assessing tectonic activity in this region through morphometric analysis appeared necessary, and the findings of this study can provide valuable insights for urban planners in the area.
The results of this research are consistent with similar morphotectonic studies conducted by Abdideh et al. (2011), Khodabakhshnezhad et al. (2015), Babaei et al. (2017), Rabeti et al. (2018), Yamani et al. (2012), Bayati Khatibi (2009), Mosadeghzadeh et al. (2020), Bull and McFadden (1980), Burbank and Anderson (2012), Melosh and Keller (2013), and El Hamdouni et al. (2008). However, some of these scholars used different quantitative indices compared to this study, and some supplemented quantitative indices with qualitative measures. Nonetheless, in comparable indices, the same methodological approaches and similar conclusions were drawn, consistent with the present research.
Based on the drainage basin asymmetry index, the activity of the Sivand fault, Sivand River fault, and Avenjan fault has caused significant tilting in most sub-basins located along these faults, reflecting moderate to high tectonic activity. Wells et al. (1988) using the asymmetry index in Costa Rica, and Bayati Khatibi (2009) in the Qarangu Chay watershed, reported asymmetry values below 15, suggesting moderate tectonic activity. Conversely, Rabeti et al. (2018) in the Sefidrud basin and Babaei et al. (2017) in the Central Alborz reported asymmetry values exceeding 15, corresponding to high tectonic activity.
Based on the relative relief index and zoning map, it was determined that due to the activity of segments of the Sivand River fault and southern faults, most basins in the region exhibited moderate to low tectonic activity. Burbank and Anderson (2012), using the relative relief index in the Manchester watershed, reported values above 2000 and classified the region as highly active. Similarly, Babaei et al. (2017) observed values close to 2000 in the Central Alborz, also indicating high tectonic activity.
The form factor index revealed that the greatest elongation of basins and the highest tectonic activity were associated with the Sivand River fault. The activity of the Avenjan and Musakhani faults, as well as faults in the southern part of the study area, also influenced the drainage basins, causing them to display high tectonic activity. El Hamdouni et al. (2008) reported form factor values of 0.11 and 0.17 in Nevada basins, classifying them as highly active. Similarly, Bull and McFadden (1980) identified values below 0.2 in California, also associated with high activity. In contrast, Bayati Khatibi (2009) reported values between 0.2 and 0.25 in Qarangu Chay, corresponding to moderate activity.
The hypsometric integral and curves showed the highest tectonic activity in areas influenced by the Sivand River fault. The Musakhani, Avenjan, and southern faults caused the basins in these areas to display moderate to low activity, indicating limited tectonic movement. Mosadeghzadeh et al. (2020), in their study of rivers east of Ramsar, reported hypsometric integral values similar to the present study, classifying their basins as moderate to low activity. Yamani et al. (2012) also reported moderate tectonic activity for several sub-basins north of Damghan. Similarly, Singh (2009) reported a hypsometric integral of 0.50 in the Himalaya, close to the present findings, and classified the basin as moderate to low activity.
The highest values of the stream length-gradient index were observed along the Sivand River fault, where the fault intersects the basin channel, creating anomalies and indicating high tectonic activity. The intersection of the Avenjan fault with channels in basins 6 and 7, the Musakhani fault with basin 8, and the Sivand fault with basins 2 and 3, as well as the activity of southern faults, caused anomalies in drainage networks and reflected moderate activity. Khodabakhshnezhad et al. (2015) reported similar values in the Karun basin, classifying the basins as moderately active. Melosh and Keller (2013), studying the Santa Barbara fold belt, and Abdideh et al. (2011), in the Dez basin, also reported comparable values, concluding moderate tectonic activity.
According to the relative active tectonic index (Iat) and its zoning map, the study area overall displayed high tectonic activity due to the influence of the Sivand River, Avenjan, and Musakhani faults, along with the impact of southern faults. This finding aligns with the results of El Hamdouni et al. (2008), Melosh and Keller (2013), Mosadeghzadeh et al. (2020), Khodabakhshnezhad et al. (2015), Babaei et al. (2017), Rabeti et al. (2018), Yamani et al. (2012), and others, who employed similar morphometric indices and reached consistent conclusions.
