Quantitative analysis of morphometry of alluvial fans developed along Dehshir strike-slip fault
Document Type : Original Article
Authors
1
shahid beheshti university
2
Department of Sedimentary Basins and Petroleum, Faculty of Earth Sciences, Shahid Beheshti University
3
ِM.Sc. of Geomorphology- Department of Physical Geography- Faculty of Earth Sciences- Shahid Beheshti Univrsity-Tehran.
10.22034/gmpj.2024.431610.1475
Abstract
Quantitative analysis of morphometry of alluvial fans developed along Dehshir strike-slip fault
Extended abstract
Introduction
Tectonic activities along the strike-slip faults can result in the formation of various landforms such as shutter ridges, pressure ridges, linear valleys, offset stream, beheaded streams, sag ponds, pull-apart basins, scarps, offset fans, and asymmetric valleys. Alluvial fans are one of the most important landforms that obviously reflect the effect of tectonic activity of strike-slip faults. Active tectonics play an important role in the alluvial fan’s morphology, morphometry, geomorphological processes (aggradation, degradation), sediment thickness, and location of sedimentation. Dehshir Fault as a 350 km long right-lateral strike-slip fault is one of the most important structural features in the Central Iran structural zone. The formation offset fans, offsets streams, and fault scarps along Dehshir Fault implies that the mentioned fault has been active in the Quaternary Period. The aim of this study is to evaluate the effect of Dehshir active tectonics on the morphometric properties of alluvial fans including area, topographic slopes, length, width, width/length ratio, sweep angle, and revised fan conicality index.
Methodology
To achieve the aim of this study, the borders of 28 alluvial fans developed along Dehshir Fault were delineated by Google Earth images. Then, the morphometric parameters of alluvial fans including area (A), topographic slope (S), length (L), width (W), width to length ratio (W/L), sweep angle (SA), and revised fan conicality index (FCIR) were calculated for each fan. The sweep angle (SA) of each fan was obtained by measuring the angle between the two outermost positions of the fan. Fan conicality index is a parameter to show that the shape of fan is conical or not. Normal or typical fans are conical in shape, but may be distorted due to the effects of parameters such as tectonic. Mukerji (1976) defined the fan conicality index as:
AIF/ATF = FCI
where ATF is the area of the fan, and AIF (area of equivalent ideal fan) is expressed as:
AIF=(〖πr〗^2 ×dFA)/360
where r (radius or length) is distance between fan apex and fan toe, and dFA is the sweep angle.
In this study, we defined the FCIR (revised fan conicality index) as:
FCIR=|FCI-1|
The lower values of FCIR (close to 0) show the ideal or normal fans, whereas the higher values of FCIR demonstrate distorted fans by external factors such as tectonic.
In this study, the means of morphometric factors were compared in two groups of large (more than 1 km2), and small (less than 1 km2) alluvial fans. To compare the means of morphometric factors in two groups of fans, the independent sample t-tests were applied. Correlations between the morphometric parameters were evaluated using Pearson correlation coefficients.
Results and discussion
Data reveal that means of sweep angle (SA) and revised fan conicality index (FCIR) are higher in the large fans group (100.3° and 1.1 respectively), compared to the small fans group (83.1° and 0.69 respectively). Nevertheless, means of topographic slope (S), and width to length (W/L) ratio are higher in the small fans group (1.78% and 1.11 respectively), compared to the large fans group (1.3% and 0.73 respectively). Higher mean values of SA and FCIR in the large alluvial fans can be attributed to the higher discharge and sediment yield as well as to the lower topographic slope of large fans so that when larger discharges with higher sediment yield, produced by the occasional intense rainfalls, reach the large and low-gradient fan surface they are easily diverted across the fan, and hence increase the sweep angle and distortion of alluvial fans (increasing the SA and FCIR indexes). Results of T-test show statistically significant differences between the means of area, length, and width parameters in large and small fans. There are statistically no significant differences between the means of topographic slope, sweep angle and revised fan conicality index in large and small fans. Pearson correlation coefficients reveal that there are strong positive correlations between A-L, and A-W pairs, and strong negative correlations between S-SA, and S-W/L pairs, implying that fans with higher topographic slopes have lower values of sweep angle and width to length ratio. There is strong negative correlation between W/L and FCIR parameters showing that fans with higher values of width to length ratio have regular or more typical form. Data reveal that the mean sweep angle of study area fans is considerably high (92°) compared to fans developed around Danehkhoshk Anticline in the Zagros Folded Belt (with mean sweep angle of 49°). The high value of SA in the study area alluvial fans can be attributed to the lateral movement of Dehshir Fault so that continuous changes in the location of aggradation on fans surfaces have resulted in increasing the sweep angles of fans. Apart from the effect of lateral movements of Dehshir Fault in distortion of fans (high values of FCIR), the formation of presser ridges along Dehshir Fault and its impact on the flood diversion are also considerable.
Conclusion
Geomorphic evidence shows that Dehshir strike-slip fault as one of the most important structural features in the Central Iran structural zone has been active in Quaternary Period. In this study, 28 alluvial fans formed along Dehshir Fault were selected and their morphometric parameters including area, topographic slope, length, width, width to length ratio, sweep angle, and revised fan conicality index were analyzed. Results show that lateral movement of Dehshir Fault have resulted in increasing sweep angle, width to length ratio, and revised fan conicality index of fans. Larger fans are characterized by higher values of sweep angle and revised fan conicality index. Higher values of SA and FCIR in larger fans can be associated with higher discharge and sediment yield of large fans with lower topographic slope resulting in the fan distortion. It can be concluded that, in addition to the effect of lateral movement of Dhshir Fault in the morphometry of alluvial fans, development of pressure ridges along fault can also cause distortion of fans.