Differences in geological conditions between regions explain why certain indices are more appropriate in some areas than others. The geological context of the Sivand watershed limited the applicability of certain quantitative morphometric indices, such as hierarchical anomaly indices, basin tilting indices, or mountain front sinuosity indices, which could not be measured effectively. Therefore, these indices were excluded from the analysis.
It is recommended that further geophysical studies, particularly magnetometry, be conducted to investigate subsurface conditions, lineaments, and hidden faults in the region, which may significantly influence regional seismotectonics.
Conclusion
The Sivand watershed exhibits moderate to high tectonic activity, with the northern, northwestern, central, and southern parts of the region being more active than other areas. This increased activity is attributed to the influence of the aforementioned faults on the region.
Acknowledgments: None reported by the author.
Ethical Permission: This article has not been published in any domestic or international journals.
Conflict of Interest: None reported by the author.
Authors’ Contributions: Khalaj M (first author): Introduction Writer/Methodologist/Principal Researcher/Statistical Analyst/Discussion writer (100%)
Funding: None reported by the author.
The purpose of tectonic geomorphology is to utilize landform features to infer the processes responsible for their formation. These landforms may have been shaped by diverse climatic and tectonic processes, themselves influenced by lithological conditions [Volker et al., 2007]. Virtually no region of the world is unaffected by tectonic changes. The Earth is a dynamic system, in which transformation is one of its defining characteristics. Many of the present-day landscapes were formed during recent tectonic events [Fossen, 2016]. Tectonic geomorphology is the science that studies landforms and landscapes shaped by tectonic mechanisms. Examining the geomorphology of drainage basins provides valuable information regarding the tectonic conditions of an area and the degree of its recent activity [Burbank & Anderson, 2012].
One of the most important advantages of employing morphometric indices lies in the relative simplicity of their calculation and the efficiency of their application for assessing tectonic activity across large regions. Their required data are typically obtained from digital elevation models (DEMs) and satellite imagery, which can reveal active tectonic structures [Keller & Pinter, 2002]. Geomorphic evidence can be evaluated both qualitatively and quantitatively; the latter, commonly referred to as morphometry, allows for quantitative comparison of different landforms and the identification of specific characteristics, such as the level of tectonic activity within a given area [Keller, 1986]. Active tectonic processes influence the morphology and behavior of rivers. Analysis of rivers, their incision, and their deviations provides significant insight into the development and evolution of regional faults [Walker, 2006]. Riverbed incision, which alters river topography, is primarily induced by tectonic processes [Snyder et al., 2000].
The use of morphometric indices for studying neotectonic activity was first introduced by Horton [Horton, 1945] and later advanced by researchers such as Strahler and Bull & McFadden [Strahler, 1952; Bull & McFadden, 1980], continuing to this day. Among related studies, Yamani et al. examined qualitative geomorphic evidence alongside quantitative morphometric indices to assess active tectonics on the alluvial fans north of Damghan, confirming Quaternary fault activity [Yamani et al., 2012]. Similarly, Abdideh et al. have studied the Dez River basin and demonstrate that continental collision between the Arabian Plate and the Iranian block has resulted in extensive tectonic activity in this region [Abdideh et al., 2011]. Further noteworthy contributions include those by Wells et al., Khodabakhshnezhad et al., Babaei et al. and Rabeti et al. [Wells et al., 1988;Rabeti et al., 2018; Khodabakhshnezhad et al., 2015; Babaei et al., 2017].
Since the relationship between active tectonics and morphometry in the Sivand Basin has not yet been investigated, examining these indices to determine the role of active tectonics in the geomorphic evolution of its drainage basins and rivers is necessary. In general, geomorphic studies of tectonic activity are conducted through qualitative and quantitative approaches. Some previous investigations in areas similar to the Sivand Basin have focused on qualitative geomorphic analyses. In such studies, the presence of geological structures alone is used to infer tectonic activity and seismic potential. While this enables the identification of regions with the capacity to store energy and the likelihood of future earthquakes, it does not allow for the quantitative estimation of seismic hazards.
Conversely, quantitative morphometric studies involve precise measurements of tectonic and geomorphic structures. The results are then analyzed using mathematical equations and specialized software, enabling the numerical estimation of seismic hazards, the level of tectonic activity, and the relationship between geomorphic phenomena and geological structures. Consequently, such studies can provide seismic hazard zonation of the area. In related studies based on morphometric analysis, various indices have been applied depending on research objectives.
The objective of this study was to quantify the extent of recent tectonic activity in the Sivand drainage basin by applying quantitative indices such as drainage basin asymmetry, relative relief, form factor, hypsometric integral and curve, and river longitudinal gradient. Ultimately, by employing the relative active tectonics index (Iat), the overall tectonic activity of the study area was quantified, and the region was zoned in terms of seismic hazard.
Methodology
The Sivand drainage basin, with an area of 685.119 km², is located in southern Iran within Fars Province and represents one of the sub-basins of the Bakhtegan–Maharlu watershed. The study area is situated between 53°15′04″ and 52°44′35″ E longitude and 29°55′41″ and 30°15′17″ N latitude. This survey-based research was conducted in 2022 in Fars Province and is of an applied nature.
Geology of the Study Area
According to the tectono-sedimentary classification of Iran, the study area lies within the High Zagros zone, characterized by the presence of several faults, including the Sivand River Fault, the Sivand Fault, the Mousikhani Fault, and the Avanjan Fault [Stocklin, 1968]. The relative movement and subsequent collision between the Arabian and Iranian plates following the closure of the Neo-Tethys Ocean resulted in the formation of the Zagros orogeny. This mountain belt extends from northwestern to south-central Iran, forming part of the Alpine–Himalayan orogenic system. According to Berberian [1995], it is bounded in the northeast by Central Iran and in the southwest by the Arabian Shield.
The High Zagros includes the highest peaks of the Zagros Mountains, such as Dena, Oshtoran Kuh, and Zard Kuh. Although relatively narrow (10–70 km wide), it exhibits prominent relief with steep escarpments that strongly influence surrounding landscapes. Numerous longitudinal faults within the High Zagros have displaced the limestone strata both vertically and horizontally, forming high mountain cliffs. In sections between Oshtoran Kuh and Dena, fault scarps have been uplifted along fractures, producing some of the largest mountains in the region. Therefore, studying morphometric indices in the Sivand Basin significantly contributes to the identification of tectonically active areas. The study area lies within the 1:250,000-scale Eqlid and Saadatshahr geological maps. Lithologically, the region includes the Amiran, Fahliyan, Gadvan, Dariyan, Kazhdomi, and Sarvak formations.
To extract precise fluvial features and morphometric indices and to analyze drainage basins, a digital elevation model (DEM) was employed [Maathuis & Wang, 2006]. In this study, the assessment of active tectonics using morphometric indices was carried out as follows:
- Using a 30 m resolution DEM in a GIS environment, together with the Arc Hydro extension, sub-basins and stream networks were extracted, and the Sivand Basin was divided into 21 sub-basins.
- Morphometric indices were calculated separately for each sub-basin. The indices employed in this study include drainage basin asymmetry (Af), relative relief (Bh), form factor (Ff), hypsometric integral and curve (Hi), and stream length-gradient index (SL). Basin-scale zonation maps were prepared for each index.
- Geological units and faults within the area were identified using 1:100,000 and 1:250,000 geological maps from the Geological Survey of Iran.
- Finally, the relative tectonic activity index (Iat) was computed, and its map was produced. The results of morphometric index measurements were then analyzed in relation to the main tectonic structures of the region.
The asymmetry index reflects the degree of tectonic tilting in drainage basins and is useful for understanding tilting in large-scale basins [Morisawa & Hack, 1985; Keller & Pinter, 2002]. It is calculated using Equation 1:
Af=100(Ar/At)
where Ar is the area of the basin to the right of the main stream (looking downstream), and At is the total basin area. This index is sensitive to tilting perpendicular to the main stream. In this study, Af values are expressed as |Af-50|, representing the absolute difference between the observed value and the neutral value of 50 [El Hamdouni et al., 2008].