Key words: strike-slip fault, sweep angle, revised fan conicality index
Extended abstract
Introduction
Tectonic activities along the strike-slip faults can result in the formation of various landforms such as shutter ridges, pressure ridges, linear valleys, offset stream, beheaded streams, sag ponds, pull-apart basins, scarps, offset fans, and asymmetric valleys. Alluvial fans are one of the most important landforms that obviously reflect the effect of tectonic activity of strike-slip faults. Active tectonics play an important role in the alluvial fan’s morphology, morphometry, geomorphological processes (aggradation, degradation), sediment thickness, and location of sedimentation. Dehshir Fault as a 350 km long right-lateral strike-slip fault is one of the most important structural features in the Central Iran structural zone. The formation offset fans, offsets streams, and fault scarps along Dehshir Fault implies that the mentioned fault has been active in the Quaternary Period. The aim of this study is to evaluate the effect of Dehshir active tectonics on the morphometric properties of alluvial fans including area, topographic slopes, length, width, width/length ratio, sweep angle, and revised fan conicality index.
Methodology
To achieve the aim of this study, the borders of 28 alluvial fans developed along Dehshir Fault were delineated by Google Earth images. Then, the morphometric parameters of alluvial fans including area (A), topographic slope (S), length (L), width (W), width to length ratio (W/L), sweep angle (SA), and revised fan conicality index (FCIR) were calculated for each fan. The sweep angle (SA) of each fan was obtained by measuring the angle between the two outermost positions of the fan. Fan conicality index is a parameter to show that the shape of fan is conical or not. Normal or typical fans are conical in shape, but may be distorted due to the effects of parameters such as tectonic. Mukerji (1976) defined the fan conicality index as:
AIF/ATF = FCI
where ATF is the area of the fan, and AIF (area of equivalent ideal fan) is expressed as:
AIF=(〖πr〗^2 ×dFA)/360
where r (radius or length) is distance between fan apex and fan toe, and dFA is the sweep angle.
In this study, we defined the FCIR (revised fan conicality index) as:
FCIR=|FCI-1|
The lower values of FCIR (close to 0) show the ideal or normal fans, whereas the higher values of FCIR demonstrate distorted fans by external factors such as tectonic.
In this study, the means of morphometric factors were compared in two groups of large (more than 1 km2), and small (less than 1 km2) alluvial fans. To compare the means of morphometric factors in two groups of fans, the independent sample t-tests were applied. Correlations between the morphometric parameters were evaluated using Pearson correlation coefficients.
Results and discussion
Data reveal that means of sweep angle (SA) and revised fan conicality index (FCIR) are higher in the large fans group (100.3° and 1.1 respectively), compared to the small fans group (83.1° and 0.69 respectively). Nevertheless, means of topographic slope (S), and width to length (W/L) ratio are higher in the small fans group (1.78% and 1.11 respectively), compared to the large fans group (1.3% and 0.73 respectively). Higher mean values of SA and FCIR in the large alluvial fans can be attributed to the higher discharge and sediment yield as well as to the lower topographic slope of large fans so that when larger discharges with higher sediment yield, produced by the occasional intense rainfalls, reach the large and low-gradient fan surface they are easily diverted across the fan, and hence increase the sweep angle and distortion of alluvial fans (increasing the SA and FCIR indexes). Results of T-test show statistically significant differences between the means of area, length, and width parameters in large and small fans. There are statistically no significant differences between the means of topographic slope, sweep angle and revised fan conicality index in large and small fans. Pearson correlation coefficients reveal that there are strong positive correlations between A-L, and A-W pairs, and strong negative correlations between S-SA, and S-W/L pairs, implying that fans with higher topographic slopes have lower values of sweep angle and width to length ratio. There is strong negative correlation between W/L and FCIR parameters showing that fans with higher values of width to length ratio have regular or more typical form. Data reveal that the mean sweep angle of study area fans is considerably high (92°) compared to fans developed around Danehkhoshk Anticline in the Zagros Folded Belt (with mean sweep angle of 49°). The high value of SA in the study area alluvial fans can be attributed to the lateral movement of Dehshir Fault so that continuous changes in the location of aggradation on fans surfaces have resulted in increasing the sweep angles of fans. Apart from the effect of lateral movements of Dehshir Fault in distortion of fans (high values of FCIR), the formation of presser ridges along Dehshir Fault and its impact on the flood diversion are also considerable.
Conclusion
Geomorphic evidence shows that Dehshir strike-slip fault as one of the most important structural features in the Central Iran structural zone has been active in Quaternary Period. In this study, 28 alluvial fans formed along Dehshir Fault were selected and their morphometric parameters including area, topographic slope, length, width, width to length ratio, sweep angle, and revised fan conicality index were analyzed. Results show that lateral movement of Dehshir Fault have resulted in increasing sweep angle, width to length ratio, and revised fan conicality index of fans. Larger fans are characterized by higher values of sweep angle and revised fan conicality index. Higher values of SA and FCIR in larger fans can be associated with higher discharge and sediment yield of large fans with lower topographic slope resulting in the fan distortion. It can be concluded that, in addition to the effect of lateral movement of Dhshir Fault in the morphometry of alluvial fans, development of pressure ridges along fault can also cause distortion of fans.
Key words: strike-slip fault, sweep angle, revised fan conicality index
Keywords
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