Relative Relief Index (Bh)
Relative relief represents the elevation difference within a basin, calculated from the highest and lowest elevations. These values were extracted from the DEM in ArcGIS for each sub-basin and calculated using Equation 2 [Keller & Pinter, 2002]:
Bh=Hmax_Hmin
where Hmax is the maximum basin elevation and Hmin the minimum.
Form Factor Index (Ff)
The form factor index was calculated using Equation 3 [Horton, 1945]:
Ff=A/L2
where A is the basin area and L the basin length, measured from the basin outlet to the highest point. Values closer to 1 indicate a square-shaped basin, while smaller values (<1) indicate elongation. Elongated basins are typically associated with tectonically active regions [Singh et al., 2014].
Hypsometric Integral and Curve (Hi)
The hypsometric integral describes the proportion of un-eroded volume in a basin and characterizes the relative distribution of elevations, particularly in drainage basins. It is calculated using Equation 4 [Strahler, 1952]:
Hi=(average elevation-min elevation)/(max elevation-min elevation)
High values indicate tectonically active regions, while low values reflect older landscapes less affected by recent tectonics [Font et al., 2010]. Higher values correspond to rugged topography and significant relief, whereas medium to low values suggest a smoother drainage network [Singh, 2009].
Stream Length-Gradient Index (SL)
The SL index is a useful tool for assessing relative tectonic activity, reflecting the impact of environmental changes on river longitudinal profiles. It is computed using Equation 5 [Hack, 1973]:
SL=(∆H/∆L)L
where ΔH/ΔL is the channel slope, ΔH the elevation difference across a river segment, ΔL the horizontal distance of that segment, and L the total channel length from the measurement point to the river source. High SL values are typically observed where streams intersect faults, marking tectonically active zones [Font et al., 2010]. SL increases in regions of high uplift rates, while it decreases when streams flow parallel to strike-slip faults or structural valleys [Keller & Pinter, 2002].
Relative Tectonic Activity Index (Iat)
To evaluate tectonic activity in the Sivand Basin, the indices Af, Bh, Ff, Hi, and SL were employed. Each index was classified according to its measured values, and finally, following the method of El Hamdouni et al. [2008], the mean class values (S/n) for each sub-basin were calculated. The final output, termed the relative tectonic activity index (Iat), was obtained, enabling zonation of the study area in terms of tectonic activity levels.
Findings
Based on the indices outlined in the methodology section, this part presents the analysis of the results obtained for each index.
Analysis of Drainage Basin Asymmetry Index
To calculate the asymmetry index using ArcGIS software, the total basin area and the right-side area of the main stream were separately measured for 21 sub-basins within the study area. Subsequently, a map of basin tilting was prepared by recording asymmetry index values along with the main structural features of the region. From the perspective of tectonic activity, this index was classified into three categories: Class 1 with very high tectonic activity (15<|Af–50|), Class 2 with moderate tectonic activity (15≥|Af–50|≥7), and Class 3 with low tectonic activity (7>|Af–50|). Numerical results showed that the asymmetry index ranged from 0.1 to 32.03. The highest value and greatest tilting belonged to basin 13 (32.03), whereas the lowest value was observed in basin 5 (0.1).
Analysis of the Relative Relief Index
High relative relief values indicated higher uplift rates, stronger water flow, and lower infiltration, all of which reflect more intense tectonic activity. In addition to tectonic factors, higher elevations were associated with reduced erosion and consequently higher tectonic activity. The zoning map of the relative relief index was prepared for the study area, and tectonic activity was classified into two categories: Class 2 (moderate activity; 2000>Bh>1000) and Class 3 (low activity; Bh<1000). Class 1 (high activity; Bh>2000) was not observed in the region, as the uplift rate was low to moderate and no evidence of high uplift was detected.
Analysis of Form Factor Index
Using the 30 m resolution DEM and the main channel of each basin, the area and length of all basins were calculated individually, and the form factor index was measured for the 21 drainage basins. A basin-scale zoning map was then prepared. Based on tectonic activity, the index was classified into three categories: Class 1 (Ff<0.2) indicating high tectonic activity, Class 2 (0.25>Ff≥0.2) indicating moderate tectonic activity, and Class 3 (Ff>0.25) indicating low tectonic activity.
Analysis of Hypsometric Integral and Curve
Using the 30 m resolution DEM, mean, minimum, and maximum elevations were determined separately for each sub-basin, and the hypsometric integral was calculated. A tectonic activity zoning map was then prepared. Based on the hypsometric integral values and the convexity or concavity of the curve, the results were categorized into three classes: Class 1 (Hi>0.5) with convex hypsometric curves indicating high topography relative to the basin mean; Class 2 (0.5≥Hi≥0.4) with convex–concave curves representing intermediate conditions; and Class 3 (Hi<0.4) with concave curves.
Hypsometric curves of selected basins with the highest index values included basin 1 (0.51), basin 8 (0.48), and basin 5 (0.47), indicating high to moderate tectonic activity.
Analysis of Stream Length-Gradient Index
This index was calculated using the 30 m resolution DEM along each flow path, the longest channel within each basin, and topographic layers. The mean values were obtained for each basin. Based on tectonic activity, this index was classified into: Class 1 (950<SL<3000) representing high activity, Class 2 (300<SL≤950) representing moderate activity, and Class 3 (0<SL≤300) representing low activity.
Analysis of Relative Active Tectonic Index
Based on the results of morphotectonic indices including drainage basin asymmetry, relative relief, form factor, hypsometric integral, and stream length-gradient, the relative tectonic activity of the 21 sub-basins was calculated and classified into three groups: Class 2 (high activity; 1.5<S/n≤2), Class 3 (moderate activity; 2<S/n≤2.5), and Class 4 (low activity; S/n>2.5).
According to the Iat results, the Sivand basin showed no Class 1 tectonic activity, meaning that none of its sub-basins fell into the very high tectonic activity category (1<S/n≤1.5).
The morphometric analysis of the Sivand watershed in this part of the Zagros Mountains revealed that, due to the influence of active faults such as the Sivand River fault, Sivand fault, Musakhani fault, and Āvenjān fault, 37.8% of the study area showed high tectonic activity, 19.8% moderate activity, and 42.4% low activity. The northern, northwestern, central, and southern parts of the region were more active compared to other areas.
Discussion
The aim of this research was to evaluate and analyze tectonic activity in the Sivand watershed using morphometric indices. Morphotectonic hazards are among the major environmental hazards that cause serious disruption to human activities worldwide. In recent years, recognition of hazardous and risk-prone areas has increased, and morphotectonic approaches have been developed as solutions to environmental issues. Since no morphometric studies had previously been conducted in the Sivand watershed, assessing tectonic activity in this region through morphometric analysis appeared necessary, and the findings of this study can provide valuable insights for urban planners in the area.
The results of this research are consistent with similar morphotectonic studies conducted by Abdideh et al. (2011), Khodabakhshnezhad et al. (2015), Babaei et al. (2017), Rabeti et al. (2018), Yamani et al. (2012), Bayati Khatibi (2009), Mosadeghzadeh et al. (2020), Bull and McFadden (1980), Burbank and Anderson (2012), Melosh and Keller (2013), and El Hamdouni et al. (2008). However, some of these scholars used different quantitative indices compared to this study, and some supplemented quantitative indices with qualitative measures. Nonetheless, in comparable indices, the same methodological approaches and similar conclusions were drawn, consistent with the present research.
Based on the drainage basin asymmetry index, the activity of the Sivand fault, Sivand River fault, and Avenjan fault has caused significant tilting in most sub-basins located along these faults, reflecting moderate to high tectonic activity. Wells et al. (1988) using the asymmetry index in Costa Rica, and Bayati Khatibi (2009) in the Qarangu Chay watershed, reported asymmetry values below 15, suggesting moderate tectonic activity. Conversely, Rabeti et al. (2018) in the Sefidrud basin and Babaei et al. (2017) in the Central Alborz reported asymmetry values exceeding 15, corresponding to high tectonic activity.
Based on the relative relief index and zoning map, it was determined that due to the activity of segments of the Sivand River fault and southern faults, most basins in the region exhibited moderate to low tectonic activity. Burbank and Anderson (2012), using the relative relief index in the Manchester watershed, reported values above 2000 and classified the region as highly active. Similarly, Babaei et al. (2017) observed values close to 2000 in the Central Alborz, also indicating high tectonic activity.
The form factor index revealed that the greatest elongation of basins and the highest tectonic activity were associated with the Sivand River fault. The activity of the Avenjan and Musakhani faults, as well as faults in the southern part of the study area, also influenced the drainage basins, causing them to display high tectonic activity. El Hamdouni et al. (2008) reported form factor values of 0.11 and 0.17 in Nevada basins, classifying them as highly active. Similarly, Bull and McFadden (1980) identified values below 0.2 in California, also associated with high activity. In contrast, Bayati Khatibi (2009) reported values between 0.2 and 0.25 in Qarangu Chay, corresponding to moderate activity.
The hypsometric integral and curves showed the highest tectonic activity in areas influenced by the Sivand River fault. The Musakhani, Avenjan, and southern faults caused the basins in these areas to display moderate to low activity, indicating limited tectonic movement. Mosadeghzadeh et al. (2020), in their study of rivers east of Ramsar, reported hypsometric integral values similar to the present study, classifying their basins as moderate to low activity. Yamani et al. (2012) also reported moderate tectonic activity for several sub-basins north of Damghan. Similarly, Singh (2009) reported a hypsometric integral of 0.50 in the Himalaya, close to the present findings, and classified the basin as moderate to low activity.
The highest values of the stream length-gradient index were observed along the Sivand River fault, where the fault intersects the basin channel, creating anomalies and indicating high tectonic activity. The intersection of the Avenjan fault with channels in basins 6 and 7, the Musakhani fault with basin 8, and the Sivand fault with basins 2 and 3, as well as the activity of southern faults, caused anomalies in drainage networks and reflected moderate activity. Khodabakhshnezhad et al. (2015) reported similar values in the Karun basin, classifying the basins as moderately active. Melosh and Keller (2013), studying the Santa Barbara fold belt, and Abdideh et al. (2011), in the Dez basin, also reported comparable values, concluding moderate tectonic activity.
According to the relative active tectonic index (Iat) and its zoning map, the study area overall displayed high tectonic activity due to the influence of the Sivand River, Avenjan, and Musakhani faults, along with the impact of southern faults. This finding aligns with the results of El Hamdouni et al. (2008), Melosh and Keller (2013), Mosadeghzadeh et al. (2020), Khodabakhshnezhad et al. (2015), Babaei et al. (2017), Rabeti et al. (2018), Yamani et al. (2012), and others, who employed similar morphometric indices and reached consistent conclusions.
Differences in geological conditions between regions explain why certain indices are more appropriate in some areas than others. The geological context of the Sivand watershed limited the applicability of certain quantitative morphometric indices, such as hierarchical anomaly indices, basin tilting indices, or mountain front sinuosity indices, which could not be measured effectively. Therefore, these indices were excluded from the analysis.
It is recommended that further geophysical studies, particularly magnetometry, be conducted to investigate subsurface conditions, lineaments, and hidden faults in the region, which may significantly influence regional seismotectonics.
Conclusion
The Sivand watershed exhibits moderate to high tectonic activity, with the northern, northwestern, central, and southern parts of the region being more active than other areas. This increased activity is attributed to the influence of the aforementioned faults on the region.
Acknowledgments: None reported by the author.
Ethical Permission: This article has not been published in any domestic or international journals.
Conflict of Interest: None reported by the author.
Authors’ Contributions: Khalaj M (first author): Introduction Writer/Methodologist/Principal Researcher/Statistical Analyst/Discussion writer (100%)
Funding: None reported by the author.
Keywords:
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